Angular and seasonal variation of spectral surface reflectance ratios: Implications for the remote sensing of aerosol over land

seasonal variability of surface reflectance using a hand-held spectrometer from a light aircraft, The data is used to test a procedure that allows us to estimate visible surface reflectance from the longer wavelength 2.1 mum channel (mid-IR), Estimating or avoiding surface reflectance in the visible is a vital first step in most algorithms that retrieve aerosol optical thi ckness over land targets. The data indicate that specular reflection found when viewing targets from the forward direction can severely corrupt the re lationships between the visible and 2.1 mum reflectance that were derived f rom nadir data. There is a month by month variation in the ratios between t he visible and the mid-IR, weakly correlated to the Normalized Difference V egetation Index (NDVI), If specular reflection is not avoided, the errors r esulting from estimating surface reflectance from the mid-IR exceed the acc eptable limit of Delta rho similar to 0.01 in roughly 40% of the cases, usi ng the current algorithm. This is reduced to 25% of the cases if specular r eflection is avoided.