Isotopic evaluation on relative contributions of major NO_x sources to nitrate of PM_(2.5) in Beijing

摘要

Nitrate (NO3-) is a key component of secondary inorganic aerosols and PM2.5. However, the contributions of nitrogen oxides (NOx) emission sources to NO3- in PM2.5 remain poorly constrained. This study measured nitrogen (N) isotopes of NO3- (hereafter as delta N-15-NO3-) in PM2.5 collected at Beijing in 2014. We observed that delta N-15-NO3- values in PM2.5 (-2.3 parts per thousand 19.7 parts per thousand; 73 +/- 5.4 parts per thousand annually) were significantly higher in winter (11.9 +/- 4.4 parts per thousand) than in summer (2.2 +/- 2.5 parts per thousand)). The delta N-15 differences between source NOx and NO3- in PM2.5 (hereafter as Delta values) were estimated by a computation module as 7.8 +/- 2.2 parts per thousand - 10.4 +/- 1.6 parts per thousand (8.8 +/- 2.4 parts per thousand). Using the Delta values and delta N-15 values of NOx from major fossil (coal combustion, vehicle exhausts) and non-fossil sources (biomass burning, microbial N cycle), contributions of major NOx sources to NO3- in PM2.5 were further estimated by the SIAR model. We found that seasonal variations of delta N-15-NO3- values in PM2.5 of Beijing were mainly caused by those of NOx contributions from coal combustion (38 10% in winter, 20 +/- 9% in summer). Annually, NOx from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 +/- 12%, 29 +/- 17%, 27 +/- 15%, and 16 +/- 7% to NO3- in PM2.5, respectively, showing actually comparable contributions between non-fossil NOx (43 +/- 16%) and fossil NOx (57 +/- 21%). These results are useful for planning the reduction of NOx emissions in city environments and for elucidating relationships between regional NOx emissions and atmospheric NO3- pollution or deposition. (C) 2019 Elsevier Ltd. All rights reserved.