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North-West University (2020)

Aerosol optical properties at a savannah grassland site in South Africa

Venter, Marcell

Titre : Aerosol optical properties at a savannah grassland site in South Africa

Auteur : Venter, Marcell

Université de soutenance : North-West University

Grade : Doctor of Philosophy in Science with Atmospheric Chemistry 2020

Résumé partiel
Atmospheric aerosols affect the earth’s radiative budget in two ways : firstly, particles directly absorb and scatter short- and long-wave radiation and, secondly, particles indirectly influence the lifetime and physical properties of clouds. There are many uncertainties associated with these effects of atmospheric aerosols on the earth’s radiative budget due to their high spatial and temporal variability of aerosol optical properties, particularly on regional scales. Consequently, high-resolution long-term, regional scale aerosol optical property measurements are required in order to decrease the uncertainties. Southern Africa is an important sub-source region of Africa where open biomass burning produces significant amounts of aerosols, especially during the dry season. Within southern Africa, South Africa is the largest economy with numerous primary and secondary sources of aerosols. Only a few papers have been published on aerosol optical properties in South Africa. Therefore, to partially address this knowledge gap, i.e. long-term ground level in situ aerosol optical data, aerosol optical properties, which include scattering and absorption coefficients (σSP and σAP), single scattering albedo (ω0), and Ångström exponent (αSP), are investigated based on in situ measurements conducted from September 2011 to November 2016 at the Welgegund measurement station. The σSP was measured with a three wavelength light scattering Nephelometer and the σAP with a multi-angle absorption photometer. The αSP and ω0 were calculated from the σSP and σSP and σAP, respectively. Relatively well-defined seasonal and diurnal patterns were observed, which indicated the influence of open biomass burning frequencies, other possible sources (e.g. industrial emissions, domestic combustion, wind-blown dust) and meteorological effects (e.g. temperature, relative humidity, planetary boundary layer daily evolution and air mass circulation patterns). Two main approaches, i.e. auto-generated source maps and defined source regions, were used to identify more unambiguously the sources and source areas that influenced the aerosol optical properties. From these two approaches, the contributions of seasonal sources (e.g. open biomass burning, domestic combustion for space heating, wind-blown dust) and continuous emission sources (e.g. industrial emissions and domestic combustion for cooking) were observed. From the auto-generated source maps, considering all aerosol optical properties for the entire measurement period, anthropogenic activities such as emissions from the Vaal Triangle, Mpumalanga Highveld, and Johannesburg-Pretoria megacity, as well as the aging and recirculation of pollution over the dominant anti-cyclonic recirculation pattern, in addition to high open biomass burning frequencies especially over eastern Zimbabwe and central Mozambique had the most significant effects on the aerosol optical properties (e.g. higher σSP and σAP), if compared to the background sector between west-southwest and south-southwest of Welgegund. The auto-generated source maps for defined periods, i.e. warmest/wettest, coldest and driest, peak open biomass burning, indicated the contributions from sources and/or source areas even better. From the warmest/wettest period for all aerosol optical properties, the contribution of air masses that had passed over industrial activities and the dominant anti-cyclonic recirculation pattern were evident.

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