LAUSR

Abstract The chemical components, air pollutants and stable isotopes of NO3− were used to determine NO3− sources in precipitation and reveal NOx transformation mechanisms from 2015 to 2017 in the cities of Hangzhou and Huzhou, which are representative of most cities in Southeastern China (SE China). The mean concentrations (monthly weighted-mean values) of NO3− were 49.52 μmol L−1 in Hangzhou, and were higher than NO3− concentrations (39.35 μmol L−1) in Huzhou. Both δ15N-NO3 values (−4.4‰–3.7‰) and δ18O-NO3 values (27.9‰–83.4‰) varied widely in the study areas. It was demonstrated that the main NO3− sources in Hangzhou and Huzhou included coal consumption, traffic emissions and soil emissions. Higher levels of traffic emissions in Hangzhou were responsible for the spatial variations in NO3− concentrations and δ15N-NO3 values between Hangzhou and Huzhou. The different NO2 oxidation pathways are particularly important in seasonal variation of oxygen stable isotope in NO3−. Due to the oxidation of NO2 via O3 forming HNO3 in the N2O5 pathway, the highest δ18O-NO3 values were observed in the winter. The lowest δ18O-NO3 values were explained by the NOx oxidation forming HNO3 via the OH pathway in the summer. The seasonal variation of δ15N-NO3 values was influenced by the changing contributions of the main NO3− sources in precipitation collected from the study areas: soil emissions increased in summer and coal consumption emissions increased because of coal consumption for winter heating from northern China in Hangzhou and Huzhou.