Mitigating NO_X emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei, China

Stringent mitigation measures have reduced wintertime fine particulate matter (PM_2.5) 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 PM_2.5, although the surface-measured NO₂ concentration has decreased by over 20%. The contributions of nitrogen oxides (NO_X) emissions mitigation to the nitrate and PM_2.5 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 NO_X emissions mitigation does not help lower wintertime nitrate and PM_2.5 concentrations under current conditions in the BTH. A 50% reduction in NO_X emissions only decreases nitrate mass by 10.3% but increases PM_2.5 concentrations by 3.2%, because the substantial O₃ increase induced by NO_X 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 PM_2.5 concentrations in the BTH under current conditions, with 35.5% and 12.7% decrease, respectively, when NH₃ emissions are reduced by 50%.