Impact of Coal Replacing Project on atmospheric fine aerosol nitrate loading and formation pathways in urban Tianjin: Insights from chemical composition and 15N and 18O isotope ratios.

• Significant decrease in the concentrations of PM 2.5 and major inorganic ions during the CRP. • Contribution of NO X sources to nitrate based on isotopes and the Bayesian mixing model. • Decreased contribution of coal combustion during the CPR. • The homogeneous formation pathway in summer and heterogeneous pathway in winter. The 'Coal Replacing Project' (CRP), replacing coal with cleaner energy like natural gas and electricity, was implemented in North China to curb PM 2.5 pollution; therefore, it is important to explore the sources and transformation mechanisms of PM 2.5 nitrate under this strategy for examining its effectiveness. In this study, daytime and nighttime PM 2.5 samples of one summer (Jul-2016, C1) and two winters (Jan-2017, C2 and Jan-2018, C3, before and during the CRP, respectively) were collected in urban Tianjin. Concentrations of PM 2.5 and water-soluble inorganic ions were analyzed, and δ15N and δ18O were used to calculate the contributions of different NO X sources to nitrate based on a Bayesian mixing model. The results showed that the average concentrations of PM 2.5 and its dominant inorganic ions (SO 4 2−, NO 3 −, NH 4 +) in C3 during the CRP, compared to C2, decreased by 62.13%, 79.69%, 55.14% and 38.84%, respectively, attesting the improvement of air quality during the CRP. According to the correlation between [NO 3 −/SO 4 2−] and [NH 4 +/SO 4 2−] as well as δ18O variations, the homogeneous formation pathway might be dominant in C1, while the heterogeneous pathway would be primary in C2 and C3 during the formation of nitrate. Moreover, the heterogeneous pathway contributed more in C3 than in C2. The dominant sources in C1 were biogenic soil emission (37.0% ± 9.9%) and mobile emission (25.7% ± 19.1%), while coal combustion (42.4% ± 13.8% in C2 and 34.9% ± 14.4% in C3) and biomass burning (31.0% ± 21.2% and 34.7% ± 22.7%) were the main sources in C2 and C3. In the winter, the contribution of coal combustion dropped by about 8% during the CRP (C3) in comparison with that in C2, suggesting the implementation of CRP played an important role in reducing NO X from coal combustion. [ABSTRACT FROM AUTHOR]