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Observed Changes in Aerosol Physical and Optical Properties before and after Precipitation Events


doi: 10.1007/s00376-016-5178-z

  • Precipitation scavenging of aerosol particles is an important removal process in the atmosphere that can change aerosol physical and optical properties. This paper analyzes the changes in aerosol physical and optical properties before and after four rain events using in situ observations of mass concentration, number concentration, particle size distribution, scattering and absorption coefficients of aerosols in June and July 2013 at the Xianghe comprehensive atmospheric observation station in China. The results show the effect of rain scavenging is related to the rain intensity and duration, the wind speed and direction. During the rain events, the temporal variation of aerosol number concentration was consistent with the variation in mass concentration, but their size-resolved scavenging ratios were different. After the rain events, the increase in aerosol mass concentration began with an increase in particles with diameter 0.8 m [measured using an aerodynamic particle sizer (APS)], and fine particles with diameter 0.1 m [measured using a scanning mobility particle sizer (SMPS)]. Rainfall was most efficient at removing particles with diameter 0.6 m and greater than 3.5 m. The changes in peak values of the particle number distribution (measured using the SMPS) before and after the rain events reflect the strong scavenging effect on particles within the 100-120 nm size range. The variation patterns of aerosol scattering and absorption coefficients before and after the rain events were similar, but their scavenging ratios differed, which may have been related to the aerosol particle size distribution and chemical composition.
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  • Andronache C., 2004: Estimates of sulfate aerosol wet scavenging coefficient for locations in the Eastern United States. Atmos. Environ., 38( 6), 795- 804.10.1016/j.atmosenv.2003.10.03521d44484d6bf8e9530a62b2cd7809a9bhttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231003009439http://www.sciencedirect.com/science/article/pii/S1352231003009439Scavenging of atmospheric aerosols by falling precipitation is a major removal mechanism for airborne particles. The process can be described by a wet scavenging coefficient (WSC), denoted L , that is dependent on the rainfall rate, R , and the collision efficiency between raindrops and aerosol particles, E . We report bulk average L values for location in the Eastern United States, estimated based on sulfate mass balance in the atmospheric domain of interest. Data used are taken from several observational networks: (a) the Atmospheric Integrated Research Monitoring Network (AIRMoN) which is part of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN); (b) the Interagency Monitoring of Protected Visibility Environments (IMPROVE); and (c) the National Climatic Data Center (NCDC). The results are fitted relatively well by L values computed using a microphysical representation of the WSC process based on collision efficiency and precipitation size distribution. Such representation leads to a simple expression L = f ( R ) for soluble aerosols, suitable for WSC description in regional scale models. The agreement between the bulk method and the microphysical representation is due in part to the predominant widespread precipitation, well represented by Marshall and Palmer raindrop distribution, and in part due to assumptions made in the bulk model. Results indicate that high-resolution rainfall rates and realistic vertical cloud structure information are needed to improve the accuracy of aerosol wet scavenging modeling for pollution studies.
    Belosi F., D. Contini, A. Donateo, G. Santachiara, and F. Prodi, 2012: Aerosol size distribution at Nansen Ice Sheet Antarctica. Atmos. Res., 107, 42- 50.10.1016/j.atmosres.2011.12.007e75d6b0bfe59cc1f67cb2548bab3c2d3http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0169809511004182http://www.sciencedirect.com/science/article/pii/S0169809511004182During austral summer 2006, in the framework of the XXII Italian Antarctic expedition of PNRA (Italian National Program for Research in Antarctica), aerosol particle number size distribution measurements were performed in the 10-500 range nm over the Nansen Ice Sheet glacier (NIS, 74°30' S, 163°27' E; 85 m a.s.l), a permanently iced branch of the Ross Sea. Observed total particle number concentrations varied between 169 and 1385 cm. A monomodal number size distribution, peaking at about 70 nm with no variation during the day, was observed for continental air mass, high wind speed and low relative humidity. Trimodal number size distributions were also observed, in agreement with measurements performed at Aboa station, which is located on the opposite side of the Antarctic continent to the NIS. In this case new particle formation, with subsequent particle growth up to about 30 nm, was observed even if not associated with maritime air masses.
    Berthet S., M. Leriche, J.-P. Pinty, J. Cuesta, and G. Pigeon, 2010: Scavenging of aerosol particles by rain in a cloud resolving model. Atmos. Res., 96, 325- 336.10.1016/j.atmosres.2009.09.0154c278eb5279c6ee787ad8d135124a24ahttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0169809509002725http://www.sciencedirect.com/science/article/pii/S0169809509002725We describe a below-cloud scavenging module of aerosol particles by raindrops for use in a three-dimensional mesoscale cloud resolving model. The rate of particle removal is computed by integrating the scavenging efficiency over the aerosol particle and the drop size distributions. Here the numerical integration is performed accurately with a Gauss quadrature algorithm. The efficiency of the scavenging module is partially confirmed with experimental data. More interestingly, it is illustrated by two numerical experiments: the simulation of a forced convective circulation in a tropical cloudy boundary layer and a two-dimensional simulation of an African squall line. The results show a very selective wet removal of the aerosol particles which clearly depends on the mode radius, the width and the vertical profile of concentration. Furthermore, the squall line case shows the importance of resolving internal circulations to redistribute layers of aerosol particles in order to improve estimates of particle removal by below-cloud scavenging.
    Charlson R. J., S. E. Schwartz, J. M. Hales, R. D. Cess, J. A. Coakley Jr., J. E. Hansen, and D. J. Hofmann, 1992: Climate forcing by anthropogenic aerosols. Science, 255, 423- 430.10.1126/science.255.5043.42317842894cb805ace-79cc-4a88-a300-471f69b35380037c5940a449fc755457924d88ef1409http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1126%2Fscience.255.5043.423refpaperuri:(bb479242f5b2a6cb4f370efe36a63098)http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM17842894Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of short-wavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.
    Chate D. M., 2005: Study of scavenging of submicron-sized aerosol particles by thunderstorm rain events. Atmos. Environ., 39, 6608- 6619.10.1016/j.atmosenv.2005.07.0634146154b01085c62fdd6f32a0d9e4130http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231005006874http://www.sciencedirect.com/science/article/pii/S1352231005006874Observed scavenging coefficients for 0.013–0.75μm particles are between 1.08×10 615 and 7.58×10 614 s 611 . Based on observed results a correlation between scavenging coefficient and rain intensity is obtained to study below thundercloud scavenging of atmospheric aerosols during thunderstorm rain events. When the rain intensity increases from 5.24 to 45.54mmh 611 , the corresponding scavenging coefficient increases from 0.5×10 615 to 4×10 615 s 611 for thunderstorm rain episodes. The overall scavenging coefficients for 0.02 – 10μm particles at different rainfall rates are estimated from contributions of Brownian diffusion, directional interception, inertial impaction, thermophoresis, diffusiophoresis and electrical forces during thunderstorms. The evolutions of PSD are predicted at different time intervals with theoretical scavenging rates. Comparison of observed evolutions of PSD during thunderstorm rain events with predicted evolutions of PSD shows an order of discrepancy between the observed and model results. Possible causes for discrepancy are discussed in terms of uncertainties in observed data and shortcomings in theoretical approach. The present results are useful for recommendations for the type of experimental setup essential for the field study of precipitation scavenging and improvements in theoretical approach close to atmospheric conditions during thunderstorm rain events.
    Chate D. M., P. S. P. Rao, M. S. Naik, G. A. Momin, P. D. Safai, and K. Ali, 2003: Scavenging of aerosols and their chemical species by rain. Atmos. Environ., 37( 18), 2477- 2484.10.1016/S1352-2310(03)00162-64bae6402ce18593b4e4451d4e647d2f5http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231003001626http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_JJ027767722Washout or scavenging coefficients have been widely used to study the wet deposition processes quantitatively. In the present theoretical study, the washout coefficients are computed for the aerosols of diameters in the range of 0.02-10 渭m having various densities in accordance with their chemical compositions for heavy rain regime. The theoretical scavenging rates are applied to the observed average particle size distributions of pre-monsoon months of the year 1998 and 1999 for Pune and 1999 for Himalayan regions. The evolution of particle size distributions at different time intervals for the non-hygroscopic particles of CaCO_3, MgCO_3, Zn and Mn indicates that the inertial impaction mechanism is the dominant one in removing particles of all sizes for the heavy rain regime. The size dependence of aerosols as a function of relative humidity is considered for the estimation of washout coefficients of hygroscopic particles such as NaCl and (NH_4)_2SO_4. The washout coefficients are found to be highly dependent on relative humidity for hygroscopic particles. The rainwater concentrations are predicted as a function of rainfall depth and a comparison is made with the observed rainwater concentrations of sequential samples collected on 27 June 2001 in a single rain event to support the results of this theoretical work. The predicted rainwater concentrations for RH = 50% are about two times larger than that for RH = 95% in the case of hygroscopic particles.
    Chate D. M., K. Ali, G. A. Momin, P. S. P. Rao, P. S. Praveen, P. D. Safai, and P. C. S. Devara, 2007: Scavenging of sea-salt aerosols by rain events over Arabian Sea during ARMEX. Atmos. Environ., 41, 6739- 6744.10.1016/j.atmosenv.2007.04.05172947d023ac39202bdfacf9e81d481d9http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231007004116http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_JJ024485370Scavenging coefficients are obtained for sea-salt particles at rainfall intensity of 5, 10, 15, 20 and 45 turn h(-1). Evolutions of size distributions for sea-salt particles by precipitation scavenging are simulated using theoretically estimated scavenging coefficients. Results indicate that below-cloud scavenging affects mainly sea-salt particles in coarse mode. Observed concentrations of Na+ and Cl- in rainwater increased with rainfall intensity and aerosol size. Comparison of predicted concentrations of Na+ and Cl- in rainwater with observed ionic concentrations of short-timed wet-only samples collected during rain events on 2 August 2002 over Arabian Sea (ARMEX-2002) supports the model result. (C) 2007 Elsevier Ltd. All rights reserved.
    Chate D. M., P. Murugavel, K. Ali, S. Tiwari, and G. Beig, 2011: Below-cloud rain scavenging of atmospheric aerosols for aerosol deposition models. Atmos. Res., 99, 528- 536.199a4cdbce2ce61a375d3510f4272a94http%3A%2F%2Fwww.sciencedirect.com%2Fscience%3F_ob%3DArticleURL%26md5%3Df07bbe68122f425f0bafb72fcafcf3cd%26_udi%3DB6V95-51TGFYC-2%26_user%3D6894003%26_coverDate%3D03%252F31%252F2011%26_rdoc%3D17%26_fmt%3Dhigh%26_orig%3Dbrowse%26_origin%3Dbrowse%26_zone%3Drslt_list_item%26_srch%3Ddoc-info%28%2523toc%25235889%25232011%252/s?wd=paperuri%3A%28d7e26845bdd3931b7135e8696b4b68c0%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%3F_ob%3DArticleURL%26md5%3Df07bbe68122f425f0bafb72fcafcf3cd%26_udi%3DB6V95-51TGFYC-2%26_user%3D6894003%26_coverDate%3D03%252F31%252F2011%26_rdoc%3D17%26_fmt%3Dhigh%26_orig%3Dbrowse%26_origin%3Dbrowse%26_zone%3Drslt_list_item%26_srch%3Ddoc-info%28%2523toc%25235889%25232011%252&ie=utf-8&sc_us=10002988413395947476
    Croft, B., Coauthors, 2009: Influences of in-cloud aerosol scavenging parameterizations on aerosol concentrations and wet deposition in ECHAM5-HAM. Atmos. Chem. Phys., 9( 5), 22041- 22101.10.5194/acpd-9-22041-20090ce25cff2ad76d3f2282b6a7ff279e99http%3A%2F%2Fwww.oalib.com%2Fpaper%2F1371891http://www.oalib.com/paper/1371891A diagnostic cloud nucleation scavenging scheme, which determines stratiform cloud scavenging ratios for both aerosol mass and number distributions, based on cloud droplet, and ice crystal number concentrations, is introduced into the ECHAM5-HAM global climate model. This scheme is coupled with a size-dependent in-cloud impaction scavenging parameterization for both cloud droplet-aerosol, and ice crystal-aerosol collisions. The aerosol mass scavenged in stratiform clouds is found to be primarily (>90%) scavenged by cloud nucleation processes for all aerosol species, except for dust (50%). The aerosol number scavenged is primarily (>90%) attributed to impaction. 99% of this impaction scavenging occurs in clouds with temperatures less than 273 K. Sensitivity studies are presented, which compare aerosol concentrations, burdens, and deposition for a variety of in-cloud scavenging approaches: prescribed fractions, a more computationally expensive prognostic aerosol cloud processing treatment, and the new diagnostic scheme, also with modified assumptions about in-cloud impaction and nucleation scavenging. Our results show that while uncertainties in the representation of in-cloud scavenging processes can lead to differences in the range of 20-30% for the predicted annual, global mean aerosol mass burdens, and near to 50% for accumulation mode aerosol number burden, the differences in predicted aerosol mass concentrations can be up to one order of magnitude, particularly for regions of the middle troposphere with temperatures below 273 K where mixed and ice phase clouds exist. Different parameterizations for impaction scavenging changed the predicted global, annual mean number removal attributed to ice clouds by seven-fold, and the global, annual dust mass removal attributed to impaction by two orders of magnitude. Closer agreement with observations of black carbon profiles from aircraft (increases near to one order of magnitude for mixed phase clouds), mid-troposphere 210Pb vertical profiles, and the geographic distribution of aerosol optical depth is found for the new diagnostic scavenging scheme compared to the prescribed scavenging fraction scheme of the standard ECHAM5-HAM. The diagnostic and prognostic schemes represent the variability of scavenged fractions particularly for submicron size aerosols, and for mixed and ice phase clouds, and are recommended in preference to the prescribed scavenging fractions method.
