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Cloud Condensation Nuclei over the Bay of Bengal during the Indian Summer Monsoon


doi: 10.1007/s00376-017-6331-z

  • The first measurements of cloud condensation nuclei (CCN) at five supersaturations were carried out onboard the research vessel "Sagar Kanya" (cruise SK-296) from the south to the head-bay of the Bay of Bengal as part of the Continental Tropical Convergence Zone (CTCZ) Project during the Indian summer monsoon of 2012. In this paper, we assess the diurnal variation in CCN distributions at supersaturations from 0.2% to 1% (in steps of 0.2%) and the power-law fit at supersaturation of 1%. The diurnal pattern shows peaks in CCN concentration (N CCN) at supersaturations from 0.2% to 1% between 0600 and 0700 LST (local standard time, UTC+0530), with relatively low concentrations between 1200 and 1400 LST, followed by a peak at around 1800 LST. The power-law fit for the CCN distribution at different supersaturation levels relates the empirical exponent (k) of supersaturation (%) and the N CCN at a supersaturation of 1%. The N CCN at a supersaturation of 0.4% is observed to vary from 702 cm-3 to 1289 cm-3, with a mean of 961 161 cm-3 (95% confidence interval), representing the CCN activity of marine air masses. Whereas, the mean N CCN of 1628 193 cm-3 at a supersaturation of 1% is higher than anticipated for the marine background. When the number of CCN spectra is 1293, the value of k is 0.57 0.03 (99% confidence interval) and its probability distribution shows cumulative counts significant at k≈ 0.55 0.25. The results are found to be better at representing the features of the marine environment (103 cm-3 and k≈ 0.5) and useful for validating CCN closure studies for Indian sea regions.
    摘要: 作为大陆热带辐合带(CTCZ)计划的组成部分, 利用“萨加尔坎亚”科考船(编号SK-296)船载仪器对2012年印度夏季风期间孟加拉湾南部到前端地区五种过饱和度下的云凝结核(CCN)进行了首次观测. 本文分析了过饱和度从0.2%到1%(间隔为0.2%)下的CCN日变化特征和在1%过饱和度下使用幂函数拟合的CCN谱分布. 日变化特征显示过饱和度从0.2%到1%时CCN数浓度(NCCN)在0600到0700 LST(当地时间, 协调世界时+0530)出现峰值, 接着在1200到1400 LST出现相对低值, 然后在1800 LST左右又出现峰值. 不同过饱和度下幂函数拟合的CCN谱分布依赖于和过饱和(%)相关的经验参数k以及1%过饱和度下的CCN数浓度. 过饱和度为0.4%时CCN数浓度的变化范围为702 cm?3到1289 cm?3, 平均值为961 ± 161 cm?3(95%置信区间), 代表了海洋性CCN特征. 然而当过饱和度为1%时, 1628 ± 193 cm?3的平均NCCN高于预期的海洋背景CCN数浓度. 当CCN谱的参数C值是1293时, 参数k值为0.57 ± 0.03(99%置信区间)并且其概率分布的累计频数显著出现在k ≈ 0.55 ± 0.25. 上述结果能够更好的代表海洋性CCN特征(C=103 cm?3和k ≈ 0.5)并且有助于验证印度洋地区的CCN闭合结果. (摘要翻译: 赵震)
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  • Andreae M. O., D. Rosenfeld, 2008: Aerosol-cloud-precipitation interactions.Part 1. The nature and sources of cloud-active aerosols. Earth-Science Reviews89,13-41,doi: 10.1016/j.earscirev.2008.03.001.10.1016/j.earscirev.2008.03.001237d50455de85f2d460a3f5965fb97f5http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0012825208000317http://linkinghub.elsevier.com/retrieve/pii/S0012825208000317Aerosol particles are produced either directly by anthropogenic and natural sources (dust, sea salt, soot, biological particles, etc.), or they are formed in the atmosphere by condensation of low-volatility compounds (e.g., sulfuric acid or oxidized organic compounds). We discuss the magnitude of these sources, and the CCN and IN characteristics of the particles they produce. In contrast to previous assessments, which focused on the aerosol mass, we are emphasizing the number of particles being produced, as this is the key variable in cloud microphysics. Large uncertainties still exist for many aerosol sources, e.g., the submicron part of the seaspray aerosol, the particles produced by the biosphere, and the secondary organic aerosol. We conclude with a discussion on what particle concentrations may have been in the pristine atmosphere, before the onset on anthropogenic pollution. Model calculations and observations in remote continental regions consistently suggest that CCN concentrations over the pristine continents were similar to those now prevailing over the remote oceans, suggesting that human activities have modified cloud microphysics more than what is reflected in conventional wisdom. The second part of this review will address the effects of changing CCN and IN abundances on precipitation processes, the water cycle, and climate.
    Andreae M. O., 2009: Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions. Atmos. Chem. Phys. 9,543-556, doi: 10.5194/acp-9-543-2009.10.5194/acpd-8-11293-20087f50e14cea046fd9a3d16cf485c8dc60http%3A%2F%2Fwww.oalib.com%2Fpaper%2F2703978http://www.oalib.com/paper/2703978A large number of published and unpublished measurements of cloud condensation nuclei (CCN) concentrations and aerosol optical thickness (AOT) measurements have been analyzed. AOT measurements were obtained mostly from the AERONET network, and selected to be collocated as closely as possible to the CCN investigations. In remote marine regions, CCNlt;subgt;0.4lt;/subgt; (CCN at a supersaturation of 0.4%) are around 110 cmlt;supgt;amp;minus;3lt;/supgt; and the mean AOTlt;subgt;500lt;/subgt; (AOT at 500 nm) is 0.057. Over remote continental areas, CCN are almost twice as abundant, while the mean AOTlt;subgt;500lt;/subgt; is ca. 0.075. (Sites dominated by desert dust plumes were excluded from this analysis.) Some, or maybe even most of this difference must be because even remote continental sites are in closer proximity to pollution sources than remote marine sites. This suggests that the difference between marine and continental levels must have been smaller before the advent of anthropogenic pollution. lt;brgt;lt;/brgt; Over polluted marine and continental regions, the CCN concentrations are about one order of magnitude higher than over their remote counterparts, while AOT is about five times higher over polluted than over clean regions. The average CCN concentrations from all studies show a remarkable correlation to the corresponding AOT values, which can be expressed as a power law. This can be very useful for the parameterization of CCN concentrations in modeling studies, as it provides an easily measured proxy for this variable, which is difficult to measure directly. It also implies that, at least at large scales, the radiative and microphysical effects of aerosols on cloud physics are correlated and not free to vary fully independently. While the observed strong empirical correlation is remarkable, it must still be noted that there is about a factor-of-four range of CCN concentrations at a given AOT, and that there remains considerable room for improvement in remote sensing techniques for CCN abundance.
    Asmi E., E. Freney, M. Hervo, D. Picard, C. Rose, A. Colomb, and K. Sellegri, 2012: Aerosol cloud activation in summer and winter at puy-de-D\ome high aLSTitude site in France.Atmos. Chem. Phys.12,11 589-11 607,doi: 10.5194/acp-12115892012.10.5194/acpd-12-23039-201270162431dd3ba23848fdcbccee2f3bd5http%3A%2F%2Fwww.oalib.com%2Fpaper%2F1365964http://www.atmos-chem-phys.net/12/11589/2012/Cloud condensation nuclei (CCN) size distributions and numbers were measured for the first time at Puy-de-D00me high altitude (1465 m a.s.l) site in Central France. Majority of the measurements were done at constant supersaturation (SS) of 0.24%, which was also deduced to be representative of the typical in-cloud SS at the site. CCN numbers during summer ranged from about 200 up to 2000 cmlt;supgt;613lt;/supgt; and during winter from 50 up to 3000 cmlt;supgt;613lt;/supgt;. Variability of CCN number was explained by both particle chemistry and size distribution variability. The higher CCN concentrations were measured in continental, in contrast to marine, air masses. Aerosol CCN activity was described with a single hygroscopicity parameter κ. Range of this parameter was 0.29 ± 0.13 in summer and 0.43 ± 0.19 in winter. When calculated using SS of 0.51% during summer, κ of 0.22 ± 0.07 was obtained. The decrease with increasing SS is likely explained by the particle size dependent chemistry with smaller particles containing higher amounts of freshly emitted organic species. Higher κ values during winter were for the most part explained by the observed aged organics (analysed from organic lt;igt;m/zlt;/igt; 44 ratio) rather than from aerosol organic to inorganic volume fraction. The obtained κ values also fit well within the range of previously proposed global continental κ of 0.27 ± 0.21. During winter, the smallest κ values and the highest organic fractions were measured in marine air masses. CCN closure using bulk AMS chemistry led to positive bias of 5% and 2% in winter and summer, respectively. This is suspected to stem from size dependent aerosol organic fraction, which is underestimated by using AMS bulk mass composition. Finally, the results were combined with size distributions measured from interstitial and whole air inlets to obtain activated droplet size distributions. Cloud droplet number concentrations were shown to increase with accumulation mode particle number, while the real in-cloud SS correspondingly decreased. These results provide evidence on the effects of aerosol particles on maximum cloud supersaturations. Further work with detailed characterisation of cloud properties is proposed in order to provide more quantitative estimates on aerosol effects on clouds.
