Aerosol Optical Properties and Associated Direct Radiative Forcing over the Yangtze River Basin during 2001-2015

The spatiotemporal variation of aerosol optical depth at 550 nm (AOD(550)), angstrom ngstrom exponent at 470-660 nm (AE(470-660)), water vapor content (WVC), and shortwave (SW) instantaneous aerosol direct radiative effects (IADRE) at the top-of-atmosphere (TOA) in clear skies obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) are quantitatively analyzed over the Yangtze River Basin (YRB) in China during 2001-2015. The annual and seasonal frequency distributions of AE(470-660) and AOD(550) reveal the dominance of fine aerosol particles over YRB. The regional average AOD(550) is 0.49 +/- 0.31, with high value in spring (0.58 +/- 0.35) and low value in winter (0.42 +/- 0.29). The higher AOD(550) (>= 0.6) is observed in midstream and downstream regions of YRB and Sichuan Basin due to local anthropogenic emissions and long-distance transport of dust particles, while lower AOD(550) (<= 0.3) is in high mountains of upstream regions. The IADRE is estimated using a linear relationship between SW upward flux and coincident AOD(550) from CERES and MODIS at the satellite passing time. The regional average IADRE is -35.60 +/- 6.71 Wm(-2), with high value (-40.71 +/- 6.86 Wm(-2)) in summer and low value (-29.19 +/- 7.04 Wm(-2)) in winter, suggesting a significant cooling effect at TOA. The IADRE at TOA is lower over Yangtze River Delta (YRD) (<=-30 Wm(-2)) and higher in midstream region of YRB, Sichuan Basin and the source area of YRB (>=-45 Wm(-2)). The correlation coefficient between the 15-year monthly IADRE and AOD(550) values is 0.63, which confirms the consistent spatiotemporal variation patterns over most of the YRB. However, a good agreement between IADRE and AOD is not observed in YRD and the source area of YRB, which is probably due to the combined effects of aerosol and surface properties.