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Effects of Additional HONO Sources on Visibility over the North China Plain

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doi: 10.1007/s00376-014-4019-1

  • The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (2) the reaction of photo-excited nitrogen dioxide (NO2*) with water vapor; (3) the NO2 heterogeneous reaction on aerosol surfaces; and (4) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%30% (34 km) in local urban areas of the NCP. Monthly-mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%52% (311 g m-3), 10%38%, 6%10%, and 6%11% (917 g m-3), respectively; and in urban areas, monthly-mean accumulation-mode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%20% and 10%20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NOx and aerosols.
    摘要: The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (2) the reaction of photo-excited nitrogen dioxide (NO2*) with water vapor; (3) the NO2 heterogeneous reaction on aerosol surfaces; and (4) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%-30% (3-4 km) in local urban areas of the NCP. Monthly-mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%-52% (3-11 g m-3), 10%-38%, 6%-10%, and 6%-11% (9-17 g m-3), respectively; and in urban areas, monthly-mean accumulation-mode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%-20% and 10%-20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NOx and aerosols.
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Manuscript received: 25 January 2014
Manuscript revised: 03 March 2014
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Effects of Additional HONO Sources on Visibility over the North China Plain

    Corresponding author: AN Junling; 
  • 1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029;
  • 2. Cloud Physics and Severe Weather Research Section, Environment Canada, Toronto, Ontario M3H 5T4, Canada
Fund Project:  This research was supported by the Beijing Natural Science Foundation (Grant No. 8144054), the Key Project of the Chinese Academy of Sciences (Grant No. XDB05030301), the National Natural Science Foundation of China (Grant No. 41175105), and the Carbon and Nitrogen Cycle project of the Institute of Atmospheric Physics, Chinese Academy of Sciences. Special thanks are given to Prof. XIE Pinhua from Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, for providing HONO observations in Beijing, and Prof. WANG Yuesi from CERN, LAPC, Institute of Atmospheric Physics, Chinese Academy of Sciences, for offering observed data of O3, PM2.5, and PM10. Thanks are also extended to the anonymous reviewers for key suggestions that helped improve the manuscript.

Abstract: The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (2) the reaction of photo-excited nitrogen dioxide (NO2*) with water vapor; (3) the NO2 heterogeneous reaction on aerosol surfaces; and (4) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%30% (34 km) in local urban areas of the NCP. Monthly-mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%52% (311 g m-3), 10%38%, 6%10%, and 6%11% (917 g m-3), respectively; and in urban areas, monthly-mean accumulation-mode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%20% and 10%20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NOx and aerosols.

摘要: The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (2) the reaction of photo-excited nitrogen dioxide (NO2*) with water vapor; (3) the NO2 heterogeneous reaction on aerosol surfaces; and (4) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%-30% (3-4 km) in local urban areas of the NCP. Monthly-mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%-52% (3-11 g m-3), 10%-38%, 6%-10%, and 6%-11% (9-17 g m-3), respectively; and in urban areas, monthly-mean accumulation-mode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%-20% and 10%-20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NOx and aerosols.

1. Introduction
  • The importance of nitrous acid (HONO) in issues related to tropospheric photochemistry has risen in recent years because of its significant contribution to the hydroxyl radical (OH), which is the key oxidant in the atmosphere (Alicke et al., 2002). Nevertheless, the formation mechanisms of HONO and the strength of its sources remain uncertain. Among known HONO chemical sources, the chemistry of electronically excited NO2 and H2O (NO2* + H2O HONO + OH; hereafter referred to as NO2* chemistry) (Crowley and Carl, 1997; Li et al., 2008; Amedro et al., 2011) has received much attention in recent years for its implication in the enhancement of OH. Studies conclude that the impact of NO2* chemistry on air quality is more significant over urban and coastal areas with high NOx (= NO + NO2) emissions, while it is minor in rural and background areas where NOx emissions are relatively low (Wennberg and Dabdub, 2008; Sarwar et al., 2009; Ensberg et al., 2010; An et al., 2011;