    Bougiatioti A., A. Nenes, C. Fountoukis, N. Kalivitis, S. N. Pand is, and N. Mihalopoulos, 2011: Size-resolved CCN distributions and activation kinetics of aged continental and marine aerosol.Atmos. Chem. Phys11,8791-8808,10.5194/acp-11-8791-2011.10.5194/acpd-11-12607-2011824efea775dbc2a6882aaf95331e5cc3http%3A%2F%2Fwww.oalib.com%2Fpaper%2F2695566http://www.atmos-chem-phys.net/11/8791/2011/We present size-segregated measurements of cloud condensation nucleus (CCN) activity of aged aerosol sampled at Finokalia, Crete, during the Finokalia Aerosol Measurement Experiment of summer 2007 (FAME07). From analysis of the data, hygroscopicity and activation kinetics distributions are derived. The CCN are found to be highly hygroscopic, (expressed by a size- and time-averaged hygroscopicity parameter amp;kappa; ~ 0.22), with the majority of particles activating at ~0.50.6% supersaturation. Air masses originating from Central-Eastern Europe tend to be associated with higher CCN concentrations and slightly lower hygroscopicity (amp;kappa; ~ 0.18) than for other airmass types. The particles were always well mixed, as reflected by the high activation ratios and narrow hygroscopicity distribution widths. Smaller particles (~30 nm) were found to be more hygroscopic (~0.1 amp;kappa; units higher) than the larger ones (~100 nm). The particles with diameters less than 80 nm exhibited a diurnal hygroscopicity cycle (with amp;kappa; peaking at ~14:00 h local time), consistent with photochemical aging and volatilization of less hygroscopic material from the aerosol. Use of bulk chemical composition and the aerosol number distribution results in excellent CCN closure when applying Khler theory in its simplest form. Using asymptotic and threshold droplet growth analysis, the aged organics present in the aerosol were found not to suppress or delay the water uptake kinetics of particles in this environment.
    CTCZ-Scientific Steering Committee, 2011: Proposal for Continental Tropical Convergence Zone (CTCZ) programme. CTCZ-Scientific Steering Committee,66 pp.
    Dinger J. E., H. B. Howell, and T. A. Wojciechowski, 1970: On the source and composition of cloud nuclei in a subsident air mass over the North Atlantic.J. Atmos. Sci.27,791-797,doi: 10.1175/1520-0469(1970)027<0791:OTSACO>2.0.CO;2.10.1175/1520-0469(1970)0272.0.CO;20ff298bc76ef0469287ae1b9b8744101http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1970jats...27..791dhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281970%29027%3C0791%3AOTSACO%3E2.0.CO%3B2Measurements of the concentration of cloud nuclei, which are activated at a supersaturation of 0.75%, were made aboard an aircraft at various altitudes in a subsident air mass over the North Atlantic Ocean and on the cast coast of Barbados, West Indies. The measurements were made on air samples at normal temperatures as well as on air samples heated to various temperatures up to 600C. In this way the volatility of the cloud nuclei was compared to the volatility as measured in a similar manner in the laboratory on nuclei artificially generated and of a known composition.The measurements at Barbados showed that 50% of the cloud nuclei were nonvolatile at the temperatures used and thus were similar to artificially generated nuclei composed of sea salt; the remaining nuclei were destroyed by temperatures 320C. The aircraft measurements showed the fraction of volatile cloud nuclei to increase with altitude with all nuclei being volatile above the inversion layer.These measurements indicate that in a subsident marine atmosphere only a fraction of the cloud nuclei at the sea surface are composed of sea salt, this fraction decreasing with altitude such that the sea salt nuclei are confined to the lower few kilometers. Based on the work of other investigators it is suggested that the volatile cloud nuclei are sulfates or sulfuric acid particles which result from the oxidation in the atmosphere of S0or HS.
    Ervens, B., Coauthors, 2010: CCN predictions using simplified assumptions of organic aerosol composition and mixing state: A synthesis from six different locations.Atmos. Chem. Phys.10,4795-4807,doi: 10.5194/acp-1047952010.10.5194/acp-10-4795-20108db06016253c56f942163b7483908ee2http%3A%2F%2Fwww.oalib.com%2Fpaper%2F2701785http://www.atmos-chem-phys.net/10/4795/2010/An accurate but simple quantification of the fraction of aerosol particles that can act as cloud condensation nuclei (CCN) is needed for implementation in large-scale models. Data on aerosol size distribution, chemical composition, and CCN concentration from six different locations have been analyzed to explore the extent to which simple assumptions of composition and mixing state of the organic fraction can reproduce measured CCN number concentrations. lt;brgt;lt;brgt; Fresher pollution aerosol as encountered in Riverside, CA, and the ship channel in Houston, TX, cannot be represented without knowledge of more complex (size-resolved) composition. For aerosol that has experienced processing (Mexico City, Holme Moss (UK), Point Reyes (CA), and Chebogue Point (Canada)), CCN can be predicted within a factor of two assuming either externally or internally mixed soluble organics although these simplified compositions/mixing states might not represent the actual properties of ambient aerosol populations, in agreement with many previous CCN studies in the literature. Under typical conditions, a factor of two uncertainty in CCN concentration due to composition assumptions translates to an uncertainty of ~15% in cloud drop concentration, which might be adequate for large-scale models given the much larger uncertainty in cloudiness.
    Fitzgerald J. W., 1973: Dependence of the supersaturation spectrum of CCN on aerosol size distribution and composition.J. Atmos. Sci.30(4),628-634,doi: 10.1175/1520-0469(1973) 030<0628:DOTSSO>2.0.CO;2.10.1175/1520-0469(1973)0302.0.CO;236c6f41625c7476185ba9016d6e4c639http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1973JAtS...30..628Fhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281973%29030%3C0628%3ADOTSSO%3E2.0.CO%3B2Simultaneous measurements of aerosol size distribution and the supersaturation spectrum of cloud condensation nuclei (CCN) made at the Second International Workshop on Condensation and Ice Nuclei are used to investigate the relationship of CCN supersaturation spectra to aerosol size distribution and composition. The measured CCN spectra are compared with those calculated from the aerosol size distributions for a wide range of values of particle solubility. For each of three natural aerosol samples considered, the shape of the calculated CCN spectra is shown to be quite insensitive to aerosol solubility and to agree quite well with the shape of the measured spectrum. These results tend to confirm the conclusion of Junge and McLaren that the shape of CCN supersaturation spectra is primarily determined by aerosol size distribution rather than by aerosol composition. The concentration of CCN is, however, critically dependent an aerosol composition.An expression is derived for the mean size of CCN active at a given supersaturation as a function of particle solubility and the slope of the CCN supersaturation spectrum. From this equation it is shown that Twomey's measurements of the mean size of CCN active at 0.35 and 0.75% supersaturation imply a mean CCN composition of greater than 70% soluble material, assuming the soluble constituent of CCN to be ammonium sulfate. A comparison of the measured aerosol size distributions with those inferred from the CCN supersaturation spectra on the assumption that the CCN component of the aerosols has a mean solubility of 70% shows that only about 35-40% of the aerosol population in the size range 0.03-0.1 m diameter is active at supersaturations between 0,2 and 1.0%. The fraction of soluble material in the natural aerosol samples used at the Workshop is estimated to be between 15 and 35%.
