Abstract: The chemical composition of precipitation serves as a crucial indicator of air pollution.As a major maritime nation,investigating the chemical composition of marine precipitation holds significant scientific importance for China's marine environmental protection and ecological civilization construction.To reveal the chemical composition and sources of atmospheric precipitation in the offshore areas of eastern China,a one-year online monitoring campaign was conducted in 2021 using the ZJX-IX all-performance automatic wet deposition monitoring system to measure pH,electrical conductivity,and major inorganic ions in precipitation in the Zhoushan Islands,located in the eastern coastal region of China.The degree of precipitation acidification was assessed,the characteristics of precipitation chemical composition were analyzed,and the main sources of chemical components in Zhoushan's precipitation were identified by integrating methods such as backward air mass trajectories and Positive Matrix Factorization (PMF).The results showed that during the observation period in 2021,the frequency of acid rain occurrence was 85%,with weak acid rain events accounting for 81%,indicating a high frequency of acid rain in Zhoushan,but the degree of acidification is relatively mild,dominated by weak acid rain.The volume-weighted mean (VWM) concentration of total anions and cations was 183.4 μmol/L,with Cl^- and Na⁺ being the dominant anion and cation,respectively.The VWM concentrations of Cl^- and Na⁺ were 59.3 and 48.1μmol/L,representing 63% of total anions and 53% of total cations,respectively.The sum of Cl^- and Na⁺ accounted for approximately 59% of total ion content.Source analysis revealed that the chemical components in Zhoushan's atmospheric precipitation are primarily influenced by marine sources.In 2021,the average contribution of marine sources to the precipitation chemical components was 54%,while contributions from sources such as dust and biomass burning were relatively minor.