Sensitivity of off-nadir zenith angles to correlation between visible and near-infrared reflectance for use in remote sensing of aerosol over land

Cloud absorption radiometer (CAR) multispectral and multiangular data, collected during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) Experiment, was used to examine the ratio technique, the official method for remote sensing of aerosols over land from the moderate resolution imaging spectroradiometer (MODIS) data, for view angles from nadir to 650 off-nadir. The strategy used is to first select a pristine, low aerosol optical thickness flight, and then to compute ratios of reflectance at 0.47 and 0.68 /spl mu/m to corresponding values at 2.20 /spl mu/m, separately for backward and forward scattering directions. Similarly, the authors analyzed data from high turbidity flights for comparison purposes. For both flights, they removed the effects of atmospheric absorption and scattering using 6S, a radiative transfer code, and then recomputed the ratios again for different values of aerosol optical thickness. Finally, they analyzed bidirectional reflection function (BRF) data to examine the dependence of the ratio technique on the relative azimuth angle. Results of this analysis show that a relationship between visible reflectance and near infrared (IR) reflectance exists for view angles from nadir to 400 off-nadir, and that simple parametric relationships can be derived.