Isotopic Observations Reveal Divergent Origins and Formation Pathways of Atmospheric Nitrate in Aerosols and Rainwater

Atmospheric nitrate (NO₃^-) serves as the primary sink for reactive nitrogen, with profound impacts on ecosystems and atmospheric chemistry. Although rainwater NO₃^- is widely assumed to originate primarily from local washout processes, its potential regional origin remains debated. To resolve this, we conducted year-round δ¹⁵N-NO₃^- and Δ¹⁷O-NO₃^- measurements in rainwater and particulate matter, supplemented by high-frequency sampling during the East Asian monsoon. Isotopic source apportionment indicates that ∼59% of rainwater NO₃^- originates from photochemical oxidation, reflecting significant aloft and regional contributions, whereas ∼73% of particulate NO₃^- forms via N₂O₅ hydrolysis and NO₃ oxidation at the surface. Rainwater NO₃^- derives more from soil, vehicle, and biomass-burning sources and is transported from broad regions, while particulate NO₃^- is mainly associated with urban coal combustion. Global data sets reveal significantly lower Δ¹⁷O and δ¹⁵N values in rainwater than in particulate NO₃^-, driven by different formation pathways, previously underestimated isotopic fractionation, and different source contributions. After accounting for isotopic fractionation, coal combustion contributions to rainwater NO₃^- were 44-47%, lower than the 62% for particulate NO₃^-. These findings provide direct evidence that rainwater and particulate NO₃^- differ in sources and formation pathways, and ignoring these differences may bias nitrogen budget assessments.