Seasonal gaseous elemental mercury fluxes at a terrestrial background site in south-eastern Australia

Abstract

Terrestrial air-surface exchange of mercury (Hg) forms an important component of the global Hg cycle, with drivers varying across spatial and temporal scales. These drivers include substrate properties, atmospheric chemistry, and meteorological factors. Vegetation uptake represents the dominant pathway of atmospheric Hg deposition to terrestrial surfaces. This study investigated the drivers of net ecosystem exchange of gaseous elemental mercury (Hg0) across multiple seasons in order to gain an understanding of the influence of vegetation and other environmental parameters on the Hg0 air-surface exchange. Measurements were made continuously using a micrometeorological aerodynamic flux gradient method at a low-vegetated background site in south-eastern Australia, over 14 months. Mean Hg fluxes and atmospheric concentrations across the entire study period were 0.002 ng m–2 h–1 (SD ± 14.23 ng m2 h–1) and 0.68 ng m–3 (SD ± 0.22 ng m–3), respectively. Variability was observed across seasons, with the highest average rate of emissions occurring in austral summer (December, January, February) (0.69 ng m–2 h–1) and the highest rate of deposition observed in autumn (March, April, May) (–0.50 ng m–2 h–1). Vegetation uptake dominated Hg flux during the winter and spring when meteorological conditions were cold and light levels were low. This is supported by CO2 flux data, with a daytime winter mean of 0.80 µmol m–2 h–1 and a spring daytime mean of 1.54 µmol m–2 h–1. Summer Hg fluxes were dominantly emission due to higher solar radiation and temperature. Climatic conditions at Oakdale allowed plant production to occur year-round, however the hot dry conditions observed in the warmer months increased evasion, allowing this site to be a small net source of Hg0 to the atmosphere.