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Optical Engineering

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

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.

Paper Details

Date Published: 1 April 2005
PDF: 17 pages
Opt. Eng. 44(4) 041005 doi: 10.1117/1.1886819
Published in: Optical Engineering Volume 44, Issue 4
Show Author Affiliations
Nickolay A. Krotkov, NASA Goddard Space Flight Ctr. (United States)
Pawan K. Bhartia, NASA Goddard Space Flight Ctr. (United States)
Jay R. Herman, NASA Goddard Space Flight Ctr. (United States)
James R. Slusser, Colorado State Univ. (United States)
Gwendolyn R. Scott, Colorado State Univ. (United States)
Gordon J. Labow, Science Systems and Applications, Inc. (United States)
Alexander P. Vasilkov, Science Systems and Applications, Inc. (United States)
Tom Eck, NASA Goddard Space Flight Ctr. (United States)
Oleg Doubovik, NASA Goddard Space Flight Ctr. (United States)
Brent N. Holben, NASA Goddard Space Flight Ctr. (United States)

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