Enhanced atmospheric oxidation and particle reductions driving changes to nitrate formation mechanisms across coastal and inland regions of north China

Abstract

Nitrate (NO3-) has surpassed sulfate as the dominant secondary inorganic ion, posing a significant challenge to air quality improvement measures in China. We utilized the WRF-CMAQ (Weather Research and Forecasting and Community Multiscale Air Quality) model and isotope analysis to investigate the nitrate formation mechanisms driving regional changes in inland and coastal cities in north China during the winters of 2013 and 2018. Among the nitrate formation pathways, the oxidation reaction of OH radicals with NO2 (OH+NO2) and the heterogeneous reaction of N2O5 (hetN2O5) were determined to be the dominant pathways (88 %-95.5 %), whereas the other pathways contributed less than 12.4 % to the total amount of nitrate formation. In inland cities, 63.7 %-85.6 % of nitrate was formed via OH+NO2 and 8.3 %-27.7 % was formed from hetN2O5. In coastal cities, approximately half of the nitrate (48.2 %-56.5 %) was produced from OH+NO2, whereas hetN2O5 contributed 37.0 %-45.7 % due to higher N2O5 concentrations and longer NO3 radical lifetimes. Compared with that in 2013, the OH+NO2 contribution in 2018 increased by 7.6 % in inland cities and 3.6 % in coastal cities due to the increased atmospheric oxidizing capacity. Scenario simulations indicated that a 60 % reduction in NOx emissions led to a 4.5 % decrease in nitrate concentrations in Beijing. The reduction reached 32.4 % reduction in Qingdao. A 60 % combined reduction in NH3, NOx, and volatile organic compounds (VOCs) yielded 44.2 % and 60.0 % reductions in nitrate in Beijing and Qingdao, respectively, underscoring the necessity of multipollutant control strategies.