Year

2017

Degree Name

Master of Research

Department

School of Earth and Environmental Science

Abstract

The GEOS-Chem biogeochemical Hg model (like every other major Hg model) has historically been developed and evaluated using observations from the northern hemisphere, which are significantly more abundant than observations in the southern hemisphere. A recent evaluation of GEOS-Chem against a global database of Hg observations found significant biases in the simulation of Hg in the southern hemisphere (Song et al., 2015; Horowitz et al., 2017); however, only three southern hemisphere sites were included and none were in Australia.

The Australian climate, its flora, fauna and soil types are unique from those in other regions of the world. There are a lack of reliable estimates for the Australian region regarding the cycling of mercury emissions (both natural and anthropogenic) between the atmosphere and terrestrial surface (Edwards & Howard, 2013). To date, limited research has been undertaken on modelling and quantifying the sources and quantities of atmospheric Hg in Australia.

Gaseous elemental mercury (Hg0) observations from five southern hemispheric sites were used to evaluate the performance of GEOS-Chem in the Australasian region. It was found that GEOS-Chem simulates an atmospheric Hg budget for the Australian region of approximately 110 to 144 Mg yr- 1. Hg exchange from the ocean accounts for approximately 22.5 to 35.3 % of this total. Once oceanic sources were removed, the budget was estimated at 93.2 Mg yr-1, slightly lower than previous estimates. The key findings are that: (i) GEOS-Chem is generally able to reproduce the observed seasonal cycle of Hg0 at southern hemisphere sites, but not the daily variability; (ii) the simulations tend to overestimate Hg0 concentrations; (iii) Australian and southern hemispheric sites are heavily influenced by oceanic emissions; and (iv) the Australian terrestrial surface may provide a net sink for atmospheric Hg.

Australian and southern hemispheric Hg simulations in GEOS-Chem may benefit from: (i) using daily averages for bromine (rather than monthly averages) for driving the atmospheric chemistry; (ii) coupling vegetation with soil emissions/deposition, which are currently treated independently but would be more accurately represented by bi-directional exchange; (iii) additional observation sites and measurements of gaseous oxidised mercury (HgII) and particulate bound mercury (HgP) to provide an improved understanding of Hg speciation, chemical redox reactions and Hg fluxes (both evasion and deposition) in the region; (iv) customization of the emission factors relating to soil, biomass burning and vegetation processes.

In summary, this work provides the first evaluation of the GEOS-Chem Hg model over Australia, paving the way for model development that will improve Hg simulation in this part of the world. Future work will specifically target inclusion of moisture limitation for emissions from tropical soils, investigation of chlorine chemistry in the marine boundary layer, focusing on scavenging in the tropical regions, and customization of emissions factors for Australian soils and vegetation.

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