Growth of nitrate contribution to aerosol pollution during wintertime in Xi'an, northwest China: Formation mechanism and effects of NH3--颗粒学报PARTICUOLOGY

Su, H., Zhang, T., Liu, S., Qu, Y., Li, H., Zhou, J., . . . Cao, J. (2024). Growth of nitrate contribution to aerosol pollution during wintertime in Xi'an, northwest China: Formation mechanism and effects of NH3. Particuology, 87, 303-315. https://doi.org/10.1016/j.partic.2023.09.014

Growth of nitrate contribution to aerosol pollution during wintertime in Xi'an, northwest China: Formation mechanism and effects of NH₃

Hui Su^{a b c}, Ting Zhang ^{a c d *}, Suixin Liu ^{a c d}, Yao Qu ^{a c e}, Huan Li ^{a b}, Jiamao Zhou ^{a c d}, Zhuzi Zhao ^f, Qiyuan Wang ^{a c d}, Lu Li ^{a c}, Minxia Shen ^a^c, Shuoyuan Chen ^a, Steven Sai Hang Ho ^g, Junji Cao ^{a c h}

a State Key Laboratory of Loess and Quaternary Geology, Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China
b Xi'an Institute for Innovative Earth Environment Research, Xi'an 710061, China
c National Observation and Research Station of Regional Ecological Environment Change and Comprehensive Management in the Guanzhong Plain, Xi'an 710061, China
d Shaanxi Key Laboratory of Atmospheric and Haze-fog Pollution Prevention, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China
e University of Chinese Academy of Sciences, Beijing 100049, China
f School of Chemical and Environmental Engineering, Jiangsu University of Technology, Changzhou 213001, China
g Division of Atmospheric Sciences, Desert Research Institute, Reno NV89512, United States
h Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

10.1016/j.partic.2023.09.014

Volume 87, April 2024, Pages 303-315

Received 25 July 2023, Revised 19 September 2023, Accepted 20 September 2023, Available online 30 October 2023, Version of Record 30 October 2023.

E-mail: zhangting@ieecas.cn

Highlights

• Homogeneous reactions between NH₃ and HNO₃ govern NO₃⁻ formation in clean periods.

• Heterogeneous hydrolysis of N₂O₅ with high relative humidities dominant in heavy pollution process.

• Neutralization reaction caused by NH₃ plays an important role in nitrate formation.

• Vehicle restrictions could effectively reduce NH₃ and NO₂ emissions.

Abstract

With the strengthened controls on SO₂ emissions and extensive increases in motor vehicles’ exhaust, aerosol pollution shifts from sulfate-rich to nitrate-rich in recent years in Xi'an, China. To further gain insights into the factors on nitrate formation and efficiently mitigate air pollution, highly time-resolved observations of water-soluble inorganic ions (WSIIs) in PM_2.5 were measured in a suburban area of Xi'an, China during wintertime. Hourly concentration of total WSIIs is 39.8 μg m−3 on average, accounting for 50.3% of PM2.5 mass. In contrast to a slight decrease in the mass fraction of SO₄²⁻, NO₃⁻ shows a significant increase of the PM_2.5 contribution with the aggravation of aerosol pollution. This suggests the importance of NO₃⁻ formation to haze evolution. Furthermore, homogeneous reactions govern the formation of NO₃⁻, while alkali metals such as calcium and sodium play an additional role in retaining NO₃⁻ in PM_2.5 during clean periods. However, the heterogeneous hydrolysis reaction contributed more to NO₃⁻ formation during the pollution periods under high relative humidity. Our investigation reveals that temperature, relative humidity, oxidant, and ammonia emissions facilitate rapid NO₃⁻ formation. Using the random forest (RF) model, NO₃⁻ concentrations were successfully simulated with measured variables for the training and testing datasets (R² > 0.95). Among these variables, CO, NH₃, and NO₂ were found to be the main factors affecting the NO₃⁻ concentrations. Compared with the period without vehicle restriction, the contributions of NO₃⁻ and NH₄⁺ to PM_2.5 mass decreased by 5.3% and 3.4% in traffic restriction periods, respectively. The vehicle restriction leads to the decreases of precursor gases of NO₂, SO₂, and NH₃ by 12.8%, 5.9%, and 27.6%, respectively. The results demonstrate collaborative emission reduction of NO_x and NH₃ by vehicle restrictions, and using new energy vehicles (or electric vehicles) can effectively alleviate particulate matter pollution in northwest China.

Graphical abstract

Keywords

PM_2.5; Water-soluble inorganic ions; Nitrate formation; Excess ammonium; Vehicle restrictions

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