[Effects of Emission Reductions of Key Sources on the PM_2.5 Concentrations in the Yangtze River Delta]

The effects of emission reductions of key sources (eight key industries and transportation) on the PM_2.5 concentrations in the Yangtze River Delta (YRD) were investigated using the weather research forecast-chemistry (WRF-Chem) model in 2013 combined with two normal and enhanced emission reduction scenarios. The SO₂, NO, PM_2.5, and NMVOC emissions in the YRD decrease by 36.3%, 26.3%, 32.0%, and 14.6% and by 51.4%, 39.6%, 37.6%, and 28.4% under the normal and enhanced emission reduction scenarios, respectively. The simulation results show that the annual mean PM_2.5 concentrations over the national environmental monitoring sites in the YRD decline by 1.4-26.7 μg·m^-3 and 2.1-32.3 μg·m^-3, reflecting a decrease of 2.7%-23.1% and 3.9%-27.5%, under the two emission reduction scenarios, respectively. The nitrate in secondary inorganic aerosols contributes the most to the reduction of the annual mean PM_2.5 concentration. The seasonal variation characteristics of the PM_2.5 and secondary inorganic aerosol concentrations reflect that the smallest and largest declining rates occur in winter and summer, respectively. With increasing emission reduction, the declining rates of PM_2.5 and the secondary inorganic aerosol concentrations in summer increase more compared with those in other seasons, resulting in a greater seasonal variation of the rates. The PM_2.5 concentrations decrease by~20% in Shanghai and the Jiangsu Province under the enhanced emission scenario in summer. The analysis of the atmospheric oxidation shows that the atmospheric oxidation capacity is enhanced to different degrees by emission reductions of key sources in all seasons; it is further enhanced with increasing emission reduction. The enhanced oxidation capacity favors the formation of secondary PM_2.5, thereby hindering the reduction of the PM_2.5 concentration. The strongest hindrance occurs in winter, resulting in the worst PM_2.5 pollution improvement. The atmospheric oxidation capacity is less affected by emission reductions of key sources in summer, making PM_2.5 pollution improvement most effective. Furthermore, the negative effects of the enhancement of the atmospheric oxidation capacity on the reduction of the PM_2.5 concentration in spring and autumn cannot be ignored.