DETECTION OF FORESTS USING MID-IR REFLECTANCE - AN APPLICATION FOR AEROSOL STUDIES

The detection of dark, dense vegetation is an important step in the re mote sensing of aerosol loading. Current methods that employ the red ( 0.64 mum) and the near-IR (0.84 mum) regions are unsatisfactory in tha t the presence of aerosols in the scene distorts the apparent reflecta nce in the visible and near-IR ranges of the spectrum. The mid-IR spec tral region is also sensitive to vegetation due to the absorption of l iquid water in the foliage, but s not sensitive to the presence of mos t aerosols (except for dust). Therefore, mid-IR channels on the AVHRR and ECS MODIS (e.g., the 3.75 mum or the 3.95 mum channels) have a uni que potential for the remote sensing of dark, dense vegetation, partic ularly in the presence of biomass burning smoke or industrial/urban ha ze. The reflective part of the 3.75 mum channel (rho3.75) is applied t o images of the AVHRR over the eastern United States. This channel was found to be correlated to reflectance at 0.64 mum (rho0.64), less sen sitive to haze than the visible channel and superior to both the 0.64 mum reflectance and the normalized difference vegetation index (NDVI) to determine forest pixels in an image. However, its application to mo nitor the seasonal evolution of vegetation is presently questionable. For the purpose of the remote sensing of aerosol over dark, dense vege tation, it is proposed that the dark, dense vegetation be determined f rom rho3.75 < 0.025. These findings may have further implications for other specific applications of the remote sensing of vegetation in haz y atmospheres.