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Sunlight Transmission through Desert Dust and Marine Aerosols: Diffuse Light Corrections to Sun Photometry and PyrheliometryDesert dust and marine aerosols are receiving increased scientific attention because of their prevalence on intercontinental scales and their potentially large effects on Earth radiation and climate, as well as on other aerosols, clouds, and precipitation. The relatively large size of desert dust and marine aerosols 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. Here we quantify these diffuse-light effects for common Sun photometer and pyrheliometer fields of view (FOV), using a data base on dust and marine aerosols derived from (1) 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 non-sphericity is important when deriving dust size distribution from both AERONET and in situ aerodynamic measurements. We express our results in terms of correction factors that can be applied to Sun photometer and pyrheliometer measurements of aerosol optical depth (AOD). We find that the corrections are negligible (less than approximately 1% of AOD) for Sun photometers with narrow FOV (half-angle eta less than degree), but that they can be as large as 10% of AOD at 354 nm wavelength for Sun photometers with eta = 1.85 degrees. For pyrheliometers (which can have eta up to approximately 2.8 degrees), corrections can be as large as 16% at 354 nm. We find that AOD correction factors are well correlated with AOD wavelength dependence (hence Angstrom exponent). We provide best-fit equations for determining correction factors from Angstrom exponents of uncorrected AOD spectra, and we demonstrate their application to vertical profiles of multiwavelength AOD.
Document ID
20030112119
Document Type
Preprint (Draft being sent to journal)
Authors
Russell, P. B. (NASA Ames Research Center Moffett Field, CA, United States)
Livingston, J. M. (SRI International Corp. Menlo Park, CA, United States)
Dubovik, O. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Ramirez, S. A. (Bay Area Environmental Research Inst. Sonoma , CA, United States)
Wang, J. (Brookhaven National Lab. Upton, NY, United States)
Redemann, J. (Bay Area Environmental Research Inst. Sonoma , CA, United States)
Schmid, B. (Bay Area Environmental Research Inst. Sonoma , CA, United States)
Box, M. (New South Wales Univ. Sydney, Australia)
Holben, B. N. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
August 21, 2013
Publication Date
January 1, 2003
Subject Category
Meteorology and Climatology
Distribution Limits
Public
Copyright
Other