Air/Surface Exchange of Gaseous Elemental Mercury at Different Landscapes in Mississippi, USA

Abstract

Mercury (Hg) is a global pollutant with human health and ecological impacts. Gas exchange between terrestrial surfaces and the atmosphere is an important route for Hg to enter and exit ecosystems. Here, we used a dynamic flux chamber to measure gaseous elemental Hg (GEM) exchange over different landscapes in Mississippi, including in situ measurements for a wetland (soil and water), forest floor, pond, mowed field and grass-covered lawn, as well as mesocosm experiments for three different agricultural soils. Fluxes were measured during both the summer and winter. Mean ambient levels of GEM ranged between 0.93–1.57 ng m−3. GEM emission fluxes varied diurnally with higher daytime fluxes, driven primarily by solar radiation, and lower and more stable nighttime fluxes, dependent mostly on temperature. GEM fluxes (ng m−2 h−1) were seasonally dependent with net emission during the summer (mean 2.15, range 0.32 to 4.92) and net deposition during the winter (−0.12, range −0.32 to 0.12). Total Hg concentrations in the soil ranged from 17.1 ng g−1 to 127 ng g−1 but were not a good predictor of GEM emissions. GEM flux and soil temperature were correlated over the forest floor, and the corresponding activation energy for Hg emission was ~31 kcal mol−1 using the Arrhenius equation. There were significant differences in GEM fluxes between the habitats with emissions for grass > wetland soil > mowed field > pond > wetland water ≈ forest ≈ agriculture soils. Overall, we demonstrate that these diverse landscapes serve as both sources and sinks for airborne Hg depending on the season and meteorological factors.

Can the MerPAS Passive Air Sampler Discriminate Landscape, Seasonal, and Elevation Effects on Atmospheric Mercury? A Feasibility Study in Mississippi, USA

“…GEM concentrations by landscape, elevation, and season are shown Figure 4, with exact values in Table A2. The levels are at or below those generally accepted as the North American background (1-2 ng m −3 ) [1], but within this range, we have previously measured using active sampling in the area [23]. These relatively low concentrations indicate that there are no Hg point sources in the local area to affect the results.…”

“…We have previously measured GEM fluxes over a mowed field at the UMFS using a dynamic flux chamber, observing a net emission (mean 4.52 ± 4.8 ng m −2 h −1 ). However, these data were limited to a couple of days during an earlier summer [23]. Others have observed relatively high GEM fluxes over grass, even at night [36].…”

“…Forest GEM concentrations were also slightly higher in the winter than in the summer. It has also been demonstrated that soil is a major contributor to atmospheric Hg and that solar radiation, which can reach the ground more frequently during the winter due to a decreased forest canopy, can impact emission fluxes [23,38,39].…”

“…GEM is slowly converted by photochemical and other reactions to GOM, which, because of its solubility and particle-reactivity, has a relatively short lifespan in the atmosphere (estimated from days to weeks) depending on atmospheric conditions [1,4]. Likewise, PBM is scavenged from the atmosphere at shorter time-scales depending We have previously examined GEM fluxes from different landscapes in Mississippi, USA using a dynamic flux chamber with active sampling [23]. The main goal of the current study was to test the feasibility of the MerPAS to measure GEM concentration gradients between some of these same landscapes, as well as by elevation (vertical profiles) and seasons.…”

Air Concentrations of Gaseous Elemental Mercury and Vegetation–Air Fluxes within Saltmarshes of the Tagus Estuary, Portugal

“…The reduction of Hg(II) to Hg(0) catalyzed by solar radiation is also an important factor in both terrestrial [20] and aquatic [21] environments. The effect of vegetation on Hg biogeochemical cycling has been examined in terrestrial species [6,[22][23][24]. Plant species, air Hg(0), and soil Hg concentrations have all been shown to influence vegetation-air Hg exchange, and terrestrial plants may act as either a source or sink for atmospheric Hg.…”