    Li et al., 2011; Jorba et al., 2012; An et al., 2013; Zhang et al., 2013). The maximum enhancement of surface ozone (O3) concentrations due to the NO2* chemistry identified in eastern Asia under contemporary emissions are more than 30 ppbv (Li et al., 2011; Jorba et al., 2012). NO2* chemistry also notably affects mass concentrations of fine particles. During a typical summer episode over the South Coast Air Basin of California, mass concentrations of particulate matter with diameter ≤2.5 μm (PM2.5) and nitrate (NO3-) near Riverside were enhanced by up to 20 μg m-3 and 25%, respectively (Wennberg and Dabdub, 2008; Ensberg et al., 2010). Some of the earlier studies performed over eastern Asia also showed that monthly-mean daytime NO3- concentrations were increased by as much as 7% over the North China Plain (NCP) during August 2007 (An et al., 2013) when the NO2* chemistry was inserted into a fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model (Grell et al., 2005; Fast et al., 2006). These works show the potentially important effects of NO2* chemistry on the air quality of areas with high NOx emissions.

    Another HONO source receiving considerable attention is the NO2 heterogeneous reaction (2NO2 + H2O HNO3 + HONO), which is believed to be the most important formation pathway of nocturnal HONO (Kleffmann et al., 1998; Finlayson-Pitts et al., 2003). Recent studies have reported that heterogeneous NO2 hydrolysis can increase PM2.5 concentrations by more than 10%, and the enhancements are attributable mainly to increased NO3- and ammonium (NH4+) concentrations, albeit with slight increases also found in sulfate (SO42-) and secondary organic aerosols (SOA) (Gonçalves et al., 2012; An et al., 2013).

    The significant contribution of HONO sources to aerosol concentrations (for hydrophilic particles, e.g., SO42-, NO3-, and NH4+) addresses the question of the relative contribution of HONO sources to visibility. Excluding rainy and foggy conditions (Gultepe et al., 2007, 2009), the dominant factor causing visibility degradation is aerosols (Chang et al., 2009; Chen et al., 2012). The NCP, a region with heavy aerosol loadings (Zhao et al., 2006; Yang et al., 2011), is one of the most populated and polluted areas of the world, suffering severely low visibility conditions in recent times. Excluding days of precipitation and fog, the annual mean visibility from 1999 to 2007 in Beijing was reported to have fallen to between 10 and 15 km, with a mean value of below 10 km in summer, the season with the lowest visibility (Zhang et al., 2010).

    It has been shown previously that the low visibility over the NCP is primarily caused by the high mass burden of PM2.5, with sulfate and nitrate as the major contributors to the total extinction coefficient (Quan et al., 2011; Cao et al., 2012; Han et al., 2013). For example, (NH4)2SO4 and NH4NO3 contributed 67% to the total extinction during the 2006 Campaign of Air Quality Research in Beijing (CAREBeijing-2006) (Jung et al., 2009).

    The main purpose of this work is to evaluate the impacts of additional HONO sources on visibility, and explore the key factors of visibility degradation due to HONO sources, which are aspects that have not been studied previously in any great detail. In this work, besides HONO gas-phase production from OH and NO, three additional HONO sources (NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions) coupled into the WRF/Chem model were used to simulate the contributions of HONO to the prediction of visibility over the NCP during August 2007. The model and observations are described in section 2. Section 3 gives an evaluation of the model simulations and discusses how the additional HONO sources impact visibility, and then proposes a conceptual model for the effects of HONO sources on visibility prediction. The conclusions of the study are presented in section 4.

