Nonlinear Nitrate Formation Governed by Aerosol Water and VOC-Sensitive Oxidation in a Humid Urban Basin

Secondary nitrate is a major driver of wintertime PM2.5 pollution in China, yet the coupled photochemical and heterogeneous controls on its formation remain poorly constrained in humid, topographically confined basins. Here, we combined wintertime field observations with a box model and thermodynamic calculations to quantify nitrate formation pathways, radical chemistry, and precursor sensitivities. During pollution episodes, nitrate became the dominant PM2.5 component, systematically co-varied with elevated NO2, Ox, and aerosol liquid water content (ALWC). Pathway-resolved modeling revealed near-equal contributions from the two dominant nitrate formation pathways, daytime OH+NO2 reactions (52%) and nighttime heterogeneous N2O5 hydrolysis (46%), with the latter increasing sharply during polluted episodes. The modeled OH budget indicated efficient HOx recycling, dominated by HO2+NO recycling. Binning regressions identified critical thresholds of RH = 87% and ALWC = 76.5 μg m-3, above which nitrate formation efficiency increased abruptly. Sensitivity analyses indicated that VOCs exerted a slightly stronger control on nitrate formation than NOx, with oxygenated VOCs and alkenes as the most influential groups, and solvent usage and vehicle exhaust as dominant sources. These results reveal a humidity-modulated dual-pathway mechanism for wintertime nitrate formation sustained by VOC-influenced radical cycling. Effective nitrate mitigation therefore requires coordinated reductions in NOx and reactive VOCs, together with measures that limit aerosol water accumulation under high-humidity conditions.