Land- atmosphere exchange of elemental mercury : new insights using a novel relaxed eddy accumulation and enclosure techniques

Anthropogenic activities, such as mining and burning of fossil fuels, have significantly increased the emissions of mercury (Hg) to the atmosphere, and the subsequent deposition onto global ecosystems. To restrain Hg emissions and reduce its accumulation in biota and human exposure, the UN’s legally binding Minamata Convention was signed by 128 countries. To estimate the potential of different ecosystems as sinks or sources for atmospheric Hg, reliable quantification of land-atmosphere exchange of gaseous elemental Hg (GEM) is crucial.
Despite extensive efforts to quantify GEM exchange and to characterize controls, large uncertainty remains due to the complexity of bi-directional GEM flux, model parameterization, and the application of different measurement techniques. The majority of flux studies were temporally biased toward summer and daytime. More than 60% of these measurements have been conducted over Hg-enriched sites and limited to small-scale studies using enclosure techniques.
The main goal of the thesis was to identify the role of boreal peatlands as net sinks or sources of Hg by calculating the first annual Hg budget including continuous measurements of peatland-atmosphere exchange of GEM. Peatlands are major mediators for the high levels of Hg in freshwater fish in Europe and North America, because the peatlands provide favorable conditions for the formation of bioavailable and highly toxic methylmercury. In high latitude regions almost all freshwater fish have Hg concentrations exceeding European limits for good chemical status (0.02 mg Hg kg-1 fish muscle, Chalmers et al., 2011, Åkerblom et al., 2014). To test the hypothesis that enough Hg evades from peat to the atmosphere to play a significant role in Hg removal, we developed a relaxed eddy accumulation (REA) system for long-term and large-scale GEM flux monitoring. The first objective was to test the system over an urban site and a boreal peatland at different heights with contrasting surface and turbulence characteristics. In addition, we aimed to inter-compare REA with dynamic flux chambers (DFCs) during a concurrent measurement campaign. DFCs represent the far most common GEM flux measurement technique mainly because they are relatively simple to use and cheaper than micrometeorological methods, while also being suitable for short-term and small-scale flux measurements. As a result they provide an efficient method to resolve confounding influences on GEM flux over a boreal peatland and to test whether GEM emission from contaminated properties constitutes a health risk for residents caused by chronic inhalation of Hg vapor.
The novel REA design features two inlets and two pairs of gold cartridges for continuous sampling of GEM in both updrafts and downdrafts for subsequent measurement on a single Hg detector. We tested the system for two weeks in the center of Basel, Switzerland, and for a period of three weeks during snowmelt above the nutrient poor, minerogenic Degerö Stormyr peatland, located about 50 km NW of Umeå, Sweden. Both environments were identified as net sources of GEM to the atmosphere, with average emission rates of 3 and 15 ng m-2 h-1, respectively. The tests revealed that our REA system reduced major uncertainties caused by the sequential sampling in previous single detector designs. Continuous and autonomous measurements were facilitated by regular monitoring of detector drift and recovery rates using a GEM reference gas and a Hg zero-air generator. Despite the very low GEM concentration difference between updraft and downdraft (0.13 ng m-3) at Degerö Stormyr, the results indicate that REA is feasible for measurements that are close to the surface over snow and/or short vegetation.
In a longer deployment we continuously monitored the GEM flux at Degerö Stormyr over a period of one year. The annual Hg mass balance was dominated by net GEM emission
(10.2 µg m-2) due to substantial evasion between May and October. The annual wet bulk deposition was 3.9 µg m-2. The annual discharge export of Hg from the peatland area (1.9 km2) amounted to 1.3 µg m-2.
The GEM evasion rate, a factor of eight higher than runoff Hg export, results most likely from recent declines in atmospheric Hg concentrations (Amos et al., 2015) that have turned the peatland from a net sink into a source of atmospheric Hg. This is consistent with the Hg concentration gradients in the superficial peat which decline from a Hg concentration peak at about 30 cm depth (110 ng g-1, corresponding to Hg emission peaks during the 1950s) towards the surface (23 ng g-1). Under the assumptions that environmental conditions remain stable and that catchment runoff is dominated by Hg from the uppermost peat layers, it will take around 80 years to deplete the entire pool of legacy Hg in the uppermost 34 cm to a background concentration level of 20 ng g-1. We suggest that the strong Hg evasion demonstrated in this study means that open boreal peatlands and thus downstream ecosystems may recover more rapidly from past atmospheric Hg deposition than previously assumed.
