资源环境科技发展态势分析平台

Nitrate (NO3-), one of the most important inorganic aerosols in the atmosphere, is mainly formed by oxidation of NO(x)by the hydroxyl radical (OH) and ozone (O-3) in urban atmospheres. However, the fractional contributions of its various oxidation pathways remain unclear. Here, we collected particulate matter with aerodynamic diameter less than 2.5 mu m (PM2.5) samples in a second-tier city in southeast China from 1 September to 31 December 2017 and measured the NO(3)(-)and nitrate isotopic compositions (delta N-15 and delta O-18). The average concentration of NO3-, delta N-15, and delta O-18 values were 14.7 +/- 11.6 mu g/m(3), (+4.3 +/- 4.3)parts per thousand, and (+71.8 +/- 14.7)parts per thousand with the ranges from 0.8 to 57.7 mu g/m(3), -10.5 parts per thousand to +12.5 parts per thousand and +34.5 parts per thousand to +91.9 parts per thousand, respectively. All three species were significantly higher in winter than in summer. Based on a Bayesian mixing model with a dual isotope array for NO3-, contributions of (37.1 +/- 33.4)%, (60.3 +/- 32.2)%, and (2.6 +/- 2.7)% to NO(3)(-)could be attributed to OH oxidation, N(2)O(5)hydrolysis, and NO3 + hydrocarbon (HC) pathways, respectively. Higher OH radical concentrations with higher ratios of OH to O(3)led to lower NO(3)(-)concentrations, while lower OH radical concentrations with higher ratios of O(3)to OH led to higher contributions of N(2)O(5)hydrolysis, forming higher NO(3)(-)concentrations in winter. Under low OH, an increased O(3)to NO(x)ratio increased the contribution of the NO3 + HC pathway. The comprehensive analysis of the isotopic compositions of nitrate helped identify the importance of major oxidation pathways of NO(x)in this city.

Plain Language Summary Nitrate formation contributes to serious haze pollution in China. However, it is difficult to quantify the contribution of its various formation pathways. We used dual isotopes of nitrate and models to evaluate the contributions of its three main formation pathways. Hydrolysis of N2O5 accounted for more than 60% of nitrate formation in a Chinese urban environment of this study. Oxidation of NOx by the OH radical dominated in summer, because of long daytimes and high temperatures, while oxidation via N2O5 hydrolysis dominated in winter, because of long nighttimes and low temperatures. Understanding the mechanisms of nitrate formation will support better control measures.