Top-of-atmosphere radiance is computed between 350 and 2500 nm for atmospheres containing one of three aerosol models (rural, maritime and dust) inherent to MODTRAN, over different surface reflectance values, and compared with those computed using a model of the same aerosol species derived from measurements by a global network of ground-based radiometers (AERONET). It is observed that even over high reflectance targets (R=0.5), care must be taken in the prescription of aerosol optical properties so as to limit uncertainty resulting from aerosols in the top-of-atmosphere radiance to less than 2%. It is found that, for grass and desert sites, using a simple power law exponent derived from measured spectral optical depths reduces uncertainty in the computed satellite radiance resulting from prescription of aerosol properties to less than approximately 1.5% for the aerosol species examined. Uncertainty in the computed top-of-atmosphere radiance during vicarious sensor calibration over desert sites that may result from this simple prescription of the aerosol size distribution is thus less than uncertainty in the TOA radiance resulting from measurements of the site reflectance. The new aerosol and multiple scattering capabilities of the most recent version of MODTRAN have made such studies possible and are promising for attempts to use MODTRAN in the vicarious calibration of airborne and spaceborne sensors.
Lidar (extinction-to-backscatter) ratios are computed at 0.55, 1 and 10 μm, based upon a recently published summary of the physicochemical properties of climatically relevant aerosol species. The results agree very well with previously measured values in the literature, indicating that low Sa values for desert dust (15-30) and maritime (30-45) aerosols are clearly distinguishable from biomass burning (55-65) and urban/industrial pollution (55-80). The results show that most aerosol types can be discriminated by their absorption and scattering characteristics through use of spectral lidar ratios, except between biomass burning and pollution aerosols. Predictions of on- and off-axis scattering in the presence of these aerosol types illustrate the range of signal that may be expected in a bistatic lidar system in such cases, and indicate that bistatic lidar may be successfully used to detect a source lidar signal and discriminate the aerosol species present. These findings strongly suggest that a combination of passive and active remote sensing systems operating simultaneously (e.g., ground-based sky radiance and bistatic lidar), would be capable of directly measuring the absorption and scattering characteristics required to describe the optical behaviour of the aerosol with vertical resolution. This is expected to be of great utility to climate researchers or other communities interested in comprehensively measuring atmospheric optical properties.