Source and formation of fine particulate nitrate in South China: Constrained by isotopic modeling and online trace gas analysis
Tao Su, Jun Li, Chongguo Tian, Zheng Zong, Duohong Chen, Gan Zhang
Abstract
NO x is an important precursor of fine particulate matter (PM 2.5 ) and O 3 and its oxidation product NO 3 − may be the main driver of PM 2.5 . In this study, 76 daily fine particle samples were collected from a site in South China, and the characteristics of NO 3 − were studied using an improved Bayesian mixing model based on δ 15 N–δ 18 O compositions and online trace gas analysis. NO 3 − concentrations ranged from 0.60 to 21.7 μg/m 3 , accounting for 6.0 ± 3.3% of total PM 2.5 on average. δ 15 N–NO 3 − values varied from −3.6 to 15.4‰ (average: 7.5 ± 3.3‰). Driven by changes in chemical pathways and environmental parameters, including equilibrium fractionation and thermodynamic effects, δ 15 N–NO 3 - values were lowest in spring and highest in winter. δ 18 O–NO 3 - values varied from 21.0 to 90.1‰ (average: 62.4 ± 14.0‰). In spring, the lowest δ 18 O–NO 3 - values were observed due to high proportions originating from ·HO 2 and ·OH reaction pathways, as well as changes in other parameters. In winter, the highest δ 18 O–NO 3 - values were observed due to cold and dry weather, along with the lowest proportion from the ·OH pathway. Moreover, the trace gases CO, O 3 , NO x , SO 2 , and N 2 O 5 were measured seasonally to determine the main drivers of seasonality in δ 15 N–δ 18 O composition. Monte Carlo simulation was used to estimate the relative contributions of the gas-phase reactions of NO 2 and ·OH radicals and the hydrolysis of N 2 O 5 . The results showed that the ·OH generation pathway was predominant throughout the year, with relative contributions of 72 ± 18%, 76 ± 16%, 63 ± 17% and 39 ± 15% in spring, summer, fall and winter, respectively. Coal combustion (50.1 ± 13.8%) was the predominant source of NO 3 − identified using the Bayesian model, and originated from central and southern Guangdong Province, as indicated by potential source contribution function analysis.