The MODIS 2.1-μm channel-correlation with visible reflectance for use in remote sensing of aerosol

A new technique for remote sensing of aerosol over the land and for atmospheric correction of Earth imagery is developed. It is based on detection of dark surface targets in the blue and red channels, as in previous methods, but uses the 2.1 μm channel, instead of the 3.75 μm for their detection. A 2.1-μm channel is present on ADEOS OCTS and GLI and planned on EOS-MODIS and EOSP, and a similar 2.2-μm channel is present on Landsat TM. The advantage of the 2.1-μm channel over the 3.75-μm channel is that it is not affected by emitted radiation. The 2.1-μm channel is transparent to most aerosol types (except dust) and therefore can be used to detect dark surface targets. Correlation between the surface reflection in the blue (0.49 μm), red (0.66 μm), and 2.1 μm is established using atmospherically corrected Landsat TM and AVIRIS aircraft images collected over the Eastern United States, Maine, and California and spectral data obtained from the ground and light aircraft near San Diego, CA. Results from a variety of surface covers show that the surface reflectance at 0.49 μm (ρ_0.49) and 0.66 μm (ρ_0.66) can be predicted from that at 2.2 μm (ρ_2.2) within Δρ=±0.06 for ρ_2.2⩽0.10, using ρ _0.49=ρ_2.2/4 and ρ_0.66=ρ_2.2/2. Error in surface reflectance of 0.006 corresponds to an error in remote sensing of aerosol optical thickness, τ, of Δτ~±0.06. These relationships were validated using spectral data taken close to the surface over vegetated areas in a different biome. This method expends application of dark targets for remote sensing of aerosol to brighter, nonforested vegetation. The higher reflection of the surface at 2.2 μm than that of 3.75 μm may even enable remote sensing of dust above surfaces with reflectivity ρ_2.2=0.15±0.05. For this reflectivity range the dust radiative effect at 2.2 μm is small, and the surface reflectance in the blue and red channels can be retrieved