Elucidate the formation mechanism of particulate nitrate based on direct radical observations in the Yangtze River Delta summer 2019

Abstract. Particulate nitrate (NO3-) is one of the dominant components of fine particles in China, especially during pollution episodes, and has a significant impact on human health, air quality, and climate. Here a comprehensive field campaign that focuses on the atmospheric oxidation capacity and aerosol formation and their effects in the Yangtze River Delta (YRD) was conducted from May to June 2019 at a regional site in Changzhou, Jiangsu Province in China. The concentrations of NO3-, OH radicals, N2O5, NO2, O3, and relevant parameters were measured simultaneously. We showed a high NO3- mass concentration with 10.6 ± 8.9 µg m−3 on average, which accounted for 38.3 % of total water-soluble particulate components and 32.0 % of total PM2.5, followed by the proportion of sulfate, ammonium, and chloride by 26.0 %, 18.0 %, and 2.0 %, respectively. This result confirmed that the heavy nitrate pollution in eastern China happened not only in winter, but also in summer. This study's high nitrate oxidation ratio (NOR) emphasized the solid atmospheric oxidation and fast nitrate formation capacity in the YRD. It was found that OH + NO2 during daytime dominated nitrate formation on clean days, while N2O5 hydrolysis vastly enhanced and became comparable with that of OH + NO2 during polluted days (67.2 % and 30.2 %, respectively). The updated observed-constraint Empirical Kinetic Modeling Approach (EKMA) was used to assess the kinetic controlling factors of both local O3 and NO3- productions, which indicated that the O3-targeted scheme (VOCs : NOx = 2:1) is adequate for mitigating the O3 and nitrate pollution coordinately during summertime in this region. Our results promote the understanding of nitrate pollution mechanisms and mitigation based on field observation and model simulation and call for more attention to nitrate pollution in the summertime.