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 (SCARB) 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 65° off-nadir. The strategy we used is to first select a pristine, low aerosol optical thickness flight, and computed ratios of reflectance at 0.47 and 0.68 μm to corresponding values at 2.20 μm, separately for backward and forward scattering directions. Similarly, we analyzed data from high turbidity flights for comparison purposes. For both flights, we 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, we 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 reflectance exists for view angles from nadir to 40° off-nadir and that simple parametric relationships can be derived. In spite of these observed relationships, the relationship R 0.47 = R 2.20 /4, used in remote sensing of aerosol over land, does not seem to hold for the cases tested, both in the forward and backward scattering directions, and that R 0.68 = R 2.20 /2 seems to hold for view angles from nadir to 45° off-nadir but only in the backward scattering direction. These ratios show little variation with azimuth view angle and this seems to hold well for backward scattering directions over dark targets.