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Assessment and quantification of NOx sources at a regional background site in North China: Comparative results from a Bayesian isotopic mixing model and a positive matrix factorization model

Zong, Zheng, Tan, Yang, Wang, Xiaoping, Tian, Chongguo, Fang, Yunting, Chen, Yingjun, Fang, Yin, Han, Guangxuan, Li, Jun, Zhang, Gan
Environmental pollution 2018 v.242 pp. 1379-1386
Bayesian theory, aerosols, biomass, burning, combustion, diurnal variation, dust, hydroxyl radicals, nitrates, nitrogen, nitrogen oxides, oxygen, soil, stable isotopes, statistical models, summer, China
Regional sources of nitrogen oxides (NOx) in North China during summer were explored using both a Bayesian isotopic mixing model and a positive matrix factorization (PMF) model. Results showed that the nitrogen isotope (δ15N) composition of particulate nitrate (NO3−) varied between −8.9‰ and +14.1‰, while the oxygen isotope (δ18O) composition ranged from +57.4‰ to +93.8‰. Based on results from the Bayesian isotopic mixing model, the contribution of the hydroxyl radical (•OH) NOx conversion pathway showed clear diurnal fluctuation; values were higher during the day (0.53 ± 0.16) and lower overnight (0.42 ± 0.17). Values peaked at 06:00–12:00 and then decreased gradually until 00:00–06:00 the next day. Coal combustion (31.34 ± 9.04%) was the most significant source of NOx followed by biomass burning (25.74 ± 2.58%), mobile sources (23.83 ± 3.66%), and microbial processes (19.09 ± 5.21%). PMF results indicated that the contribution from mobile sources was 19.83%, slightly lower as compared to the Bayesian model (23.83%). The PMF model also reported a lower contribution from coal combustion (28.65%) as compared to the Bayesian model (31.34%); however, the sum of biomass burning and microbial processes in the Bayesian model (44.83%) was lower than the aggregate of secondary inorganic aerosol, sea salt, and soil dust in PMF model (51.52%). Overall, differences between the two models were minor, suggesting that this study provided a reasonable source quantification for NOx in North China during summer.