    Fitzgerald J. W., 1991: Marine aerosols: A review.Atmospheric Environment. Part A. General Topics25(3-4),533-545,doi: 10.1016/0960-1686(91)90050-H.10.1016/0960-1686(91)90050-Hhttp://linkinghub.elsevier.com/retrieve/pii/096016869190050H
    Gras J. L., 1990: Cloud condensation nuclei over the Southern Ocean.Geophys. Res. Lett.17,1565-1567,doi: 10.1029/ GL017i010p01565.10.1029/GL017i010p01565http://doi.wiley.com/10.1029/GL017i010p01565
    Hegg D. A., P. V. Hobbs, 1992: Cloud condensation nuclei in the marine atmosphere: A Review, Proceedings of the Thirteenth International Conference on Nucleation and Atmospheric Aerosols, Hampton, VA, Deepak Publishing, 181- 192.
    Hegg D. A., D. S. Covert, and H. H. Jonsson, 2008: Measurements of size-resolved hygroscopicity in the California coastal zone. Atmos. Chem. Phys., 8, 7193-7203, doi:10.5194/acp-8-7193-2008.
    Hegg D. A., D. S. Covert, D. S., H. H. Jonsson, H. H., and R. Woods, R., 2009: Differentiating natural and anthropogenic cloud condensation nuclei in the California coastal zone.Tellus61B,669-676,http://dx.doi.org/10.1111/j.1600-0889.2009.00435.x.10.1111/j.1600-0889.2009.00435.xa16c95684e5c7aae8bff427b05f92083http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093250535.htmlhttp://www.cabdirect.org/abstracts/20093250535.htmlAerosol samples were collected and cloud condensation nuclei (CCN) concentrations at five supersaturations were measured along and off the central California coast within the cloud-topped, marine boundary layer from aircraft flights during August 2007. Receptor modelling has been applied to estimate the natural versus anthropogenic source contribution of cloud condensation nuclei in this region, a region of climatically important marine stratocumulus. The results suggest that anthropogenic CCN accounted for about 50&#37; of the CCN active at 0.3&#37; supersaturation in this region during the measurement period.
    Hegg D. A., L. F. Radke, L. F., and P. V. Hobbs, 1991: Measurements of Aitken nuclei and cloud condensation nuclei in the marine atmosphere and their relation to the DMS-cloud-climate hypothesis.J. Geophys. Res.96,18 727-18 733,doi: 10.1029/91JD01870.10.1029/91JD018702fbd07dd29efe8f6ceefb0682619562chttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F92JD00448%2Fpdfhttp://doi.wiley.com/10.1029/91JD01870New airborne measurements provide support for the hypothesis that layers of high concentrations of Aitken nuclei near the tops of marine clouds are due to photochemical nucleation. They also reveal a significant correlation between cloud condensation nucleus (CCN) concentrations in the boundary layer and mean cloud droplet concentration in stratus clouds topping a marine boundary layer. Nonsea salt sulfate mass and the concentration of CCN active at 1% supersaturation are also significantly correlated. These results provide quantitative support for some facets of the DMS-cloud-climate hypothesis.
    Hudson J. G., 2007: Variability of the relationship between particle size and cloud-nucleating ability. Geophys. Res. Lett., 34,L08801, doi: 10.1029/2006GL028850.10.1029/2006GL028850254bbc89f20169fcd2ddfaefa7b72ee0http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2006GL028850%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2006GL028850/fullCloud condensation nuclei (CCN) are characterized by their critical supersaturation (S), which is a function of particle size and chemistry, namely water solubility. Measurements that relate particle size to Scan thus be used to determine CCN solubility. A sufficiently small degree of variability of size-Smeasurements has been cited as evidence that CCN can be deduced from particle size measurements alone. Since particle size is so much easier to measure than particle chemistry or CCN this would have significant advantages for investigations of the largest climate uncertainty, the indirect aerosol effect; e.g., remote sensing of CCN. However, we present size-Smeasurements with a greater range of variability, which appears to at least limit or cast doubts on the practicality of deducing CCN from particle size measurements.
    Hudson J. G., T. J. Garrett, P. V. Hobbs, S. R. Strader, Y. H. Xie, and S. S. Yum, 2000: Cloud condensation nuclei and ship tracks.J. Atmos. Sci.57,2696-2706,doi: 10.1175/1520-0469(2000)057<2696:CCNAST>2.0.CO;2.10.1175/1520-0469(2000)0572.0.CO;2758200d1e793c0dbfb9095412f081cdchttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2000JAtS...57.2696Hhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0469%282000%29057%3C2696%3ACCNAST%3E2.0.CO%3B2Enhancements of droplet concentrations in clouds affected by four ships were fairly accurately predicted from ship emission factors and plume and background cloud condensation nucleus (CCN) spectra. Ship exhausts thus accounted for the increased droplet concentrations in these `ship tracks.' Derived supersaturations were typical of marine stratus clouds, although there was evidence of some lowering of supersaturations in some ship tracks closer to the ships where CCN and droplet concentrations were very high.Systematic differences were measured in the emission rates of CCN for different engines and fuels. Diesel engines burning low-grade marine fuel oil produced order of magnitude higher CCN emissions than turbine engines burning higher-grade fuel. Consequently, diesel ships burning low-grade fuel were responsible for nearly all of the observed ship track clouds. There is some evidence that fuel type is a better predictor of ship track potential than engine type.
    Krüger, M. L., Coauthors, 2014: Assessment of cloud supersaturation by size-resolved aerosol particle and cloud condensation nuclei (CCN) measurements.Atmospheric Measurement Techniques7,2615-2629,doi: 10.5194/amt-7-2615-2014.10.5194/amtd-6-10021-2013b72e0b8233d4ce0a1f93fb622d9d2010http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014AMT.....7.2615Khttp://www.atmos-meas-tech.net/7/2615/2014/In this study we show how size-resolved measurements of aerosol particles and cloud condensation nuclei (CCN) can be used to characterize the supersaturation of water vapor in a cloud. The method was developed and applied during the ACRIDICON-Zugspitze campaign (17 September to 4 October 2012) at the high-Alpine research station Schneefernerhaus (German Alps, 2650ma.s.l.). Number size distributions of total and interstitial aerosol particles were measured with a scanning mobility particle sizer (SMPS), and size-resolved CCN efficiency spectra were recorded with a CCN counter system operated at different supersaturation levels. During the evolution of a cloud, aerosol particles are exposed to different supersaturation levels. We outline and compare different estimates for the lower and upper bounds (Slow, Shigh) and the average value (S-avg) of peak supersaturation encountered by the particles in the cloud. A major advantage of the derivation of Slow and S-avg from size-resolved CCN efficiency spectra is that it does not require the specific knowledge or assumptions about aerosol hygroscopicity that are needed to derive estimates of S-low, S-high, and S-avg from aerosol size distribution data. For the investigated cloud event, we derived S-low approximate to 0.07-0.25 %, S-high approximate to 0.86-1.31% and S-avg approximate to 0.42-0.68 %.
    Leena P. P., G. Pand ithurai, V. Anilkumar, P. Murugavel, S. M. Sonbawne, and K. K. Dani, 2016: Seasonal variability in aerosol,CCN and their relationship observed at a high aLSTitude site in Western Ghats. Meteor. Atmos. Phys., 128, 143-153, doi: 10.1007/s00703-015-0406-0-.
    McFiggans G., andCoauthors, LST2006: The effect of physical and chemical aerosol properties on warm cloud droplet activation. Atmos. Chem. Phys., 6, 2593-2649, doi: 10.5194/acp-62593-2006.
