Role of black carbon in modulating aerosol direct effects driven by air pollution controls during 2013-2017 in China

Aerosol direct effects (ADEs) can modulate shortwave radiation as well as atmospheric dynamics and air quality. As the key absorbing component of aerosol, the black carbon (BC) largely determines the aerosol optical properties. Therefore, it is expected that BC emission controls might gain co-benefits from the simultaneous reduction of ADEs. To demonstrate such synergy, here we quantified the ADEs changes and the role of BC controls in China during 2013-2017 using a regional two-way coupled meteorology chemistry transport model. Simulated results suggest that the control action effectively reduced the wintertime PM_2.5 concentration (-26.0 μg m^-3) and associated ADEs. In January, the influence of ADEs on surface shortwave radiation, 2-meter temperature, and planetary boundary layer height was weakened from -16.7 W m^-2, -0.20 °C, and -15.4 m in 2013 to -11.3 W m^-2, -0.06 °C, and -10.7 m in 2017, respectively. The enhancement of SO₂, NO₂, and PM_2.5 concentrations due to ADEs was reduced from +3.1%, +5.2%, and +5.4% in 2013 to +2.6%, +4.5%, and +3.3% in 2017, respectively, demonstrating the extra benefit of air pollution controls for improving air quality by reducing ADEs. Meanwhile, the BC emission reduced by 12.5% simultaneously along with the effective controls on SO₂ and NO₂ emissions during 2013-2017, mainly from domestic combustion (-11.7%), resulting in 30.3% (-0.9 μg m^-3) reduction of BC concentration. Such BC controls contributed 15.6-60.2% of such changes in the ADEs influence on meteorological variables, and 32.6-41.1% on air pollutants. More specially, the effectiveness of collaborative reduction of BC further reduced surface shortwave radiation in China by 3.6 W m^-2 in January and 1.0 W m^-2 in July, leading to a more weakened ADEs that bring extra benefits in reducing PM_2.5 concentrations by 1.8 μg m^-3 in January and 0.3 μg m^-3 in July. Apparently, BC played an important role in modulating the ADEs and associated influences on meteorology and air quality, suggesting a wise control strategy by targeting absorbing component of PM_2.5 reduction to address both air pollution and climate change in the future.