Abstract
The chemical composition of the troposphere in the tropics and Southern Hemisphere (SH) is significantly influenced by gaseous emissions released from African biomass burning (BB). Here we investigate how various emission estimates given in bottom-up BB inventories (GFEDv2, GFEDv3, AMMABB) affect simulations of global tropospheric composition using the TM4 chemistry transport model. The application of various model parameterizations for introducing such emissions is also investigated. There are perturbations in near-surface ozone (O3) and carbon monoxide (CO) of ~60–90% in the tropics and ~5–10% in the SH between different inventories. Increasing the update frequency of the temporal distribution to eight days generally results in decreases of between ~5 and 10% in near-surface mixing ratios throughout the tropics, which is larger than the influence of increasing the injection heights at which BB emissions are introduced. There are also associated differences in the long range transport of pollutants throughout the SH, where the composition of the free troposphere in the SH is sensitive to the chosen BB inventory. Analysis of the chemical budget terms reveals that the influence of increasing the tropospheric CO burden due to BB on oxidative capacity of the troposphere is mitigated by the associated increase in NOx emissions (and thus O3) with the variations in the CO/N ratio between inventories being low. For all inventories there is a decrease in the tropospheric chemical lifetime of methane of between 0.4 and 0.8% regardless of the CO emitted from African BB. This has implications for assessing the effect of inter-annual variability in BB on the annual growth rate of methane.
Highlights
The release of large quantities of trace gas pollutants into the tropical troposphere from human burning practices and wildfires in Africa exerts a significant influence on global atmospheric composition, most importantly on the relatively pristine Southern Hemisphere (SH), tropicalAtlantic [1,2,3] and around the burning regions
In this paper we investigate the uncertainties introduced towards simulating global tropospheric composition associated with differences between bottom-up biomass burning (BB) emission estimates when applied in a global 3D CTM
nitrogen oxides (NOx) emissions have on mitigating the long range transport of carbon monoxide (CO) we perform a simulation using the AMMABB inventory where the BB NOx is reduced by 50% (AMMABB_LOWNOX)
Summary
The release of large quantities of trace gas pollutants into the tropical troposphere from human burning practices and wildfires in Africa exerts a significant influence on global atmospheric composition, most importantly on the relatively pristine Southern Hemisphere (SH), tropical. Recent multi-model intercomparison studies estimate that in large-scale global models τCO, τO3 and τCH4 have typical values of ~2 months, ~22 days and ~8.7 years, respectively [15,16] These chemical lifetimes control the impact of emission sources on air quality far away from the source area by changing the resident surface concentrations [17]. In this paper we investigate the uncertainties introduced towards simulating global tropospheric composition associated with differences between bottom-up BB emission estimates when applied in a global 3D CTM For this purpose we examine the influence on global air quality, the long range transport of pollutants out of BB source regions and the perturbations introduced in estimating global atmospheric lifetimes of dominant trace species. We show that there is a compensating effect towards the perturbations introduced towards the oxidative capacity of the troposphere due to the CO/N ratio defined in different BB emission inventories, which constrains the effects on the global lifetimes for dominant trace gases
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