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

摘要

Nitrate (NO_3~-) is becoming a significant contributor to acid deposition in many cities in China. Based on the chemical compositions and stable isotopes of NO_3~- in precipitation (δ~(15)N-NO_3~- and δ~(18)O-NO_3~-, the NO_3~- 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 SO_4~(2-)/NO_3~- ratio was 1.07. The δ~(18)O-NO_3~- (49.5‰-82.8‰) and δ~(15)N-NO_3~- 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 δ~(18)O-NO_3~- values were mainly controlled by the NO_3~- formation pathways, following the OH • pathway during the warm season and N_2O_5 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 N_2O_5 pathway. The improved Bayesian model incorporating nitrogen (N) isotopic fractionation (ε = 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.