Mitigating NO emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei, China.

Stringent mitigation measures have reduced wintertime fine particulate matter (PM) 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 PM, although the surface-measured NO concentration has decreased by over 20%. The contributions of nitrogen oxides (NO) emissions mitigation to the nitrate and PM 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 NO emissions mitigation does not help lower wintertime nitrate and PM concentrations under current conditions in the BTH. A 50% reduction in NO emissions only decreases nitrate mass by 10.3% but increases PM concentrations by 3.2%, because the substantial O increase induced by NO mitigation offsets the HNO 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 NO concentration. Mitigation of NH emissions constitutes the priority measure to effectively lower the nitrate and PM concentrations in the BTH under current conditions, with 35.5% and 12.7% decrease, respectively, when NH emissions are reduced by 50%.