Source characterisation of atmospheric trace metal deposition around Australia

• Main Author: Strzelec, M
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: iron; aerosols; atmospheric deposition; dust; trace metals; Garden Island; Southern Ocean; Western Shelf
• Online Access: https://eprints.utas.edu.au/35895/; https://eprints.utas.edu.au/35895/1/Strzelec_whole_thesis.pdf

Atmospheric aerosol deposition is an important pathway for the delivery of micronutrient trace elements to the ocean. This includes the atmospheric supply of iron (Fe) to either high nutrient low chlorophyll regions (where Fe is needed for phytoplankton primary production) or to low nutrient low chlorophyll regions (where Fe is needed for nitrogen fixation). The atmospheric supply of micronutrients therefore has the potential to significantly alter global carbon and nitrogen biogeochemical cycles, reduce carbon dioxide content in the atmosphere and consequently impact climate change. Australia is one of the major sources of aerosols to the oceans of the Southern Hemisphere. Australian emissions from mineral dust, bushfires and anthropogenic activities supply micronutrients to surrounding waters. Consequently, atmospheric micronutrient deposition may be utilised by marine biota directly, in the world’s greatest HNLC area - the Southern Ocean (SO) - or in-directly in oligotrophic waters such as the Great Barrier Reef (GBR) and the Western Shelf (WS). Despite Australia being important for the atmospheric delivery of micronutrients, the coastal regions around Australia are poorly studied and, in contrast to the Northern Hemisphere, Southern Hemisphere atmospheric trace metal data is limited, with a pressing need for new data for global models. This study focuses on atmospheric trace metal deposition near three ocean regions around Australia where micronutrient delivery is expected to have an effect on marine biological processes. The focus was mostly on Fe, however other trace elements were used as proxies for fingerprinting different aerosol sources. Data on fractional solubilities of micronutrient trace elements and their atmospheric concentrations and fluxes are also reported. Chapter 1 provides a literature review of the project and study area. Chapter 2 discusses the analytical and field methods used in this study which enables us to collect the data for chapters 3, 4 and 5. Chapter 3 focuses on atmospheric deposition near the GBR during the dry season of 2016. Dry and wet deposition samples were collected during the Atmospheric Integrated Research on Burdens and OXidative capacity (AIRBOX) campaign ‘Reef to Rainforest’. Chapter 4 describes seasonal trends in relative contributions of natural versus anthropogenic sources of aerosols and implications on solubility and deposition fluxes near the WS of Australia. Seasonal cycling was measured near Gingin in Western Australia (approx. 70 km north of Perth) and additional short-term observations were conducted during the AIRBOX campaign ‘Near surface aerosol characterisation’ nearby on Garden Island in Western Australia. Chapter 5 describes the temporal observations of trace metal deposition near the SO, measured on Mount Wellington in southern Tasmania. Chapter 6 summarises the main findings from this thesis and presents the important directions for future research. Atmospheric concentrations and fluxes of trace metals, differentiating between total and potentially bioavailable forms were determined. For this purpose, a 3-step leaching protocol was applied, consisting of: (i) an ultrapure water leach, (ii) an ammonia-acetate buffer at pH 4.7, and (iii) a digestion in aggressive acids (HF, HNO3) to determine (i) ‘soluble’, (ii) ‘leachable’ and (iii) ‘refractory’ fractions. The ‘soluble’ fraction is indicative of the amount available to marine biota immediately after aerosol deposition to seawater. The ‘leachable’ fraction represents trace metal forms which require more time to dissolve and may be consumed by marine biota. Consequently, the sum of (i) and (ii) is called ‘labile’ fraction and is considered as a proxy of the total bioavailable amount of an element. The sum of (i), (ii) and (iii) is called ‘total’ and represents the total element concentration in the sampled atmosphere. In addition to the leaching experiments, ‘dissolvable’ major ion contents were determined to study the influence of aging processes and mixing with combustion gases. Aerosol morphology and chemical composition were determined by scanning electron microscopy to gain insights into aerosol types and single particle chemistry. Overall, the major source of total Fe was mineral dust to all analysed regions. Total Fe was reversely correlated with the fraction of labile Fe, a similar finding to many other regions of the world. This indicates that mineral dust, as a source of less soluble Fe, dominates to varying degrees more soluble Fe sources. Highly soluble Fe aerosols were linked to anthropogenic activities, particularly combustion processes. However, the highest fraction of bioavailable (labile) Fe was found in bushfire emissions. The three regions studied varied greatly in terms of mean dry flux of total Fe which was highest near the WS, between 2.2 and 3.4 times more than near GBR and the SO, respectively. However, greater fraction of the mean (±SD) labile Fe fraction was observed for aerosols collected in coastal areas of GBR (8.0±2.0%), the SO (7.6±6.2%) when compared to WS (2.5±1.3%). Consequently, the highest dry deposition flux of labile Fe fraction was observed near GBR (0.123±0.50 μmol m\(^{-2}\) d\(^{-1}\)), while values near the SO and WS were similar, 0.084±0.075 and 0.079±0.067 μmol m\(^{-2}\) d\(^{-1}\) , respectively. Most of the data on fractional Fe solubility, bioavailable and total Fe dry fluxes provided in this study agree with findings from the current (sparse) literature for the Southern Hemisphere and with predictions from global atmospheric aerosol deposition models. However, important seasonal variations in aerosol source types was observed. A six-fold higher mineral atmospheric dust concentration in the warm compared to the cool season near WS was observed. This may be explained by variations in the activity of the regional mineral dust sources. This was accompanied by a shift in the fractional Fe solubility and relative contribution of anthropogenic emissions to total aerosol deposition. Similarly, variations in the atmospheric concentration of mineral dust observed near the SO reflect the in-situ observations on dust events on the Australia mainland. This works also highlights the role of anthropogenic activities, such as fossil fuel combustion and bushfire emissions, which were found to positively correlate with fractional Fe solubility (e.g., emissions from ships crossing GBR was positively correlated with fractional Fe solubility). The contribution of wet deposition to the total atmospheric flux of Fe was found to exceed dry deposition near the GBR and has the potential to be the dominant pathway of micronutrient supply. This dataset will greatly improve atmospheric and biogeochemical models which have relied on sparse datasets for the Southern Hemisphere.