Sensitivity of tropospheric loads and lifetimes of short lived pollutants to fire emissions
Abstract. The capability of global chemistry and transport models (CTMs) to simulate atmospheric composition and its spatial and temporal changes highly relies on the input data used by the models, in particular the emission inventories. Biomass burning emissions show large spatial, diurnal, seasonal and year-to-year variability. In the present study, we applied a global 3-D CTM to evaluate uncertainties in the computed atmospheric composition associated with the use of different biomass burning emissions and identify areas where observational data can help to reduce these uncertainties. We find the emission inventory choice to lead to regional differences in the calculated load of aerosols up to a factor of 4. Assumptions on the injection height of the biomass burning emissions are found to produce regionally up to 30% differences in the calculated tropospheric lifetimes of pollutants. Computed changes in lifetimes point to a strong chemical feedback mechanism between emissions from biomass burning and isoprene emissions from vegetation that are linked via NOx-driven oxidant chemistry, NOx-dependent changes in isoprene oxidation products, aerosol emissions and atmospheric transport. These interactions reduce isoprene load in the presence of biomass burning emissions by 15%, calculated for the same amount of isoprene emitted into the troposphere. Thus, isoprene load and lifetime are inversely related to the quantities of pollutants emitted by biomass burning. These interactions are shown to be able to increase the global annual secondary aerosol yield from isoprene emissions, defined as the ratio of tropospheric loads of secondary aerosol from isoprene oxidation to isoprene emissions, by up to 18%.