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Influence of Humidity On the Aerosol Scattering Coefficient and Its Effect on the Upwelling Radiance During ACE-2Aerosol scattering coefficients (sigma(sub sp)) have been measured over the ocean at different relative humidities (RH) as a function of altitude in the region surrounding the Canary Islands during the Second Aerosol Characterization Experiment (ACE-2) in June and July 1997. The data were collected by the University of Washington passive humidigraph (UWPH) mounted on the Pelican research aircraft. Concurrently, particle size distributions, absorption coefficients and aerosol optical depth were measured throughout 17 flights. A parameterization of sigma(sub sp) as a function of RH was utilized to assess the impact of aerosol hydration on the upwelling radiance (normalized to the solar constant and cosine of zenith angle). The top of the atmosphere radiance signal was simulated at wavelengths corresponding to visible and near-infrared bands of the EOS (Earth Observing System) AM-1 (Terra) detectors, MODIS (Moderate Resolution Imaging Spectroradiometer) and MISR (Multi-angle Imaging Spectroradiometer). The UWPH measured sigma(sub sp) at two RHs, one below and the other above ambient conditions. Ambient sigma(sub sp) was obtained by interpolation of these two measurements. The data were stratified in terms of three types of aerosols: Saharan dust, clean marine (marine boundary layer background) and polluted marine aerosols (i.e., two- or one-day old polluted aerosols advected from Europe). An empirical relation for the dependence of sigma(sub sp) on RH, defined by sigma(sub sp)(RH) = k.(1 - RH/100)(sup gamma), was used with the hygroscopic exponent gamma derived from the data. The following gamma values were obtained for the 3 aerosol types: gamma(dust) = 0.23 +/- 0.05, gamma(clean marine) = 0.69 +/- 0.06 and gamma(polluted marine) = 0.57 +/- 0.06. Based on the measured gammas, the above equation was utilized to derive aerosol models with different hygroscopicities. The satellite simulation signal code 6S was used to compute the upwelling radiance corresponding to each of those aerosol models at several ambient humidities. For the prelaunch estimated precision of the sensors and the assumed viewing geometry of the instrument, the simulations suggest that the spectral and angular dependence of the reflectance measured by MISR is not sufficient to distinguish aerosol models with various different combinations of values for dry composition, gamma and ambient RH. A similar behavior is observed for MODIS at visible wavelengths. However, the 2100 nm band of MODIS appears to be able to differentiate between at least same aerosol models with different aerosol hygroscopicity given the MODIS calibration error requirements. This result suggests the possibility of retrieval of aerosol hygroscopicity by MODIS.
Document ID
20020066595
Acquisition Source
Ames Research Center
Document Type
Reprint (Version printed in journal)
Authors
Gasso, S.
(Washington Univ. Seattle, WA United States)
Hegg, D. A.
(Washington Univ. Seattle, WA United States)
Covert, D. S.
(Washington Univ. Seattle, WA United States)
Collins, D.
(California Inst. of Tech. Pasadena, CA United States)
Noone, K. J.
(Stockholm Univ. Sweden)
Oestroem, E.
(Stockholm Univ. Sweden)
Schmid, B.
(Bay Area Environmental Research Inst. San Francisco, CA United States)
Russell, P. B.
(NASA Ames Research Center Moffett Field, CA United States)
Livingston, J. M.
(SRI International Corp. Menlo Park, CA United States)
Durkee, P. A.
(Naval Postgraduate School Monterey, CA United States)
Date Acquired
August 20, 2013
Publication Date
January 1, 2000
Publication Information
Publication: Tellus
Publisher: Munksgaard
Volume: 52B
Issue: 2
ISSN: 0280-6509
Subject Category
Environment Pollution
Funding Number(s)
CONTRACT_GRANT: NCC2-1094
Distribution Limits
Public
Copyright
Other

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