Urban aerosol radiative properties: Measurements during the 1999 Atlanta Supersite Experiment

calculated with a Mie code yielding Eap = 9.5 ± 1.5 m 2 g 1 , while EC mass summed from the impactor stages in comparison to measured sap gives Eap = 9.3 ± 3.2 m 2 g 1 .M ie light-scattering calculations using inputs of measured mass and EC size distributions give geometric mean light scattering and absorption Dp = 0.54 and 0.13 mm, respectively, and show the dominance of the submicrometer diameter particles to light extinction in the urban environment. Based on the measured aerosol optical depth in Atlanta, da (500 nm) = 0.44 ± 0.22, and other radiative measurements, a best estimate of the average direct aerosol radiative forcing at the top of the atmosphere (a measure of the climate significance) is F= 11 ± 6 W m 2 in Atlanta. This value is an order of magnitude greater than global mean estimates for aerosols underscoring the influence of aerosol particles on radiative transfer in the urban environment. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0394 Atmospheric Composition and

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