2. Method
  • Version 3.2.1 of the WRF/Chem model (Grell et al., 2005; Fast et al., 2006) has been updated to include three additional HONO sources (Li et al., 2011; An et al., 2011, 2013). The NO2* chemistry was added into the Carbon-Bond Mechanism Z (CBM-Z) (Zaveri and Peters, 1999) gas-phase chemical mechanism. The rate constant for the reaction of NO2* with H2O was estimated as 9.1× 10-14 cm3 molecule-1 s-1 (Li et al., 2011). HONO heterogeneous formation on aerosol surfaces followed the recommendations of (Jacob, 2000), where the total aerosol surface (S a) per volume of air is one of the key factors for computing the reaction rate. In this study, S a was derived from aerosol mass concentrations and the number density in each bin set by the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) (Zaveri et al., 2008), which was fully coupled in the WRF/Chem (Fast et al., 2006). Eight bins for the aerosol size distribution were employed (Table 1). Aerosols in MOSAIC were composed of SO42-, NO3-, NH4+, chloride, sodium, other unspecified inorganics, organic carbon, elemental carbon, and water.

    HONO direct emissions were estimated as 0.8% for the HONO/NOx emission ratio (Kurtenbacha et al., 2001). Besides direct emissions, HONO emissions in this study also included 2.3% of the NOx emitted in diesel exhaust converted to HONO via the heterogeneous reaction with semivolatile organics (Gutzwiller et al., 2002). The final ratio for HONO/NOx as HONO emissions was computed as 1.18% in the urban center of Beijing (Li et al., 2011; An et al., 2013).

    For the WRF/Chem configuration, three nested domains were employed, with the third domain covering the NCP (Fig. 1). The finest horizontal resolution was 9 km, with 28 levels ranging from the surface to 50 hPa. Initial and boundary conditions for meteorological fields were obtained from the US National Centers for Environmental Prediction (NCEP) analysis data applied to nudging every 6 h, and those for chemical fields were constrained every 6 h from the outputs of the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4) (Emmons et al., 2010). Monthly anthropogenic emissions used in this study were derived from (Zhang et al., 2009).

    The detailed physical and chemical schemes for the simulations can be found in (Li et al., 2011). An evaluation of the updated model has been conducted for the NCP during August 2007 (Li et al., 2011), in which it was stated that simulations of HONO, O3 and NOx concentrations were improved when the additional HONO sources were considered. Diurnal variations of PM2.5 and PM10 concentrations were satisfactorily reproduced. The simulated aerosol number (N a) and S a were found to be within the observed range (Li et al., 2011; An et al., 2013; Tang et al., 2014).

    Figure 1.  Modeling domains and meteorological stations providing the observed visibility data.

  • One of the important objectives of this paper is to evaluate the model's performance in predicting atmospheric visibility. Daytime visibility (V d) can be calculated by the Koschmieder equation (Koschmieder, 1924):

    where β ext is the extinction coefficient and ε is the brightness contrast threshold assumed as 0.02 in this work (Gultepe et al., 2006). The MOSAIC aerosol module calculated the aerosol β ext based on Mie theory (Ackerman and Toon, 1981; Barnard et al., 2010), and values at 550 nm were adopted in this study.

    Nighttime visibility (V n) was obtained using the simplified Allard's law (Gultepe et al., 2008):

    where C DB=0.084 miles-1 and I0=25 candela. Comparing Eqs. (2) and (3), a simplified equation for the relationship of V d and V n was given by (Gultepe et al., 2008):

  • The observed visibility data were collected at more than 600 meteorological stations in China. Those stations located on the NCP are shown in Fig. 1(D3). Visibility was observed by trained observers under standard procedures of the Chinese Meteorological Administration (Chang et al., 2009; Lin et al., 2012). Observations were available at local times of 0200, 0800, 1400 and 2000, except for the stations of Zhangbei, Datong, Taiyuan, Fengning, Miyun, Tianjin and Raoyang, which conduct hourly visibility observations. In this study, observations during periods of precipitation or relative humidity (RH) exceeding 90% were discarded (Mebust et al., 2003; Chang et al., 2009) to prevent hydrometeors from biasing the evaluation.

