3.7
Aerosol retrievals over land surfaces (the advantages of polarization)
Brian Cairns, NASA/GISS, New York, NY; and L. Travis, M. I. Mishchenko, and J. Chowdhary
Measurements and theoretical analyses indicate that the polarized reflectance of natural surfaces is almost spectrally invariant. If true this means that measurements at a wavelength of 2250nm, where most aerosols have a small effect on observed radiances, can be used as a proxy for the surface polarization at shorter wavelengths.
To test the validity of this approach, ground-based sunphotometry measurements were obtained simultaneously with co-located aircraft measurements of the upwelling polarized radiances at various locations in California and Oklahoma. Model calculations of the upwelling polarized radiances were performed using the sunphotometry measurements to provide optical depth and aerosol size information and using the 2250nm polarization measurements as a proxy for the surface polarization.
An excellent fit between measured and modeled upwelling polarized radiances was found at wavelengths of 410, 470, 555, 670 and 865 nm. Moreover, at a pixel level, large variations in polarized reflectance occur from one pixel to the next due to the inhomogeneity of the underlying surface, which variability is entirely captured by using the 2250 nm measurements as a proxy for the surface polarized reflectance. This agreement between model and measurements demonstrates that polarization measurements can provide information about aerosol properties over land surfaces), provided a longer wavelength (2250nm) measurement is available, because although the surfaces are bright and inhomogeneous their polarized reflectance is grey (spectrally invariant.
Current satellite retrievals of aerosols over land surfaces from intensity measurements make use of empirical regressions that relate surface albedo at 450nm and 670nm to intensity measurements at 2250nm (MODIS). The capability of polarization measurements at longer wavelengths (2250nm) to characterize the surface polarization is much more robust than such regressions which are not only dependent on spatial scale but also on the nature of the underlying surface. An alternative satellite retrieval method for aerosol properties over land surfaces uses multi-angle intensity measurements over heterogeneous surfaces to retrieve aerosol properties (MISR). We present examples of aerosol retrievals over a 2000 km transect from Santa Barbara CA. to Ponca City OK. and compare and contrast the utility of intensity only approaches to retrieving aerosols over land, such as those proposed for use with MODIS and MISR with the capabilities provided by polarization.
Session 3, The Role of Satellites in Tropospheric Chemistry Measurements
Tuesday, 16 January 2001, 9:00 AM-10:58 AM
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