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University of Cape Town (2013)

Effects of biomass-burning aerosol loading on Southern African climate

Maúre, Genito Amós

Titre : Effects of biomass-burning aerosol loading on Southern African climate

Auteur : Maúre, Genito Amós

Université de soutenance : University of Cape Town.

Grade : Doctor of Philosophy 2013

Résumé
Aerosols are known to directly influence the climate system by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei (CCN) and/or ice nuclei (CN) and thereby affecting the lifetime and extent of clouds. In this work, a state–of–the–art Global Circulation Model (GCM) incorporating both effects was used to assess how local communities are impacting the seasonality of regional climate by contributing to the release of massive amounts of aerosols through seasonal biomass burning. Except for localised biases (e.g. especially for the desert areas and over areas where observed data is scarce) overall validation of the model performance has shown that it was capable of capturing, with a good approximation, the observed climate variability in the study region. Simulations revealed that the largest pollution sources contributing to the aerosol burden found over Southern African atmosphere are local emissions. Minor external contributions come from the African tropical forest north of the Equator as well as south American pollution (especially during two seasons : March–to–May and December–to–February). The largest biomass burning aerosol concentrations tend to be found over low–pressure systems over the subcontinent. Model results have shown that aerosol impact in rainfall was small during the June–October period, as this period falls largely within the austral dry season. Additionally, results indicated not only negative changes of surface temperature can be induced, as previous studies have suggested, but positive changes can also be induced by the inclusion of biomass burning sulfur dioxide and sulfate aerosols nucleated thereofrom, which implied increased lower–level convergence over these regions, and, consequently, increased precipitation. The most significant impacts are an 80% increase in daily precipitation, for December–to–February, over the Atlantic Ocean (near the coast of Angola) and a prominent reduction (between 40–60%) found during the same season over central and northern Mozambique and Tanzania. This study highlights the importance of including all biomass burning emissions, from the aerosols directly released from fires to the precursor gases, as different radiative forcings will be obtained from different chemical species, and, therefore, different circulation patterns are likely to be induced over all seasons, regardless of how large the emissions loading and/or concentrations are.

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