Inter-annual variations of wet deposition in Beijing during 2014-2017: 1 implications of below-cloud scavenging of inorganic aerosols

16 Wet scavenging is an efficient pathway for the removal of particulate matter (PM) from 17 the atmosphere. High levels of PM have been a major cause of air pollution in Beijing 18 but have decreased sharply under the Air Pollution Prevention and Control Action Plan 19 launched in 2013. In this study, four years of observations of wet deposition have been 20 conducted using a sequential sampling technique to investigate the detailed variation in 21 chemical components through each rainfall event. We find that the major ions, SO 42- , 22 Ca 2+ , NO 3- and NH 4+ , show significant decreases over the 2013-2017 period (decreasing 23 by 39%, 35%, 12% and 25%, respectively), revealing the impacts of the Action Plan. 24 An improved sequential sampling method is developed and implemented to estimate 25 the contribution of below-cloud and in-cloud wet deposition over the four-year period. 26 Overall, below-cloud scavenging accounts for between half and two thirds of wet 27 deposition of the four major ions, with the highest contribution for NH 4+ at 65% and 28 lowest for SO 42- at 50%. The contribution of below-cloud scavenging for Ca 2+ , SO 42- 29 and NH 4+ decreases from above 50% in 2014 to below 40% in 2017. This suggests that 30 the Action Plan has mitigated PM pollution in the surface layer and hence decreased 31 scavenging due to the washout process. In contrast, we find little change in the annual 32 volume weighted average concentration for NO 3- where the contribution from below- 33 cloud scavenging remains at ~44% over the period 2015-2017. While highlighting the 34 importance of different wet scavenging processes, this paper presents a unique new 35 perspective on the effects of the Action Plan and clearly identifies oxidized nitrogen 36 species as a major target for future air pollution controls.

It is important to recognize the contribution of below-cloud scavenging to total wet 70 deposition. However, many studies have found that it is difficult to separate the two wet 71 scavenging processes based on measurement methods alone (Huang et al., 1995;Wang 72 and Wang, 1996;Goncalves et al., 2002;Bertrand et al., 2008;Xu et al., 2017). A 73 commonly used approach to separating below-cloud scavenging from total wet 74 deposition is through sequential sampling (Aikawa et al., 2014;Ge et al., 2016;Aikawa 75 and Hiraki, 2009;Wang et al., 2009;Xu et al., 2017). In this way, precipitation 76 composition during different stages of a rainfall event can be investigated separately in 77 the lab after sampling. The chemical components in later increments of rainfall are 78 thought to be less influenced by the below-cloud scavenging process than by the in-79 cloud rainout process (Aikawa et al., 2014;2009). Xu et al. (2017) applied this approach 80 to summer rainfall in Beijing in 2014 and found that more than 50% of deposited sulfate, 81 nitrate and ammonium ions were from below-cloud scavenging. In this study, an 82 innovated method based on exponential curve to chemical ions in rainfall by sequential 83 sampling is developed and implemented to estimate the ratio of below-cloud to in-cloud   140 Previous studies have shown that the concentration of chemical ions in precipitation 141 decreases through the progression of a rainfall event and eventually stabilizes at low 142 levels (Aikawa and Hiraki, 2009;2014;Ge et al., 2016;Xu et al., 2017). The rainout and 143 washout contributions to total wet deposition are estimated based on the assumption 144 that the concentrations in later increments can be attributed to scavenging by rainout 145 only. This assumption relies on the efficient scavenging of air pollutants below cloud 146 through the evolution of precipitation. However, the concentration of chemical ions in 147 precipitation may also be affected by many other factors in addition to below-cloud air 148 pollutant concentrations and in-cloud rainout processes. For example, the precipitation 149 intensity may affect the scavenging efficiency of air pollutants below cloud and hence 150 influence wet deposition (Andronache, 2004b;Wang et al., 2014;Xu et al., 2017;2019). 151 Yuan et al. (2014) reported that in central North China high intensity rainfall events of 152 short duration (lasting less than 6 h) are dominant rather than long-duration rainfall that 153 is more common in the Yangtze River Valley. Therefore, the time window for the 154 definition of in cloud stage is very important for estimating the below cloud and in 155 cloud contributions. Previous studies have estimated the concentration of chemical ions 156 scavenged in-cloud based on the subjective judgment that 5 mm of accumulated concentration of NO3and SO4 2in cloud in Japan was found to be 0.70 and 1.30 mg/L, 160 respectively (Aikawa and Hiraki, 2009). In Beijing, high concentration of NH4 + , SO4 2-161 and NO3during 2007 were found at 2.1~5.5, 3.1~14.9, 1.5~5.9 mg/L, respectively 162 (Wang et al., 2009;Xu et al., 2017).

