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, coll ected during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) Experiment, w as used to examine the ratio technique, the official method for remote sens ing of aerosols over land from the moderate resolution imaging spectroradio meter (MODIS) data, for view angles from nadir to 65 degrees off-nadir. The strategy we used is to first select a pristine, low aerosol optical thickn ess flight, and we then computed ratios of reflectance at 0.47 and 0.68 mum to corresponding values at 2.20 mum, separately for backward and forward s cattering directions. Similarly, we analyzed data from high turbidity fligh ts for comparison purposes. For both flights, we removed the effects of atm ospheric absorption and scattering using 6S, a radiative transfer code, and then recomputed the ratios again for different values of aerosol optical t hickness. Finally, we analyzed bidirectional reflection function (BRF) data to examine the dependence of the ratio technique on the relative azimuth a ngle. Results of this analysis show that a relationship between visible ref lectance and near infrared (TR) reflectance exists for view angles from nad ir to 400 off-nadir, and that simple parametric relationships can be derive d. 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 fo r 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 degrees off-nadir but only in the backward scattering direction. These rati os show little variation with azimuth view angle and this seems to hold wel l for backward scattering directions over dark targets.