    Duhanyan N., Y. Roustan, 2011: Below-cloud scavenging by rain of atmospheric gases and particulates.. Atmos. Environ, 45, 7201- 7217.10.1016/j.atmosenv.2011.09.002ee67c727a742ba724d2abb3d95a9dc48http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231011009344http://www.sciencedirect.com/science/article/pii/S1352231011009344Below-cloud scavenging (BCS) by rain is one of the phenomena that control the removal of atmospheric pollutants from air. The present work introduces a detailed review of the most literature referred theories and parameterisations to describe the below-cloud scavenging by rain in air quality modelling. The theories and parameterisations in question concern the raindrop size distribution (RSD), the terminal velocity of raindrops, and the below-cloud scavenging coefficient for gaseous and particulate pollutants. 0D computations are run to calculate the latter coefficient with the help of the current theories and parameterisations thus extracted from the literature. As a result to improve the atmospheric modelling studies, it can be mentioned that the choice of the raindrop terminal velocity among the available parameterisations does not matter much and therefore, the practice of the most simple formulae is advised. On the other hand, a great dispersion on the scavenging coefficient (several orders of magnitude) is observed related to the variations of the RSD. Therefore, a great care is recommended in the choice of the RSD with respect to the type of rain and sampling duration involved (e.g. thunderstorm, widespread, shower, etc.; long or instantaneous sampling duration). Many uncertainties do remain due to the lack of precision in the experimental records after which the RSD parameterisations are established or to the poor level of accuracy of the theoretical models.
    Flossmann A. I., H. R. Pruppacher, and J. H. Topalian, 1987: A theoretical study of the wet removal of atmospheric pollutants. Part II: The uptake and redistribution of (NH4) 2SO4 particles and SO2 gas simultaneously scavenged by growing cloud drops. J. Atmos. Sci., 44( 20), 2912- 2923.10.1175/1520-0469(1987)0442.0.CO;223ebd949-086c-47a7-99c4-5395cfe3b3691642913e6e578dce0be5c1e850de7b45http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F249608966_Theoretical_study_of_the_wet_removal_of_atmospheric_pollutants._Part_II_The_uptake_and_redistribution_of_%28NH%29SO_particles_and_SO_gas_simultaneously_scavenged_by_growing_cloud_dropsrefpaperuri:(88d88cb7c12ee91b19a97ecfb56f9ce1)http://www.researchgate.net/publication/249608966_Theoretical_study_of_the_wet_removal_of_atmospheric_pollutants._Part_II_The_uptake_and_redistribution_of_(NH)SO_particles_and_SO_gas_simultaneously_scavenged_by_growing_cloud_dropsABSTRACT A theoretical model has been formulated which allows the processes which control the wet deposition of atmospheric aerosol particles and pollutant gases to be included in cloud dynamic models. The cloud considered in the model was allowed to grow by condensation and collision--coalescence, to remove aerosol particles by nucleation and impaction scavenging, and to remove pollutant gases by convective diffusion. The model was tested by using a simple air-parcel model as the dynamic framework. In this form the model was used to determine the fate of ammonium sulfate ((NHâ)âSOâ) particles and sulfur dioxide (SOâ) gas as they became scavenged by cloud and precipitation drops. Special emphasis was placed on determining 1) the evolution with time of the mass of total sulfur as S(IV) and S(VI) inside the drops, 2) the evolution with time of the acidity of the cloud water as a function of various oxidation rates and as a function of drop size, 3) the relative importance of sulfur scavenging from SOâ as compared to sulfur scavenging from (NHâ)âSOâ particles, and 4) the effect of cloud drop evaporation on the aerosol particle size distribution in the air.
    Gon\ccalves, F. L. T., A. M. Ramos, S. Freitas, M. A. S. Dias, O. Massambani, 2002: In-cloud and below-cloud numerical simulation of scavenging processes at Serra Do Mar region, SE Brazil. Atmos. Environ., 36( 33), 5245- 5255.10.1016/S1352-2310(02)00461-2dc015c612bd8666d889ac4af3dfd1081http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231002004612http://www.sciencedirect.com/science/article/pii/S1352231002004612Atmospheric scavenging processes have been investigated, taking into consideration a numerical simulation through the model Regional Atmospheric Modeling System (RAMS), the below-cloud scavenging model, local atmospheric conditions and local emissions in the Serra do Mar region in southeastern Brazil. The RAMS modeling was coupled with a one-dimensional (1-D) below-cloud scavenging model in order to simulate the in-cloud and below-cloud scavenging processes. RAMS modeling was also used in order to simulate the cloud structures. The aim of the modeling was to predict the average concentration of three chemical species found in rainwater: SO , NO and NH , scavenged from the atmosphere. The concentrations of gases and particles in the samplings, as well as the meteorological parameters obtained during the March 1993 Campaign, were the input data in both models. Another objective was to compare the modeled and the observed rainwater and determine the variability in concentration. Rainwater was obtained by using fractionated rain samplers. Variability was determined through chemical analysis. Urban and rural aerosol spectra modeling were also used in order to compare the rainwater concentration species variability. When both in-cloud and below-cloud processes are included, the general result of the March 1993 events presents a better agreement between modeled and observed data sets than only below-cloud . Preliminary results lead us to conclude that the rainwater variability of nitrate is explained by the scavenging of particles from urban spectrum size distribution, whereas rural spectra explain ammonium rainwater variability-攊ndicating the different sources of those species. Comparing to the March 1992 events, these case studies present a significant contribution from the in-cloud scavenging, supported by the Weather Radar maps and RAMS modeling. In particular, the almost constant rainwater concentrations on 16 March (an indication of strong in-cloud contribution) are related to the rainfall event, which crossed the study area on that day. These results add an important understanding to the atmospheric wet removal processes in the region studied.
    Gon\ccalves, F. L. T., W. N. Morinobu, M. F. Andrade, A. Fornaro, 2007: In-cloud and below-cloud scavenging analysis of sulfate in the metropolitan area of S\ ao Paulo, Brasil. Revista Brasileira De Meteorologia, 22, 94- 104.10.1590/S0102-778620070001000102ffdb0cf-9db8-4af1-84d5-5a2f35642dcac07307674fb7e6d18656b5224c9e762fhttp%3A%2F%2Fwww.oalib.com%2Fpaper%2F1022587refpaperuri:(baea71269674dbfa8b54b87340f7484a)http://www.oalib.com/paper/1022587The Metropolitan Area of So Paulo (MASP) is one of the largest urban centers in the world. The significant atmospheric concentrations of ozone, inhalable particles and other pollutants in the MASP raise serious air-quality concerns. In this study, we consider gases, particulate matter (PM) and cloud processes, with a focus on sulfate chemistry. The Regional Atmospheric Modeling System mesoscale numerical model was used in conjunction with detailed scavenging models to compare varying PM mass spectra and size distributions. Field data were collected during the July 1989-May 1990 and February-October 2000 campaigns. Adjusted-urban and rural spectra seem to fit better with observed results which improved the scavenging numerical modeling. Correlations between modeled and observed concentrations were better when the model included rural and adjusted-urban spectra, suggesting locally dominant below-cloud scavenging. Spatial variability analysis and numerical modeling also revealed that the varying sulfate rainwater concentrations indicate below-cloud removal process dominance.
    Hu M., J. Zhang, and Z. J. Wu, 2005: Chemical compositions of precipitation and scavenging of particles in Beijing. Science in China Series B: Chemistry, 48, 265- 272.10.1360/042004-49444d2c0433d849390eb9910aae3e100dhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1360%2F042004-49http://d.wanfangdata.com.cn/Periodical_zgkx-eb200503012.aspxWet deposition is the scavenging approach of pollutants from the atmosphere. Rain in summer and snow in winter are the main scavenging processes of air pollutants by wet deposition in Beijing and key factors of changing air pollutants concentrations[1].
    Jiang Q., Y. Yin, Y. S. Qin, K. Chen, and S. Y. Yang, 2013: Numerical simulation study on hygroscopic growth of aerosols in Huangshan area. Journal of the Meteorological Sciences, 33( 3), 237- 245. (in Chinese)a3554963c8eeb8b10f1b53e6344d9ae0http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-QXKX201303000.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-QXKX201303000.htmBy using adiabatic air parcel method of multi-component aerosols,this paper analyzes hygroscopic growth characteristics of aerosols in Huangshan area in 2008.Some calculations show that The hygroscopic growth factor f defined as d p,wet/d p,dry,where d p,wet is the particle mobility diameter under a humidified condition,is closely bound up with particle radius,relative humidity(RH),particle chemical components,vertical velocity and lifting height.The hygroscopic growth of small particles is more significant than that of the large ones.Relative humidity is positively related with the hygroscopic growth factor f.The more relative humidity approaches the critical relative humidity of particles,the more the hygroscopic growth factor f changes with relative humidity.Through increasing the solute soluble inorganic aerosol influences critical saturated ratio so as to make the hygroscopic growth factor f increase.If the role of unsolvable nucleation doesn't be taken into account,the hygroscopic growth of particles will be overestimated.With the velocity rising,the hygroscopic growth factor reduces,and the reduction degree depends on initial relative humidity.The lifting height can influence the hygroscopic growth factor through the change of relative humidity.
    Jung C. H., Y. P. Kim, and K. W. Lee, 2003: A moment model for simulating raindrop scavenging of aerosols. J. Aerosol Sci., 34, 1217- 1233.10.1016/S0021-8502(03)00098-3fd5133fbca76a2201e9df1565b300089http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0021850203000983http://www.sciencedirect.com/science/article/pii/S0021850203000983ABSTRACT The dynamics of a polydispersed aerosol size distribution, scavenged by precipitation, are numerically studied. The collision efficiency formula proposed by Slinn (Precipitation Scavenging in Atmospheric Sciences and Power Production—1979, Division of Biomedical Environmental Research, US Department of Energy, Washington, DC, USA, 1983, Chapter 11) and the moment method were introduced to represent the particle removal mechanism by raindrops and the aerosol size distribution, respectively. Consequently, the dynamics of the particle size distribution were reduced to a set of ordinary differential equations using the moment approach. A generalized raindrop distribution, including two widely used distributions; the Marshall–Palmer (MP) and Krigian–Mazin (KM) raindrop distributions, was adopted.Our model results have shown that raindrops with smaller diameters, and narrower distributions, collect aerosols more efficiently. Further, it was shown, in the small particle size region that the geometric mean diameter increases, while in the large particle region it decreases. For the two size ranges, the geometric standard deviations decrease with time, and a scavenging gap, the minimum particle removal efficiency region, exists between these particle size ranges.The dynamics of the particle size distributions, the MP and KM raindrop distributions, in the small particle range, show that the effects of the overestimation in the MP distribution were not as great as expected. Also, this study ascertained that the conventional parameterization of the constant collision efficiency introduces significant errors for estimating the particle size distribution dynamics by wet scavenging.
    Jung C. H., S. Y. Bae, and Y. P. Kim, 2011: Approximated solution on the properties of the scavenging gap during precipitation using harmonic mean method. Atmos. Res., 99, 496- 504.10.1016/j.atmosres.2010.11.0230e331f004970e5c877d69ba968823bf7http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0169809510003376http://www.sciencedirect.com/science/article/pii/S0169809510003376Wet deposition refers to both natural and artificial processes where particles are scavenged by atmospheric hydrometeors. Below-cloud atmospheric particles are removed by raindrops via Brownian diffusion, interception, and impaction. The overall scavenging coefficient has a broad and distinctive minimum for aerosol penetration between 0.1 and several micrometers in diameter. In this study, the approximated analytical solution for most penetrating particle size during precipitation was obtained. Brownian diffusion and interception were considered under the assumption of the inertial impaction can be neglected in this study conditions. Both the minimum collection efficiency and minimum scavenging coefficient particle size were estimated using the harmonic mean type approximation, with the solution compared to the numerically calculated results. The approximated results were comparable with the numerical solutions. The results showed that collection efficiency diameter is a function of terminal velocity and the collection mechanisms included. When considering Brownian diffusion and interception, most penetrating particle size increases as drop diameter increases, which shows a contrary to the study of Wang (1978) and this shows that most penetrating particle size depends on collection efficiency mechanism, flow velocity and collector diameter. Consequently, this study analytically approximated general type-solutions for scavenging gap particle size and minimum collection efficiency during precipitation.
    Kaufman Y. J., D. Tanrè, and O. Boucher, 2002: A satellite view of aerosols in the climate system. Nature, 419, 215- 223.10.1038/nature0109112226676d31470b9-32fc-498d-a61a-6c737a7a6c20e8c6ea1f8402360d05af57e982e4758ahttp%3A%2F%2Fwww.nature.com%2Fnature%2Fjournal%2Fv419%2Fn6903%2Fabs%2Fnature01091.htmlrefpaperuri:(e4e2fb0c2868c479bfb1b7209c0ecae2)http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM12226676Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.
    Khain A. P., M. B. Pinsky, 1997: Turbulence effects on the collision kernel. II: Increase of the swept volume of colliding drops. Quart. J. Roy. Meteor. Soc., 123, 1543- 1560.10.1002/qj.49712354205afeefa4843c5dbf8dfef2589f7cacfa3http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.49712354205%2Fcitedby/s?wd=paperuri%3A%282fe6472b232e7db1e1def2e7a043d082%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.49712354205%2Fcitedby&ie=utf-8&sc_us=1853754466474395562Not Available
    Khoshsima M., F. Ahmadi-Givi, A. A. Bidokhti, and S. Sabetghadam, 2014: Impact of meteorological parameters on relation between aerosol optical indices and air pollution in a sub-urban area. J. Aerosol Sci., 68, 46- 57.10.1016/j.jaerosci.2013.10.008f9ffaff5-551c-4fef-9df6-39ae6687516d2f1cf9e93caf7e371363e9b058f3d070http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0021850213002206refpaperuri:(f05186a12c15be18c659da1c9c2215fc)http://www.sciencedirect.com/science/article/pii/S0021850213002206ABSTRACT Aerosol optical depth (AOD) provides a useful characterization of the total absorption and scattering effect of particles in direct or scattered sunlight, and can be derived from sun spectra measured directly by sun photometers. In this paper, atmospheric optical properties (e.g. AOD440–1020 nm, α and β, the coefficients in Angstrom formula) and meteorological conditions are presented for: summer (July-August-September) and winter (December-January-February-March) of 2009–2010 over Zanjan (36.41° N, 48.29° E) in northwestern Iran. The diurnal variation of AOD in Zanjan is approximately 15%. An exponential dependence of α on AOD in winter indicates that dust aerosols are major contributions of atmospheric turbidity in this region. AOD regressed against PM10 to establish prediction models. The role of three meteorological parameters on the correlation of AOD and PM10 are analyzed. Results show that there is a high correlation between AOD440 and PM10 in wintertime, and β is a better indicator of air quality in winter than in summer for the study region considered here. Hourly analysis shows that this correlation is highest in the afternoon when the atmospheric mixed layer is at its highest thickness. A similar behavior for AOD-PM10 and a correlation between optical properties with NO2 and PM10 are detected. A sensitivity study was designed to quantify the role of meteorological properties, such as relative humidity, wind speed, and temperature, on the correlation between AOD and PM10 concentration.
