Pandemic-Related Anthropogenic Influences on the Dissolved Organic Matter Chemical Character in São Paulo State Wet Deposition by Ultrahigh-Resolution Mass Spectrometry

Rainwater dissolved organic matter (DOM) is a complex mixture of organic compounds, the composition of which remains largely unknown. This is despite its central role in a host of fundamentally important atmospheric processes (e.g., aerosol hygroscopicity, light absorption, etc.). This work illustrates the molecular composition of DOM in rainwater collected from February 2020 to June 2021 (n = 32 of rain samples) in Ribeirão Preto, SP (21°09′41.0″S 47°51′29.9″W), using complementary methods of traditional ion chromatography and Orbitrap mass spectrometry with novel statistical analysis. This approach provides a detailed, ultrahigh-resolution, high-throughput method for rainwater DOM investigations, demonstrated here with ultra performance liquid chromatography coupled with Orbitrap mass spectrometer, positive-mode electrospray ionization. Using this method, 2788 unique molecular formulas were identified over the mass-to-charge ratio range of 80–800. DOM character is revealed to be largely influenced by organic nitrogen, as 86.0% (2397 species) of unique organic compounds identified contained nitrogen, many likely peptides, or amino acid derivatives. Recent pandemic-related influences on anthropogenic activity in Ribeirão Preto, SP region, are observed through variations in rainwater DOM characteristics. Rainwater prior to the COVID-19 pandemic’s emergence in Brazil is distinct from reduced anthropogenic activity rainwater in the DOM character─seen as an increase in primary compounds and amino acid contribution, wider range of O/C ratios, and absence of atmospheric NOx-related CHON oligomers─and a major ion content─showing a reduction in sulfate, nitrate, potassium, and formate. Clustering analysis shows that these distinctions are mostly driven by changes in anthropogenic reactive nitrogen emissions as well as seasonal influences.