Direct aerosol radiative forcing in the Yangtze delta region of China: Observation and model estimation

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Abstract
The Yangtze delta region of China is a key agricultural area that experiences relatively high aerosol loadings [Xu et al., 2002]. In order to characterize the aerosol radiative properties and estimate direct aerosol radiative forcing in this region, measurements of the multiwavelength aerosol optical depth, τλ, light scattering coefficient, σsp, and absorption coefficient, σap, as well as the downward photosynthetically active radiation (400–700 nm), DPAR, were conducted during November 1999 in Linan, China. The direct aerosol radiative forcing for photosynthetically active radiation (PAR) and total solar radiation (0.2–4.0 μm, TSR) at both the surface and top of atmosphere (TOA) are estimated based on the measurements using two radiative transfer models. The model estimates indicate that the mean cloud‐free instantaneous direct aerosol radiative forcing efficiency (for solar zenith angle < 70°) at the surface for PAR is −73.5 W m−2, which is in agreement with the value of −74.4 W m−2 derived directly from the measurements of DPAR and 500 nm aerosol optical depth (τ500). On the basis of the measured mean τ500 of 0.61, and the estimated cloud optical depth and cloud coverage of 5.0 and 50%, respectively, the 24‐hr mean direct aerosol radiative forcing at the surface for PAR is estimated to be approximately −11.2 W m−2. This suggests that the amount of PAR reaching the surface over the Yangtze delta region is reduced by ∼16% as a result of the direct radiative effect of aerosols. The model results also indicate that the cloud‐free 24‐hr average direct aerosol radiative forcing efficiency at the TOA for TSR is −30.4 W m−2. When the presence of clouds is considered, the mean direct aerosol radiative forcing at the TOA for TSR is estimated to be approximately −12.1 W m−2. This value is roughly an order of magnitude greater than the estimated global mean aerosol radiative forcing of −0.3 to −1.0 W m−2 suggested by the Intergovernmental Panel on Climate Change [1996]. Overall, this study indicates that aerosols have a substantial impact on the amount of radiation reaching the surface as well as the radiation balance at the TOA in the Yangtze delta region.

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