Aerosol ultraviolet absorption experiment (2002 to 2004), part 2: absorption optical thickness, refractive index, and single scattering albedo





Citation of Original Publication

Nickolay A. Krotkov, Pawan K. Bhartia, Jay R. Herman, James R. Slusser, Gwendolyn R. Scott, Gordon J. Labow, Alexander P. Vasilkov, Tom Eck, Oleg Doubovik, Brent N. Holben, "Aerosol ultraviolet absorption experiment (2002 to 2004), part 2: absorption optical thickness, refractive index, and single scattering albedo," Opt. Eng. 44(4) 041005 (1 April 2005)


This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Compared to the visible spectral region, very little is known about aerosol absorption in the UV. Without such information it is impossible to quantify the causes of the observed discrepancy between modeled and measured UV irradiances and photolysis rates. We report results of a 17-month aerosol column absorption monitoring experiment conducted in Greenbelt, Maryland, where the imaginary part of effective refractive index k was inferred from the measurements of direct and diffuse atmospheric transmittances by a UV-multifilter rotating shadowband radiometer [UV-MFRSR, U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network]. Colocated ancillary measurements of aerosol effective particle size distribution and refractive index in the visible wavelengths [by CIMEL sun-sky radiometers, National Aeronautics and Space Administration (NASA) Aerosol Robotic Network (AERONET)], column ozone, surface pressure, and albedo constrain the forward radiative transfer model input, so that a unique solution for k is obtained independently in each UV-MFRSR spectral channel. Inferred values of k are systematically larger in the UV than in the visible wavelengths. The inferred k values enable calculation of the single scattering albedo ω, which is compared with AERONET inversions in the visible wavelengths. On cloud-free days with high aerosol loadings [τₑₓₜ(440)>0.4], ω is systematically lower at 368 nm (<ω₃₆₈>=0.94) than at 440 nm (<ω₄₄₀>=0.96), however, the mean ω differences (0.02) are within expected uncertainties of ω retrievals (~0.03). The inferred ω is even lower at shorter UV wavelengths (<ω₃₂₅>~<ω₃₃₂>=0.92), which might suggest the presence of selectively UV absorbing aerosols. We also find that decreases with decrease in aerosol loading. This could be due to real changes in the average aerosol composition between summer and winter months at the Goddard Space Flight Center (GSFC) site. Combing measurements of τₑₓₜ and ω, the seasonal dependence of the aerosol absorption optical thickness, τₐbₛ=τₑₓₜ(1 - ω) is derived in the UV with an uncertainty of 0.01 to 0.02, limited by the accuracy of UV-MFRSR measurement and calibration. The τₐbₛ has a pronounced seasonal dependence with maximum values ~0.1 occurring in summer hazy conditions and <0.02 in the winter and fall seasons, when aerosol loadings are small. The measured τₐbₛ is sufficient to explain both the magnitude and seasonal dependence of the bias in satellite estimates of surface UV irradiance previously seen with ground-based UV measurements.