    The HONO concentrations were measured by Differential Optical Absorption Spectroscopy (DOAS) at the Meteorological Tower (39.8°N, 116.3°E) in Beijing during August of 2007 (Zhu et al., 2009). Observed hourly concentrations of O3, PM2.5 and PM10 across the NCP were taken from the Beijing Atmospheric Environmental Monitoring Action (Li et al., 2011). Daily-mean concentrations of SO42-, NO3-, and NH4+ were measured at Peking University (40.0°N, 116.3°E) during 2-31 August 2007 as part of the CAREBeijing-2007 Experiment (Ianniello et al., 2011).

  • Five simulations were conducted in this study. The first one was a "reference case" (Case R), which was performed using the standard CBM-Z mechanism and the MOSAIC module. The other three cases included the NO2* chemistry, NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions, respectively. The last simulation, called the "enhanced case" (Case E) included all of the three HONO sources. Sensitivity simulations were performed for 1-31 August 2007 with a spin-up period of seven days.

3. Results and discussion
  • An evaluation of the WRF/Chem model in terms of the spatial pattern of monthly-mean visibility at 1400 LST (local standard time) is shown in Fig. 2. The visibility observations ranged from 9 km to 29 km across the NCP during August 2007; visibility was down to <15 km in the south. The simulated values of visibility were 2-7 km larger than observations in the south of the NCP. Apart from its overestimation in the southern region, the WRF/Chem model generally reproduced the observed regional-scale pattern well (Fig. 2). The mean observed and simulated visibility, mean bias (MB), normal mean bias (NMB), normal mean error (NME), and correlation coefficient (RC) were 17 km, 20 km, 2.6 km, 15%, 24%, and 0.74, respectively. Hourly visibility comparisons at urban sites are shown in Fig. 3. The observed visibility is relatively good in the afternoon because the higher temperatures reduce the RH. The higher planetary boundary layer height also provides suitable conditions for vertical mixing of pollutants, leading to improved visibility. The WRF/Chem simulations captured the trend of diurnal variations and magnitudes of the observed visibility. The simulated mean visibility at the seven urban sites was 10.9 km, very close to the observed mean value of 11.1 km.

    Figure 2.  Monthly-mean visibility simulations for the reference case and observations at 1400 LST August 2007. Station observations over the NCP are indicated by colored circles.

    Figure 3.  Simulated and observed visibility at urban stations of the NCP in August 2007.

    When the NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions were considered in the simulations, visibility at 1400 LST was decreased by up to 20%-30% (3-4 km) during August 2007 in local urban areas (Fig. 4a). Inclusion of the additional HONO sources led to improvements in the visibility simulations. The simulated mean visibility, MB, NMB, and NME for the meteorological stations shown in Fig. 2 were 19.5 km, 2.1 km, 12%, and 22%, respectively. Figure 4a also shows the maximum percentage decreases of 40%-55% were located over Bohai Bay (to the east of Tianjin). The shallow marine boundary layer and abundant moisture over the Bohai Sea are favorable for the NO2 heterogeneous reaction and NO2* chemistry (Sarwar et al., 2009; Jorba et al., 2012; An et al., 2013). Up to 10% (1-2 km) decreases in visibility were found in the rural areas (Fig. 4a).

    Figure 4.  (a) Maximum changes of visibility at 1400 LST and (b) enhancements of monthly-mean total aerosol extinction coefficients over the NCP in August 2007 due to the inclusion of NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions.

    Among the three HONO sources, the NO2 heterogeneous reaction was the largest contributor to the visibility degradation, with 10%-20% decreases in urban areas and 20%-30% decreases over Bohai Bay. The NO2* chemistry resulted in 4%-10% decreases in visibility over local polluted regions and 8%-10% decreases along Bohai Bay. The contribution of HONO emissions to visibility was minor, with less than 6% reductions in urban areas (not shown). Figure 4b shows the enhancements of monthly-mean total aerosol β ext near the ground due to the three additional HONO sources. Increases of 10%-21% β ext were found in Beijing, Tianjin, south of Hebei Province, and Bohai Bay. Accordingly, the values of monthly-mean visibility decreased by 2%-6% (0.5-1 km) over the NCP during August 2007.

