AFRI - aerosol free vegetation index

Aircraft measurements using a field spectrometer over variety of ground sur faces in Israel reveals that under clear sky conditions the shortwave infra red (SWIR) spectral bands around 1.6 and 2.1 mum are highly correlated with the visible-blue, green, and red- spectral bands. Empirical linear relatio nships, such as rho (0.469) = 0.25(rho2.1); rho (0.555) = 0.33(rho2.1); rho (0.645) = 0.5(rho2.1); and rho (0.645) = 0.66(rho1.6), were found to be st atistically significant and consistent with previous findings. Based on the above relationships, a modified vegetation index (VI) is proposed and name d Aerosol Free Vegetation Index (AFRI). Two versions of this VI are formula ted as: AFRI(1.6) = (rho (NIR) - 0 66 rho (1.6))/(rho (NIR) + 0.66 rho (1.6 )) and AFRI(2.1) = (rho (NIR) - 0.5 rho (2.1))/(rho (NIR) + 0.5 rho (2.1)). It is shown that under clear sky conditions, the AFRIs (and especially AFR I(2.1)) closely resemble the Normalized Difference Vegetation Index (NDVI) and their values are almost identical. The advantage of the derived AFRIs, based on the ability of the SWIR bands is to penetrate the atmospheric colu mn even when aerosols such as smoke or sulfates exist. Consequently, these indices have a major application in assessing vegetation in the presence of smoke, anthropogenic pollution, or volcanic plumes. This was demonstrated by applying the AFRI for a biomass burned forest in Brazil. Limited success of these indices is expected in case of dust due to presence of larger par ticles that are of similar size to the wavelength and therefore not transpa rent at 2.1 mum. The AFRIs can be implemented to data from any sensor that has the SWIR bands. Currently the most commonly used of such instruments ar e the Landsat-Thematic Mapper (TM) and Advanced Thematic Mapper Plus (ETM+) , Moderate Resolution Imaging Spectrometer (MODIS), Advanced Spaceborne The rmal Emission and Reflection (ASTER), and Japanese Earth Resources Satellit e-Optical System (JERS-OPS). Although the AFRI(2.1) was demonstrated to per form better than the AFRI(1.6), the latter still can be used for the same a pplication in conjunction with instruments that have onboard only the 1.6-m um band, such as Systeme Probatoire d'Observation del la Terre (SPOT4)-VEGE TATION, Indian Remote Sensing (IRS-IC/D), and Resource-21. (C) 2001 Elsevie r Science Inc. All rights reserved.