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

•Significant decrease in the concentrations of PM2.5 and major inorganic ions during the CRP.•Contribution of NOX 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.