163
In this study, a new method based on fitting a curve to the chemical ion 164 concentrations with successive rainfall increments has been developed to estimate the 165 contribution of the rainout process. As shown in Figure 1, an exponential curve is fitted 166 to the median, 25 th and 75 th percentiles of the chemical ion concentrations in each 167 fraction through the rainfall increments. In theory, the concentration of chemical ions 168 stabilize at higher rainfall increments and this represents the concentration in cloud.

169
However, the decrease during each rainfall event is distinctly different, and this 170 regression method is not fully applicable to all rainfall events in practice. Therefore, the 171 exponential regression method is used to estimate the in-cloud concentration under 172 most circumstances, but where the decreasing trend with the increment of rainfall is not 173 significant, the average value of rainfall increments 6-8 of the event is used. The below 174 cloud contributions to wet deposition of each species are then calculated using the 175 following equations (1-2): (2)

178
Where, C i , and C ̅ represent the concentration of each chemical ion in fraction i and in 179 cloud and P i represents the volume of rainfall.    Table 1. This indicates that the concentration of chemical ions in precipitation at the 237 start of rainfall is more greatly influenced by aerosols below the cloud. As rainfall 238 continues and below-cloud concentrations are reduced, there is an increased 239 contribution from in-cloud scavenging, which is less influenced by aerosols in the 240 surface layer. This is confirmed by the substantial difference in the two R coefficients 241 for the cation ion Ca 2+ (0.85 for the first fraction, 0.47 for the VWA), which often exists 242 in coarse particles below cloud. For the fine particle SO4 2which is present both in and 243 below clouds (Xu et al., 2017), the difference in the two R coefficients is small.

244
The slope of the linear fits in Figure 3 can be used to calculate the scavenging ratio 245 H, which is the ratio of the ions concentration in precipitation (mg/L) and in air (μg/m 3 ).

247
respectively. This is similar to that reported for rainfall events in 2014 in Beijing  from the exponential fit of the observed rainwater concentrations. Table 2   where other factors such as precipitation intensity are important, see Table 2. Similar 292 results are found for most ions with the exponential and average approach except for 293 NH4 + , F -, K + and Mg 2+ , where the maximum difference is less than 20% (Table 2). Thus, 294 the replacement of in-cloud concentration by the average value is acceptable for SO4 2-, 295 NO3 -, Ca 2+ , Cland Na + but much uncertainty for the other ions. It is worth noting that 296 for all ions the average approach gives higher estimates of in-cloud concentrations, and 297 this can be recognized as an upper limit for in-cloud concentrations.

298
Following Eq (2), the contribution of below-cloud scavenging to wet deposition in  other systems. Figure 6 shows the contributions of below-cloud scavenging for the two and are characterized by short heavy rainfall (Zhang et al., 2008;Liu et al., 2016;Zheng 355 et al., 2020). This is common during the summer months in Beijing with deep 356 convective clouds (Yu et al., 2011;Gao and He, 2013), and suggests that there is a large 357 contribution from in-cloud scavenging to the total wet deposition.