Strict control of biomass burning inhibited particulate matter nitrate pollution over Tianjin: Perspective from dual isotopes of nitrate

Abstract

Although nitrate (NO3−) has more importance to haze formation in the North China Plain, extensive research mainly aimed at a typical haze episode or season to explore its formation mechanisms and sources. This study aimed to better understand the formation mechanisms and sources of NO3− and their seasonal variations based on a one-year continuous sample observation at Tianjin. The water-soluble ions, δ15N–NO3− and δ18O–NO3− values were determined in PM2.5 samples collected from September 1, 2017 to August 31, 2018 in Tianjin. The results showed that NO3− was the most predominant water-soluble species and promoted the rapid accumulation of PM2.5. Both δ15N and δ18O of NO3− showed higher values in winter than in summer. The Bayesian mixing model was applied to quantify the formation pathways and sources of NO3− based on the NO3− dual-isotope of PM2.5. The model results revealed that homogeneous oxidation and heterogeneous processes contributed equally to producing NO3− in Tianjin. The heterogeneous contribution to NO3− increased from 34.6% in summer to 66.0% in winter, from 48.0% during clean days to 79.0% during heavily polluted days. The contribution fractions of biomass burning (BB), coal combustion (CC), biogenic soil emissions (SE), and vehicle emissions (VE) to NO3− of PM2.5 in Tianjin were 34.2 ± 21.6%, 24.5 ± 14.6%, 10.7 ± 7.6%, and 30.6 ± 18.1%, respectively. In addition, the contribution of CC was 42.6 ± 13.6% during the heating periods, while the fractional contribution of BB to NO3− exceeded CC during pollution days and heavy pollution days. The results highlighted that, in addition to controlling CC in winter, we should also pay more attention to preventing and controlling NOx emissions from BB for whole years in Tianjin.