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Original Research Papers

Influence of humidity on the aerosol scattering coefficient and its effect on the upwelling radiance during ACE-2

Authors:

S. Gassó ,

Geophysics Program, Box 351650, University of Washington, Seattle, WA, US
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D. A. Hegg,

Atmospheric Science Department, University of Washington, Seattle, WA, US
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D. S. Covert,

Atmospheric Science Department, University of Washington, Seattle, WA, US
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D. Collins,

California Institute of Technology, Pasadena, CA, US
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K. J. Noone,

Geophysics Program, Box 351650, University of Washington, Seattle, WA, US
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E. Öström,

Geophysics Program, Box 351650, University of Washington, Seattle, WA, US
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B. Schmid,

Bay Area Environmental Research Institute, San Francisco, CA, US
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P. B. Russell,

NASA Ames Research Center, Moffett Field, CA, US
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J. M. Livingston,

SRI International, Menlo Park, CA, US
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P. A. Durkee,

Naval Postgraduate School, Monterey, CA, US
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H. Jonsson

Naval Postgraduate School, Monterey, CA, US
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Abstract

Aerosol scattering coefficients (σsp) 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 σsp 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-AM) “Terra” (detectors, MODIS and MISR. The UWPH measured σsp at 2 RHs, one below and the other above ambient conditions. Ambient σsp was obtained by interpolation of these 2 measurements. The data were stratified in terms of 3 types of aerosols: Saharan dust, clean marine (marine boundary layer background) and polluted marine aerosols (i.e., 2- or 1-day old polluted aerosols advected from Europe). An empirical relation for the dependence of σsp on RH, defined by σsp(RH)=k. (1−RH/100)−γ, was used with the hygroscopic exponent γ derived from the data. The following γ values were obtained for the 3 aerosol types: γ(dust)=0.23±0.05, γ(clean marine)= 0.69±0.06 and γ(polluted marine)=0.57±0.06. Based on the measured γ’s, 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 pre-launch 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, γ 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.

How to Cite: Gassó, S., Hegg, D.A., Covert, D.S., Collins, D., Noone, K.J., Öström, E., Schmid, B., Russell, P.B., Livingston, J.M., Durkee, P.A. and Jonsson, H., 2000. Influence of humidity on the aerosol scattering coefficient and its effect on the upwelling radiance during ACE-2. Tellus B: Chemical and Physical Meteorology, 52(2), pp.546–567. DOI: http://doi.org/10.3402/tellusb.v52i2.16657
  Published on 01 Jan 2000
 Accepted on 27 Sep 1999            Submitted on 1 Mar 1999
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