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

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 air pollution in Beijing but have decreased sharply under the Air Pollution Prevention and Control Action Plan launched in 2013. In this study, 4 years of observations of wet deposition have been conducted using a sequential sampling technique to investigate the detailed variation in chemical components through each rainfall event. We find that the major ions, SOinline-formula $M1inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimgfa6a3d1c655e3a1a9feb31b6a98317a5 acp-21-9441-2021-ie00001.svg13pt17ptacp-21-9441-2021-ie00001.png , Cainline-formula²⁺, NOinline-formula $M3inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg0aaab3ee324d7a9ba8e4b96f67d8036e acp-21-9441-2021-ie00002.svg9pt16ptacp-21-9441-2021-ie00002.png , and NHinline-formula $M4inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg310866e2e55ea172cf2d52bb7208ba35 acp-21-9441-2021-ie00003.svg8pt15ptacp-21-9441-2021-ie00003.png , show significant decreases over the 2013–2017 period (decreasing by 39 %, 35 %, 12 %, and 25 %, respectively), revealing the impacts of the Action Plan. An improved method of estimating the below-cloud scavenging proportion based on sequential sampling is developed and implemented to estimate the contribution of below-cloud and in-cloud wet deposition over the four-year period. Overall, below-cloud scavenging plays a dominant role to the wet deposition of four major ions at the beginning of the Action Plan. The contribution of below-cloud scavenging for Cainline-formula²⁺, SOinline-formula $M6inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg4a53e7d1f00f4334c934356877052515 acp-21-9441-2021-ie00004.svg13pt17ptacp-21-9441-2021-ie00004.png , and NHinline-formula $M7inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimga896672a5a2c6d19cc4695ffc843bfde acp-21-9441-2021-ie00005.svg8pt15ptacp-21-9441-2021-ie00005.png decreases from above 50 % in 2014 to below 40 % in 2017. This suggests that the Action Plan has mitigated PM pollution in the surface layer and hence decreased scavenging due to the washout process. In contrast, we find little change in the annual volume weighted average concentration for NOinline-formula $M8inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimg7de8959157e6c258409d4c11688ca166 acp-21-9441-2021-ie00006.svg9pt16ptacp-21-9441-2021-ie00006.png where the contribution from below-cloud scavenging remains at inline-formula∼ 44 % over the 2015–2017 period. While highlighting the importance of different wet scavenging processes, this paper presents a unique new perspective on the effects of the Action Plan and clearly identifies oxidized nitrogen species as a major target for future air pollution controls.