NDVI converted into at-surface values for atmospheric attenuation by using MODTRAN for four atmospheric temperature and moisture soundings made over Pennsylvania on differing days (see keys) and the corrected NDVI based on the radiative transfer model simulations (Nsim) described in the text (solid rectangles) versus the uncorrected (apparent) NDVI for values used in the simulations: (a) 15-km visibility; (b) 5-km visibility.