Isotopic evaluation on relative contributions of major NO sources to nitrate of PM in Beijing.

Nitrate (NO) is a key component of secondary inorganic aerosols and PM. However, the contributions of nitrogen oxides (NO) emission sources to NO in PM remain poorly constrained. This study measured nitrogen (N) isotopes of NO (hereafter as δN-NO) in PM collected at Beijing in 2014. We observed that δN-NO values in PM (-2.3‰ - 19.7‰; 7.3 ± 5.4‰ annually) were significantly higher in winter (11.9 ± 4.4‰) than in summer (2.2 ± 2.5‰). The δN differences between source NO and NO in PM (hereafter as Δ values) were estimated by a computation module as 7.8 ± 2.2‰ - 10.4 ± 1.6‰ (8.8 ± 2.4‰). Using the Δ values and δN values of NO from major fossil (coal combustion, vehicle exhausts) and non-fossil sources (biomass burning, microbial N cycle), contributions of major NO sources to NO in PM were further estimated by the SIAR model. We found that seasonal variations of δN-NO values in PM of Beijing were mainly caused by those of NO contributions from coal combustion (38 ± 10% in winter, 20 ± 9% in summer). Annually, NO from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 ± 12%, 29 ± 17%, 27 ± 15%, and 16 ± 7% to NO in PM, respectively, showing actually comparable contributions between non-fossil NO (43 ± 16%) and fossil NO (57 ± 21%). These results are useful for planning the reduction of NO emissions in city environments and for elucidating relationships between regional NO emissions and atmospheric NO pollution or deposition.