Shifts in Nitrate Formation Pathways and Precursor Sensitivity During Autumn PM 2.5 Pollution in Shijiazhuang, China

Abstract Fine particulate matter (PM 2.5 ) have significant impacts on regional air quality, climate, and human health. Among its chemical components, nitrate has shown a rapidly increasing contribution in recent years with the trend of dominating over sulfate. In this study, a field observation was conducted from October to November 2024 at an urban site in Shijiazhuang, Hebei Province in China. The results showed that the rapid increase in particulate nitrate was the main driver of PM 2.5 pollution events, with nitrate accounting for up to 53.8% of PM 2.5 mass during pollution episodes, compared to 39.6% on clean days. The average daily nitrate formation potential on clean days was 21.0 ppb day −1 , dominated by the OH + NO 2 pathway (49.1%), while on polluted days it increased to 32.4 ppb day −1 (approximately 54% higher), with N 2 O 5 heterogeneous uptake (46.4%–49.0%) becoming the dominant pathway. Importantly, NO 2 heterogeneous uptake consistently contributed 10.1%–13.6% to nitrate formation, with its reaction rate increasing with PM 2.5 concentrations. The relative incremental reactivity (RIR) analysis further revealed that nitrate formation was VOC‐sensitive, with alkenes contributing the most, and that NO x reduction may exacerbate nitrate pollution at current NO x levels, while CO also played a notable role in local nitrate production.