Terrestrial mercury biogeochemical cycling within the Australian environment
thesisposted on 28.03.2022, 19:34 authored by Katrina Macsween
Mercury (Hg) biogeochemical cycling is a complicated and highly variable process, particularly in the terrestrial environment. Despite many decades of research, the mechanisms involved in controlling Hg are still poorly understood. A clear understanding of the controlling mechanisms is vital for accurate representation in the global Hg models, particularly regarding re-emission. Behaviour of Hg in the environment varies considerably across temporal and spatial scales due to variation in environmental conditions such as meteorological drivers, plant production and atmospheric chemistry. Few long-term field experiments exist, and none in Australia, to verify the roll of these parameters on seasonal time scales. The primary focus of this thesis was to determine key drivers of Hg's natural cycle within an Australian context. This thesis presents research looking at Hg fluxes over a 14-month period (3 April 2017 to 21 June 2018) at a vegetated background site in Australia (Oakdale, New South Wales). This flux study will allow for a clearer understanding of Australia's contribution to global natural sources of mercury and exploitation of environmental influences on these emission rates. Experimentation was undertaken using aerodynamic gradient micrometeorological flux method. Meteorological and atmospheric chemistry variables were measured concurrently throughout the duration of the study. Emissions from biomass burning were also measured using a combination of combustion wind-tunnel and field measurements. Hg emission flux average over the duration of the study were close to zero, however, there was significant seasonal variation. Highest net surface emission occurred during austral summer, while net deposition occurred during winter and early spring. This variation was largely attributed to seasonal changes in net radiation and surface temperatures, which control the volatilisation of Hg from the substrate. Soil moisture change due to rainfall appeared to have little prolonged influence on Hg surface emissions, beyond an initial interstitial release. Atmospheric Hg concentrations followed a similar seasonal trend to the Hg fluxes, where highest concentrations occurred in summer and lowest in winter. The study average was 0.68 ng m-3, below currently assumed Southern Hemisphere background concentrations (ranging between 0.85 ng m-3 and 1.05 ng m-3). Atmospheric concentrations at the study site are thought to be the product of atmospheric transport, with highest concentrations occurring when air masses had first travelled through the Sydney metropolitan region. Atmospheric Hg release during biomass burning was less than previously assumed, largely due to low Hg concentrations in both vegetation and soils. These values are ~62% lower than the global average used previously to estimate Australian biomass burning mercury release. Australia's climate has a significant impact on the rate and timing of terrestrial emissions and atmospheric transport to and from the continent. Measurements of Hg concentrations in ambient air support the conclusion that that Hg deposited in remote areas may originate from anthropogenic sources far away. This study clearly demonstrated the complexities of understanding the drivers and influencing parameters affecting emission and deposition in the environment and found that Australia is largely influenced by global Hg circulation --abstract.