Field studies and radiative transfer model calculations have shown that brightness temperature at high microwave frequencies is strongly affected by vegetation. We have analysed the daytime observations for six consecutive years (1979 to 1984) over the Sahara, Senegalese Sahel, Burkina Fasso (Upper Volta), and U.S. Southern Great Plains at 37 GHz frequency of the Scanning Multichannel Microwave Radiometer (SMMR) on board the Nimbus-7 satellite and found a high correlation with the normalized difference vegetation index derived from the Advanced Very High Resolution Radiometer (AVHRR) on board the NOAA-7 satellite. The SMMR data appear to provide a valuable new long-term global data set for monitoring vegetation. In particular, the differing responses of vegetation (for example, annual grasses versus woody plants) to drought and the stability of the desert/steppe boundary of northern Africa might be studied using the time series data.
The effects of soil optical properties on vegetation index imagery are analysed with ground-based spectral measurements and both simulated and actual AVHRR data from the NOAA satellites. Soil effects on vegetation indices were divided into primary variations associated with the brightness of bare soils, secondary variations attributed to 'colour' differences among bare soils, and soil-vegetation spectral mixing. Primary variations were attributed to shifts in the soil line owing to atmosphere or soil composition. Secondary soil variance was responsible for the Saharan desert 'artefact' areas of increased vegetation index response in AVHRR imagery. The impact of soil effects is discussed with a transect of vegetation index data derived from NOAA data from desert to equatorial forest.
A new method has been developed for the global assessment of trace gases and particulates emission from tropical biomass burning. The method is based on remote sensing of one emitted product - particulates. It uses daily meteorological satellite data with resolution of one km2. The visible (0.63 rim) and near Infrared (0.84 rim) bands are used to determine the mass of particulates in the emitted smoke and to estimate the relative contribution of flaming and smoldering fires to the resulting smoke. The mid-IR (3.5-3.9 rim) and the thermal infrared (1 0.5-1 1 .4 ji.m) bands are used to detect and count fires in order to integrate the smoke result with the whole season and for the whole area of interest. The thermal channels are sensitive enough to detect flaming fires as small as lOm*lOm and smoldering fires as small as 30m*30m. The detected mass of emitted particulates is converted into a mass of emitted trace gases using published relations between the emitted particulates and trace gases for the flaming and smoldering phases. The.technique can be applied to regions where intensive biomass burning takes place. It is capable of monitoring the extent of current biomass burning, to discover new deforestation frontiers (unknown otherwise), and to estimate quantitative contribution of biomass burning to changes in atmospheric composition. The method has been applied to a limited area where substantial deforestation has taken place. Analysis of the 1987 burning season shows that in Brazil (in a limited area between 6.5-15.5 south and 55-67 west) during the three months of the dry season (July 1 till Sept. 30) there are up to 8000 fires a day (observed from space) each contributing 4,500 tons of C02, 750 tons of CO and 26 tons of CH4 to the atmosphere. During the dry season of 1 987, it is estimated that 240,000 fires were burning in this area resulting in the emission of 1.1013 of particulates, 71 012 g of CH4, 21 014 g of CO and 1 •1 015 g of CO2. A comparison to estimates of global emissions is also given.
