Exploring Atmospheric Nitrate Formation Mechanisms during the Winters of 2013 and 2018 in the North China Region via Modeling and Isotopic Analysis

Abstract. Nitrate (NO₃^-) has surpassed sulfate as the dominant secondary inorganic ion, posing a significant challenge to air quality improvement in China. We utilized the WRF-CMAQ model and isotopic analysis to investigate nitrate formation mechanisms in inland and coastal cities in North China during the winters of 2013 and 2018. Among the seven nitrate formation pathways, the oxidation reaction of OH radicals with NO₂ (OH + NO₂) and the heterogeneous reaction of N₂O₅ (hetN₂O₅) were dominant pathways (88 %–95.5 % NO₃^-), while others contributed less than 12.4 %. In inland cities, 63.7 %–85.6 % of nitrate formed via OH + NO₂, and 8.3 %–27.7 % from hetN₂O₅. In coastal cities, about half of nitrate (48.2 %–56.5 %) was produced from OH + NO₂, while hetN₂O₅ contributed 37.0 %–45.7 % due to higher N₂O₅ concentrations and longer NO₃ radical lifetimes. Compared with 2013, the OH + NO₂ contribution in 2018 increased by 7.6 % in inland cities and 3.6 % in coastal cities, driven by greater atmospheric oxidizing capacity. Scenario simulations showed that a 60 % reduction in NO_x emissions could lower nitrate levels by 38.4 %, while combined reductions in NH₃, NO_x, and VOCs led to a 59.8 % decrease, from 14.6 μg/m³ to 5.9 μg/m³. These results highlight the need for comprehensive strategies targeting NH₃, NO_x, and VOCs to reduce nitrate pollution.