    Kulshrestha U. C., L. A. K. Reddy, J. Satyanarayana, and M. J. Kulshrestha, 2009: Real-time wet scavenging of major chemical constituents of aerosols and role of rain intensity in Indian region. Atmos. Environ., 43( 32), 5123- 5127.10.1080/1058725010802571990462d7202ef7ab00fc3e177d3c4f29chttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231009006098/s?wd=paperuri%3A%284f513cb7058df9eebe471b17f13065c4%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fmed.wanfangdata.com.cn%2FviewHTML%2FPeriodicalPaper_JJ0211339642.aspx&ie=utf-8&sc_us=12866250165894002850Real-time simultaneous studies on chemical characteristics of rainwater and PM_(10) aerosols were carried out to understand the scavenging of major chemical components in Indian region. The concentrations of Ca~(2+), NH_4~+, SO_4~(2-) and NO_3~- were observed to be lower in the aerosol samples collected during rain as compared to before and after rain events. The most significant reduction was noticed for Ca~(2+) (74%) during rain which showed highest scavenging ratio (SR) and indicated that below-cloud scavenging is an effective removal process for Ca~(2+) in Indian region. Among non-sea salt components, Ca~(2+) had highest SR at Hyderabad indicating typical characteristics of crustal influence as abundance of calcium carbonate in soil dust has been reported in India. However, the levels of these major chemical components gradually got build-up in due course of time. After rain events, the levels of SO_4~(2+) aerosols were noticed to be substantially higher (more than double) within 24 h. In general, scavenging ratios for all components (except Ca~(2+), NH_4~+ and K~+) were higher over BOB as compared to Hyderabad. The maximum fall in aerosol levels (BR minus AR) was observed during continuous and low intensity rain events that did not allow building up of aerosol concentrations.
    Li, Z. K, Y. X. Pan, R. Q. Sun, 1985: The Principle and Application of Air Pollution Meteorology. China Meteorological Press, 598 pp. (in Chinese)
    Loosmore G. A., R. T. Cederwall, 2004: Precipitation scavenging of atmospheric aerosols for emergency response applications: Testing an updated model with new real-time data. Atmos. Environ., 38, 993- 1003.10.1016/j.atmosenv.2003.10.0551989d1f32145496103ad0cb9842f2ea0http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231003009506http://www.sciencedirect.com/science/article/pii/S1352231003009506Precipitation scavenging can effectively remove particulates from the atmosphere. Interest in the phenomena waxed in the 1980s, but models developed at that time remain limited by the lack of both detailed, time-resolved wet deposition pattern measurements for model confirmation and real-time rain data for model execution. Recently, new rain products have become available that can revolutionize real-time use of precipitation scavenging models on the regional scale. We have utilized a 4-km, hourly resolution precipitation data set from the Arkansas Red-Basin River Forecast Center. A standard below-cloud aerosol scavenging model has been modified to incorporate the potentially larger scavenging in heavy rain events. This paper demonstrates the model on a sample rainfall data set. The simulations demonstrate the concentrating effect of rainfall, especially heavy rain, on deposition patterns. Wet deposition played an important role in the simulated fate and transport, removing as much as 70% of the released aerosol.
    Menon S., J. Hansen, L. Nazarenko, and Y. F. Luo, 2002: Climate effects of black carbon aerosols in China and India. Science, 297, 2250- 2253.10.1126/science.107515912351786ee899009eba3a609764201f2cf3bd918http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FPMED%3Fid%3D12351786http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM12351786In recent decades, there has been a tendency toward increased summer floods in south China, increased drought in north China, and moderate cooling in China and India while most of the world has been warming. We used a global climate model to investigate possible aerosol contributions to these trends. We found precipitation and temperature changes in the model that were comparable to those observed if the aerosols included a large proportion of absorbing black carbon ("soot"), similar to observed amounts. Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the large-scale circulation and hydrologic cycle with significant regional climate effects.
    Mircea M., S. Stefan, and S. Fuzzi, 2000: Precipitation scavenging coefficient: Influence of measured aerosol and raindrop size distributions. Atmos. Environ., 34, 5169- 5174.10.1016/S1352-2310(00)00199-009f273f3987876d8f1b26862d801f03dhttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231000001990http://www.sciencedirect.com/science/article/pii/S1352231000001990Precipitation scavenging coefficients, widely used in pollution studies, are derived from microphysical parameterisations of aerosol particles and raindrop populations and parameterisations of their interactions. The present study investigates the effects of measured aerosol and raindrop size distributions in a microphysical polydisperse framework. The interactions between aerosol and raindrops parameterised as collision efficiency are explicitly included to account for Brownian diffusion, inertial impaction and interception. Estimated values of the polydisperse scavenging coefficients exhibit variations of orders of magnitude depending on the aerosol type and almost no variation with the raindrop size distributions. For practical use, linear relationships between the scavenging coefficients and rain intensity for different aerosol types are derived.
    Pan, L., Coauthors, 2010: Aerosol optical properties based on ground measurements over the Chinese Yangtze Delta Region. Atmos. Environ., 44, 2587- 2596.10.1016/j.atmosenv.2010.04.013ba1810c4990db075e7b7ce473d51cb78http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231010002955http://www.sciencedirect.com/science/article/pii/S1352231010002955The analysis results between aerosol optical properties and wind measurement at Pudong showed that the wind speed from the east correlates with the lower observed AOD. The back trajectory analysis indicates that more than 50% airmasses were from the marine area at Pudong, while back trajectories distribution is relatively homogeneous at Lin’an.
    Pinsky M., M. Shapiro, A. P. Khain, and A. Pokrovsky, 2000: Investigation of the process of in- and below cloud aerosol scavenging from the atmosphere. J. Aerosol Sci., 31, 295- 296.10.1016/S0021-8502(00)90305-7e6d686908716442b7844175605629705http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0021850200903057http://www.sciencedirect.com/science/article/pii/S0021850200903057
    Qiu J. H., D. R. Lv, H. B. Chen, G. C. Wang, and G. Y. Shi, 2003: Modern research progresses in atmospheric physics. Chinese J. Atmos. Sci., 27, 628- 652. (in Chinese)8a763ca5f3bd1839076215b4b7c23810http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-DQXK200304013.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-DQXK200304013.htmThis paper briefly surnmarizes modern research content of the atmospheric physics and its development history, and it emphatically treats out research progresses and results (especially innovate results) in five fields of the atmospheric physics, contributed by scientists in the Institute of Atmospheric Physics, Chinese Academy of Sciences. These research fields include interaction between cloud and radiation, aerosol optical properties and its radiative forcing, atmospheric radiative transfer, and atmospheric composition measurements.
    Ramanathan V., P. J. Crutzen, J. T. Kiehl, and D. Rosenfeld, 2001: Aerosols, climate, and the hydrological cycle. Science, 294, 2119- 2124.10.1126/science.10640341748932652025203185d349b87b243e399b7948b4f89ce84fbhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FPMED%3Fid%3D11739947http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM11739947Human activities are releasing tiny particles (aerosols) into the atmosphere. These human-made aerosols enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.
    Rasch, P. J., Coauthors, 2000: A comparison of scavenging and deposition processes in global models: results from the WCRP Cambridge Workshop of 1995. Tellus B, 52, 1025- 1056.10.1034/j.1600-0889.2000.00980.xe3c95670-ebc2-4f38-86dd-cc8f3e892801cb720760ebb7e06f00c2340597cee413http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1034%2Fj.1600-0889.2000.00980.x%2Ffullrefpaperuri:(51425b8d632cdeb43fac0246fd489916)http://onlinelibrary.wiley.com/doi/10.1034/j.1600-0889.2000.00980.x/fullWe report on results from a World Climate Research Program workshop on representations of scavenging and deposition processes in global transport models of the atmosphere. 15 models were evaluated by comparing simulations of radon, lead, sulfur dioxide, and sulfate against each other, and against observations of these constituents. This paper provides a survey on the simulation differences between models. It identifies circumstances where models are consistent with observations or with each other, and where they differ from observations or with each other. The comparison shows that most models are able to simulate seasonal species concentrations near the surface over continental sites to within a factor of 2 over many regions of the globe. Models tend to agree more closely over source (continental) regions than for remote (polar and oceanic) regions. Model simulations differ most strongly in the upper troposphere for species undergoing wet scavenging processes. There are not a sufficient number of observations to characterize the climatology (long-恡erm average) of species undergoing wet scavenging in the upper troposphere. This highlights the need for either a different strategy for model evaluation (e.g., comparisons on an event by event basis) or many more observations of a few carefully chosen constituents.
    Santachiara G., F. Prodi, and F. Belosi, 2013: Atmospheric aerosol scavenging processes and the role of thermo- and diffusio-phoretic forces. Atmos. Res., 128, 46- 56.10.1016/j.atmosres.2013.03.00498906d53bb5b5753a9897c368dea3592http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0169809513000884http://www.sciencedirect.com/science/article/pii/S0169809513000884A decrease in scavenging efficiency as a function of crystal diameter is reported both theoretically and experimentally. By comparing aerosol scavenging by drops and snow, most studies agree that, in terms of equal mass of precipitation, snow is more efficient at scavenging atmospheric particles than rain.
    Schumann T., 1991: Aerosol and hydrometeor concentrations and their chemical composition during winter precipitation along a mountain slope II. Size-differentiated in-cloud scavenging efficiencies. Atmospheric Environment. Part A: General Topics, 1991, 25, 809- 824.
    Sparmacher H., K. Fülber, and H. Bonka, 1993: Below-cloud scavenging of aerosol particles: Particle-bound radionuclides閳ユ柡锟芥摗xperimental. Atmos. Environ., 27, 605- 618.
    Textor, C., Coauthors, 2006: Analysis and quantification of the diversities of aerosol life cycles within AeroCom. Atmos. Chem. Phys.,6, 1777-1813, doi: 10.5194/acp-6-1777-2006.10.5194/acp-6-1777-2006ab6ce4b961186d785d340d441ec32677http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.5194%2Facpd-5-8331-2005http://onlinelibrary.wiley.com/resolve/reference/XREF?id=10.5194/acpd-5-8331-2005Simulation results of global aerosol models have been assembled in the framework of the AeroCom intercomparison exercise. In this paper, we analyze the life cycles of dust, sea salt, sulfate, black carbon and particulate organic matter as simulated by sixteen global aerosol models. The differences among the results (model diversities) for sources and sinks, burdens, particle sizes, water uptakes, and spatial dispersals have been established. These diversities have large consequences for the calculated radiative forcing and the aerosol concentrations at the surface. Processes and parameters are identified which deserve further research. The AeroCom all-models-average emissions are dominated by the mass of sea salt (SS), followed by dust (DU), sulfate (SO4), particulate organic matter (POM), and finally black carbon (BC). Interactive parameterizations of the emissions and contrasting particles sizes of SS and DU lead generally to higher diversities of these species, and for total aerosol. The lower diversity of the emissions of the fine aerosols, BC, POM, and SO4, is due to the use of similar emission inventories, and does therefore not necessarily indicate a better understanding of their sources. The diversity of SO4-sources is mainly caused by the disagreement on depositional loss of precursor gases and on chemical production. The diversities of the emissions are passed on to the burdens, but the latter are also strongly affected by the model-specific treatments of transport and aerosol processes. The burdens of dry masses decrease from largest to smallest: DU, SS, SO4, POM, and BC. The all-models-average residence time is shortest for SS with about half a day, followed by SO4 and DU with four days, and POM and BC with six and seven days, respectively. The wet deposition rate is controlled by the solubility and increases from DU, BC, POM to SO4 and SS. It is the dominant sink for SO4, BC, and POM, and contributes about one third to the total removal of SS and DU species. For SS and DU we find high diversities for the removal rate coefficients and deposition pathways. Models do neither agree on the split between wet and dry deposition, nor on that between sedimentation and other dry deposition processes. We diagnose an extremely high diversity for the uptake of ambient water vapor that influences the particle size and thus the sink rate coefficients. Furthermore, we find little agreement among the model results for the partitioning of wet removal into scavenging by convective and stratiform rain. Large differences exist for aerosol dispersal both in the vertical and in the horizontal direction. In some models, a minimum of total aerosol concentration is simulated at the surface. Aerosol dispersal is most pronounced for SO4 and BC and lowest for SS. Diversities are higher for meridional than for vertical dispersal, they are similar for the individual species and highest for SS and DU. For these two components we do not find a correlation between ve
    Wang X., L. Zhang, and M. D. Moran, 2010: Uncertainty assessment of current size-resolved parameterizations for below-cloud particle scavenging by rain. Atmos. Chem. Phys.,10, 5685-5705, doi: 10.5194/acp-10-5685-2010.10.5194/acp-10-5685-201024a2156dfe8cb74d82b35fb1d6f116a6http%3A%2F%2Fwww.oalib.com%2Fpaper%2F2696397http://www.oalib.com/paper/2696397Current theoretical and empirical size-resolved parameterizations of the scavenging coefficient (Λ), a parameter commonly used in aerosol transport models to describe below-cloud particle scavenging by rain, have been reviewed in detail and compared with available field and laboratory measurements. Use of different formulations for raindrop-particle collection efficiency can cause uncertainties in size-resolved Λ values of one to two orders of magnitude for particles in the 0.01–3 μm diameter range. Use of different formulations of raindrop number size distribution can cause Λ values to vary by a factor of 3 to 5 for all particle sizes. The uncertainty in Λ caused by the use of different droplet terminal velocity formulations is generally small than a factor of 2. The combined uncertainty due to the use of different formulations of raindrop-particle collection efficiency, raindrop size spectrum, and raindrop terminal velocity in the current theoretical framework is not sufficient to explain the one to two order of magnitude under-prediction of Λ for the theoretical calculations relative to the majority of field measurements. These large discrepancies are likely caused by additional known physical processes (i.e, turbulent transport and mixing, cloud and aerosol microphysics) that influence field data but that are not considered in current theoretical Λ parameterizations. The predicted size-resolved particle concentrations using different theoretical Λ parameterization can differ by up to a factor of 2 for particles smaller than 0.01 μm and by a factor of >10 for particles larger than 3 μm after 2–5 mm of rain. The predicted bulk mass and number concentrations (integrated over the particle size distribution) can differ by a factor of 2 between theoretical and empirical Λ parameterizations after 2–5 mm of moderate intensity rainfall.
