Mitigating NOX emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei, China
2021
Li, Xia | Bei, Naifang | Hu, Bo | Wu, Jiarui | Pan, Yuepeng | Wen, Tianxue | Liu, Zirui | Liu, Lang | Wang, Ruonan | Li, Guohui
Stringent mitigation measures have reduced wintertime fine particulate matter (PM₂.₅) concentrations by 42.2% from 2013 to 2018 in the Beijing-Tianjin-Hebei (BTH) region, but severe PM pollution still frequently engulfs the region. The observed nitrate aerosols have not exhibited a significant decreasing trend and constituted a major fraction (about 20%) of the total PM₂.₅, although the surface-measured NO₂ concentration has decreased by over 20%. The contributions of nitrogen oxides (NOX) emissions mitigation to the nitrate and PM₂.₅ concentrations and how to alleviate nitrate aerosols efficiently under the current situation still remains elusive. The WRF-Chem model simulations of a persistent and heavy PM pollution episode in January 2019 in the BTH reveal that NOX emissions mitigation does not help lower wintertime nitrate and PM₂.₅ concentrations under current conditions in the BTH. A 50% reduction in NOX emissions only decreases nitrate mass by 10.3% but increases PM₂.₅ concentrations by 3.2%, because the substantial O₃ increase induced by NOX mitigation offsets the HNO₃ loss and enhances sulfate and secondary organic aerosols formation. Our results are further consolidated by the occurrence of severe PM pollution in the BTH during the COVID-19 outbreak, with a significant reduction in NO₂ concentration. Mitigation of NH₃ emissions constitutes the priority measure to effectively lower the nitrate and PM₂.₅ concentrations in the BTH under current conditions, with 35.5% and 12.7% decrease, respectively, when NH₃ emissions are reduced by 50%.
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