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

Abstract 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 NOx emissions in Ningbo, an important port city. The acid rain frequency in Ningbo was 67%, and the mean SO42−/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 NOx emissions in Ningbo. The results indicate that the influence of isotopic fractionation on source apportionment must be considered in a Bayesian model.