Formation Mechanisms of Secondary Inorganic Components in Fine Particulate Matter in a Typical City of the Fenwei Plain

• • SNA averaged 33.19 ± 18.72 μg m−3, constituting 47.13% of PM2.5 mass in Taiyuan winter, underscoring its dominance and the need for targeted control of precursor gases. • • Daytime nitrate formation was favored at RH > 55%, indicating that high humidity promotes gas-to-particle conversion of HNO3, a critical factor for predicting nitrate episodes. • • Nighttime nitrate production correlated with RH and AWC, with higher AWC enhancing NO2-to-nitrate conversion, highlighting the role of aqueous-phase chemistry in nocturnal pollution. • • PM2.5 pH ranged from 4.3 to 5.2, and sulfate formation was dominated by H2O2 oxidation, with NO2 oxidation secondary, providing quantitative constraints for modeling sulfate production.