Stratospheric aerosol perturbing effect on the remote sensing of vegetation: operational method for the correction of AVHRR composite NDVI

Stratospheric aerosols produced by the eruption of the Mount Pinatubo in the Philippines (6 June, 1991) have a detectable effect on NOAA AVHRR data. Following the eruption, a longitudinally homogeneous dust layer was observed between 20°N and 20°S. The largest optical thickness observed for the dust layer was 0.4 - 0.6 at 0.5 microns. The amount of aerosols produced by Mount Pinatubo was two to three times greater than that produced by El Chichon and the Stratospheric Aerosol and Gas Experiment (SAGE) on-board the Earth Radiation Budget Experiment was not able to give quantitative estimate of aerosol optical thickness because of saturation problem. The monthly composite Normalized Difference Vegetation Index (NDVI) (generally bounded between -0.1 and 0.6) has systematically decreased by approximately 0.15 two months after the eruption. Such atmospheric effect has never been observed on composite product and is related to the persistence and spatial extent of the aerosol layer causing the composite technique to fail. Therefore, long term monitoring of vegetation using the NDVI necessitates correction of the effect of stratospheric aerosols. In this paper we present an operational stratospheric aerosol correction scheme adopted by the Laboratory for Terrestrial Physics, NASA/GSFC. The stratospheric aerosol distribution is assumed to be only variable with latitude. Each 9 days the latitudinal distribution of the optical thickness is computed by inverting radiances observed in AVHRR channel 1 (0.63 microns) and channel 2 (0.83 microns) over the Pacific Ocean. This radiance data set is used to check the validity of model used for inversion by checking consistency of the optical thickness deduced from each channel as well as optical thickness deduced from different scattering angles. The deduced optical thickness and spectral dependence are compared to Mauna Loa observation from 1991 to end of 1992 for validation. Using the optical thickness profile previously computed and radiative transfer code assuming lambertian boundary condition, each pixel of channel 1 and 2 are corrected prior to computation of NDVI. Comparison between corrected, non corrected, and years prior to Pinatubo eruption (1989, 1990) NDVI composite, shows the necessity and the accuracy of the operational correction scheme. The same technique is applied to the afternoon satellite AVHRR archive (NOAA7,9,11) from 1981 to 1993. The stratospheric profile derived over ocean shows that the El Chichon eruption was of less importance than Pinatubo. The stratospheric aerosol optical depth distribution computed from AVHRR data during the El Chichon period compared well to latitudinal monthly profile based on SAGE observations.