Evolution and determinants of precipitation chemistry in a plateau city, China: Insight from the pollutants source identification and environmental effect

Rainfall-related chemical ions deposition processes have important environment effects on earth-surface, which is also a vital sink of urban air pollutants. Therefore, identifying the rainwater chemical evolution and determinants, and source is essential. The daily-based rainwater samples during 2022– 2023 and the historical rainwater data in a typical plateau city (Kunming) were investigated. The Ca2+ and NH4+, SO42− and NO3− dominated the rainwater cations and anions, with the cationic and anionic percentages of 78% and 10%, 50% and 33%. The historical variations in ions percentages revealed the achievement of environmental protection. The precipitation amount and the seasonal changes of source emission were critical determinants for the variations of rainwater chemistry. Using the stoichiometry and receptor model, the contributions of potential origins to rainwater chemicals were assessed. 86% of NO3− and 40% SO42− were from combustion-related anthropogenic source, volatilization-related source contributed 85% of NH4+ (e.g., agricultural/municipal wastes emissions), whereas marine input was the main contributor of Cl− (59%) and Na+ (52%). Mg2+ (58%) and Ca2+ (41%) were primary from terrestrial input source. The risk of wet deposition-caused acidification was limited due to the high rainwater neutralization level, whereas the impact of nitrogen-related deposition on gross primary productivity (GPP) was non-negligible and further attention was need. Overall, the present study clarified the rainfall chemical features of plateau city region and further provided decision support for more effective urban air environmental management programs.