    Xia X. G., H. B. Chen, Z. Q. Li, P. C. Wang, and J. K. Wang, 2007a: Significant reduction of surface solar irradiance induced by aerosols in a suburban region in northeastern China. J. Geophys. Res., 112,D22S02, doi: 10.1029/2006JD007562.10.1029/2006JD0075628f978fce6132b75461c004670183714fhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2006JD007562%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1029/2006JD007562/abstractIn the spring of 2005, a Sun photometer and a set of broadband pyranometers were installed in Liaozhong, a suburban region in northeastern China. Aerosol properties derived from Sun photometer measurements and aerosol-induced changes in downwelling shortwave surface irradiances are analyzed in this paper. It is shown that the mean aerosol optical depth (AOD) at 500 nm is 0.63. The day-to-day variation of aerosol optical depth is dramatic, with a maximum daily AOD close to 2.0 and a minimum value close to the background level. Dust activities generally produce heavy aerosol loading characterized by larger particle sizes and less absorption than those observed under normal conditions. The reduction of instantaneous direct shortwave surface irradiance per unit of AOD is 404.5 W m. About 63.8% of this reduction is offset by an increase in diffuse irradiance; consequently, one unit increase in AOD leads to a decrease in global surface irradiance of 146.3 W m. The diurnal aerosol direct radiative forcing efficiency is about -47.4 W m. Overall, aerosols reduce about 30 W mper day of surface net shortwave irradiance in this suburban region.
    Xia X. G., Z. Q. Li, B. Holben, P. C. Wang, T. Eck, H. B. Chen, M. Cribb, and Y. X. Zhao, 2007b: Aerosol optical properties and radiative effects in the Yangtze Delta region of China. J. Geophys. Res., 112,D22S12, doi: 10.1029/2007JD008859.10.1029/2007JD00885908288d96398caab233bebec4e60460c0http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2007JD008859%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2007JD008859/fullOne year's worth of aerosol and surface irradiance data from September 2005 to August 2006 were obtained at Taihu, the second supersite for the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE). Aerosol optical properties derived from measurements by a Sun photometer were analyzed. The aerosol data were used together with surface irradiance data to quantitatively estimate aerosol effects on surface shortwave radiation (SWR) and photosynthetically active radiation (PAR). The annual mean aerosol optical depth at 500 nm is 0.77, and mean ngstrom wavelength exponent is 1.17. The annual mean aerosol single scattering albedo and mean aerosol asymmetry factor at 440 nm are 0.90 and 0.72, respectively. Both parameters show a weak seasonal variation, with small values occurring during the winter and larger values during the summer. Clear positive relationships between relative humidity and aerosol properties suggest aerosol hygroscopic growth greatly modifies aerosol properties. The annual mean aerosol direct radiative forcing at the surface (ADRF) is -38.4 W mand -17.8 W mfor SWR and PAR, respectively. Because of moderate absorption, the instantaneous ADRF at the top of the atmosphere derived from CERES SSF data is close to zero. Heavy aerosol loading in this region leads to -112.6 W mand -45.5 W mreduction in direct and global SWR, but 67.1 W mmore diffuse SWR reaching the surface. With regard to PAR, the annual mean differences in global, direct and diffuse irradiance are -23.1 W m, -65.2 W mand 42.1 W mwith and without the presence of aerosol, respectively.
    Yang J., B. Zhu, and Z. H. Li, 2001: Physicochemical properties of atmospheric aerosol particles at Zetang and Jinghong of China. Acta Meteorologica Sinica, 59, 795- 802. (in Chinese)36018008a92c969d4de551d99018c190http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-QXXB200106015.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-QXXB200106015.htmAtmospheric aerosol particles were measured at Jinghong (Yunnan Province)and Zetang (Ti bet Region )meteorological observatories in the winter 1997/1998, and their physicochemical properties, such as mass concentration, size distribution, optical absorption coefficient and chemical composition, were analyzed. Results show that aerosol particles at the two sites have significant physical and chemical difference and also different from the results at other locations. Zetang has a low number density with a relative high mass concentration. This study has great practical importance to add to our knowledge of Chinese aerosol distribution and their effects on the regional climate.
    Yin Y., C. Chen, K. Chen, J. L. An, W. W. Wang, Z. Y. Lin, J. D. Yan, and J. Wang, 2010: An observational study of the microphysical properties of atmospheric aerosol at Mt. Huang. Transactions of Atmospheric Sciences, 33, 129- 136. (in Chinese)10.1016/j.enpol.2013.04.025b62c0309-d2b8-4ad1-91f8-3c022f69f2d43bd6e19219d303702d7fd055f790f582http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-NJQX201002003.htmrefpaperuri:(4061b891e5923ceea68d9fdf8fa3813a)http://en.cnki.com.cn/Article_en/CJFDTotal-NJQX201002003.htmBased on observational data of atmospheric aerosol from April to July 2008 at the top of Mt.Huang,the characteristics of aerosol particles such as number concentration,size distribution and its relationship with meteorological factors were analyzed and a comparison was made between cloudy/foggy and clear weather conditions.The results show that the mean number concentration reaches 3.14×103 cm-3 in spring,and 1.80×103 cm-3 in summer,respectively,and ultra fine particles(smaller than 0.1 μm in diameter) account for 79% and 68%,respectively,in the total number concentration in spring and summer.It is also shown that the size distribution of aerosol particles all appear as a single mode spectrum in spring and summer,with the peak value concentrating at particle sizes of 0.04—0.12 μm and that the accumulation mode particles(0.1—1.0 μm in diameter) dominate in the volume and surface distributions.It is found that the concentration of fine particles is higher under non-foggy weather conditions as compared with foggy periods and that particle concentration is positively correlated with the air temperature and negatively correlated with relative humidity.The results also show that while northwest and southerly winds dominate in spring,the particle concentration is highest when it is northwest.In summer,high particle concentration is observed when the wind blows from the east.
    Yoo, J.-M., Coauthors, 2014: New indices for wet scavenging of air pollutants (O3, CO, NO2, SO2, and PM10 by summertime rain. Atmos. Environ., 82, 226- 237.cb0561fb-09a4-422b-bac8-43ec0ebfbd92a10590fbf7b0b21859a05d1b80394eb5http%3A%2F%2Fwww.dbpia.co.kr%2FArticle%2FNODE02408961http://www.dbpia.co.kr/Article/NODE02408961This study has analyzed the concentration variation of four air pollutants (PM, NO60, CO, and SO60) during the typhoon periods over 10 years (2002~2011). In this study, 10 typhoon events which had rainfalls in Korean Peninsula were selected during the study period. The analysis was performed using the observation data of both the air pollutants and rainfall. In order to examine and compare the concentrations of the air pollutants between normal periods and typhoon periods, we have obtained monthly average concentrations from July to September and daily average concentrations during typhoon periods. For the period from July to September, 34% of the total rainfalls can be explained by typhoons, and the concentration of air pollutants during the typhoon period was lower than the normal period. In addition, the concentration variations of the pollutants during the typhoon period were analyzed according to two categories: differences in the concentrations between the day before and the day of the typhoon (Case 1) and between the day before and after the typhoon (Case 2). The results indicated that the reduction rate of PM, NO60, CO, and SO60 was 30.1%, 17.9%, 11.6%, 9.7% (Case 1) and 22.8%, 21.0%, 9.0%, 8.0% (Case 2), respectively. This result suggested that air quality was significantly improved during the typhoon period than after the typhoon period by the rainfall.
    Zhang L. M., D. V. Michelangeli, and P. A. Taylor, 2004: Numerical studies of aerosol scavenging by low-level, warm stratiform clouds and precipitation. Atmos. Environ., 38, 4653- 4665.10.1016/j.atmosenv.2004.05.04258dde0ed0f8b364a4ea91e83fd17129dhttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231004005369http://doi.med.wanfangdata.com.cn/10.1007/978-0-387-75865-7_26Numerical studies have been performed to investigate aerosol scavenging by low-level, warm stratiform clouds and precipitation using a one-dimensional model with detailed cloud microphysics and size resolved aerosol particles and hydrometeors. Activation processes remove most aerosol mass within the cloud layer despite the very low supersaturation, since a large fraction of the aerosol mass is associated with large aerosols which can be quickly activated into cloud droplets. Impaction scavenging inside the cloud layer removes little aerosol mass; however, this process removes aerosols as high as 50% in number during a period of a few hours. Total in-cloud scavenging removes more than 70% of aerosols in number and more than 99% in mass. Below cloud scavenging is linked to aerosol concentration and size distribution, precipitation intensity and droplet spectra. During a 4-h period, weak precipitation having less than 0.1 mm h intensity can remove 50-80% of the below-cloud aerosol in both number and mass. Scavenging coefficients for large particles vary significantly with precipitation rates and/or droplet mean radii while for small particles such variation is not apparent. As a result, bulk aerosol mass-scavenging coefficients depend strongly on precipitation intensity while bulk number scavenging coefficients have less dependence. A dependence of scavenging coefficients for all size particles on total droplet surface area is found to be possible and such dependence is stronger for smaller particles. With the same precipitation amount, precipitation with more small droplets can remove aerosols more effectively due to larger total droplet surface area. Size-resolved scavenging coefficients have to be used in order to correctly track both aerosol number and mass distributions. It is suggested that parameterizations for bulk or size-resolved scavenging coefficients should be a function of other precipitation properties as well as precipitation intensity.
    Zhang X. L., Y. Huang, and R. Rao, 2012: Aerosol characteristics including fumigation effect under weak precipitation over the southeastern coast of China. Journal of Atmospheric and Solar-Terrestrial Physics,84-85, 25- 36.10.1016/j.jastp.2012.05.00577578e614e51c33e9a3d2b1dcedd717fhttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1364682612001289http://www.sciencedirect.com/science/article/pii/S1364682612001289Aerosol size distribution, total number concentration, scattering coefficient and absorption coefficient were measured in Quanzhou on the southeastern coast of China, from December 13, 2010 to January 16, 2011. Five light-rain events were analyzed for statistical study and one typical light-rain process was chosen as the case study for analysis in detail. The study focuses on the influence of weak precipitation on aerosol light-scattering and absorption properties as well as the size distribution. Similar size distributions were observed between clear-day regime and light-rain regime. The scavenging coefficient in the scavenging gap was about 10swith the mean precipitation intensity of 0.5 mm/h, which were significantly larger than those of model estimations but close to those from other field measurements. Fumigation effect was also observed in the light-rain day due to the downward flow from the clouds at the beginning of precipitation in this measurement.
    Zhao H. B., C. G. Zheng, 2006: Monte Carlo solution of wet removal of aerosols by precipitation. Atmos. Environ., 40, 1510- 1525.10.1016/j.atmosenv.2005.10.04301721c20be4c91666eaa6b5ba3a0bb1dhttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231005010216http://www.sciencedirect.com/science/article/pii/S1352231005010216The time evolution of aerosol size distribution (ASD) during precipitation describes quantitatively aerosols wet scavenging process. Scavenging coefficient, which takes account of the three most important wet removal mechanisms: Brownian diffusion, interception and inertial impaction, is used to parameterize wet scavenging process. A new multi-Monte Carlo method (MMC) is promoted to solve general dynamic equation for wet removal of aerosols. Two special cases in which analytical solutions exist are adopted to validate computation precision of MMC method. Furthermore, the influence of precipitation type on aerosols wet scavenging process is investigated by numerical simulation of the method. The results show that for lognormal raindrop size distribution and lognormal ASD (1) the increase of rainfall intensity (from light precipitation to moderate precipitation and then to heavy precipitation) can help scavenge aerosols with any size; (2) any precipitation type scavenges large aerosols (>2 渭m) more effectively than small aerosols (0.01 渭m and <2 渭m) (in that order); (3) the three precipitation types have a weak effect of wet scavenging on intermediate aerosols.
    Zheng B., D. Wu, F. Li, and T. Deng, 2013: Variation of aerosol optical characteristics in Guangzhou on a backgroud of South China Sea summer monsoon. Journal of Tropical Meteorology, 29, 207- 214. (in Chinese)83166399ed6949a8d546bbdce61da6a5http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-RDQX201302003.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-RDQX201302003.htmIn order to study the variation of Guangzhou’s aerosol optical characteristics on a large-scale background of South China Sea summer monsoon(SCSSM) and its possible cause,aerosol data derived at Panyu Atmospheric Composition Watch Station in Guangzhou and National Centers for Environmental Prediction/National Center for Atmospheric Research(USA) reanalysis data are used to take composite analysis and do physical diagnoses.Main results showed that aerosol extinction in Guangzhou increases first and then decreases during the active period of SCSSM.The data analyses indicate that stratification variation of the planetary boundary layer and environmental winds play important roles in affecting Guangzhou’s aerosol optical characteristics.To a great extent,stratification of the planetary boundary layer is modified by regional diabatic heating and anomalous cyclonic circulation excited by monsoon convection would induce the environmental winds anomalies.
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Manuscript received: 17 August 2015
Manuscript revised: 11 March 2016
Manuscript accepted: 07 April 2016
通讯作者: 陈斌, bchen63@163.com
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Observed Changes in Aerosol Physical and Optical Properties before and after Precipitation Events

