The trace gases and particulate matter emitted during biomass burning play significant roles in tropospheric and stratospheric chemistry and significantly affect the Earth's radiative balance. The impact of biomass burning is particularly significant in the tropics where the extent of land burned annually is quite large. For these reasons, it is important to develop accurate estimates of the spatial and temporal distributions of the trace gases and particulate matter emitted during biomass burning. These estimates require the use of data that range from FAO statistics to satellite images and involve multiple levels of analysis.

Fires are used widely for land use practices in most tropical countries and involve the burning of several types of vegetation. The largest amount of biomass burned in the tropics, roughly half, is combusted during savanna fires, followed by shifting cultivation, and deforestation, according to estimates based on FAO survey data of forest resources and several other sources. The spatial distributions of biomass burning can be obtained from satellite images. However, it is significantly more difficult to estimate the size of the burned area, the amount of vegetation that is burned, and the trace gas emissions that result from burning.

Satellite images provide information on the patterns of biomass burning in a region. However, the calculation of the amount of vegetation burned in that area requires several additional steps. The first step is to determine the vegetation type in each pixel. Each of these vegetation types is characterized by different burning patterns. The areas that are burned include tropical rain forest, and wet and dry savanna. Dry savannas are burned very infrequently because there is insufficient vegetation to support the spread of fires; thus these areas do not contribute significantly to the annual biomass burning emissions. In the rain forest, some areas are burned during deforestation and during shifting cultivation. For the most part, however, burning activity is concentrated in the transition zone between rain forest and humid savanna.

For each of these regions, the amount of biomass available for burning must be estimated. For forested areas, these estimates are based on the amount of aboveground biomass (derived from FAO statistics). In order to calculate the amount of biomass available for burning in humid savannas, rainfall is used to estimate the aboveground biomass. For savannas, the frequency of burning is also used to calculate the annual exposure of biomass to fires. Using these techniques, the amount of biomass burning in a geographic region can be estimated.

Following determination of the amount of biomass burned in each pixel, it is necessary to estimate the emission factors for trace gases (amounts of trace gases emitted per unit weight of biomass burned). The emission factors vary considerably between the flaming and smoldering phases of burning and are different for different types of fuels. Consideration of these aspects of biomass burning are important in estimating trace gas emissions. During the flaming phase, there is usually ample oxygen to oxidize the biomass but during the smoldering phases, oxygen diffusion to the site of combustion becomes limited. This can then influence the relationship between combustion efficiency and trace gas emission (the emission factor).

To calculate emission factors, field measurements are used in which combustion efficiencies are compared to measured or estimated emission factors for each trace gas and vegetation type. For methane emissions, there is generally a linear correlation between the combustion efficiency and the emission factor with a general pattern of decreased hydrocarbon emissions with increased fire efficiency (see Figure 6.1). Methane emissions can be used to estimate the emissions of other hydrocarbons such as the non-methane hydrocarbons (NMHCs) as there is usually a positive linear relationship between the NMHCs and methane.

The global contribution of biomass burning to trace gas budgets is significant. For carbon monoxide (CO), biomass burning contributes more to the global budget than industrial emissions. In determining global budgets, it is important to consider, however, that all biomass burning is not equivalent and the relative contribution of different types of fires (e.g., forest, savanna, fuelwood, agricultural residues) depends on both the extent of burning and the emission factors associated with the burning. Table 6.1 shows various estimates of different types of biomass burning. For methane, biomass burning contributes about 8% to the global annual budget (comparable to the industrial sources). The amount of benzene emitted from biomass burning is about twice the amount emitted from industrial activities. Emissions of benzene are of particular interest because it is toxic and may affect human health in tropical countries.

The atmospheric importance of emissions of trace gases from biomass burning depends largely on the transport of these gases away from the site of burning. The biomass burning footprint for ozone covers much of Central and Southern Africa and much of the Amazon basin and some emissions of long-lived trace gases, such as CO, can be transported to the Northern Hemisphere.

There is significant uncertainty in the estimates of trace gas emissions from biomass burning. Most of this uncertainty is associated with estimates of the amount of land that is cleared and the amount of vegetation that is burned following clearing. Improving these estimates is critical to our understanding of the effects of biomass burning on global atmospheric chemistry and the radiative balance of the Earth.