Assessment of the seasonal cycle of nitrate in PM<inf>2.5</inf> using chemical compositions and stable nitrogen and oxygen isotopes at Nanchang, China: Fid-Bau Portal

Assessment of the seasonal cycle of nitrate in PM2.5 using chemical compositions and stable nitrogen and oxygen isotopes at Nanchang, China

Luo, Li / Pan, Yuan-Yuan / Zhu, Ren-Guo / Zhang, Zhong-Yi / Zheng, Neng-Jian / Liu, Yong-Hui / Liu, Cheng / Xiao, Hong-Wei / Xiao, Hua-Yun

Abstract

Abstract Nitrate (particulate ${NO}_{3}^{-}$ and gas-phase HNO3) are the primary acidic ions in the atmosphere and play important roles in regional air pollution. However, quantification of the formation pathways of atmospheric nitrate are still poorly understood. Samples of PM2.5 were collected at Nanchang, China, from September 2017 to August 2018. The concentrations of water-soluble ions and the stable isotopic compositions of δ¹⁸O- ${NO}_{3}^{-}$ and δ¹⁵N- ${NO}_{3}^{-}$ were measured. Seasonal values of δ¹⁵N- ${NO}_{3}^{-}$ (autumn:4.4 ± 2.8‰; winter: 6.6 ± 2.4‰; spring: 4.4 ± 2.0‰; summer: 3.1 ± 2.4‰) suggest that seasonal cycles of ${NO}_{3}^{-}$ in PM2.5 are mainly affected by NOx sources. The highest concentrations of ${NO}_{3}^{-}$ , aerosol liquid water content (ALWC), Cl⁻, ${NH}_{4}^{+}$ , and the lowest concentration of OH were showed during winter. These parameters exhibited opposite trends during summer. δ¹⁸O- ${NO}_{3}^{-}$ values in PM2.5 during autumn (79.2 ± 6.7‰) and spring (73 ± 6.3‰) were between those observed in winter (86.4 ± 5.9‰) and summer (60.7 ± 3.9‰). Values of δ¹⁸O- ${HNO}_{3}^{}$ endmembers produced via pathway of NO2 + OH (PNO2+OH), pathway of N2O5 + H2O (PN2O5+H2O) and pathways of NO3 + VOCs, N2O5 + Cl⁻, ClNO3 + H2O and 2NO2 + H2O (Pother) were evaluated during four seasons. The possible fractional contributions of different ${HNO}_{3}^{}$ formation pathways to PM2.5 ${NO}_{3}^{-}$ were assessed using the Bayesian isotope mixing model. The results show that the PNO2+OH contributes to 59 ± 5% and 12 ± 6% of HNO3 production during summer and winter, respectively. The possible fractional contributions of PN2O5+H2O in all seasons are comparable, but the contributions of Pother in winter are higher than those during summer. This study suggested that Pother may be important for wintertime ${NO}_{3}^{-}$ formation and provides potential information for using chemical models to simulate atmospheric ${NO}_{3}^{-}$ formation.

Graphical abstract Display Omitted

Highlights • ${NO}_{3}^{-}$ concentration and its dual isotopes in PM2.5 were observed at Nanchang for one year. • Seasonal variations of ${NO}_{3}^{-}$ were mainly affected by NOx sources. • Production of ${NO}_{3}^{-}$ by NO2 + OH pathway and other pathways change seasonally.