Dominance of Heterogeneous Chemistry in Summertime Nitrate Accumulation: Insights from Oxygen Isotope of Nitrate (δ18O-NO3-)

The role of nitrate in fine-mode particulate (PM2.5) is increasingly important in polluted environments of urban China, although mitigation strategies have focused on NOx (NO + NO2) reductions. Understanding the source and transformation of NOx is crucial for further alleviation of PM pollution in the context of clean air action in China, especially in the North China Plain (NCP) that experiences severe haze pollution. Daily fine-mode particulate (PM2.5) was collected during summer 2018 (May to August) in urban Jinan. High levels of nitrate (NO3-) were observed (average 6.6 +/- 4.4 mu g/m(-3), from 0.6 to 26.2 mu g/m(-3)), while delta O-18- NO3- in PM2.5 ranged between 53.4 parts per thousand and 83.7 parts per thousand (67.3 +/- 5.0 parts per thousand) during the campaign. In contrast to the previous perspective of strong summertime photochemistry, we suggested that the accumulation of summertime nitrate was sensitive to HNO3 formation, under which the heterogeneous nitrogen chemistry dominated the oxidation of NOx (i.e., uptake of dinitrogen pentoxide). The heterogeneous pathway accounted for approximately 60.9% (uncertainty <1%) of the overall nitrate loadings, evaluated using a simple two-end-member isotopic mixing model. The inferred results can be attributed to the increasing summertime O-3 levels (45.7 ppbv) and substantial aerosol water content (30.5 mu g/m 93 ), which significantly facilitated the nocturnal heterogeneous nitrogen chemistry. The phenomenon that NO3- was sensitive to atmospheric oxidation capacity has been observed over China and other regions worldwide. To alleviate the nitrate pollution, mitigation strategies in the near future should focus on O-3 controls as well as its precursors of NOx and volatile organic compounds (VOCs).