  • 1. Meteorological Institute of Shaanxi Province, Xi'an 710014
  • 2. Beijing Normal University, Beijing 100875

Abstract: Precipitation scavenging of aerosol particles is an important removal process in the atmosphere that can change aerosol physical and optical properties. This paper analyzes the changes in aerosol physical and optical properties before and after four rain events using in situ observations of mass concentration, number concentration, particle size distribution, scattering and absorption coefficients of aerosols in June and July 2013 at the Xianghe comprehensive atmospheric observation station in China. The results show the effect of rain scavenging is related to the rain intensity and duration, the wind speed and direction. During the rain events, the temporal variation of aerosol number concentration was consistent with the variation in mass concentration, but their size-resolved scavenging ratios were different. After the rain events, the increase in aerosol mass concentration began with an increase in particles with diameter 0.8 m [measured using an aerodynamic particle sizer (APS)], and fine particles with diameter 0.1 m [measured using a scanning mobility particle sizer (SMPS)]. Rainfall was most efficient at removing particles with diameter 0.6 m and greater than 3.5 m. The changes in peak values of the particle number distribution (measured using the SMPS) before and after the rain events reflect the strong scavenging effect on particles within the 100-120 nm size range. The variation patterns of aerosol scattering and absorption coefficients before and after the rain events were similar, but their scavenging ratios differed, which may have been related to the aerosol particle size distribution and chemical composition.

