Insights into factors affecting size-segregated nitrate formation in a coastal city through measurements of dual isotopes

Abstract

Due to the effective control of SO2 emissions in China, nitrate becomes a key component in atmospheric aerosols, especially in haze episodes. Therefore, it is important to elucidate the formation pathways and sources of ambient NO3−. In this study, size-segregated particles were collected in Xiamen urban area from August 2019 to February 2021. Water-soluble inorganic ions and the dual isotope compositions (δ15N and δ18O) of NO3− were measured. A Bayesian isotope mixing model was used to investigate the formation pathways and contributions of different NOx sources to NO3−. The results showed that NO3− exhibited pronounced seasonal variations in concentrations and size distributions. The δ15N–NO3- and δ18O–NO3- values in different modes were lower in the warm season (May to November) than in the cold season (December to Feburary), attributing largely to the source and formation pathway changes. Bayesian model indicated that the contribution of the NO2 + ⋅OH pathway in the warm season was higher than in the cold season, and the contribution decreased with the increase of particle size. However, opposite seasonal trends were observed for N2O5 + H2O, N2O5 + Cl− and NO3 + HC pathways. The size distribution of δ15N–NO3- values was more influenced by the formation pathway than the isotopic fractionation. Results also indicated that the average contribution of soil emission (13.5%) to ambient NO3− was significantly lower than that of coal combustion (30.4%), ship emission (26.6%) and vehicle exhaust (29.6%). The contribution of soil emisison was almost double in warm season (17.7%) compared to cold season (9.3%) whereas coal combustion and vehicle exhaust decreased and ship emission remained relatively stable. Our results highlight the influence of formation pathways on the size distribution of δ15N–NO3- and the importance of after emission treatment of ship exhaust in reducing particle pollution in port cities.