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 (delta N-15-NO3- and delta O-18-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 delta O-18-NO3- (49.5 parts per thousand-82.8 parts per thousand) and delta N-15-NO3- values (-4.3 parts per thousand-2.7 parts per thousand) both varied seasonally, with high values during the cold season and low values during the warm season. The seasonal variations in the delta O-18-NO3- values were mainly controlled by the NO3- formation pathways, following the OH center dot pathway during the warm season and N2O5 pathway during the cold season. The Monte Carlo simulation results indicated that the contributions of the OH center dot 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 (epsilon = 4 parts per thousand) 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. (C) 2021 Elsevier B.V. All rights reserved.