The method comparison study investigating differences between a Teflon® PFA DFC (TDFC), a new type DFC (NDFC) and REA was conducted over four days in July 2014. This revealed that the variability in GEM flux increased in the following order: TDFC < NDFC < REA. The average ± SD fluxes were 0.7 ± 1.3 ng m-2 h-1, 1.9 ± 3.8 ng m-2 h-1 and 2 ± 24 ng m-2 h-1, respectively. Compared to conventional chamber designs the NDFC is able to account for the effect of wind and yielded cumulative flux values similar to the turbulent fluxes measured by REA (< 2% difference). This result indicates the potential of the NDFC to bridge the gap between turbulent and enclosure-based flux measurements. While the REA flux was rather variable within a day, alternate DFC measurements revealed a distinct diel pattern with highest GEM emission in the early afternoon. Spatial heterogeneity in peatland surface characteristics introduced by total Hg concentrations in the uppermost 34 cm (48 - 67 ng g-1), vascular plant cover (18 - 60%), water table level (4 - 18 cm) or dissolved gaseous Hg concentrations (20 - 82 pg L-1) did not appear to significantly influence GEM flux. We conclude that for short-term mechanistic studies DFCs are the preferred tool while the NDFC is suitable for quantitative flux estimations over short vegetation.
The comparison of peatland-atmosphere exchange of GEM from 16 experimental plots determined using a shaded polycarbonate DFC revealed significantly lower flux rates, occasionally indicating Hg uptake, from plots subjected to sulfur additions at rates of
20 kg ha-1 yr-1. These deposition rates were typical during the 1980s in southern Sweden which are approximately seven times faster than contemporary deposition rates in northern Sweden. Enhanced nitrogen deposition and greenhouse treatment had no significant effect on GEM fluxes. The suppressed GEM evasion from the sulfur-treated plots was most likely related to Hg binding to S in organic matter, making Hg less susceptible to evasion, and more prone to transport in runoff at the start of the S additions 15 years earlier.
The thesis was completed with shaded NDFC flux measurements over industrially polluted properties in Switzerland. Topsoil (0 - 10 cm) THg concentrations from 27 measurement plots at nine properties ranged from 0.2 to 390 µg g-1. We found that atmospheric GEM concentrations at 1 m height over the parcels were up to 14 times higher than northern hemispheric background concentrations (~1.5 ng m-3), however, they did not appear to reach harmful levels. The parcel averaged fluxes ranged from 38 to 1258 ng m-2 h-1 and were clearly driven by total Hg concentrations in the soil (r2 = 0.77, p < 0.01). GEM emission from the entire area of 8.6 km2 was estimated to 4.5 kg yr-1. This accounts for 0.5% of the total Hg emission in Switzerland, as estimated by emission inventories (BAFU, 2015).
It is emphasized that GEM evasion dominates the flux during the growing season over the studied peatland and that elevated sulfur concentrations in the peat also influence flux magnitudes. Spatial heterogeneity in peat characteristics did not explain the variations in flux. The all-season REA measurements identified peatlands as sources of GEM to the atmosphere. Release of Hg that accumulated earlier in the uppermost peat layers may continue for the next half century. This implies that Hg contamination to aquatic ecosystems and food webs will decrease in parts of Europe and North America with substantial areas covered by peatlands. The variation may be even greater between sites located in different climate zones. A combination of long-term GEM flux measurements, Hg tracer and Hg isotope experiments are necessary to further elucidate the complex biogeochemical cycle of Hg in peatlands, especially to detect potential peak flux events, identify mechanisms of Hg reduction in the soils and to pinpoint pathways of GEM transport from soils to the atmosphere.