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 understood 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 40.9 % of samples, continental secondary aerosols are highly aged, lacking a fine-mode NO₃^– peak. Gaseous HNO₃ evaporated from continental secondary aerosols and anthropogenic NO_x react with natural dust and sea spray aerosols (SSA), 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, which is an important source of reactive halogens that affect ozone chemistry. Shipping emissions contribute to ~20 % of SO₄^2– in spring and summer before the International Maritime Organization’s 2020 regulation, but this contribution likely decreases 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.