Changes of Emission Sources to Nitrate Aerosols in Beijing After the Clean Air Actions: Evidence From Dual Isotope Compositions

Abstract

Nitrate (NO3−) is a major contributing species to haze formation in Northern China. So far, formation processes and source apportionments of nitrate aerosols during haze pollution have not yet been well understood. In this study, the PM2.5 samples were collected in Beijing from 13 November to 24 December 2018. In addition to water‐soluble ions, oxygen (δ18O‐NO3−) and nitrogen (δ15N‐NO3−) isotopes in particulate NO3− were also determined, in order to investigate the formation pathways and potential sources of NO3− aerosols. The results showed that NO3− was a dominant species (43%) of secondary inorganic aerosols during the sampling period. The δ18O‐NO3− and δ15N‐NO3− values averaged at 83.8 ± 13.4 and 11.5 ± 5.0‰, respectively. Combining isotope compositions and Bayesian isotope mixing model, we found that heterogeneous reaction and gas phase oxidation contributed equally to nitrate formation during the sampling period. However, the contribution of heterogeneous processes to nitrate increased from 39% during the clean period to 64% during the haze period. On average, coal combustion, biomass burning, vehicle emissions, and soil emission contributed 50%, 26%, 20%, and 4%, respectively, to nitrate aerosols during the sampling period. Compared to the result in 2013, the significant decrease (~21%) of relative contribution of coal combustion to nitrate was due to strict reduction of coal consumption in Beijing. Finally, the relative contribution of traffic emissions to nitrate increased from 18% during the clean period to 30% during the haze period, suggesting that control of traffic emissions would be an important way to decrease nitrate concentrations and improve the air quality in Beijing.

Long‐Term Evolution of Particulate Nitrate Pollution in North China: Isotopic Evidence From 10 Offshore Cruises in the Bohai Sea From 2014 to 2019

“…Results show that the average contributions of biomass burning, coal combustion, mobile sources, and microbial processes were 22.1 ± 5.8%, 46.6 ± 15.9%, 19.4 ± 6.2%, and 11.9 ± 6.0%, respectively, in the two seasons from 2014 to 2019 (Table S4 in Supporting Information ). These proportions indicate that coal combustion was still the primary source of NO 3 − in North China, especially in winter (59.3 ± 14.2%) (Fan et al., 2020). However, the dominance of coal combustion as a source of NO 3 − progressively decreased over the study period (Figure 5).…”

“…However, Fan et al. (2020) finds that the contribution of microbial processes to NO x increased significantly during the winter haze in Beijing, which could be doubled compared with the clean period. All these evidences illustrate the increasing importance of microbial processes to air pollution.…”

“…(2017) for details. According to the NO x emission inventory and NO 3 − sources reported in North China, biomass burning (+0.3 ± 3.5‰), coal combustion (+13.7 ± 4.6‰), mobile sources including vehicle exhaust and ship emissions (−12.6 ± 2.2‰), and microbial processes (−33.8 ± 12.2‰) were considered to be end‐members in the Bayesian simulations (Table S1 in Supporting Information ) (Fan et al., 2020; Liu et al., 2017; Luo et al., 2019; Luo et al., 2021; Song et al., 2019; Zhang, Zheng, et al., 2020; Zong, Tian, et al., 2020).…”

“…Wang et al (2007) shows that the microbial source of NO x peaked in summer, accounting for as much as 43% of the combustion sources, while it was not important in winter. However, Fan et al (2020) finds that the contribution of microbial processes to NO x increased significantly during the winter haze in Beijing, which could be doubled compared with the clean period. All these evidences illustrate the increasing importance of microbial processes to air pollution.…”

“…With the implementation of strict air‐pollution control policies in North China, the concentration of atmospheric fine particulate matter (PM 2.5 ) has declined in recent years (Zhang & Geng, 2019). However, few studies on multiple‐year observation data have shown that the composition of PM 2.5 in Beijing and the several other cities in the North China Plain has undergone significant changes; the nitrate (NO 3 − ) concentration and its proportion in PM 2.5 have increased despite reductions of nitrogen oxides (NO x ) emissions (Fan et al., 2020; Fu et al., 2020; Li, Gao, et al., 2021; Li, Ma, et al., 2021; Li, Wu, et al., 2021). In fact, NO 3 − has replaced sulfate (SO 4 2− ) as the most important component in particulate pollution in North China (Fu et al., 2020; Luo et al., 2019, 2021; Song et al., 2019).…”