    Murugavel P., D. M. Chate, 2011: Volatile properties of atmospheric aerosols during nucleation events at Pune,India. Journal of Earth System Science, 120, 1-17, doi: 10.1007/s12040-011-0072-7.10.1007/s12040-011-0072-7af06df48bf5dd98f68a1ffc18c430ac2http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs12040-011-0072-7http://link.springer.com/article/10.1007/s12040-011-0072-7Continuous measurements of aerosol size distributions in the mid-point diameter range 20.5–50002nm were made from October 2005 to March 2006 at Pune (18°32′N, 73°51′E), India using Scanning Mobility Particle Sizer (SMPS). Volatilities of atmospheric aerosols were also measured at 40°, 125°, 175°, 300° and 350°C temperatures with Thermodenuder–SMPS coupled system to determine aerosol volatile fractions. Aerosols in nucleated, CCN and accumulated modes are characterized from the measured percentage of particles volatized at 40°, 125°, 175°, 300° and 350°C temperatures. Averaged monthly aerosol concentration is at its maximum in November and gradually decreases to its minimum at the end of March. The diurnal variations of aerosol concentrations gradually decrease in the night and in early morning hours (0400–080002hr). However, concentration attains minimum in its variations in the noon (1400–160002hr) due to higher ventilation factor (product of mixing height and wind speed). The half an hour averaged diurnal variation of aerosol number concentration shows about 5 to 10-fold increase despite the ventilation factor at higher side before 120002hr. This sudden increase in aerosol concentrations is linked with prevailing conditions for nucleation bursts. The measurement of volatile fraction of ambient aerosols reveals that there are large number of highly volatile particles in the Aitken mode in the morning hours and these volatile fractions of aerosols at temperatures <150°C are of ammonium chloride and ammonium sulfate, acetic and formic acids.
    Petters M. D., S. M. Kreidenweis, 2007: A single parameter representation of hygroscopic growth and cloud condensation nucleus activity.Atmos. Chem. Phys.7,1961-1971,doi: 10.5194/acp-7-1961-2007.10.5194/acp-7-1961-200747da980e3dda255ca9c75ce3c2ab72b4http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.5194%2Facp-7-1961-2007http://www.atmos-chem-phys.net/7/1961/2007/The ability of a particle to serve as a cloud condensation nucleus in the atmosphere is determined by its size, hygroscopicity and its solubility in water. Usually size and hygroscopicity alone are sufficient to predict CCN activity. Single parameter representations for hygroscopicity have been shown to successfully model complex, multicomponent particles types. Under the assumption of either complete solubility, or complete insolubility of a component, it is not necessary to explicitly include that component's solubility into the single parameter framework. This is not the case if sparingly soluble materials are present. In this work we explicitly account for solubility by modifying the single parameter equations. We demonstrate that sensitivity to the actual value of solubility emerges only in the regime of 2×10611–5×10614, where the solubility values are expressed as volume of solute per unit volume of water present in a saturated solution. Compounds that do not fall inside this sparingly soluble envelope can be adequately modeled assuming they are either infinitely soluble in water or completely insoluble.
    Pruppacher H. R., J. D. Klett, 2010: Microphysics of Clouds and Precipitation: Atmospheric and Oceanographic Sciences Library. Springer,954 pp.
    Ramana M. V., A. Devi, 2016: CCN concentrations and BC warming influenced by maritime ship emitted aerosol plumes over southern Bay of Bengal. Sci. Rep., 6,30416, doi: 10.1038/srep30416.10.1038/srep30416ebe935ca13fd0b4286c5957635eafadchttp%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC4969613%2Fhttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC4969613/Significant quantities of carbon soot aerosols are emitted into pristine parts of the atmosphere by marine shipping. Soot impacts the radiative balance of the Earth-atmosphere system by absorbing solar-terrestrial radiation and modifies the microphysical properties of clouds. Here we examined the impact of black carbon (BC) on net warming during monsoon season over southern Bay-of-Bengal, using surface and satellite measurements of aerosol plumes from shipping. Shipping plumes had enhanced the BC concentrations by a factor of four around the shipping lane and exerted a strong positive influence on net warming. Compiling all the data, we show that BC atmospheric heating rates for relatively-clean and polluted-shipping corridor locations to be 0.06 and 0.16塊/day respectively within the surface layer. Emissions from maritime ships had directly heated the lower troposphere by two-and-half times and created a gradient of around 0.1塊/day on either side of the shipping corridor. Furthermore, we show that ship emitted aerosol plumes were responsible for increase in the concentration of cloud condensation nuclei (CCN) by an order of magnitude that of clean air. The effects seen here may have significant impact on the monsoonal activity over Bay-of-Bengal and implications for climate change mitigation strategies.
    Roberts G. C., A. Nenes, 2005: A continuous-flow streamwise thermal-gradient CCN chamber for atmospheric measurements.Aerosol Science and Technology39,206-221,doi: 10.1080/027868290913988.10.1080/027868290913988b3350a0e9c4553b5e38eed270604b620http%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Ffull%2F10.1080%2F027868290913988http://www.tandfonline.com/doi/abs/10.1080/027868290913988We have addressed the need for improved measurements of cloud condensation nuclei (CCN) by developing a continuous-flow instrument that provides in situ measurements of CCN. The design presented in this article can operate between 0.1 and 3% supersaturation, at sampling rates sufficient for airborne operation. The design constitutes a cylindrical continuous-flow thermal-gradient diffusion chamber employing a novel technique of generating a supersaturation: by establishing a constant streamwise temperature gradient so that the difference in water vapor and thermal diffusivity yield a quasi-uniform centerline supersaturation. Our design maximizes the growth rate of activated droplets, thereby enhancing the performance of the instrument. The temperature gradient and the flow through the column control the supersaturation and may be modified to retrieve CCN spectra. The principle of the CCN instrument was validated in controlled laboratory experiments at different operating conditions using a monodisperse aerosols with known composition and size. These experiments yield sharp activation curves, even for those kinetically limited particles that have not exceeded their critical diameter. The performance of the CCN instrument was also assessed using polydisperse laboratory-generated aerosol of known composition and size distributions similar to ambient particulate matter. In all tests, the measured CCN concentrations compared well with predicted values and highlight the instrument's ability to measure CCN at various size distributions. The full potential of the new design has yet to be explored; however, model simulations suggest that direct measurements in the climatically important range of supersaturations of less than 0.1% (certainly down to 0.07%) are possible. The new instrument clearly offers a unique level of design simplicity, robustness, and flexilibity (temperature control, large range of supersaturations without flow reversal, and multiple configurations for same supersaturation) necessary for atmospheric studies.
    Rose D., S. S. Gunthe, E. Mikhailov, G. P. Frank, U. Dusek, M. O. Andreae, and U. Pöschl, 2008: Calibration and measurement uncertainties of a continuous-flow cloud condensation nuclei counter (DMT-CCNC): CCN activation of ammonium sulfate and sodium chloride aerosol particles in theory and experiment.Atmos. Chem. Phys.8,1153-1179,doi: 10.5194/acp-8-1153-2008.10.5194/acp-8-1153-2008http://www.atmos-chem-phys.net/8/1153/2008/
