Sunlight transmission through desert dust and marine aerosols: Diffuse light corrections to Sun photometry and pyrheliometry

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Read Full Article (file size: 3912823 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D08207, doi:10.1029/2003JD004292, 2004 Sunlight transmission through desert dust and marine aerosols: Diffuse light corrections to Sun photometry and pyrheliometry P. B. Russell NASA Ames Research Center, Moffett Field, California, USA J. M. Livingston SRI International, Menlo Park, California, USA O. Dubovik Goddard Earth Sciences and Technology Center, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA S. A. Ramirez Bay Area Environmental Research Institute, Sonoma, California, USA J. Wang Brookhaven National Laboratory, Upton, New York, USA J. Redemann Bay Area Environmental Research Institute, Sonoma, California, USA B. Schmid Bay Area Environmental Research Institute, Sonoma, California, USA M. Box School of Physics, University of New South Wales, Sydney, New South Wales, Australia B. N. Holben Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Abstract Desert dust and marine aerosols are receiving increased scientific attention because of their prevalence on intercontinental scales and their potentially large effects on Earth radiation, climate, other aerosols, clouds, and precipitation. The relatively large size of dust and marine aerosol particles produces scattering phase functions that are strongly forward peaked. Hence Sun photometry and pyrheliometry of these aerosols are more subject to diffuse light errors than is the case for smaller aerosols. We quantify these diffuse light effects for common Sun photometer and pyrheliometer fields of view (FOV), using data on dust and marine aerosols from (1) Aerosol Robotic Network (AERONET) measurements of sky radiance and solar beam transmission and (2) in situ measurements of aerosol layer size distribution and chemical composition. Accounting for particle nonsphericity is important when deriving dust size distribution from both AERONET and in situ aerodynamic measurements. We obtain correction factors that can be applied to Sun photometer or pyrheliometer results for aerosol optical depth (AOD) or direct beam transmission. The corrections are negligible (less than ∼1% of AOD) for Sun photometers with narrow FOV (half-angle η < ∼1°), but they can be as large as 10% of AOD at 354 nm wavelength for Sun photometers with η = 1.85°. For pyrheliometers (which can have η up to ∼2.8°), corrections can be as large as 16% at 354 nm. AOD correction factors are well correlated with AOD wavelength dependence (hence Ångström exponent). We provide best fit equations for determining correction factors from Ångström exponents of uncorrected AOD spectra, and we demonstrate their application to vertical profiles of multiwavelength AOD. Received 27 October 2003; accepted 26 February 2004; published 27 April 2004. Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0394 Atmospheric Composition and Structure: Instruments and techniques. Read Full Article (file size: 3912823 bytes) Cited by Citation: Russell, P. B., J. M. Livingston, O. Dubovik, S. A. Ramirez, J. Wang, J. Redemann, B. Schmid, M. Box, and B. N. Holben (2004), Sunlight transmission through desert dust and marine aerosols: Diffuse light corrections to Sun photometry and pyrheliometry, J. Geophys. Res., 109, D08207, doi:10.1029/2003JD004292. Copyright 2004 by the American Geophysical Union.

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