Anthropogenic air pollutants strongly interact with natural aerosols over the eastern China seas: key processes, size distributions, and seasonalities

Abstract. Marine aerosols play important roles in climate, marine biogeochemistry, and coastal air quality. Over the eastern China seas adjacent to densely populated East Asia, aerosols are mutually affected by anthropogenic pollution and natural emissions. However, the impacts of anthropogenic–natural interactions on aerosol composition and properties are not well constrained, due to limited systematic observations. Here we characterized the composition of size-resolved aerosols over this region across four seasonal cruise campaigns, identifying major aerosol sources and influencing processes. Aerosol mass concentrations typically show trimodal distributions, with fine-mode mass dominating in spring and winter, due to strong influences of continental pollution. However, in 50.9 % of samples, continental secondary aerosols are highly aged, lacking a fine-mode NO3- peak. Gaseous HNO3 evaporated from continental secondary aerosols and anthropogenic NOx react with natural dust and sea spray aerosols (SSAs), forming coarse-mode NO3-, which contributes 43.2 % and 12.7 % of total NO3-, respectively. This shifts NO3- from fine to coarse mode, altering the spatial pattern of nitrogen deposition and its ecological effects. Additionally, 27.7 % of SSA Cl− is depleted on average, reaching 40.8 % in summer; this is an important source of reactive halogens that affect ozone chemistry. Shipping emissions contributed to more than 20 % of SO42- in spring and summer before the International Maritime Organization's 2020 regulation, but this contribution probably decreased by one order of magnitude thereafter. This analysis highlights the importance of anthropogenic–natural interactions over coastal seas, underscoring the need for further studies to assess their subsequent environmental impacts.