  • The dominant factor causing visibility degradation under cloud- and fog-free conditions is aerosols. Specifically, the aerosol mass concentration, chemical composition, and size distribution are important parameters affecting visibility (Cheng et al., 2008; Jung et al., 2009; Deng et al., 2011; Han et al., 2013). In the following sections, we summarize the effects of additional HONO sources on aerosol mass concentrations and aerosol sizes.

    3.2.1 Impacts of additional HONO sources on aerosol mass concentrations

    In this section we first discuss the temporal evolution of HONO, O3, PM10, and the main inorganic components of PM10 in Beijing. These results are shown in Fig. 5. Coupling additional HONO sources into the model significantly improved HONO simulations compared to the reference case (Fig. 5a). The mean values were enhanced from 0.08 ppb to 1.12 ppb; and the MB, NMB, and NME changed from -0.94 ppb, -92% and 92% to 0.29 ppb, 29%, and 56%, respectively. These results are comparable to the results of (An et al., 2013).

    Figure 5.  Comparison of simulated HONO, O3, PM10, SO42-, NO3- and NH4+ concentrations with observations in Beijing. Measurements of HONO, O3 and PM10 were taken at the Meteorological Tower and those of sulfate, nitrate and ammonium concentrations were performed at Peking University in August 2007 (Ianniello et al., 2011).

    The enhanced HONO concentrations as well as the NO2* chemistry increased OH concentrations in the boundary layer, thus enhanced O3 production, especially for its peak values (Wennberg and Dabdub, 2008; Li et al., 2011). As shown in Fig. 5b, the maximum O3 increases (10-21 ppb) due to the additional HONO sources largely occurred around noon. An increase in OH and O3 concentrations as well as the NO2 heterogeneous reaction on aerosol surfaces can further enhance particulate matter production (An et al., 2013). The simulated mean NO3- and NH4+ changed from 19.3 μg m-3 and 8.5 μg m-3 to 25.3 μg m-3 and 10.3 μg m-3, respectively. Increases in NO3- and NH4+ concentrations largely contributed to increases of PM10 (Figs. 5c-f). Enhancements of PM2.5 concentrations due to the three additional HONO sources were also found over the NCP region. As shown in Fig. 6, the maximum increase of PM2.5 concentrations at the monitoring stations of Baoding (38.9°N, 115.5°E), Cangzhou (38.3°N, 116.8°E), Shijiazhuang (38.0°N, 114.5°E) and Tangshan (39.6°N, 118.2°E) were 49.6 μg m-3 (16.2%), 73.0 μg m-3 (27.5%), 44.0 μg m^-3 (16.9%) and 81.9 μg m-3 (44.0%), respectively.

    Figure 6.  Simulated and measured PM2.5 concentrations (units: μg m-3) at the stations of Baoding, Cangzhou, Shijiazhang and Tangshan, which are located in urban areas of the NCP. Case R is a reference case; Case E includes NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions.

    Percentage increases in monthly-mean concentrations of NO3-, NH4+, SO42-, and PM2.5 over the NCP due to the additional HONO sources are shown in Fig. 7. Remarkable enhancements were found in both NO3- and NH4+ concentrations. NO3- and NH4+ in the polluted areas increased by 20%-52% (3-11 μg m-3) and 10%-38% (1-4 μg m-3), respectively. SO42- concentrations were enhanced by 6%-10%. The NO3-, NH4+ and SO42- enhancements were largely responsible for PM2.5 increases. Up to 17 μg m-3 (11%) increases in PM2.5 concentrations were found in the southern NCP (Fig. 7d).

    Figure 7.  Percentage increases of monthly-mean concentrations of (a) NO3-, (b) NH4+, (c) SO42-, (d) PM2.5, (e) OH, (f) HNO3, (g) H2O2, and (h) H2SO4 over the NCP during August 2007 due to the inclusion of NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions.

