Effects of aerosols on the surface solar radiation in a tropical urban area

Surface radiation measurements at an urban site in southern Taiwan during the dry winter season, December 2003 to March 2004, are used to investigate the aerosol properties and the effects of aerosols on the surface solar radiation in clear skies. The AERONET‐retrieved monthly mean aerosol optical thickness at 0.5 μm ranges from 0.52 to 1.18 with a mean of 0.73 for the four winter months. The single‐scattering albedo and asymmetry factor remain nearly constant with a mean of ∼0.95 for the former and ∼0.70 for the latter. Thus the air of Tainan during the winter is highly polluted, and the aerosols are moderately absorbing. A new approach is developed to estimate the effect of aerosols on the surface radiation from surface observations. The relationship between the surface radiation and the aerosol optical thickness is established for narrow intervals of the solar zenith angle by regression of the atmospheric path transmission against the aerosol optical path. It is found that the relationship is surprisingly linear. On the basis of the relationship the effect of aerosols on the surface radiation and the sensitivity of surface radiation to the aerosol optical thickness are computed. For the four‐month average, the effect of aerosols is to reduce the incident surface solar radiation by ∼39 W m −2 , and the sensitivity of the surface radiation to aerosols undergoes a reduction of ∼53 W m −2 per unit increase of the aerosol optical thickness at 0.5 μm. These results are in good agreement with radiative transfer model calculations.