Detection of forests using mid-IR reflectance: an application for aerosol studies

The detection of dark, dense vegetation is an important step in the remote sensing of aerosol loading. Current methods that employ the red (0.64 μm) and the near-IR (0.84 μm) regions are unsatisfactory in that the presence of aerosols in the scene distorts the apparent reflectance in the visible and near-IR ranges of the spectrum. The mid-IR spectral region is also sensitive to vegetation due to the absorption of liquid water in the foliage, but is not sensitive to the presence of most aerosols (except for dust). Therefore, mid-IR channels on the AVHRR and EOS-MODIS (e.g., the 3.75 μm or the 3.95 μm channels) have a unique potential for the remote sensing of dark, dense vegetation, particularly in the presence of biomass burning smoke or industrial/urban haze. The reflective part of the 3.75 μm channel (ρ_3.75) is applied to images of the AVHRR over the eastern United States. This channel was found to be correlated to reflectance at 0.64 μm (ρ_0.64), less sensitive to haze than the visible channel and superior to both the 0.64 μm reflectance and the normalized difference vegetation index (NDVI) to determine forest pixels in an image. However, its application to monitor the seasonal evolution of vegetation is presently questionable. For the purpose of the remote sensing of aerosol over dark, dense vegetation, it is proposed that the dark, dense vegetation be determined from ρ_3.75<0.025. These findings may have further implications for other specific applications of the remote sensing of vegetation in hazy atmospheres