The Effects of Aerosol from Pinatubo on Remotely Sensed Global Vegetation Measurement

The Normalized Difference Vegetation Index (NDVI) is one of important indicators of vegetation activities on land surface. There are several global NDVI time-series products available. The most popular one with longest temporal coverage is the NOAA/NASA Pathfinder AVHRR Land product (PAL) available from 1981 to 1999 with 8-km spatial resolution and 10-day time interval. The NDVI in PAL was calculated from data collected by AVHRR instruments on four satellites, NOAA-7 from 1981 to 1985, NOAA-9 from 1985 to 1988, NOAA-11 from 1989 to 1994, and NOAA-14 from 1995 to 1999. Studies on PAL or other similar products from AVHRR showed the global NDVI increases linearly with the aging of the NOAA satellites and drops when a new satellite replaces an old one. This pattern was caused by the orbital drifting of NOAA satellites. This study found the pattern was interrupted in the year of 1991, when Mt. Pinatubo erupted, and the following two years (1992, and 1993). Based on the pattern, it was estimated that the mean values of the annual global maximum dropped 0.029, 0.066, and 0.027 NDVI units from the normal pattern for years 1991, 1992, and 1993. This study proved that the most likely cause of the NDVI drops was not the reduction of global vegetation activities during those years but the aerosol brought into the stratosphere by the eruption of Mt. Pinatubo. Since the effects of volcanic aerosols on global vegetation measurement are very large, they have to be removed from the global NDVI datasets.