It has been estimated that the scattering of shortwave radiation by anthropogenic aerosols during clear sky conditions creates a global radiative perturbation of comparable magnitude but opposite sign to the forcing by anthropogenic greenhouse gases. This `direct' radiative aerosol forcing effect depends primarily on the aerosol optical depth (AOD), which is the integral of the aerosol extinction coefficient, b_ext, with height. The aerosol extinction coefficient is the sum of the aerosol scattering , b_sp, and absorption, b_ap, coefficients. We present comparison of AOD estimates made by sun photometer and nephelometer measurements on April 10, 1996 at the SGP ARM site. Direct solar irradiance was measured with a Cimel sun photometer at 6 wavelengths from 339 nm to 1021 nm. These data are used to estimate AOD's by Langley analyses for clear sky conditions. The aerosol scattering coefficient, b_sp, was measured at wavelengths of 450 nm, 550 nm, and 700 nm with a TSI Integrating Nephelometer. In addition the aerosol absorption coefficient, b_ap, was measured at 550 nm. The AOD estimates are compared from 18:00 to 21:00 UTC on April 10, a period of time that was characterized by generally clear sky conditions and very stable values of b_sp and b_ap. Radiosonde temperature profiles indicated a well mixed layer in the lower 1200 m of the troposphere. During this time period the Angstrom exponent (slope of b_sp or AOD vs. wavelength curve on a log scale) based on nephelometer measurements is 2.2 (for b_sp values at 450 nm and 700 nm) compared to a value of 1.2 from sun photometer measurements (for AOD's at 437 nm and 669 nm). This discrepancy may be largely due to differences in the RH between the nephelometer (20%) and the atmosphere (35% - 60%) which can lead to significant differences in the aerosol particle diameters due to hygroscopic growth. The aerosol optical depth estimates from sun photometer and nephelometer measurements are within 40 %. The reason for this discrepancy will be discussed, as well as the uncertainties in the factors that are used to estimate aerosol optical depth from nephelometer measurements.
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