1. Introduction
  • The study of aerosol optical properties, radiative forcing, and climate effects is an acrive area of research in the atmospheric sciences field (Kaufman et al., 2002; Xia et al., 2007a; Pan et al., 2010) due to the critical roles aerosols play in regional and global air quality and climatic changes (Charlson et al., 1992; Ramanathan et al., 2001). Studies have shown that aerosol climate effects are closely related to aerosol physical and optical properties (Menon et al., 2002; Xia et al., 2007b). The particle number and/or mass concentration, particle size spectrum, scattering and absorption coefficients, and other aerosol properties, are associated with a set of factors including temperature, humidity and emission sources, as well as atmospheric chemical and physical processes (Belosi et al., 2012; Khoshsima et al., 2014). The study of wet removal processes remains a crucial task for understanding the fate of airborne particulate matter (Andronache, 2004). Wet deposition is divided into in-cloud and below-cloud scavenging processes. Both of these important wet removal processes should be included and accurately represented in atmospheric chemical transport models, atmospheric general circulation models, and mesoscale numerical models (Pinsky et al., 2000; Croft et al., 2009; Wang et al., 2010). Recent reports have shown that the parameterization of rain scavenging removal processes in current aerosol transport models is a significant source of uncertainty (Rasch et al., 2000). These uncertainties can cause large differences in predicted bulk and size-resolved particle concentrations that are undergoing precipitation scavenging, and large discrepancies between theoretical and observed results. Rain scavenging of aerosols can occur within and below a cloud. Scavenging processes involve interactions between raindrops and snow and atmospheric pollutants. Therefore, to fully understand the wet removal process, a complete description of the meteorology, coupled with gas and aerosol physics and chemistry, and a description of the cloud microphysics and removal processes, are needed (Jung et al., 2003). The collection of field data under different rain conditions and other environmental conditions is also needed (Wang et al., 2010).

    Many studies have investigated scavenging mechanisms and the removal efficiency of precipitation on aerosols (Andronache, 2004; Jung et al., 2011). For wet removal by precipitation, atmospheric particles can come into cloud droplets by the in-cloud nucleation scavenging process, or can be collected by falling raindrops by the below-cloud scavenging process. It has been shown that nucleation scavenging is a dominant process at the beginning of cloud formation (Flossmann et al., 1987; Schumann, 1991), while below-cloud scavenging dominates in stratiform precipitation events in polluted urban areas (Gon\ccalves et al., 2007). Below-cloud scavenging mechanisms include Brownian diffusion, directional interception, inertial impaction, thermophoresis, diffusiophoresis, electroscavenging, and electrical effects during thunderstorm rain (Chate et al., 2011). (Wang et al., 2010) assessed the uncertainty of current size-resolved parameterizations for below-cloud particle scavenging by rain and found that the total raindrop-particle collection efficiency varies according to particle size because of the combined action of different microphysical processes. The collection efficiency is highest for ultrafine particles (with particle diameter (d p<0.01 μm) due to Brownian diffusion, and for large particles (d p>3 μm) due to inertial impaction. However, for particles in the diameter range of 0.01-3 μm, more microphysical mechanisms are at play, e.g., Brownian diffusion, interception, diffusiophoresis, thermophoresis, and electric charges. Because of the complexity of scavenging mechanisms, models have been developed to describe these mechanisms. (Loosmore and Cederwall, 2004) modified the standard below-cloud aerosol scavenging model——developed for emergency release scenarios at the Department of Energy's National Atmospheric Release Advisory Center at the Lawrence Livermore National Laboratory——to incorporate the potentially larger scavenging in heavy rain (≥25 mm h-1) events. (Berthet et al., 2010) described a below-cloud scavenging module of aerosol particles and demonstrated selective wet removal of aerosol particles, which depends on the mode radius, the width, and the vertical profile of concentration.

    Another study (Duhanyan and Roustan, 2011) showed that the efficiency of below-cloud scavenging depends on the type of rain, e.g., thunderstorm, widespread, showers. The scavenging coefficient increases linearly with rainfall intensity (Mircea et al., 2000; Chate et al., 2007). Under different rain intensities (1 mm h-1, 10 mm h-1 and 100 mm h-1), precipitation scavenges large aerosols (>2 μm) more effectively than small aerosols (<0.01 μm) and intermediate aerosols (>0.01 μm and <2 μm) (Zhao and Zheng, 2006). Considering the duration of the rain, short periods of high intensity rain can effectively remove relatively coarse mode particles, while low intensity rain that can last for a couple of hours to days is responsible for the removal of relatively fine-mode particles by the below-cloud scavenging process (Kulshrestha et al., 2009). In terms of equal mass of precipitation, snow is more efficient at scavenging atmospheric particles than rain (Santachiara et al., 2013).

    The physical and chemical properties of atmospheric pollutants, such as the particle size distribution, particle number concentration, hygroscopicity, solubility, condensation, and adsorption, also influence the cleansing effect of precipitation (Li et al., 1985). (Chate et al., 2003) studied the removal effect of below-cloud scavenging by rain on aerosol particles and their chemical components and found that scavenging coefficients are highly dependent on relative humidity for hygroscopic particles with diameter less than 5 μm. (Gon\ccalves et al., 2002) compared three chemical species found in rainwater in urban and rural areas (SO42-, NO3- and NH4+) and found that the scavenging effect of rain on each aerosol particle chemical component is different. The relative effect of rainfall washout on air pollutant concentrations is estimated to be SO2> NO2> CO> O3, from a correlation analysis between the hourly observations of pollutants and rainfall intensity at the surface (Yoo et al., 2014). (Mircea et al., 2000) found that estimated values of polydisperse scavenging coefficients show variations of orders of magnitude depending on the aerosol type and almost no variation with raindrop size distributions, and derived the linear relationships between the scavenging coefficients and rain intensity for different aerosol types. (Zheng et al., 2013) found that the wet deposition of precipitation significantly reduces aerosol particles in the metropolitan area of Guangzhou, leading to a dramatic decrease in the aerosol scattering coefficient after the activity of the summer South China Sea monsoon peaks.

    The above studies have investigated the effects of precipitation scavenging on aerosol optical, physical, and chemical properties. However, such studies are limited by issues such as limited observations (Qiu et al., 2003), uncertainties in observed data, and shortcomings in the theoretical approach taken (Chate, 2005). The specific mechanisms and impacts of wet scavenging on aerosols are still not fully understood. More theoretical and field studies are needed to better understand particle removal mechanisms. In this context, the present study investigates the changes in aerosol physical and optical properties before and after rain events using in situ observations made during four rain events in the summer of 2013 at the Xianghe comprehensive atmospheric observation station in China. The goal is to gain insight into the particle wet scavenging mechanism and the impacts of precipitation scavenging on aerosols. Section 2 describes the observation site and instruments used to collect data. The effects of rain on aerosol mass concentration and variations in particle size distribution, and scattering and absorption coefficients, before and after rainfall, are presented in section 3. A discussion is given in section 4 and conclusions in section 5.

2. Observations and instruments
  • The Xianghe observation site (39.75°N, 116.96°E) is located in a mainly plain-like area 70 km southeast of Beijing. It is a comprehensive atmospheric and environmental observation station under the direction of the Institute of Atmospheric Physics, Chinese Academy of Sciences. The site is surrounded by agricultural land and densely populated residences with low buildings. No large factories are located in the area. The climate is a north temperate continental monsoon climate. Summer is the main rainy season, when the surface is covered with green vegetation. Winter and early spring are cold and dry with barren ground. Measurements used in the study were made from 1 June to 5 July 2013.

    An aerodynamic particle sizer (APS) spectrometer (model 3321, TSI Inc., Shoreview, Minnesota, USA) was used to measure particle size distributions from 0.5 μm to 20 μm. The aerodynamic size of a particle is determined by the time of flight between the instrument's two laser beams. Time-of-flight is recorded and converted to aerodynamic diameter using a calibration curve. Aerosol number concentration is measured in 52 size bins and a complete particle size distribution may be determined in seconds or minutes. Other aerosol properties, such as particle mass concentration and volume concentration, can be calculated from these data. A scanning mobility particle sizer (SMPS, model 3034, TSI Inc., Shoreview, Minnesota, USA) was used to measure particle size distributions in the range of 10-487 nm by separating particles based on their electromobility. Particle number concentration is measured in 54 size bins and particle mass concentration, volume concentration, and surface area can be calculated from these measurements. The sampling rate for both instruments was five minutes. An integrating nephelometer (model 3563, TSI Inc., Shoreview, Minnesota, USA) was used to measure the scattering and backscattering coefficients of aerosol particles at 450, 500 and 700 nm with a sampling interval of five minutes. A particle soot absorption photometer (model , Radance Research Inc., Seattle, Washington, USA) was used to measure the absorption coefficient of particles at 470, 522 and 660 nm with a sampling interval of one minute. These instruments were set up in a large container placed on the ground. Environmental sampling systems were set up on the top of the container (2.5 m above the ground). All measurements have undergone strict quality control. Precipitation, wind speed, and other meteorological data were collected from the automatic weather station installed at the Xianghe site. The details of the four rain events during the observation period analyzed in this paper are given in Table 1.

  • The rainfall scavenging ratio (SR) in percentage units is defined as: $$ {SR}=\left(1-\dfrac{x_{a}}{x_0}\right)\times 100 , $$ where x a is the average value of a given variable, such as mass concentration, number concentration or scattering or absorption coefficient, during the hour after precipitation ends; and x0 is the average value of a given variable during the hour before precipitation starts. The scavenging ratio is used to represent by how much rainfall removal affects the physical and optical properties of aerosol particles.

3. Results and discussion
  • Figure 1 shows time series of total mass concentration (0.5-20 μm) measured by the APS, along with the rainfall amount, during the four rain events. The mass concentrations first increased as rain began, which was possibly related to the hygroscopic properties of aerosol particles and the evaporation of falling droplets (Zhang et al., 2004), then decreased, during the rain events on three dates (16-17 June, 22 June, and 1-2 July). The rainfall intensity was smallest during the 16-17 June rain event, with a total rainfall amount of 1.3 mm. On that day, rain started at 1340 LST (local standard time). The aerosol mass concentration increased and reached a peak of 208.6 μg m-3 four hours later, but quickly decreased to 143.4 μg m-3 at 1840 LST, and then slowly increased to a secondary peak of 176.1 μg m-3 six hours later (Fig. 1b and Table 1). The mass concentration increased at the beginning of the rain event on 22 June, and reached a peak of 143.6 μg m-3 at 0945 LST. With the rain continuing and the rain intensity increasing to 0.2 mm (10 min)-1, the mass concentration quickly decreased to 17 μg m-3 (Fig. 1c). There was a gap in mass concentration observations (1640-2200 LST) during the 1-2 July rain event because of instrument failure, but it is clear that the aerosol mass concentration first increased from 154.4 μg m-3 to 377.1 μg m-3 during 1610-2300 LST 1 July (Fig. 1d), then decreased as rain continued. The mass concentration dramatically decreased to 107.1 μg m-3 after an intense shower at around 0110 LST 2 July. During the two to three days after the rain event, the total mass concentration remained constant at 10 μg m-3. The rain intensity at the beginning of the 9-10 June rainfall event was 0.3 mm (10 min)-1, flushing aerosols out of the atmosphere. Two hours later, the aerosol mass concentrations decreased from 171 μg m-3 to 10 μg m-3 (Fig. 1a).

