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

• Main Authors: D. Howard, G. C. Edwards
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/129/2018/acp-18-129-2018.pdf
• ISSN: 1680-7316; 1680-7324

Aerodynamic gradient measurements of the air–surface exchange of gaseous elemental mercury (GEM) were undertaken over a 40 ha 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 ± 14.5 ng m⁻² h⁻¹ and mean nocturnal fluxes of −1.5 ± 7.8 ng m⁻² h⁻¹ compared to diurnal fluxes of 1.8 ± 18.6 ng m⁻² h⁻¹. Deposition velocities ranged from −2.2 to 2.9 cm s⁻¹, whilst ambient GEM concentrations throughout the study were 0.59 ± 0.10 ng m⁻³. Cumulative GEM fluxes correlated well with 24 h 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.