Abstract:Atmospheric dispersion conditions during the November Beijing Asia Pacific Economic Cooperation (APEC) meeting are analyzed and compared with those in October when four fog haze processes occurred. Stable weather index (SWI) during APEC was slightly smaller than that during the four fog haze processes in October. Numerical simulations show that emission control of Beijing and its surrounding areas is the main reason for a low level of aerosol concentrations in Beijing during APEC. Both the pollution transmission from neighboring regions and the diurnal variations of mountain valley breeze under special terrain conditions contributed to the space distribution and time variation of pollution over Beijing. The north wind at night caused by topographic distribution eliminated pollution near ground obviously. Compared to heavy pollution weather, during the APEC, because of the low aerosol concentration and the corresponding enlarged diurnal variations of surface air temperature, the valley wind circulation in the Beijing area was obvious and hence enhanced daytime mixing layer height, producing more conductive conditions to spread the pollutants. Emission reduction led to low aerosol concentration which improved dispersion condition further, forming a negative feedback for pollutants to accumulate, contributing to “APEC blue” as well.