    Su, H., Coauthors, 2010: Hygroscopicity distribution concept for measurement data analysis and modeling of aerosol particle mixing state with regard to hygroscopic growth and CCN activation.Atmos. Chem. Phys.10,7489-7503,doi: 10.5194/acp-10-7489-2010.10.5194/acp-10-7489-201002be343d62edb44ffeb1356acbe0b8efhttp%3A%2F%2Fwww.oalib.com%2Fpaper%2F1366316http://www.atmos-chem-phys.net/10/7489/2010/This paper presents a general concept and mathematical framework of particle hygroscopicity distribution for the analysis and modeling of aerosol hygroscopic growth and cloud condensation nucleus (CCN) activity. The cumulative distribution function of particle hygroscopicity, H(, Dd) is defined as the number fraction of particles with a given dry diameter, Dd, and with an effective hygroscopicity parameter smaller than the parameter . From hygroscopicity tandem differential mobility analyzer (HTDMA) and size-resolved CCN measurement data, H(, Dd) can be derived by solving the -Khler model equation. Alternatively, H(, Dd) can be predicted from measurement or model data resolving the chemical composition of single particles. A range of model scenarios are used to explain and illustrate the concept, and exemplary practical applications are shown with HTDMA and CCN measurement data from polluted megacity and pristine rainforest air. Lognormal distribution functions are found to be suitable for approximately describing the hygroscopicity distributions of the investigated atmospheric aerosol samples. For detailed characterization of aerosol hygroscopicity distributions, including externally mixed particles of low hygroscopicity such as freshly emitted soot, we suggest that size-resolved CCN measurements with a wide range and high resolution of water vapor supersaturation and dry particle diameter should be combined with comprehensive HTDMA measurements and size-resolved or single-particle measurements of aerosol chemical composition, including refractory components. In field and laboratory experiments, hygroscopicity distribution data from HTDMA and CCN measurements can complement mixing state information from optical, chemical and volatility-based techniques. Moreover, we propose and intend to use hygroscopicity distribution functions in model studies investigating the influence of aerosol mixing state on the formation of cloud droplets.
    Varghese, M., Coauthors, 2015: Airborne and ground based CCN spectral characteristics: Inferences from CAIPEEX- 2011.Atmos.Environ.,125,324-336,doi:10.1016/j.atmosenv. 2015.06.041.10.1016/j.atmosenv.2015.06.041d66c9a9f3f77d15cc196fed0ab3daceahttp%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS1352231015301825http://www.sciencedirect.com/science/article/pii/S1352231015301825A first time comprehensive study of Cloud Condensation Nuclei (CCN) and associated spectra from both airborne and ground campaigns of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) conducted over the rain shadow region of Western Ghats during September and October 2011 is illustrated. Observations of CCN spectra during clean, polluted and highly polluted conditions indicated significant differences between airborne and ground observations. Vertical variation of CCN concentration is illustrated from airborne observations in the clean, polluted and highly polluted conditions with different air mass characteristics. The cloud base CCN number concentrations are three times less than that of the surface measurements at different supersaturations. Diurnal variations of the ground based CCN number concentration and activation diameter showed bimodality. Atmospheric mixing in the wet conditions is mainly through mechanical mixing. The dry conditions favored convective mixing and were dominated by more CCN than the wet conditions. New particle formation and growth events have been observed and were found more often on days with convective mixing. The average critical activation diameter (at 0.6% SS) observed at the ground is approximately 60nm and availability of a large number of particles below this limit was due to the new particle formation. Observations give convincing evidence that the precipitable water and liquid water path is inversely proportional to surface CCN number concentration, and this relationship is largely dictated by the meteorological conditions.
  • [1] FENG Junqiao, HU Dunxin, YU Lejiang, 2013: Role of Western Pacific Oceanic Variability in the Onset of the Bay of Bengal Summer Monsoon, ADVANCES IN ATMOSPHERIC SCIENCES, 30, 219-234.  doi: 10.1007/s00376-012-2040-9
    [2] Ruth GEEN, Marianne PIETSCHNIG, Shubhi AGRAWAL, Dipanjan DEY, F. Hugo LAMBERT, Geoffrey K. VALLIS, 2023: The Relationship between Model Biases in East Asian Summer Monsoon Rainfall and Land Evaporation, ADVANCES IN ATMOSPHERIC SCIENCES, 40, 2029-2042.  doi: 10.1007/s00376-023-2297-1
    [3] LI Wei-Wei, WANG Chunzai, WANG Dongxiao, YANG Lei, DENG Yi, 2012: Modulation of Low-Latitude West Wind on Abnormal Track and Intensity of Tropical Cyclone Nargis (2008) in the Bay of Bengal, ADVANCES IN ATMOSPHERIC SCIENCES, 29, 407-421.  doi: 10.1007/s00376-011-0229-y
    [4] Xiaoli ZHOU, Wen ZHOU, Dongxiao WANG, Qiang XIE, Lei YANG, Qihua PENG, 2024: Westerlies Affecting the Seasonal Variation of Water Vapor Transport over the Tibetan Plateau Induced by Tropical Cyclones in the Bay of Bengal, ADVANCES IN ATMOSPHERIC SCIENCES, 41, 881-893.  doi: 10.1007/s00376-023-3093-7
    [5] HE Jinhai, JU Jianhua, WEN Zhiping, L\"U Junmei, JIN Qihua, 2007: A Review of Recent Advances in Research on Asian Monsoon in China, ADVANCES IN ATMOSPHERIC SCIENCES, 24, 972-992.  doi: 10.1007/s00376-007-0972-2
    [6] Lu Riyu, Chan-Su Ryu, Buwen Dong, 2002: Associations between the Western North Pacific Monsoon and the South China Sea Monsoon, ADVANCES IN ATMOSPHERIC SCIENCES, 19, 12-24.  doi: 10.1007/s00376-002-0030-z
    [7] DING Yihui, LIU Yanju, SONG Yafang, ZHANG Jin, 2015: From MONEX to the Global Monsoon: A Review of Monsoon System Research, ADVANCES IN ATMOSPHERIC SCIENCES, 32, 10-31.  doi: 10.1007/s00376-014-0008-7
    [8] Jiefan YANG, Hengchi LEI, Yuhuan LÜ, 2017: Airborne Observations of Cloud Condensation Nuclei Spectra and Aerosols over East Inner Mongolia, ADVANCES IN ATMOSPHERIC SCIENCES, 34, 1003-1016.  doi: 10.1007/s00376-017-6219-y
    [9] Xiaofei LI, Qinghong ZHANG, Huiwen XUE, 2017: The Role of Initial Cloud Condensation Nuclei Concentration in Hail Using the WRF NSSL 2-moment Microphysics Scheme, ADVANCES IN ATMOSPHERIC SCIENCES, 34, 1106-1120.  doi: 10.1007/s00376-017-6237-9
    [10] Xue Feng, Bi Xunqiang, Lin Yihua, 2001: Modelling the Global Monsoon System by IAP 9L AGCM, ADVANCES IN ATMOSPHERIC SCIENCES, 18, 404-412.  doi: 10.1007/BF02919319
    [11] Xu Jianjun, Wu Guoxiong, 1999: Dynamic Features and Maintenance Mechanism of Asian Summer Monsoon Subsystem, ADVANCES IN ATMOSPHERIC SCIENCES, 16, 523-536.  doi: 10.1007/s00376-999-0028-x
    [12] Jong-Suk KIM, ZHOU Wen, Ho Nam CHEUNG, Chak Hang CHOW, 2013: Variability and Risk Analysis of Hong Kong Air Quality Based on Monsoon and El Nino Conditions, ADVANCES IN ATMOSPHERIC SCIENCES, 30, 280-290.  doi: 10.1007/s00376-012-2074-z
    [13] CHEN Guanghua, HUANG Ronghui, 2008: Influence of Monsoon over the Warm Pool on Interannual Variation on Tropical Cyclone Activity over the Western North Pacific, ADVANCES IN ATMOSPHERIC SCIENCES, 25, 319-328.  doi: 10.1007/s00376-008-0319-7
    [14] Lanqiang Bai, Dan Yao, Zhiyong Meng, Yu Zhang, Xianxiang Huang, Zhaoming Li, 2023: Influence of Irregular Coastlines on a Tornadic Mesovortex in the Pearl River Delta during Monsoon Season. Part II: Numerical Experiments, ADVANCES IN ATMOSPHERIC SCIENCES.  doi: 10.1007/s00376-023-3096-4
    [15] Qian Weihong, Zhu Yafen, Xie An, Ye Qian, 1998: Seasonal and Interannual Variations of Upper Tropospheric Water Vapor Band Brightness Temperature over the Global Monsoon Regions, ADVANCES IN ATMOSPHERIC SCIENCES, 15, 337-345.  doi: 10.1007/s00376-998-0005-9
    [16] Lanqiang BAI, Dan YAO, Zhiyong MENG, Yu ZHANG, Xianxiang HUANG, Zhaoming LI, Xiaoding YU, 2024: Influence of Irregular Coastlines on a Tornadic Mesovortex in the Pearl River Delta during the Monsoon Season. Part I: Pre-storm Environment and Storm Evolution, ADVANCES IN ATMOSPHERIC SCIENCES.  doi: 10.1007/s00376-023-3095-5
    [17] P. L. S. Rao, 2000: The Influence of Systematic Errors on the Asian Summer Monsoon Circulation, ADVANCES IN ATMOSPHERIC SCIENCES, 17, 576-586.  doi: 10.1007/s00376-000-0021-x
    [18] Lonnie Hudgins, Jianping Huang, 1996: Bivariate Wavelet Analysis of Asia Monsoon and ENSO, ADVANCES IN ATMOSPHERIC SCIENCES, 13, 299-312.  doi: 10.1007/BF02656848
    [19] Jiang Shangcheng, Ye Qian, Yang Xifeng, An Gang, Xiangqiang Wu, 2000: Climatological Features of the Global Tropical Subsidence Region Based on Satellite Observations, ADVANCES IN ATMOSPHERIC SCIENCES, 17, 391-402.  doi: 10.1007/s00376-000-0031-8
    [20] Wang Huijun, 2002: The Mid-Holocene Climate Simulated by a Grid-Point AGCM Coupled with a Biome Model, ADVANCES IN ATMOSPHERIC SCIENCES, 19, 205-218.  doi: 10.1007/s00376-002-0017-9