    The increment of nitrate due to the additional HONO sources was related to the enhanced OH level. As shown in Fig. 7e, the monthly-mean concentrations of OH were increased by 20%-40% over Beijing, Tianjin and south of Hebei Province. The enhanced OH as well as the NO2 heterogeneous reaction resulted in the enhancements of HNO3 (Fig. 7f). Gas phase HNO3 could then partition into the aerosol phase, or be absorbed onto existing aerosols to form NO3-. The HONO-related enhancement of ammonium was largely due to the increases in HNO3 and H2SO4 (Figs. 7f and h) that could be neutralized by NH3 to form NH4+. The enhanced H2SO4 could also condense to form SO42-. In addition, the increment of SO42- due to the additional HONO sources may have been mainly related to the enhanced H2O2 (Fig. 7g) that depended indirectly on the OH concentrations. The aqueous-phase oxidation of dissolved SO2 is considered to be the main source of SO42-, with H2O2 being the most effective oxidant followed by O3 (Ianniello et al., 2011). Therefore, the increased H2O2 and O3 due to the additional HONO sources contributed to the SO42- enhancements. The HONO-related enhancements of PM2.5 and its main inorganic components presented in this study are consistent with previous studies conducted in Mexico City (Li et al., 2010) and Madrid, Spain (Gonçalves et al., 2012). (Elshorbany et al., 2014) recently conducted a global simulation of HONO effects and the results showed that NO3-, NH4+ and SO42- were significantly enhanced, especially in regions that are rich in both NOx and ammonia, e.g., eastern China.

    Figure 8.  Monthly-mean aerosol number (N a) enhancements spatially averaged over the NCP in August 2007 due to the inclusion of additional HONO sources. (a) Box and whisker plots of aerosol number concentration enhancements for the eight diameter size bins shown in Table 1. For each box, the central mark is the median; the upper and lower edges of the box are the 25th and 75th percentiles, respectively; the whiskers extend to the most extreme data points not considered outliers; and outliers are plotted individually. (b) Averaged diurnal variation of increases in N a with aerosol diameter in the range of 0.08-1.25 μm, which corresponds to the bins from the 2nd to 5th set by MOSAIC. Error bars are the standard deviations.

    The additional HONO sources also increased daily aerosol mass concentrations. The maximum daily-mean enhancements of NO3- and NH4+ during August 2007 were up to 100% in urban areas. The largest enhancements of daily mean SO42- and PM2.5 were about 10%-40% and 5%-22%, respectively (not shown). The significant increases in hourly, daily and monthly mean concentrations of PM2.5 and its hydrophilic components indicate that the additional HONO sources can reduce visibility through enhancing aerosol concentrations, particularly for NO3- and SO42-. These are identified as the main aerosol components contributing to the total extinction over the NCP (Jung et al., 2009; Han et al., 2013).

    3.2.2 Impacts of additional HONO sources on aerosol size distribution

    Aerosol size distribution is a key parameter in describing light extinction for cloud-free conditions. Aerosols with a diameter range of 0.1-1.0 μm, corresponding to the accumulation mode, scatter light most efficiently (Seinfeld and Pandis, 2006). The effects of the three additional HONO sources on N a over different size bins are shown in Fig. 8a. Aerosols within the diameter range of 0.078-1.25 μm (covering 2-5 bins) showed significant enhancements, indicating that additional HONO sources can increase the accumulation mode number concentrations (AMNC). The N a enhancements for 2-5 bins were 9.0%, 4.1%, 8.5%, and 1.2%, respectively.

    Figure 9.  Percentage increases of monthly mean (a) aerosol number and (b) aerosol surface area in the 2nd-5th diameter size bins over the NCP in August 2007 due to the inclusion of NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions. The aerosol diameter size bins are shown in Table 1.

    Figure 10.  A proposed conceptual model for the effects of additional HONO sources on visibility and haze.

