Felix, J. D.Seymore, Jackson2022-08-022022-08-022022-05https://hdl.handle.net/1969.6/93581Rainwater dissolved organic matter (DOM) is a complex mixture of organic compounds, the composition of which remains to a large extent unknown. This is despite its central role in a host of fundamentally important atmospheric processes (e.g. aerosol hygroscopicity, light absorption, etc.). The molecular composition of DOM has been used to infer emission sources and investigate atmospheric reactions that produce secondary organic aerosols (SOA), which comprise the main contributor of uncharacterized compounds in rainwater organics. 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 (21.166 S, 47.845 W) using complimentary methods of traditional ion chromatography cooperatively with Orbitrap mass spectrometry and novel statistical analysis. This approach provides a detailed, ultra-high resolution, high throughput method for future rainwater DOM investigations which is demonstrated here with direct-injection, positive mode electrospray ionization. Using this method, 41,383 total molecular formulas in 32 samples were identified over the mass range m/z+ range of 80 to 800; among them 2,788 molecular formulas were unique. DOM character in São Paulo rainwater is revealed to be largely influenced by organic nitrogen, as 2,397 of the unique formulas identified contained nitrogen. This represents 86.0% of the total variety of organic compounds identified, with many of these likely peptides or amino acid derivatives. These findings also show that in terms of variety and number the general proportions of elemental formula classes remain relatively consistent over many samples, but the composition of the individual classes varies as function of its constituent emission sources. Recent pandemic related influences on anthropogenic activity as well as biomass burning in the São Paulo region of Brazil are seen through variations in rainwater DOM characteristics. Rainwater collected prior to the COVID-19 pandemic’s emergence in Brazil is distinct from reduced anthropogenic activity rainwater in both DOM character—seen in the increase in primary compounds and direct amino acid contribution, wider range of O:C ratios, and the absence of atmospheric NOx related CHON oligomers—and major ion content—marked by 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 in part to seasonal influence. This research highlights the potential for public policy to reduce the emission of air pollutants and other biologically relevant anthropogenic emissions, particularly through reducing traffic loading and expanding remote working.108 pagesen-USAttribution-NonCommercial-ShareAlike 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-sa/4.0/aerosolsdissolved organicsDOMrainwaterText