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Manuscript received: 28 December 2016
Manuscript revised: 30 March 2017
Manuscript accepted: 10 April 2017
通讯作者: 陈斌, bchen63@163.com
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Cloud Condensation Nuclei over the Bay of Bengal during the Indian Summer Monsoon

  • 1. Indian Institute of Tropical Meteorology, Pune 411008, India
  • 2. Marine, Earth and Atmospheric Science, North Carolina State University, Raleigh, NC 27695, USA
  • 3. Amity Centre for Ocean-Atmospheric Science and Technology (ACOAST) & Amity Centre for Environmental Science and HeaLSTh (ACESH), Amity University Haryana, Gurgaon-Manesar 122413, India

Abstract: The first measurements of cloud condensation nuclei (CCN) at five supersaturations were carried out onboard the research vessel "Sagar Kanya" (cruise SK-296) from the south to the head-bay of the Bay of Bengal as part of the Continental Tropical Convergence Zone (CTCZ) Project during the Indian summer monsoon of 2012. In this paper, we assess the diurnal variation in CCN distributions at supersaturations from 0.2% to 1% (in steps of 0.2%) and the power-law fit at supersaturation of 1%. The diurnal pattern shows peaks in CCN concentration (N CCN) at supersaturations from 0.2% to 1% between 0600 and 0700 LST (local standard time, UTC+0530), with relatively low concentrations between 1200 and 1400 LST, followed by a peak at around 1800 LST. The power-law fit for the CCN distribution at different supersaturation levels relates the empirical exponent (k) of supersaturation (%) and the N CCN at a supersaturation of 1%. The N CCN at a supersaturation of 0.4% is observed to vary from 702 cm-3 to 1289 cm-3, with a mean of 961 161 cm-3 (95% confidence interval), representing the CCN activity of marine air masses. Whereas, the mean N CCN of 1628 193 cm-3 at a supersaturation of 1% is higher than anticipated for the marine background. When the number of CCN spectra is 1293, the value of k is 0.57 0.03 (99% confidence interval) and its probability distribution shows cumulative counts significant at k≈ 0.55 0.25. The results are found to be better at representing the features of the marine environment (103 cm-3 and k≈ 0.5) and useful for validating CCN closure studies for Indian sea regions.

摘要: 作为大陆热带辐合带(CTCZ)计划的组成部分, 利用“萨加尔坎亚”科考船(编号SK-296)船载仪器对2012年印度夏季风期间孟加拉湾南部到前端地区五种过饱和度下的云凝结核(CCN)进行了首次观测. 本文分析了过饱和度从0.2%到1%(间隔为0.2%)下的CCN日变化特征和在1%过饱和度下使用幂函数拟合的CCN谱分布. 日变化特征显示过饱和度从0.2%到1%时CCN数浓度(NCCN)在0600到0700 LST(当地时间, 协调世界时+0530)出现峰值, 接着在1200到1400 LST出现相对低值, 然后在1800 LST左右又出现峰值. 不同过饱和度下幂函数拟合的CCN谱分布依赖于和过饱和(%)相关的经验参数k以及1%过饱和度下的CCN数浓度. 过饱和度为0.4%时CCN数浓度的变化范围为702 cm?3到1289 cm?3, 平均值为961 ± 161 cm?3(95%置信区间), 代表了海洋性CCN特征. 然而当过饱和度为1%时, 1628 ± 193 cm?3的平均NCCN高于预期的海洋背景CCN数浓度. 当CCN谱的参数C值是1293时, 参数k值为0.57 ± 0.03(99%置信区间)并且其概率分布的累计频数显著出现在k ≈ 0.55 ± 0.25. 上述结果能够更好的代表海洋性CCN特征(C=103 cm?3和k ≈ 0.5)并且有助于验证印度洋地区的CCN闭合结果. (摘要翻译: 赵震)

