Optical Properties of Aerosols and Implications for Radiative Effects in Beijing During the Asia-Pacific Economic Cooperation Summit 2014

An intensive measurement campaign was conducted in Beijing during the Asia-Pacific Economic Cooperation (APEC) Summit 2014 to investigate the effectiveness of stringent emission controls on aerosol optical properties and direct radiative forcing (DRF). Average values of PM_2.5, light scattering (b_scat), and light absorption (b_abs) coefficients decreased by 40, 64, and 56%, respectively, during the APEC control period compared with noncontrol periods. For the APEC control period, the PM_2.5 mass scattering and absorption efficiencies were both smaller than the noncontrol period by a factor of ~2. Calculations based on a revised IMPROVE method and linear regression showed that sulfate, nitrate, organic matter, elemental carbon, and fine soil contributed comparably to the light extinction coefficient (b_ext) in both periods, but the b_ext values were 27–64% lower during the APEC period. A positive matrix factorization receptor model showed that b_ext from two secondary aerosol sources, biomass burning, traffic-related emissions, and coal burning decreased by 26–87% during the APEC control period. The average DRF calculated from the Tropospheric Ultraviolet and Visible radiation model was −11.9 and −4.6 W m⁻² at the surface during the noncontrol and APEC control periods, respectively, suggesting an overall cooling effect. The reduction of DRF from each emission source ranged from ~30–80% during the APEC control period. The results suggest that the pollution control measures implemented for APEC substantially reduced air pollution and could help mitigate the cooling effects of aerosols at the surface in Beijing.