    Figure 1.  Time series of aerosol total mass concentration (solid lines) and precipitation (grey shaded areas) on (a) 9-10 June, (b) 16-17 June, (c) 22 June, and (d) 1-2 July.

    Of the four rainfall events, the rain events that took place on 9-10 June and 1-2 July were the most intense, with the highest total amounts of rain. Total mass concentrations before and after the 9-10 June and 1-2 July rain events dropped from 173.1 μg m-3 and 124.9 μg m-3, respectively, to 11.8 μg m-3 and 17.0 μg m-3, respectively (Figs. 1a and d; Table 1). Total mass concentrations before and after the 16-17 June and 22 June rain events dropped from 128.4 μg m-3 and 57.6 μg m-3, respectively, to 38.7 μg m-3 and 34.0 μg m-3, respectively (Figs. 1b and c; Table 1). Rain scavenging ratios for the aerosol mass concentrations during the 9-10 June, 1-2 July, 16-17 June and 22 June rain events were 93.2%, 86.4%, 69.9% and 41.0%, respectively. The aerosol number concentrations changed during the rainfall events in a similar way as the mass concentrations, but their size-resolved scavenging ratios were different.

    The changes in mass concentration during the rain event on 10 June were different from those on 2 July, even though both featured high rainfall intensity [≥0.2 mm (10 min)-1] at their beginnings. The particle mass concentration decreased sharply two hours later, after the onset of rain, on 9 June. In contrast, there was an increase in particle mass concentration at the beginning of the rain event on 1 July. The wind direction was mostly north-northwest (NNW) throughout the rain event on 9-10 June, and the averaged wind speed was 2 m s-1. The colder air mass coming from a cleaner region could have been helpful in combatting the pollution through dilution, causing a rapid decrease in particle mass concentration. The prevailing wind was generally east-northeast (ENE) on 1 July, and the averaged wind speed was 1.5 m s-1. In this case, ENE air flow was not favorable for the removal of the pollution, resulting in a much slower decrease in particle mass concentration on 1 July. As rain continued to fall, the rain intensity increased to 5 mm (10 min)-1 at 0110 LST 2 July, and the wind direction changed from ENE to NNW. The particle mass concentrations dramatically decreased from 377.1 μg m-3 to 27.2 μg m-3 after the rain stopped. The precipitation on 2 July stopped at 0250 LST, and there were no emissions associated with traffic and no obvious changes in anthropogenic aerosol sources during this period. In addition, the prevailing winds after the rain stopped on 2 July were generally from the NNW, which was conducive to the maintenance of clean air for a longer time (Fig. 1d).

    Although the rainfall intensity during the 22 June rain event was stronger than that of the 16-17 June rain event, the removal of aerosols was weaker, due partly to the lighter wind and its direction during the rain. Winds near the ground may have caused turbulent flow fluctuations, and these may in turn have increased the relative motion between particles and smaller collector droplets, thereby enhancing the collection efficiency (Khain and Pinsky, 1997). Winds from the north (N), northeast (NE) and ENE prevailed during the 22 June rain event, while NE and NNW winds dominated during the 16-17 June rain event. All of this evidence suggests that the synoptic pattern during the 16-17 June event was more favorable for the dispersal of pollution, causing a higher scavenging ratio on that day.

    The results suggest that the impact of precipitation on particle concentrations is associated with wind speed as well as the advection of air from different source regions.

  • Figures 2 and 3 show the time series of particle number size distributions measured by the APS (for particles ranging in size from 0.5-20 μm) and the SMPS (for particles ranging in size from 10-487 nm), respectively, during the six hours before and after each of the four rainfall events. For the 9-10 June event, the number concentration of particles measured by the APS decreased dramatically during the two hours after rain began. Eleven hours after the onset of rain, the number concentration for particles with diameter <0.8 μm began to increase (Fig. 2a). The number concentration of fine particles ranging in size from 20 to 50 nm increased greatly two hours after rain began, indicating new particle formation (Fig. 3a), then decreased in magnitude. Ten hours later (2000 LST), the number concentration of particles with diameter <200 nm increased, especially particles with diameter ranging from 30 to 90 nm. One hour after rainfall ended, the number concentrations of APS-measured particles with diameters from 0.5 to 0.8 μm, and SMPS-measured particles with diameter from 50 to 100 nm, began to increase.

    The number concentration of particles with diameters <0.8 μm increased significantly during the first 13 hours after rain began on 16 June (Fig. 2b), then decreased gradually and remained constant for the next four hours. This was generally consistent with the changes in mass concentration seen during that rain event. For fine particles (Fig. 3b), the number concentration of particles with diameters around 100 nm decreased by 60% during the first hour after the start of rain. This low level of number concentration remained constant during the rest of the rain event. The number concentration of particles with diameters ranging from 50 to 100 nm slowly increased after the end of the rain event. During the rain event on 22 June, the number concentration of particles with diameters <1.1 μm increased dramatically after the onset of rain, especially particles with diameters <0.8 μm (Fig. 2c). Five hours after the onset of rain, the number concentration of all particles began to decrease, and remained at low levels until the end of the rain event. The number concentration of fine particles decreased gradually during the rain event (Fig. 3c). One hour after rain ended, the number concentration of particles with diameters ranging from 50 to 100 nm began to increase. During the 1-2 July rain event, the number concentration of particles with diameters <2 μm (measured by APS) increased after rain began. Low number concentrations were seen two hours before the rain event ended (Fig. 2d). The five-hour gap in data occurred because of instrument failure. The number concentration of fine particles with diameters <300 nm (measured by SMPS) began to decrease after rain started, especially particles with diameter ranging from 60 to 200 nm (Fig. 3d). At the end of the rain event, number concentrations were very low. The number concentration of particles with diameters <100 nm started to gradually increase three hours after rain ended. New fine particle formation in the 10-30 nm size range was seen 5-6 hours after the rain event was over (Fig. 3d).

    Figure 2.  Time series of aerosol particle number size distribution measured by the APS on (a) 9-10 June, (b) 16-17 June, (c) 22 June, and (d) 1-2 July. The vertical dashed lines indicate the beginning and end of the rain event.

    In summary, the temporal variations of aerosol number and mass concentration during the four rain events were consistent. At the beginning of a rain event, the increase in aerosol mass concentration was mainly caused by the increase in particles with diameters <0.8 μm, measured by the APS, which was possibly related to the hygroscopic growth of aerosol particles, the evaporation of falling droplets, and the wind speed, as well as the advection of air masses from different source regions. The number concentration of fine particles with diameters <500 nm——measured by the SMPS——generally decreased as rain continued to fall. The increase in mass concentration after rain ended was mainly caused by the increase in particles with diameters <0.8 μm, which was mainly related to anthropogenic aerosol emissions. The precipitation on 10 June, 17 June and 22 June stopped at 0700 LST, 0640 LST and 1810 LST, respectively. Those periods overlapped with the morning rush hour or evening rush hour. While the precipitation on 2 July stopped at 0250 LST, there were no emissions associated with traffic and no obvious changes in anthropogenic aerosol sources during this period. New particles with sizes of <50 nm appeared several hours after the end of a rainfall event.

    Figure 3.  As in Fig. 2, but for SMPS measurements.

    Figure 4.  Mean aerosol number concentration as a function of particle size measured by the APS during the one-hour period before (purple dotted lines) and after (red triangle-marked lines) the rain events on (a) 9-10 June, (b) 16-17 June, (c) 22 June and (d) 1-2 July.

    According to Fig. 4, the greatest change in the aerosol number concentration size distribution was caused by the rain event on 9-10 June. The total aerosol mass concentration decreased by 93.2% during this rain event (Table 1). The number concentration of particles with diameters >0.63 μm decreased by over 85%. The largest decreases were seen in the 0.835-1.382 μm and >6.732 μm size categories. The scavenging ratio of coarse-mode particles with diameters >3.5 μm was over 82%, so they were effectively removed from the atmosphere during this rain event.

    There was less change in the mean particle number concentration measured by the APS one hour before and after the rain events (Fig. 4c). The size-resolved scavenging ratios of the rain event on 22 June——measured by the APS——were between 19.2% and 67.6% (Fig. 5), and the scavenging ratio of the total mass concentration was the smallest among these four rain events (Table 1). The change in the particle number concentration before and after the rain event on 16-17 June was more obvious than that of the rain event on 22 June (Fig. 4b). Larger change could be seen for the particles with diameters between 0.5 and 1.6 μm, and those with diameters larger than 3.0 μm, during the 1-2 July rain event (Fig. 4d). The size-resolved scavenging ratios of the particles with diameters between 0.5 and 1.6 μm were 63%-94%, and the size-resolved scavenging ratios of the particles with diameters larger than 3.0 μm were 70%-100% (Fig. 5).

    Figure 5 denotes the size-resolved wet scavenging ratio observed by the APS. The scavenging ratio of the particles with diameters >3 μm was higher than that of fine particles, agreeing well with scavenging coefficients from theoretical parameterizations (Wang et al., 2010). For the particles with diameters <3 μm, the scavenging ratio changed dramatically with different precipitation processes, suggesting the changes in the particle concentration not only depended on the raindrop-particle collection efficiency, but also on other factors, such as the air mass source and local emissions during the field observation. This is different to laboratory observations and theoretical modeling results (Jung et al., 2011; Zhang et al., 2012).

    Figure 6 shows that number concentrations of fine aerosol particles with diameters ranging from 10 to 487 nm, measured by the SMPS after the rain events, were lower than those before the rain events, demonstrating the scavenging effect of rainfall on aerosols. The number concentration peak in particle size distribution was in the 100-120 nm range before rain started. This peak shifted toward smaller particle sizes (40-70 nm) after rain ended on 16-17 June, 22 June and 1-2 July (Figs. 6b-d). During the 9-10 June rain event, the peak number concentration did not shift to smaller particles size, and the bulk of the fine-sized particles were removed from the atmosphere (Fig. 6a). For the rain events on 16-17 June and 22 June, the number concentration of particles in the 10-40 nm size range after rain ended was greater than before rain started. Examining the wind directions and relative humidity during these four rainfall events, it is clear that the particles with diameters <40 nm could have been slightly removed only when N wind dominated during a large-scale precipitation process, and without local emissions (Fig. 6d). This indicates that the impact of precipitation on particles with diameters <40 nm was also associated with the advection of air and local emissions, besides the rain intensity and rainfall duration. These effects contribute to discrepancies between field observations and theoretical modeling results (Berthet et al., 2010).

    Figure 5.  Scavenging ratio (SR, %) as a function of particle diameter, for the four rain events.

    Figure 6.  Mean aerosol number concentration as a function of particle size, measured by the SMPS, during the one-hour period before (purple dotted lines) and after (red triangle-marked lines) the rain events on (a) 9-10 June, (b) 16-17 June, (c) 22 June and (d) 1-2 July.

    Figure 7.  Time series of aerosol scattering coefficients for the rain events on (a) 9-10 June, (b) 16-17 June, (c) 22 June and (d) 1-2 July. The colored lines represent the wavelength band.

