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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 1 | Copyright
Atmos. Chem. Phys., 18, 129-142, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Jan 2018

Research article | 05 Jan 2018

Mercury fluxes over an Australian alpine grassland and observation of nocturnal atmospheric mercury depletion events

Dean Howard and Grant C. Edwards Dean Howard and Grant C. Edwards
  • Department of Environmental Sciences, Macquarie University, Sydney, New South Wales, 2109, Australia

Abstract. Aerodynamic gradient measurements of the air–surface exchange of gaseous elemental mercury (GEM) were undertaken over a 40ha alpine grassland in Australia's Snowy Mountains region across a 3-week period during the late austral summer. Bi-directional GEM fluxes were observed throughout the study, with overall mean value of 0.2±14.5ngm−2h−1 and mean nocturnal fluxes of −1.5±7.8ngm−2h−1 compared to diurnal fluxes of 1.8±18.6ngm−2h−1. Deposition velocities ranged from −2.2 to 2.9cms−1, whilst ambient GEM concentrations throughout the study were 0.59±0.10ngm−3. Cumulative GEM fluxes correlated well with 24h running mean soil temperatures, and one precipitation event was shown to have a positive impact on diurnal emission fluxes. The underlying vegetation had largely senesced and showed little stomatal control on fluxes. Nocturnal atmospheric mercury depletion events (NAMDEs) were observed concomitant with O3 depletion and dew formation under shallow, stable nocturnal boundary layers. A mass balance box model was able to reproduce ambient GEM concentration patterns during NAMDE and non-NAMDE nights without invoking chemical oxidation of GEM throughout the column, indicating a significant role of surface processes controlling deposition in these events. Surface deposition was enhanced under NAMDE nights, though uptake to dew likely represents less than one-fifth of this enhanced deposition. Instead, enhancement of the surface GEM gradient as a result of oxidation at the surface in the presence of dew is hypothesised to be responsible for a large portion of GEM depletion during these particular events. GEM emission pulses following nights with significant deposition provide evidence for the prompt recycling of 17% of deposited mercury, with the remaining portion retained in surface sinks. The long-term impacts of any sinks are however likely to be minimal, as cumulative GEM flux across the study period was close to zero.

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Short summary
Air–surface exchange of mercury is an important process for the movement of this toxic metal through the environment. This study presents observations of nocturnal depletion of atmospheric mercury, with surface deposition playing a large role. This deposited mercury is more labile, with up to ~17% re-released the following morning. This study is the first of its kind taken in Australia. Comparison with studies in the Northern Hemisphere shows reasonably good agreement for deposition velocities.
Air–surface exchange of mercury is an important process for the movement of this toxic metal...