Characteristics and transport of atmospheric aerosols over the Namibian coast

Abstract

It is well understood that on various scales in the atmosphere, the transport, dispersion, accumulation, transformation and deposition of aerosols is driven by atmospheric dynamics, meteorological conditions and topographical controls. This thesis presents the results of a series of inter-disciplinary research endeavours in environmental and atmospheric sciences to describe the spatial- and temporal variability in vertical atmospheric structures, transport pathways, and aerosol character over the Namibian coast. The unique topography, atmospheric conditions, variety of aerosol sources, and the presence of a semi-permanent stratocumulus cloud deck, make the Namibian coast of particular interest for this study and this region is a type of natural laboratory for studying the variability in these conditions. Recent international research efforts in the region include the Observations of aerosol and cloud interactions (Nasa-ORACLES: Zuidema et al., 2016) and the Aerosols, Clouds and Radiation in southern Africa (AEROCLO-sA: Formenti et al., 2019) campaigns. For the aforementioned reasons, the ground-based, Henties Bay Aerosol Observatory (HBAO; 22.090619°S, 14.272424°E), Namibia, makes in situ measurements of various physical and chemical properties of aerosols. These measurements within the Namibian boundary layer (BL) include black carbon (BC) intermittently from July 2012 to December 2015, and filter sampling of PM10 aerosols from February 2016 to December 2017 (over 26 sampling weeks). These filters were analysed for element- and ion-concentrations. Additionally, publicly available datasets were used, such as the NOAA HYSPLIT kinematic back-trajectory model, radiosonde data, remote-sensed satellite data from MODIS, CALIPSO and COSMIC missions, synoptic charts, reanalysis meteorological datasets, and statistical- and mathematical models. Boundary layer height (BLH), characterised by the bulk Richardson number, at Walvis Bay, were radiosondes were released was linked to cloud- and surface cover, surface heating effects, and easterly wind components. The surface of MG across the region of interest was largely affected by changes in atmospheric moisture and cloud, and was not consistent with BLH as defined by the bulk Richardson number. Low-level inversions, identified by GPS-RO, were lowest, deepest and strongest in the spring. Their character was linked to surface-radiation interactions with cold sea surfaces, warm arid landscapes and low-level clouds in the region, and macroscale circulation, such as the southeast Atlantic and continental high-pressure systems. The effects of diurnal variability in surface temperatures extended deep into the atmosphere with stability peaking just before dawn. The character of low-level inversions along the coast also indicated the presence of the night-time Benguela jet stream. The geographical variability and springtime maximums coincided with maximum closed cloud cover over the SEA Ocean and biomass burning aerosol (BBA) plumes over the region. The character of elevated inversions was also linked to macroscale circulation, as well as the spatial and temporal distribution of radiation-absorbing aerosols emitted by biomass burning. The transport pathways to the site, conceptualised with the use of synoptic charts and trajectory analyses, helped to identify several major transport pathways to the aerosol observatory, from marine and continental atmospheres or recirculated locally. An early peak in equivalent black carbon (eBC) in comparison to literature and satellite measurements were explained by the influence of localised low-pressure systems that either enhance (July) or inhibit transport (September) into the Namibian BL. In addition, lower eBC in the BL in September may be linked to increased stability when co-occurring inversions were at a maximum for the year. In addition to the biomass burning source, five major aerosol source types were identified from the apportionment by comparisons of mass ratios to known literature values and positive matrix factorisation (PMF). These sources are sea salt (component contributes 74.7 ± 1.9% of the total mass, characterised by Na+, Cl-, Mg2+, K+, Ca2+ and SO42-), mineral dust (15.7 ± 1.4%, Si, Al, Fe, Ti, Ca2+, Mn, P, F- and V), ammonium neutralised (6.1 ± 0.7%, SO42-, NH4+, MSA, oxalate, and nitrate), fugitive dust (2.6 ± 0.2%, V, Cd, Pb, Nd and Sr) and industry (0.9 ± 0.7%, As, Zn, Cu, Ni and Sr). High fluoride concentrations (25 μg m-3) could pose a serious health problem to the affected populations. Back-trajectory analyses showed how aerosols were circulated locally and transported over great distances from the ocean and subcontinent. Even though the HBAO is set in a remote environment, the transport and contribution of aerosols generated by both natural and anthropogenic processes mean that the site cannot be considered as pristine. The combined interdisciplinary methods used in this study, have proven useful in investigating the influence of vertical structures in the atmosphere, and the local- and long-range transport of air masses on the aerosol character and load at HBAO.