  • Aerosol scattering coefficients varied in a manner similar to the mass and number concentrations. The average scattering coefficients at blue, green and red wavelengths about one hour before rain started on 9 June (Fig. 7a) were 775.8, 611.5, and 467.3 Mm-1, respectively. Six hours into the rain event, average scattering coefficients at blue, green and red wavelengths dropped to 18.5, 12.1 and 7.7 Mm-1, respectively. The average scattering coefficients at blue, green and red wavelengths gradually increased during the hour after rain stopped, reaching values of 140.7, 94.9 and 61.6 Mm-1, respectively. The scavenging ratios were 82%, 84% and 87%, respectively, for the three bands (Table 2), which was less than the scavenging ratio of aerosol mass concentration (93.2%).

    The average scattering coefficients at blue, green and red wavelengths one hour before the onset of rain on 16 June (Fig. 7b) were 946.6, 694.2 and 490.1 Mm-1, respectively. These values increased slightly during the first 10 hours after rain started, then decreased to 328.4, 248 and 180.1 Mm-1, respectively, and remained at that level during the following three hours. After rain ended, the scattering coefficients continued to drop to their lowest levels. The mean scattering coefficients at blue, green and red wavelengths were 341.3, 254.8 and 182.9 Mm-1, respectively, one hour after rain ended. The scavenging ratios were 64%, 64% and 63% for the three bands (Table 2), which were less than the scavenging ratio of aerosol mass concentration (69.9%). This suggests that this rain event had a larger impact on the magnitude of aerosol mass concentration than on aerosol scattering coefficients. As shown in Table 1, the ratio of PM2.5 to total mass concentration increased from 0.85 to 0.91 after the rain event, suggesting the scavenging ratio of coarse particles was higher than that of fine particles. In addition, fine-mode particles have a stronger effect on light extinction and scattering, resulting in a stronger impact of precipitation on the mass concentration compared to that on the scattering coefficient.

    The scattering coefficients increased during the four hours before rain started on 22 June and continued to increase until about three hours after rain started (Fig. 7c). This was consistent with the trend in the number concentration of particles <0.8 μm before the rain event started and during the rain event (Fig. 2c). The scattering coefficients at blue, green and red wavelengths began to decrease around nine hours after rain started until reaching their lowest values of 183.4, 125.2 and 80.51 Mm-1, respectively. After rain ended, the scattering coefficients began to increase in the same manner as the mass concentration. The average scattering coefficients at blue, green and red wavelengths one hour before the onset of rain were 777.3, 568.9 and 393.8 Mm-1, respectively. They dropped to 302.0, 210.2 and 138.5 Mm-1 one hour after rain ended, with scavenging ratios of 61%, 63% and 65%, respectively (Table 2). These values were larger than the scavenging ratio of the mass concentration.

    Figure 8.  Time series of aerosol absorption coefficients for the rain events on (a) 16-17 June and (b) 22 June. The colored lines represent the wavelength band.

    The scattering coefficients at blue, green and red wavelengths one hour before rain started on 1 July were 1023.4, 819.3 and 637.7 Mm-1, respectively (Fig. 7d). After rain began, there was a long intermission (>3 hours) near midnight. During this period, there was no precipitation, and diminished anthropogenic activities caused a gradual decrease in scattering coefficients. As the rain started again and continued, the magnitudes of these scattering coefficients decreased and reached low values of 46.21, 33.46 and 22.99 Mm-1, respectively. One hour after the end of the rain event, the average scattering coefficients at blue, green and red wavelengths were still low (33.0, 23.4 and 16.2 Mm-1, respectively). The scavenging ratios at all wavelengths were the same (97%), and greater than the scavenging ratio of the mass concentration (86.4%).

    Studies have shown that aerosol particles with diameters in the visible wavelength range (0.4-0.7 μm) have the strongest effect on light extinction, and aerosol particles with diameters ranging from 0.1 to 1.0 μm (the accumulation mode) have the largest effect on atmospheric visibility (Yin et al., 2010). From Figs. 4 and 6, the number concentration changed the most at 0.6-0.8 μm and at 100-120 nm, so the impact on scattering coefficients was the greatest across these particle size ranges too. When rain begins, these small aerosol (<1.0 μm) particles undergo hygroscopic growth with increasing humidity, which enhances their scattering properties (Jiang et al., 2013; Zheng et al., 2013). Therefore, trends in the variation of scattering coefficients are consistent with number and mass concentrations. As rain continues to fall, the number of aerosol particles significantly decreases through the wet scavenging effect of rain, resulting in a decrease in aerosol scattering coefficient.

    Since the scavenging ratios varied with particle size, the changes in scattering coefficient and mass concentration observed before and after the rain processes should also be different. The changes in scattering coefficient were highly dependent on the scavenging ratio of particles with diameters <0.8 μm. During the rain event on 17 June, the scavenging ratio of particles with diameters >0.8 μm, and with diameters >3.0 μm, exceeded 70% and 80%, respectively. However, the scavenging ratios of particles with diameters <0.8 μm were 51%-69%. The scavenging ratio of coarse particles was much higher than that of fine particles in this event. As a result, the impact of the precipitation on the mass concentration was stronger compared to that on the scattering coefficient. Another notable rain event was on 22 June. The scavenging ratio of particles with diameters from 0.5 to 0.6 μm was the highest and the magnitude was between 60% and 68%, while the scavenging ratio of particles with diameters >0.8 μm was between 33% and 60%. The scavenging ratio of fine particles, which are closely related to the scattering coefficient, was much higher than that of coarse particles in this event. As a result, the impact of the precipitation on the mass concentration was weaker compared to the scattering coefficient.

    Figure 9.  Spatial distributions of 24-h (0800-0800 LST) total rainfall amounts (units: mm) on (a) 9-10 June, (b) 16-17 June, (c) 22 June and (d) 1-2 July. The red dot indicates the location of Xianghe. The Yellow River is shown as a blue line.

  • Aerosol absorption coefficients were measured during the 16-17 June and 22 June rain events only because of instrument failure on the other days. The average absorption coefficients at blue, green and red wavelengths one hour before rain started on 16 June (Fig. 8a) were 187.8, 166.3 and 137.9 Mm-1, respectively. The absorption coefficients decreased as the rain event progressed. The average absorption coefficients at blue, green and red wavelengths one hour after rain ended were 71.5, 64.1 and 55.0 Mm-1, respectively. The scavenging ratios at the three wavelengths were 62%, 61% and 60%, respectively. These values were slightly lower than the scavenging ratios for the scattering coefficients.

    For the rain event on 22 June (Fig. 8b), the average absorption coefficients at blue, green and red wavelengths one hour before the onset of rain were 95.8, 85.5 and 72.6 Mm-1, respectively. These values decreased to 76.2, 67.2 and 55.1 Mm-1 during the middle of the rain event and dropped to 44.3, 39.0 and 31.2 Mm-1 one hour after the end of the rain event. The scavenging ratios at the three wavelengths were 54%, 54% and 57%, respectively. These values were lower than the scavenging ratios for the scattering coefficient. This suggests that the impact of rainfall on scattering coefficients is greater than that on absorption coefficients, which would be related to the relative size of the absorbing aerosols.

    The 0.532 μm single scattering albedo was 0.81 and 0.80 (0.87 and 0.84) before and after the 16-17 June (22 June) rain event, respectively. The decrease in single scattering albedo after rain ended suggests an increase in absorbing aerosols and a change in aerosol optical properties. However, the aerosol absorption coefficient is affected by aerosol chemical composition, particle constitution and spatiotemporal distribution. It is thus challenging to determine which process plays the dominant role in causing changes in the aerosol absorption coefficient before and after rain (Yang et al., 2001).

4. Discussion
  • The impact of rain scavenging on the physical and optical properties of aerosols is a complex process. Variations in aerosols are not only related to local sources, but are also affected by transport from neighboring regions. The wet scavenging effect on aerosols is not only associated with the local rainfall intensity, duration, and precipitation type, but also with precipitation coverage. The 24-hour total rainfall amounts for the four rain events are shown in Fig. 9. The rain events during 9-10 June and 1-2 July had not only stronger intensities and total rainfall amounts, but they also covered a much wider area than the other two events. This led to near-complete removal of aerosols over the observation sites. This suggests long-duration and large-scale precipitation have a much stronger impact on the physical and optical properties of aerosols. The impact of rain on the scattering coefficients during the 16-17 June and 22 June rain events was greater than that on the absorption coefficients, which might have been due to the differences in aerosol chemical composition and particle size distribution (Chate et al., 2003; Hu et al., 2005).

    Studies have shown that size-resolved precipitation scavenging parameterizations have large uncertainties (Rasch et al., 2000; Textor et al., 2006). (Wang et al., 2010) found that the discrepancies between field-observed and theoretically predicted scavenging coefficient values were more than one order of magnitude for particles in the 0.1-3 μm diameter range. These large discrepancies are likely caused by additional known physical processes (i.e., turbulent transport and mixing advection, cloud and aerosol microphysics) that influence field data, but may not be included in current below-cloud scavenging parameterizations. Turbulence and vertical diffusion should have a larger impact on the raindrop scavenging of small particles compared with that of large particles (Khain and Pinsky, 1997). This also explains why theoretical scavenging coefficient values for particles >3 μm in diameter agree well with most field measurements, but are as much as one to two orders of magnitude smaller for particles <3 μm when compared to field measurements. (Sparmacher et al., 1993) reported an exception to this based upon a controlled experiment. However, in the case of very intense rainfall or cases of long-lasting rain with low or moderate intensities, the below-cloud scavenging of particles in the 0.01-3 μm diameter range can become important.

5. Summary and conclusions
  • (1) At the beginning of the rainfall events examined in this study, aerosol mass concentration first increased, then decreased as the rain events progressed. The rain scavenging effect was related to the rainfall intensity, duration, areal coverage, and wind speed or turbulence, as well as the advection of air from different source regions.

    (2) During the rain events, the temporal variation of aerosol number concentration was consistent with the variation in mass concentration, but their size-resolved scavenging ratios were different. Before a rain event began, the increase in aerosol mass concentration was mainly caused by the increase in particles with diameters <0.8 μm. After the rain event ended, the aerosol number concentration began to increase as the number of small particles with diameters <0.8 μm (measured by APS) and fine particles with diameters <100 nm (measured by the SMPS) increased. This was mainly caused by human activities or local emissions sources. New fine particles in the 10-30 nm size range could begin to form 5-6 hours after a rain event ended.

    (3) Changes in aerosol number concentration occurred for particles with diameters around 0.6 μm (mainly urban aerosols) and >3.5 μm (measured by APS) before and after rainfall, indicating that rainfall was most effective at removing particles of these sizes. Changes in the number size distribution measured by the SMPS before and after rain reflected the efficient rain removal scavenging effects on particles within the 100-120 nm size range. The impact of precipitation on particles with diameters smaller than 40 nm was different from the particles within the 100-120 nm size range. There represents a discrepancy between field observations and theoretical modeling results for the scavenging effect of particles with diameters <3 μm.

    (4) The scattering coefficient and absorption coefficient were reduced by more than 61% and 54% after the rain, respectively. When the mass concentration reduced by more than 85%, the scavenging ratio of the scattering coefficient exceeded 80% simultaneously, because of the significant decrease in fine-mode particles, which are closely related to the scattering coefficient. However, for some rain events, the reduction in the magnitude of scattering coefficients was greater than that of mass concentration; while for other rain events, the reduction in the magnitude of scattering coefficients was less than that of mass concentration. The influence of rainfall on the scattering coefficient was greater than that on the absorption coefficient, which was related to the particle size distribution and their chemical compositions.

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