Impacts of aerosol direct effects on PM2.5 and O3 respond to the reductions of different primary emissions in Beijing-Tianjin-Hebei and surrounding area

In recent years, the rapid pollution emission reductions significantly decreased the PM2.5, which weakened the aerosol radiation feedback effect and in turn influenced the changes of PM2.5 and O3. To promote our understanding of this effect, we used an online-coupled meteorology-chemistry model (WRF-Chem) to investigate the influence of aerosol direct effects (ADE) on PM2.5 and O3 responding to different pollutant emissions reduction for a summer PM2.5-O3 compound pollution month in Beijing-Tian-Hebei region (BTH) and surrounding area. We performed scenario simulations with turning on/off both aerosol influences on the meteorology (ADE_MET) and photolysis (ADE_PHO), and turning on only ADE_PHO with different reduction ratios for primary PM2.5 (100%), SO2 (80%) and NOx (40%) emissions. Our results showed that total reduction of primary PM2.5, SO2 and NOx emissions led to overall declines of surface PM2.5 in BTH and surrounding area. The SO2 and NOx emission reductions both furtherly decreased PM2.5 via the changed ADE, while the primary PM2.5 emission reduction increased the PM2.5 by its induced ADE. Different from PM2.5, O3 significantly increased in BTH after cutting the three types of emissions. The primary PM2.5 emission reduction led to O3 increase over the whole region mostly by the induced ADE, while the NOx emission reduction directly enhanced O3 in the plain areas mainly due to incompatible reduction of NOx and VOCs emissions. The O3 enhanced by emission-reduction-induced ADE accounted for 56.1% of the total increase. The ADE_PHO caused by PM2.5 primary emissions reduction was the dominant contribution to the total ADE induced increases of PM2.5 and O3, which enhanced the photolysis of gases and led to overall increase of oxidants that strengthened secondary aerosol formation, and also promoted the production of O3 via photochemical processes. The results implicate that with the continuously strengthened abatement of PM2.5 primary emissions, it may offset part of the reduction effect on PM2.5 decrease by the changed ADE, and also increase O3 over the whole region. The results also suggested that while controlling both PM2.5 and O3 in BTH in summer, not only the cooperative emission reduction of NOx and VOCs is essential, the induced ADE effect by PM2.5 primary emissions reduction should also be considered.