In this study, a global three-dimensional chemical transport model (CTM) is used to assess the impact of widespread fires in the tropics on the composition of the troposphere. Sensitivity simulations are conducted to illustrate the changes in tropospheric ozone and its precursors associated with the 1997 Indonesian fires. This study is mainly aimed at testing the ability of a state-of-the-art CTM to reproduce the observed change in tropospheric ozone in a specific biomass burning case. A simple approach is adopted to account for the additional emissions from the Indonesian fires in the model based of the total C02 release estimated by Levine et al. (1998), and the effect of smoke particles on photolysis rates. Two emission scenarios are considered in our simulations and tested. In one case, we assume that the ecosystem is composed of 100 % of tropical forests, and in the second scenario 70% of forest fires and 30% of peat fires are assumed.
Our simulations show that the model results significantly underestimate the change in total ozone observed from space when the 100 % forest fire scenario is assumed. For the second emission scenario, because of the higher carbon content of peat, the ozone change is more pronounced and in line with actual observations over the source region (Sumatra, Kalimantan). The MOZART off-line chemical transport model used for the simulations is driven by the dynamical fields calculated by the NCAR Community Climate Model. Therefore, the simulations are characterized by a mean climatological state instead of actual meteorological conditions prevailing during the 1997 fire period. As a consequence, the transport of species in the model is mainly dominated by a strong export from the source region to the Pacific Ocean in the free troposphere when export to India was actually observed in 1997. A version of MOZART driven by assimilated dynamical and physical fields is currently under development and should help to address this issue.
Despite these limitations, the results clearly show that the budget of carbon monoxide, NMHCs, nitrogen species, and ozone are profoundly affected by the fires in the boundary layer, but also in the free troposphere due to rapid upward transport and subsequent redistribution of pollutants by large scale transport processes in the tropics. The simulations indicate a strong impact of this local event on the composition of the free troposphere on a regional scale, stressing the need for accurate estimates of biomass burning emissions (i.e., fire location and extent, burned area, type of ecosystem burned, timing of the fires) in chemical transport models. In the future, biomass fires determined by remote sensing techniques hopefully will provide more insight into this increasingly important aspect of the human impact on the global environment.
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