Combining dual isotopes and a Bayesian isotope mixing model to quantify the nitrate sources of precipitation in Ningbo, East China

Nitrate (NO₃^-) is becoming a significant contributor to acid deposition in many cities in China. Based on the chemical compositions and stable isotopes of NO₃^- in precipitation (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), the NO₃^- sources and their formation pathways were determined to aid in reducing NO_x emissions in Ningbo, an important port city. The acid rain frequency in Ningbo was 67%, and the mean SO₄^2-/NO₃^- ratio was 1.07. The δ¹⁸O-NO₃^- (49.5‰-82.8‰) and δ¹⁵N-NO₃^- values (-4.3‰-2.7‰) both varied seasonally, with high values during the cold season and low values during the warm season. The seasonal variations in the δ¹⁸O-NO₃^- values were mainly controlled by the NO₃^- formation pathways, following the OH· pathway during the warm season and N₂O₅ pathway during the cold season. The Monte Carlo simulation results indicated that the contributions of the OH· pathway ranged from 28.3% to 75.4%, with the remainder contributed by the N₂O₅ pathway. The improved Bayesian model incorporating nitrogen (N) isotopic fractionation (Ԑ = 4‰) indicated that mobile sources, including ship emissions (35.0%) > coal combustion (26.0%) > biomass burning (20.0%) > soil emissions (19.0%), were the major sources of NO_x emissions in Ningbo. The results indicate that the influence of isotopic fractionation on source apportionment must be considered in a Bayesian model.