Pivotal role of oxidation processes in rainwater dissolved organic matter: A three-year study

Understanding the composition, sources, and transformation of rainwater dissolved organic matter (DOM) is critical for elucidating surface-atmosphere material transport and associated ecosystem impacts. This three-year (2020–2022) Hangzhou study characterized anthropogenically influenced rainwater DOM via integrated spectroscopic and modeling approaches. Results revealed strong anthropogenic imprints, with mean dissolved organic carbon (DOC) of 2.9 mg C L –1 and secondary ions dominating rainwater chemistry. Post-pandemic economic recovery in 2022 amplified fossil-fuel-combustion signatures and sulfate dominance, alongside a doubled DOC deposition flux (1.0 g C m⁻ ( Iavorivska et al., 2016 )). Summer DOC flux peaked at 0.32 g C m –2 , driven by increased rainfall and secondary processes. Rainwater DOM featured optical indices FI, BIX, and HIX of 1.75 ± 0.17, 1.09 ± 0.31, and 1.32 ± 0.80, indicating low humification and aging degrees with anthropogenic and biological contributions. Parallel Factor Analysis (PARAFAC) identified two humic-like substances (HULIS) and two protein-like substances (PRLIS) with distinct oxidation states. Intense oxidation drove summer peaks of highly oxidized HULIS/PRLIS, with the contribution of oxidized PRLIS increasing from 10% (2020-2021) to 26% (2022). Principal Component Analysis (PCA) emphasized strengthened fossil-fuel emissions and ozone oxidation impacts in 2022, while Positive Matrix Factorization (PMF) modeling confirmed secondary processes as dominant DOM sources, with ozone oxidation accounting for 60.7% in 2022, inducing molecular condensation and fluorescence spectrum redshift. These findings advance understanding of wet-deposition DOM in megacities, clarify the dual influences of anthropogenic activities and environmental processes, and provide quantitative constraints to inform air quality management strategies in anthropogenically perturbed regions.