Unveiling the urban rainfall chemistry in summer frequent-rainy area: Variation and source identification of air pollutants based on two rainy seasons’ observation

Rainfall chemistry is a significant indicator of anthropogenic/natural inputs to air quality, and the rainfall process serves as a major sink for atmospheric pollutants, attracting widespread attention from community. The chemical components deposited via rainfall mainly occur in the rainy season, generally summer in China, contributing to the majority of air pollution sink. Here, two rainy seasons-based observation were conducted in a typical summer-frequent-rainfall city in southwestern China, to clarify the variations, controls, and sources of rainwater chemistry. The major ions were SO₄²⁻, NO₃⁻, Ca²⁺ and NH₄⁺, with the VWM concentration (in μeq/L) of SO₄²⁻ > NO₃⁻ > F⁻ > Cl⁻ for anions in two rainy seasons (2022 and 2023), while Ca²⁺ > NH₄⁺ > Mg²⁺ > Na⁺ > K⁺ in 2022 and NH₄⁺ > Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ in 2023 for cations. High rainfall amount in rainy season was the main factor influencing the rainwater ion concentration, while air pollutants also played a non-negligible role. Additionally, some elements (e.g., Ca and Mg) can be accumulated in the atmosphere during the non-rainy days, and the number of dry days thus alters the rainwater ion concentration by the subsequent rainfall. Source apportionment by stoichiometric analysis and positive matrix factorization confirmed that the human-induced sources, including urban waste volatilization and combustion-related emissions, were responsible for 30.3 % of NH₄⁺, 68.6 % of NO₃⁻, and 50.8 % of SO₄²⁻ in rainwater. Atmospheric dust and particles were the dominant contributors to the rainwater Ca²⁺ (50.4 %) and Mg²⁺ (43.7 %), and the substantial contributor to SO₄²⁻ (49.2 %), NO₃⁻ (22.3 %), and K⁺ (19.3 %). Marine inputs were the primary source of Cl⁻ (56 %), Na⁺ (77.7 %), and Mg²⁺ (33.1 %). This study elucidated the rainfall chemical features and their sources in summer-frequent rainy area, offering valuable insights for sustainable urban planning in response to climate change.