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Title: Regional modelling of air quality and aerosol-interactions over southern Africa : impact of aerosols and regional-scale meteorology
Author: Wiston, Modise
ISNI:       0000 0004 5993 9346
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2016
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Atmospheric trace components play a critical role in the earth–atmosphere system through their interaction and perturbation to global atmospheric chemistry. They perturb the climate through scattering and absorbing of solar radiation (direct effects), thereby impacting on the heat energy balance of the atmosphere, and alter cloud microphysical properties affecting cloud formation, cloud lifetime and precipitation formation (indirect effects). These trace components can also have adverse effects on human health, visibility and air quality (AQ) composition, including various feedback processes on the state of the atmosphere. As well as their direct and indirect effects, aerosols are important for cloud formation. They serve as cloud condensation and ice nuclei (CCN and IN) during cloud droplet and ice crystal formations. Although many connections between clouds and aerosol effects have been established in cloud physics and climate modelling, aerosol–cloud interaction (ACI) is still one of the areas of large uncertainties in modern climate and weather projections. Different models have been developed placing much emphasis on ACIs, to have robust and more consistent description processes within the meteorological and chemical variables to account for ACIs and feedback processes. Because pollutant distributions are controlled by a specific meteorology that promotes residence times and vertical mixing in the atmosphere, reliable chemical composition measurements are required to understand the changes occurring in the earth–atmosphere system. Also, because atmospheric pollution is a combination of both natural and man-made (anthropogenic) sources, to direct controlled and/or mitigation procedures efficiently, contributions of different sources need to be considered. Occasionally these are explored from a particular region or global environment, depending on a specific area of interest. A fully coupled online meteorology–chemistry model framework (WRF-Chem) is used to investigate atmospheric ACIs over southern Africa –a region characterized by a strong and intense seasonal biomass burning (BB) cycle. The large transport of aerosol plumes originating from the seasonal burning from agriculture, land-use management and various activities give rise to a unique situation warranting special scrutiny. Simulations are conducted for the 2008 dry season BB episode, implementing a chemical dataset from various emission sources (anthropogenic, BB, biogenic, dust and sea salt) with the meteorological conditions. A base line (CNTRL) simulation was conducted with all emission sources from 26 August to 10 September 2008. To probe the contribution of BB on the regional pollution and influence on ACIs, a sensitivity (TEST) simulation was conducted without BB emissions and compared to the base line. The impact of natural and anthropogenic aerosol particles is studied and quantified for the two simulations, focusing on aerosol concentration and cloud responses under different model resolutions. A statistical analysis of pollutant concentration of major regulated species and cloud variables is conducted and the percentage difference used to assess the contribution due to BB emissions. Results confirm the high variability of spatial and temporal patterns of chemical species, with the greatest discrepancies occurring in the tropical forests whereas the subtropics show more urban/industrial related emissions. Whilst CO and O3 show statistically significant increases over a number of cities/towns, the trend and spatial variability is much less uniform with NO2 and PM in most urban and populous cities. Statistical analysis of major chemical pollutants was mainly influenced by BB emissions. O3, NOx, CO and PM increase by 24%, 76%, 51%, 46% and 41% over the main source regions, whereas in the less affected regions concentrations increased by 5%, 5%, 5%, 3% and 2% when BB emissions are included. This study sheds new light on the response of cloud processes to changing aerosol concentrations and different model resolutions. In the parameterised case (dx = 20 km), clouds become more cellular, correlated with high supersaturations, whereas in the resolved case (dx = 4 km), they become more faint with relatively lower supersaturations. Aerosol effects on cloud properties were further studied and statistical analysis conducted on CCN, cloud droplet number concentration (CDNC), supersaturation and aerosol optical depth (AOD) at two different grid spacings. Most clouds occur to the west of the domain coincident with increase in aerosol concentration and AOD, while single scattering albedo (SSA) decreases. A considerable cloud ‘burn-off’ occurs in tropical west Africa, where aerosols can also be lofted up to 500-hPa level when BB emissions are included in the simulation. Due to BB, absorbing aerosol increased by 76% and 23% over tropical west and subtropical southeast, while tropical east shows no change. The study shows that tropical central Africa is characterized by an increased build-up in biomass burning aerosols (BBAs), forming a regional haze with high AOD; this becomes stronger near active burning areas with a significant proportion occurring to the west. AOD enhancement increases up to 38%, 31% and 11% in the west, east and south respectively. Although CDNC increased in areas with high aerosol concentration, supersaturation decreases (in the small domains) since increase in aerosol number concentration decreases maximum supersaturation Smax. Changes in absorbed radiation increased by +56 Wm-2, +23 Wm-2 and +14 Wm-2 in the west, east and southeast. To further evaluate the model sensitivity and its skill, an analysis was conducted by comparing the model performance with measurement data. Simulated AOD, surface concentrations of CO and O3, ozonesondes and liquid water path (LWP) were compared with measured data from MODIS satellite, SAFARI2000 field study and Cape Point WMO. The model shows a good skill in capturing and reproducing the trends as that measured. However, a severe lack of measurement data over southern Africa makes it more difficult to effectively evaluate WRF-Chem over southern Africa. There is a need for increased availability of measurements to adequately compare with models. This study is one of the first WRF-Chem studies conducted over southern Africa to simulate the weather and pollution interaction. The novelty of the present study is the combined analysis of ACI sensitivity to aerosol loading and cloud response in a regime-based approach. The study concludes with a brief discusssion of future directions for work on AQ and modelling interactions between pollution and weather over southern Africa.
Supervisor: Mcfiggans, Gordon ; Schultz, David Sponsor: University of Botswana
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: air quality ; pollution ; WRF-Chem ; regional modelling ; aerosol-cloud interactions ; seasonal biomass burning ; meteorology