Using nitrogen and oxygen stable isotopes to analyze the major NOx sources to nitrate of PM2.5 in Lanzhou, northwest China, in winter-spring periods

Abstract

Nitrate (NO3−) is a standout amongst the essential inorganic aerosols in the atmosphere. Stable isotopic constraint is a robust means to identify the oxidation mechanisms for atmospheric particulate nitrate (NO3−) production. However, rarely studies noted the heavily polluted environments of northwest China. In this study, fine particulate matter (PM2.5) samples were gathered starting from December 2017 till April 2018 in the urban zone of Lanzhou, northwest China, and water-soluble ions, δ15N– NO3− and δ18O– NO3−, were analysed to explore the possible sources of NO3− aerosols. The average concentration of PM2.5 was 63.1 ± 22.6 μg m−3, indicating severe fine PM pollution. The formation of secondary pollutants NO3−, SO42−, and NH4+ concentrations were higher in winter than in spring. The Ca2+, Na+, and Mg2+ concentrations were much higher in spring than in winter, and the concentration of Ca2+ was higher than those in other cities, which implies that the PM2.5 concentration is significantly affected by dust. The δ15N and δ18O values were lower in warmer months, confirming that the contribution of each nitrate source and the oxidation pathways change similarly as the season transforms from cold to warm. The nitrogen sources were analysed using stable isotope analysis in R (SIAR). The results showed that coal combustion, biomass burning, vehicle exhausts, and soil microbial emissions account for 42.2 ± 9.9%, 27.8 ± 16.2%, 22.2 ± 12.3%, and 7.7 ± 5.2% of the nitrate in PM2.5 in winter and 30.7 ± 11.4%, 28.3 ± 15.7%, 26.5 ± 14.4%, and 14.4 ± 6.9% in spring, respectively. The fractional contributions of coal combustion gradually increased in winter. These results are useful for reducing NOx emissions in urban environments and clarifying the relationship between regional NOx emissions and atmospheric NO3− pollution or deposition.