Aerosol optical properties at a savannah grassland site in South Africa

Abstract

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. The coldest period mostly indicated the contribution of higher population densities (more domestic combustion for space heating) in addition to industrial activities that contribute year-round. From the driest period, the contributions of open biomass burning frequencies of air masses that had passed over eastern Zimbabwe and central Mozambique were evident for all aerosol optical properties. The defined source regions approach was subdivided into three different methods that further improved the understanding of possible sources and source regions influencing aerosol optical properties. The predefined eastern and western sectors method allowed the comparison of aerosol optical properties for air masses that had passed over the eastern (higher industry population densities and open biomass burning frequencies), and western (very few industries, lower population densities and lower open biomass burning frequencies) sectors during the warmest/wettest and driest periods. From this comparison, the significant differences between the two defined sectors and contributions of open biomass burning to all aerosol optical properties during the driest period were evident. The predefined source regions approach allowed the comparison of aerosol optical properties for air masses that had passed over the anthropogenic source regions in the eastern sector. From this approach, it was evident that the aerosol optical properties were significantly altered (if compared to the regional background) in air masses that had passed over the anthropogenic influenced source regions in the South African interior. For example, the σSP and αSP indicated the extent of pollution of air masses that had passed over the Vaal Triangle (VaalT), as well as the occurrence of wind-blown dust that travelled over the VaalT from the eastern Free State to Welgegund. In the last method, information obtained by the predefined and auto-generated source map region methods was applied, which allowed the comparison between two separate background regions, i.e. Karoo and Kalahari, and two anthropogenically influenced regions, i.e. anti-cyclonic recirculation pattern and the industrial hub, during different periods, i.e. warmest/wettest, coldest and driest periods. From these aerosol optical properties results, it was evident that air masses that had passed over the Karoo were typically cleaner (e.g. lower σSP and σAP) than air masses that had passed over the Kalahari, and that air masses that had passed over the industrial hub during the coldest period were the most polluted (e.g. highest σSP and σAP). To contextualise the aerosol optical properties measured at Welgegund, mean values were compared with other sites. The highest mean aerosol optical property values during all periods were lower than the mean values reported for polluted sites. The mean aerosol optical property values measured in air masses that had passed over the Karoo region during all periods were similar and/or higher to mean values reported for true background sites. The mean ω0 for Welgegund over the entire measurement period was comparable with the mean ω0 reported for Elandsfontein in the Mpumalanga Highveld. However, it was not that straightforward to contextualise the Welgegund ω0 values with other sites, since the climatological effect of ω0 depends on the albedo of the underlying surface. Lastly, a statistical approach, i.e. multi-linear regression analysis, was applied to serve as an additional, independent analysis to support the source deductions made in the earlier chapters. Although the interpretations of the meaning of parameters included in the optimum multi-linear regression equations, and the signs (positive or negative) associated with them, were somewhat speculative, it did indicate that the earlier source deductions were plausible.