1. Introduction
  • Cloud condensation nuclei (CCN) are fractions of atmospheric aerosols that grow to the size of cloud droplets at a specified supersaturation level in marine, continental and in-cloud environments. Low and high CCN concentrations (N CCN) are generally observed in the marine background and polluted regions, respectively (Petters and Kreidenweis, 2007; Andreae, 2009; Bougiatioti et al., 2011; Leena et al., 2016). Field observation campaigns (e.g. Andreae and Rosenfeld, 2008) have improved our understanding of CCN activity (Ervens et al., 2010). CCN formation is an important phenomenon in cloud physics (Fitzgerald, 1973, 1991) and the activation of hygroscopic aerosols to the size of CCN largely depends on the size, source, chemical composition and mixing state of particles that nucleate as CCN and grow into droplets (Su et al., 2010; Asmi et al., 2012). The aerosol hygroscopicity in CCN formation depends on the air mass that prevails over the regions of observation (Pruppacher and Klett, 2010). Though the chemical composition of nuclei that activate into CCN distributions is difficult to measure in real time (Murugavel and Chate, 2011; Krüger et al., 2014), a power-law fit for CCN distributions, as a function of supersaturation with the exponent (k) of supersaturation and C (the N CCN in cm-3 at a supersaturation of 1%), represents the hygroscopicity of particles that nucleate to CCN size (McFiggans et al., 2006; Hegg et al., 2009). For inland stations in India, the CCN distributions along with power-law fits are reported as a function of specified supersaturation over short time scales (order of seconds) using commercially available CCN-100 counters (Leena et al., 2016; Varghese et al., 2015). Similar measurements of CCN distributions at various supersaturation values over Indian sea regions during the southwest summer monsoon can result in a better power-law formulation with C and k. Atmospheric measurements of CCN distributions at a wide range of supersaturation levels from the south to the head-bay region of the Bay of Bengal (BoB) have largely been neglected (CTCZ-Scientific Steering Committee, 2011), except shipborne observations reported by (Ramana and Devi, 2016) for the southernmost tail-bay region of the BoB.

    The present work focuses on an analysis of the in-situ measurements of CCN distributions at a wide range of supersaturation over the barely explored region of the BoB from the south to the head-bay region using CCN-100 counters deployed onboard the research vessel "Sagar Kanya" (cruise SK-296) during the Indian summer monsoon of 2012. During this season, the prevailing air mass maintained a symmetrical supply of moisture over the BoB. Therefore, the measured CCN distributions at a wide range of supersaturation in the study region motivated us to quantify the power-law parameters for southwesterly clean air masses, and to compare them with other marine environments. Also, this work aims to showcase the first observations of CCN distributions at various levels of supersaturation carried out in the marine sector spread over an area from the south to the head-bay region of the BoB during the monsoon season, and to investigate the power-law fit for the empirical parameters C and k for the monsoonal air mass. Moreover, the results from the CCN distribution measurements at a wide range of supersaturation levels are expected to improve our understanding of the CCN activity for monsoonal clouds over Indian sea regions.

2. Instrumentation
  • Atmospheric particles that transform into CCN were measured with a CCN counter (model: CCN-100). This instrument is a continuous flow thermal gradient CCN counter proposed and designed by (Roberts and Nenes, 2005) and manufactured by Droplet Measurement Technologies. The working principle of this CCN counter is to expose the aerosol to a fixed supersaturation at a certain time and to measure the number of activated particles with an inbuilt optical particle counter. Aerosols continuously flow through the center part of a cylinder with a wetted wall. Between the aerosol flow and the wall there is a particle-free sheath flow. By controlling the temperature of the wall as well as keeping it wet (ensuring that the relative humidity is 100% just outside the cylinder wall), the movement of heat and water vapor towards the middle of the cylinder and the supersaturation value can be maintained. The supersaturation values are altered in a cycle for measurement of the activated N CCN. The working principle (thermophoresis) of the instrument depends on the water molecules that diffuse towards the center faster than the heat added across the wall (water molecules diffuse mainly via heavier nitrogen and oxygen molecules, hence giving saturation ratios above 100%). Details on the measurement uncertainties and operational error are discussed elsewhere (Rose et al., 2008; Krüger et al., 2014). Full details on the operation, maintenance and calibration procedure of the CCN-100 counter can be found at http://www.dropletmeasurement.com. Also, (Ramana and Devi, 2016) described the deployment of CCN-100 onboard the research vessel "Sagar Nidhi" during a cruise over the BoB.

3. Study region
  • Figures 1a and b show the ORV Sagarkanya-296 track positions along with wind-rose diagrams, based on observed winds over the BoB during the cruise period from 10 July to 8 August 2012. As seen in Fig. 1a, since the departure of SK-296 from Chennai port on 10 July 2012, it sailed till 13 July 2012 almost parallel to the entire coastline of the Indian peninsula and reached the head-bay region of the BoB on 19 July 2012. The research vessel "Sagar Kanya" remained stationary in the head-bay region from 19 July to 2 August 2012 (Fig. 1a) and thereafter started its return expedition on 2 August 2012 towards the south of the bay, parallel but relatively far away from the Indian coastline, and arrived at Chennai port on 8 August 2012. The Indian summer monsoon season generally extends from June to September when the ITCZ shifts its position over India, maintaining monsoonal cloud cover and moisture supply over the entire country. The dominant circulation pattern is southwesterly clean air masses from June to September, with strong near-surface winds over the Ocean. CCN distributions were continuously monitored at a wide range of supersaturation levels over a period that included the expedition of the SK-296 cruise from the south to the head-bay region of the BoB (10 July to 8 August 2012). A very high total N CCN (∼ 7500 cm-3) was recorded on 17 July 2012 when SK-296 was at Paradeep port in the BoB, and also on 5 August 2012 (due to rain). The prevailing southwesterly air mass maintains the symmetric moisture supply over the BoB, as evident from the wind-rose diagram (Fig. 1b). Several rain showers were encountered during the campaign period, while typical monsoonal clouds passed over the SK-296 track positions across the BoB. The prevailing weather conditions in the marine environment of the BoB during July to August 2012 are described in (Ramana and Devi, 2016). Over the south to the head-bay region of the BoB, the sampling period (10 July to 7 August 2012) of the SK-296 cruise was long enough to represent the monsoonal pattern of CCN distributions at various supersaturation levels.

    Figure 1.  (a) SK-296 track positions. (b) Wind-rose diagram (black shadings mark the southwest and south-southwest winds). (c) Frequency distribution of winds.

4. CCN distribution data
  • The CCN distributions at supersaturations of 0.2%, 0.4%, 0.6%, 0.8% and 1% (covering typical range of supersaturation of the marine to the in-cloud environment) were monitored over the region from the south to the head-bay region of the BoB, round the clock, during 10 July to 7 August 2012. The activated N CCN is given with a temporal resolution of one second and, since it takes a few minutes for the system to come to equilibrium state with the supersaturation, a measurement cycle of 30 minutes is considered for the aforementioned levels of supersaturation (Krüger et al., 2014). The data obtained for CCN distributions as a function of supersaturation have been averaged on an hourly basis for the entire period of the SK-296 cruise campaign to showcase the results on the diurnal scale for the Indian summer monsoon of 2012. Figure 2 shows the diurnal pattern of the CCN distribution for the measurement period from 10 July to 7 August 2012 at each level of supersaturation from 0.2% to 1% (in steps of 0.2%). The diurnal cycle includes the peaks in CCN distributions between 0600 and 0700 LST (local standard time, UTC+0530) of about 634, 1122, 1425, 1619 and 1857 cm-3, followed by lower concentrations of about 543, 736, 874, 1100 and 1428 cm-3 between 1200 and 1400 LST and subsequent peaks at 1800 LST of 784, 1262, 1587, 1754 and 2027 cm-3, for supersaturations of 0.2%, 0.4%, 0.6%, 0.8% and 1%, respectively (Fig. 2). The diurnal cycle for CCN distributions at different values of supersaturation are believed to follow the monsoonal pattern of ventilation coefficients (product of mixing height and wind speed), which diurnally modulates the nucleating particle concentrations over Indian sea regions (Murugavel and Chate, 2011), including the southernmost tail-bay region of the BoB (Ramana and Devi, 2016). Information on the composition of the aerosol population fraction is embedded in the empirical parameter k, which can be extracted from the power-law fit of CCN distributions at various supersaturation levels (SK-296 cruise) over the south to the head-bay region of the BoB.

