Haze events at different levels in winters: A comprehensive study of meteorological factors, Aerosol characteristics and direct radiative forcing in megacities of north and central China

Abstract The changes in aerosol optical and microphysical properties, and sub-band shortwave direct radiative forcing (DARF) in Beijing and Wuhan were compared at different haze levels in winters. The occurrence of haze is found to be governed by the wind circulation and boundary layer in Beijing where the ground wind speed and the height of boundary layer decreased significantly with the development of haze. Compared with the boundary layer and wind, the relative humidity has a stronger impact on haze in Wuhan. Especially, hygroscopic growth of aerosol particles is observed in Wuhan. The increase of fine-mode non-absorbing particles is the main characteristic of aerosol change during haze periods. With the development of haze, the larger increase in DARF at the top of atmosphere is found in Beijing, while the change in DARF at the atmosphere is more obvious in Wuhan. Efficiency of the DARF shows that the change of DARF depends highly on the single scattering albedo in Beijing due to the obvious enhancement of particle scattering, while it depends much more on particle radius over Wuhan. The increase in particle size can also change DARF proportion of each sub-band in shortwave that the DARF proportion in ultraviolet and visible decreased. However, the DARF proportion in near-infrared increased during haze, as the light in shorter wavelength is more sensitive to the change of the fine particle radius.