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. Wet scavenging is an efficient pathway for the removal of particulate matter(PM) from the atmosphere. High levels of PM have been a major cause of airpollution in Beijing but have decreased sharply under the Air PollutionPrevention and Control Action Plan launched in 2013. In this study, 4years of observations of wet deposition have been conducted using asequential sampling technique to investigate the detailed variation inchemical components through each rainfall event. We find that the majorions, SO42-, Ca2+, NO3-, and NH4+, showsignificant decreases over the 2013–2017 period (decreasing by 39 %,35 %, 12 %, and 25 %, respectively), revealing the impacts of theAction Plan. An improved method of estimating the below-cloud scavengingproportion based on sequential sampling is developed and implemented toestimate the contribution of below-cloud and in-cloud wet deposition overthe four-year period. Overall, below-cloud scavenging plays a dominantrole to the wet deposition of four major ions at the beginning of the ActionPlan. The contribution of below-cloud scavenging for Ca2+,SO42-, and NH4+ decreases from above 50 % in 2014 tobelow 40 % in 2017. This suggests that the Action Plan has mitigated PMpollution in the surface layer and hence decreased scavenging due to thewashout process. In contrast, we find little change in the annual volumeweighted average concentration for NO3- where the contributionfrom below-cloud scavenging remains at ∼ 44 % over the2015–2017 period. While highlighting the importance of different wetscavenging processes, this paper presents a unique new perspective on theeffects of the Action Plan and clearly identifies oxidized nitrogen speciesas a major target for future air pollution controls.

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