Nonlinear effect of NO_x concentration decrease on secondary aerosol formation in the Beijing-Tianjin-Hebei region:...

During the COVID-19 lockdown in the Beijing-Tianjin-Hebei (BTH) region in China, large decrease in nitrogen oxides (NO_x) emissions, especially in the transportation sector, could not avoid the occurrence of heavy PM_2.5 pollution where nitrate dominated the PM_2.5 mass increase. To experimentally reveal the effect of NO_x control on the formation of PM_2.5 secondary components (nitrate in particular), photochemical simulation experiments of mixed volatile organic compounds (VOCs) under various NO_x concentrations with smog chamber were performed. The proportions of gaseous precursors in the control experiment were comparable to ambient conditions typically observed in the BTH region. Under relatively constant VOCs concentrations, when the initial NO_x concentration was reduced to 40% of that in the control experiment (labelled as NO_x,0), the particle mass concentration was not significantly reduced, but when the initial NO_x concentration decreased to 20 % of NO_x,0, the mass concentration of particles as well as nitrate and organics showed a sudden decrease. A "critical point" where the mass concentration of secondary aerosol started to decline as the initial NO_x concentration decreased, located at 0.2-0.4 NO_x,0 (or 0.18-0.44 NO_2,0) in smog chamber experiments. The oxidation capacity and solar radiation intensity played key roles in the mass concentration and compositions of the formed particles. In field observations in the BTH region in the autumn and winter seasons, the "critical point" exist at 0.15-0.34 NO_2,0, which coincided mostly with the laboratory simulation results. Our results suggest that a reduction of NO_x emission by >60% could lead to significant reductions of secondary aerosol formation, which can be an effective way to further alleviate PM_2.5 pollution in the BTH region.