    (Elshorbany et al., 2014) showed that elevated HONO concentrations can produce significantly higher particle number concentrations in both the hydrophilic accumulation mode and the hydrophilic Aitken mode. In the present study, however, N a in the Aitken mode (1st bin with particle diameters 0.039-0.078 μm; Table 1) were reduced by up to 26% when the additional HONO sources were included. This may be attributable mainly to the condensation and hydrolysis of the enhanced gas-phase HNO3 and H2SO4 (Figs.7f and h) as well as the coagulation of the soluble new aerosol particles in the Aitken mode, and this finally resulted in the enhancements of AMNC. (Elshorbany et al., 2014) also showed that enhanced HONO induces the transfer from hydrophobic to hydrophilic aerosol modes, which is mainly related to the condensation of H2SO4 on the hydrophobic particles. For example, over eastern China the particle number concentration in the hydrophobic Aitken mode is reduced on average by 6%, while that of the hydrophilic accumulation mode is enhanced by 14% (Elshorbany et al., 2014). Therefore, the enhanced aerosol hygroscopicity due to the additional HONO sources may also contribute to the AMNC enhancements, and this will be studied in future work by differentiating the aerosol size distribution in the WRF/Chem model into the hydrophilic and hydrophobic modes.

    Figure 8b shows the averaged diurnal variation of AMNC enhancements. Because HONO photolysis is one of the most significant sources of OH in the early morning (Alicke et al., 2002), AMNC enhancements due to the three additional HONO sources were larger in the morning and peaked at 0800 LST, corresponding to high HONO concentrations during that period (Fig. 5a). The AMNC enhancements during the night were mainly related to the NO2 heterogeneous reaction on aerosol surfaces, which produces NO3- in the presence of NH3. Figure 8 suggests that HONO sources can cause decreases in visibility via increasing AMNC.

    The spatial distribution of increases in monthly-mean N a in the range of 2-5 bins is shown in Fig. 9a. The additional HONO sources increased AMNC in extended areas of the NCP, especially for regions with high emissions of NOx, e.g., Beijing, Tianjin and Shijiazhuang. In those big cities, 15%-20% enhancements were found for AMNC. Among the three additional HONO sources, the NO2 heterogeneous reaction occurring on aerosol surfaces was the largest contributor for increasing AMNC (by 5%-15%). During summer, aerosol hygroscopic growth with increasing RH results in an increase of aerosol size and a decrease of the real part of the aerosol refractive index (Yoon and Kim, 2006; Pan et al., 2009; Chen et al., 2012). Shown in Fig. 9b are the enhancements of monthly-mean Sa in the range of 2-5 bins due to the additional HONO sources. Similar to Fig. 9a, the accumulation mode S a also increased by up to 10%-20% in polluted areas.

  • A conceptual model (Fig. 10) is proposed to describe how the additional HONO sources impact atmospheric visibility prediction. The impact of HONO sources on the air quality is more significant in polluted areas than in rural or background regions (Sarwar et al., 2009; Li et al., 2011; Jorba et al., 2012; An et al., 2013). When HONO concentrations increase in the atmosphere, OH is additionally produced by HONO photolysis. For instance, monthly-mean daytime OH concentrations increase by up to 84% over the NCP when the additional HONO sources are considered in the WRF/Chem model (An et al., 2013). The NO2* chemistry not only produces OH directly, but also enhances OH production from other formation pathways, e.g., the HONO photolysis and the reaction of NO with HO2 (Sarwar et al., 2009). Monthly mean daytime OH concentrations increase by up to 21% in the eastern U.S. and 28% in the western U.S. due to NO2* chemistry. The enhanced OH concentrations contribute to more O3 production from the catalytic oxidation of volatile organic compounds (VOCs) when NOx is present. Photolysis of the increased O3 can further produce OH, which can oxidize NO2 and SO2 in the presence of NH3 to produce sulfate and nitrate. Furthermore, OH enhancements also lead to more H2O2 production (Fig. 7g), which contributes to sulfate formation through the aqueous phase oxidation.

