Temporal variability of the visibility, particulate matter mass concentration and aerosol optical properties over an urban site in Northeast China

Visibility, particulate matter (PM) mass concentration, and aerosol optical properties data from June 2009 to December 2011 were obtained at Shenyang in Northeast China. The characteristics and relationships between these parameters were statistically analyzed. The results demonstrate that the monthly averaged visibility over Shenyang was higher in spring and autumn but lower in summer and winter, and had an inverse trend to PM and aerosol optical depth (ADD). Higher AOD at 500 nm was found year by year, with the maximum value (1.31 +/- 0.45) occurring in June 2011, and the minimum in June 2010 (0.72 +/- 0.31). The mean value of the Angstrom exponent underwent a notable reduction during the period of 2011, with values less than 1.0 from February to September. The single scattering albedo was consistently around 0.90 during 2009-2011, which was higher in summer but lower in winter. The higher absorption aerosol optical depth at 440 nm in 2011 indicates that there were more absorbing aerosol particles in this period compared with the corresponding absorption Angstrom exponent in the same year (about 0.75). The direct radiative forcing at the bottom of the atmosphere increased to 200 W/m(2) in 2011, compared with 150 W/m(2) from June 2009 to December 2010, suggesting a stronger cooling effect of aerosols at the surface. The positive radiative forcing at the top of the atmosphere in November and December 2009 could have been due to snow cover, which has a large surface albedo that reflects shortwave radiation to the atmosphere. (C) 2015 Elsevier B.V. All rights reserved.