It's not uncommon to see some negative values in the results of atmospheric correction. As long as they are not really large negative values, or every pixel is negative, then it's probably not anything to worry about. If you would like to set your negative values to values of 0, you could do that in ENVI by building and applying a mask, using Basic Tools > Masking > Build Mask. Under Options, you could choose Import Data Range, and specify a Data Max Value of -0.0000001 (or something else that captures all of your negative values, but not zero). Then you can choose Basic Tools > Masking Apply Mask, to use that mask image to change all of the pixels in the mask to have values of 0. - Peg