Stringent mitigation measures have reduced wintertime PM2.5 concentrations by 42.2% from 2013 to 2018 in the BTH. 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 level has decreased by over 20%. It still remains elusive about contributions of nitrogen oxides (NOx) emissions mitigation to the nitrate and PM2.5 level. The WRF-Chem model simulations of a persistent haze 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, 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 occurrence of the severe PM pollution in the BTH during the COVID-19 outbreak with a significant reduction of NO2.