Sensitivity of Satellite-derived Critical Reflectance to Aerosol Physical and Optical Properties

An accurate determination of the direct aerosol forcing over land is dependent upon the availability of reliable information on aerosol absorption. Quantifying aerosol absorption over land surfaces using single-image remote sensing methods is complicated, however, by the often large surface signal. The critical reflectance, which can be derived using pairs of images from satellite-borne passive radiometers such as MODIS, is one method that can be used to estimate aerosol absorption over bright land surfaces. The critical reflectance refers to the surface (or top-of-atmosphere) reflectance at which the earth-atmosphere reflectance is relatively insensitive to the aerosol optical depth for an aerosol of fixed optical properties. Using a DISORT-type radiative transfer code, we explore the parameter space upon which the critical reflectance depends to test its sensitivity to aerosol physical and optical properties as a function of solar and viewing geometry, as well as wavelength. Using phase functions derived from two AERONET stations influenced by different aerosol types, we find that the spectral dependence of the critical reflectance weakens as solar zenith angle increases for biomass burning particles, whereas the opposite is true for dust aerosol. The magnitude of this change varies as a function of the satellite view angle. The sensitivity of the critical reflectance to aerosol single-scattering albedo (SSA) is larger for fine-mode particles, although this also varies as a function of satellite view angle. For all particle-types examined, the critical reflectance becomes less sensitive to SSA at large viewing angles. This is especially the case for dust particles in the near-IR wavelengths. Additionally, for all particle types, the sensitivity of the critical reflectance to SSA increases with increasing solar zenith angle. The sensitivity of the critical reflectance to the real part of the refractive index is largest in the backscattering direction, as is the sensitivity to particle shape. The application of the critical reflectance method to retrieve spectral aerosol absorption over bright land surfaces is both helped and hindered by these sensitivities. The spectral dependence of critical reflectance offers some information about particle size, but sensitivities to shape and the real part of the refractive index add to uncertainties in the retrieved absorption. We retrieved SSA from MODIS-derived critical reflectance over the Banizoumbou AERONET station in 2006, and estimate the uncertainty in the results due to the choice of aerosol model.