Quantifying major NOx sources of aerosol nitrate in Hangzhou, China, by using stable isotopes and a Bayesian isotope mixing model

Nitrogen oxides (NOx) from anthropogenic emissions oxidized to form nitrate (NO3−) and play a key role in air pollution. The aim of this study is to identify the NOx sources and their seasonal variations in Hangzhou, a typical city in East China. To achieve this, the chemical characteristics of water-soluble inorganic ions (WSIIs) and the stable isotopes of NO3− (δ15N–NO3- and δ18O–NO3-) were measured in fine particulate matter (PM2.5) and total suspended particle (TSP) aerosol samples that were collected between December 2018 and November 2019. It was found that the annual average concentration of PM2.5 (69.28 ± 34.69 μg/m3) in Hangzhou exceeded the Chinese air quality limit of 35.00 μg/m3. The results showed that NO3− accounted for the largest proportion of the WSIIs in both PM2.5 and TSP, and that major ions were mainly in the form of NH4NO3, (NH4)2SO4, Ca(NO3)2, CaSO4 in aerosol particles. High ratios of NO3−/SO42− in PM2.5 and TSP indicated that vehicle emissions became increasingly important source of aerosol pollution in Hangzhou during the study period. The δ15N–NO3- values ranged from +1.1‰ to +9.2‰ in PM2.5 and from −1.0‰ to +8.5‰ in TSP in Hangzhou. A Bayesian isotopic mixing model (stable isotope analysis in R (SIAR)) was applied to quantify the NOx sources in Hangzhou. This revealed that vehicle emissions, coal combustion and biomass burning were the main NOx sources of PM2.5 and TSP. The seasonal variation of the δ15N–NO3- values in PM2.5 and TSP in the study area were due to seasonal changes in the NOx contributions from different sources. The SIAR model results indicated that coal combustion increased in association with heating in north China during the winter (winter: 38.4% for PM2.5 and 36.4% for TSP, summer: 30.6% for PM2.5 and 29.8% for TSP) and the microbial N cycle increased with a higher application of fertilizer to crops during the summer (winter: 5.3% for PM2.5 and 5.6% for TSP, summer: 10.7% for PM2.5 and 11.6% for TSP). The improved SIAR model combined with the equilibrium fractionation reported the lower contributions from coal combustion (15.5–20.2%) and the higher contributions from Microbial N cycle (22.9–29.3%), as compared to the SIAR model. The seasonal variation of the δ18O–NO3- values was influenced by differences in the oxidation of NO2 via O3 (forming HNO3), whereby the N2O5 pathway dominated during the winter and the OH pathway dominated during the summer.