    Both SO42- and NO3- are the dominant contributors to the total extinction parameter in many regions of the world, e.g., the eastern coast of the U.S. (Debell et al., 2006), Beijing and the Pearl River Delta region of China (Cheung et al., 2005; Jung et al., 2009), and northwest England (Colbeck and Harrison, 1984). Increased NO3- and SO42- can lead to PM2.5 enhancements, and increase aerosol hygroscopicity, which result in high aerosol extinction estimations. Furthermore, the three additional HONO sources, especially the NO2 heterogeneous reaction occurring on aerosol surfaces, can induce high aerosol numbers in the accumulation mode. In this mode, aerosols scatter light most efficiently. It is emphasized that the NO2 heterogeneous reaction as a positive feedback mechanism enhances the conversion of primary gas pollutants to secondary aerosols. Shown as red lines in Fig. 10, the heterogeneous reaction increases aerosol number concentrations and NO3- concentrations. These further increase the surface area of aerosols affecting heterogeneous reactions, which lead to the conversion of primary pollutants to secondary pollutants.

    In summary, increases in concentrations of PM2.5, its hydrophilic components, and aerosol numbers in the accumulation mode due to the additional HONO sources reduce visibility significantly, and this may result in more favorable haze formation conditions under stagnant meteorological situations. The basic reason for the visibility impairment due to the additional HONO sources is that the additional HONO sources lead to the enhancements in the concentrations of oxidants (OH, H2O2 and O3), which subsequently enhance the atmospheric oxidizing capacity. It should be noted that, in this study, both rainy and foggy days were excluded from the analysis. Previous observations show that AMNC presents a close relationship with cloud droplet number concentration (CNDC) (Hegg et al., 2012). As a result, the enhanced AMNC due to the additional HONO sources may increase CNDC during foggy days, which can further reduce visibility, but this was not studied here.

4. Conclusions
  • Three additional HONO sources (NO2* chemistry, the NO2 heterogeneous reaction on aerosol surfaces, and HONO emissions) were added into a fully coupled meteorology-chemistry numerical forecasting model to investigate the impacts of the increased HONO concentrations on visibility over the NCP during August 2007. The main conclusions of the study can be summarized as follows:

    (2) The spatial pattern and diurnal variations of visibility were generally reproduced well by the WRF/Chem model over the NCP, except that an overestimation was found in the southern NCP region.

    (3) The three additional HONO sources exerted a significant effect on visibility estimation. The largest decreases in Vis at 1400 LST during August 2007 were about 40%-55% over Bohai Bay, 20%-30% (3-4 km) in local urban areas, and 10% (1-2 km) in rural areas.

    (4) Among the three HONO sources, the NO2 heterogeneous reaction was the largest contributor to the visibility decrease. Monthly mean total aerosol extinction coefficients were increased by 10%-21% in most areas of the NCP due to the additional HONO sources.

    (4) The three HONO sources reduced visibility through enhancing aerosol concentrations, particularly for hydrophilic components. Monthly mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%-52% (3-11 μg m-3), 10%-38%, 6%-10%, and 6%-11% (9-17 μg m-3) in urban areas of the NCP, respectively. Increases in daily mean concentrations of NO3- and NH4+ were up to approximately 100% during August 2007.

    (5) The additional HONO sources caused visibility degradation via increasing accumulation-mode aerosol numbers and surface areas. Monthly-mean AMNC values were enhanced by 15%-20% in large areas of the NCP because of these sources. Accordingly, S a in the accumulation mode was increased by 10%-20%.

    (6) A conceptual model of the effects of HONO sources on visibility was proposed and the effect of the NO2 heterogeneous reaction as a positive feedback mechanism that enhances the conversion of primary gas pollutants to secondary aerosols was also emphasized.

    Overall, the results suggest that visibility estimation in forecasting models should consider the enhanced atmospheric oxidizing capacity due to additional HONO sources, especially in areas with high concentrations of NOx and aerosols.

Reference

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