Column aerosol optical properties and aerosol radiative forcing during a serious haze-fog month over North China Plain in 2013 based on ground-based sunphotometer measurements

In January 2013, North China Plain experienced several serious haze events. Cimel sunphotometer measurements at seven sites over rural, suburban and urban regions of North China Plain from 1 to 30 January 2013 were used to further our understanding of spatial-temporal variation of aerosol optical parameters and aerosol radiative forcing (ARF). It was found that Aerosol Optical Depth at 500 nm (AOD(500nm)) during non-pollution periods at all stations was lower than 0.30 and increased significantly to greater than 1.00 as pollution events developed. The Angstrom exponent (Alpha) was larger than 0.80 for all stations most of the time. AOD(500nm) averages increased from north to south during both polluted and non-polluted periods on the three urban sites in Beijing. The fine mode AOD during pollution periods is about a factor of 2.5 times larger than that during the non-pollution period at urban sites but a factor of 5.0 at suburban and rural sites. The fine mode fraction of AOD(675nm) was higher than 80% for all sites during January 2013. The absorption AOD(675nm) at rural sites was only about 0.01 during pollution periods, while similar to 0.03-0.07 and 0.01-0.03 during pollution and non-pollution periods at other sites, respectively. Single scattering albedo varied between 0.87 and 0.95 during January 2013 over North China Plain. The size distribution showed an obvious tri-peak pattern during the most serious period. The fine mode effective radius in the pollution period was about 0.01-0.08 mu m larger than during non-pollution periods, while the coarse mode radius in pollution periods was about 0.06-0.38 mu m less than that during non-pollution periods. The total, fine and coarse mode particle volumes varied by about 0.06-0.34 mu m(3), 0.03-0.23 mu m(3), and 0.03-0.10 mu m(3), respectively, throughout January 2013. During the most intense period (1-16 January), ARF at the surface exceeded -50 W m(-2), -180 W m(-2), and -200 W m(-2) at rural, suburban, and urban sites, respectively. The ARF readings at the top of the atmosphere were approximately -30 W m(-2) in rural and -40-60 W m(-2) in urban areas. Positive ARF at the top of the atmosphere at the Huimin suburban site was found to be different from others as a result of the high surface albedo due to snow cover.