    The power-law N CCN,S=CSk of CCN distributions at different values of supersaturation describes the CCN activation, where N CCN,S is the concentration of CCN at a specified supersaturation S, C is the CCN concentration at a supersaturation of 1%, and k is the slope of the power-law fit curve. The diurnal variation of k is plotted in Fig. 2b, and shows a first peak at around 0600 LST, a low at 1200 LST, and another peak at 2000 LST. Thus, the diurnal pattern of k is purely due to ventilation conditions.

    Figure 2.  Diurnal patterns of (a) the CCN distributions at each supersaturation level from 0.2% to 1% (in steps of 0.2%) and (b) k, for the measurement period from 10 July to 7 August 2012.

    The variations in the CCN distribution along with the standard deviation (Fig. 3) show an increase in N CCN with the level of supersaturation from 0.2% to 1%. The variations in the slope (k) in Fig. 3 seem to be synchronous with C, where k contains information about the source and mixing state of particles analogous to that of the hygroscopicity of nucleated particles. The results suggest that, during the monsoon season, there is a dominance of hygroscopic particles over the BoB. Furthermore, the average values of C and k in Fig. 3 are C=1659 29 cm-3 (at a supersaturation of 1%) and k=0.57 0.03 (R2=0.99) for the entire dataset (number of CCN distribution spectra ≈ 1300). For N CCN measured along and off the central Californian coast during August 2007, (Hegg et al., 2009) reported the power-law fit parameters as C≈ 328 10 cm-3 and k≈ 0.72 0.06 (R2=0.99). For the present dataset for the SK-296 observation period between 10 July and 7 August 2012, the mean N CCN of 1628 193 cm-3 (at a supersaturation of 1%) appears to be higher than the anticipated value for the marine background, for which C values are more typically of the order of a few hundred CCN cm-3 and k≈ 0.5, as suggested by (Hegg and Hobbs, 1992) and (Hegg et al., 2008). Many observational studies (Dinger et al., 1970; Gras, 1990; Pruppacher and Klett, 2010) have reported a value of k≈ 0.5 for the maritime environment. (Hegg et al., 1991) suggested a value of k>0.5 for N CCN during monsoon. For the marine region, off the central Californian coast, (Hudson et al., 2000) measured background C and k values of about 450 cm-3 and 0.65, respectively; while in June and late July, (Hudson, 2007) reported a value of C≈ 103 cm-3. Thus, our LSTCCN distribution results from the SK-296 expedition corroborate reasonably well with the C and k values reported for marine environments.

    (Ramana and Devi, 2016) reported N CCN at a supersaturation of 0.4% of about 1245-2225 cm-3 (mean N CCN ≈1801 486 cm-3), 191-938 cm-3 (mean N CCN ≈ 418 161 cm-3) and 64-1420 cm-3 (mean N CCN ≈ 291 209 cm-3) for coastal (21 July 2012), clean marine (23 July to 11 August 2012) and shipping lane ranges (13-16 August 2012), respectively, in the southernmost tail-bay region of the BoB. Furthermore, for the sampling period from 21 July to 16 August 2012, they reported the N CCN as 837285 cm-3 at a supersaturation of 0.4% which is a mean of 1801, 418 and 291 cm-3. Similarly, for the entire dataset of the SK-296 expedition (10 July to 7 August 2012), N CCN at a supersaturation of 0.4% varied from 702 to 1289 cm-3, with a mean of 961 151 cm-3. The mean N CCN (837 285 cm-3) at a supersaturation of 0.4% from the southernmost tail-bay region of the BoB reported by (Ramana and Devi, 2016) was lower, by about 15%, than the mean N CCN of the present study (961 151 cm-3 at a supersaturation of 0.4%) for the south to the head-bay region of the BoB over the sampling period from 10 July to 7 August 2012 during the SK-296 cruise.

    Figures 4a and b illustrate the probability distribution of k, with its cumulative counts in percent, for the period of observations from 10 July to 7 August 2012. Figure 4a shows the cumulative counts increase with k for the entire dataset (number of CCN distribution spectra = 1293) obtained during the SK-296 cruise campaign. It is evident from Fig. 4a that the probability counts are significant at k≈ 0.55 0.25. A clear influence of marine-type air masses on the CCN distributions and power-law fit parameters C and k can be seen from the aforementioned analyses of the entire dataset obtained during the SK-296 cruise campaign. This is likely linked to increased natural sources of CCN in the south to the head-bay region of the BoB over the sampling period from 10 July to 7 August 2012 (SK-296) due to enhanced marine-derived aerosols in southwesterly air masses. The parameters C and k with a power-law fit on the entire dataset of the SK-296 cruise, and also from the probability distributions, show the best estimates for a typical marine environment in the tropics, and hence may be applicable to most cloud microphysical studies, including CCN closure studies.

    Figure 3.  Variations in CCN distribution along with standard deviations as a function of supersaturation (%) for the power-law fit on the entire dataset for the period 10 July to 7 August 2012.

    Figure 4.  Probability distributions of k (a) cumulative counts and (b) counts.

5. Summary and conclusions
  • As part of the CTCZ programme, CCN distributions at supersaturations from 0.2% to 1% (in steps of 0.2%) were continuously monitored onboard the research vessel "Sagar Kanya" (SK-296 expedition) during the Indian summer monsoon of 2012. The results of the hourly mean CCN distributions at supersaturations of 0.2% to 1% for the entire dataset on the diurnal scale, and the power-law fit with empirical constants C and k, are discussed in a comparative analysis. The peaks in N CCN appear during morning and evening hours, with lower N CCN during noon hours, at supersaturations of 0.2%, 0.4%, 0.6%, 0.8% and 1%. For the entire dataset from the SK-296 cruise campaign (number of CCN distribution spectra = 1293), the mean CCN concentrations are 1628 193 cm-3 and 961 151 cm-3 at supersaturations of 1% and 0.4%, respectively; while from the power-law fit, k=0.57 0.03 (R2=0.99), and probability distributions, cumulative and probability counts show significance at k=0.55 0.25. Though the mean N CCN at a supersaturation of 1% is higher than expected for the marine background, the mean N CCN at a supersaturation of 0.4%, as well as the C and k, broadly corroborate the results of marine environments. Knowledge of the parameter k is routinely considered to be sufficient for many cloud microphysical applications, while for the SK-296 dataset, k≈ 0.57 0.03 represents the CCN distributions in the marine environments of Indian sea regions. The values of C and k in the present study suggest that the track positions of SK-296 may be impacted by sources other than the sea surface in the case of a few events during the campaign. The quantitative evaluation of the contributing sources to the CCN distributions for Indian sea regions is beyond the scope of this study and can be addressed separately. Also, no significant trend in the monthly (July and August) arithmetic mean N CCN was found for the sampling period, and the conclusion is that more shipborne CCN distribution data are expected to enable a more robust analysis of possible trends. The availability of shipborne data should facilitate an increase in our understanding of the processes linking N CCN, aerosol concentrations and cloud droplet number concentrations (and cloud albedo) for the BoB region.

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