Seasonal variation in dissolved gaseous mercury and total mercury concentrations in Juam Reservoir, Korea

Dissolved gaseous mercury (DGM) and total mercury (TM) concentrations were measured in Juam Reservoir, Korea. DGM concentrations were higher in spring (64+/-13pgL(-1)) and summer (109+/-15pgL(-1)), and lower in fall (20+/-2pgL(-1)) and winter (23+/-6pgL(-1)). In contrast, TM concentrations were higher in fall (3.2+/-0.1ngL(-1)) and winter (3.3+/-0.1ngL(-1)) than in spring (2.3+/-0.1ngL(-1)) and summer (2.2+/-0.4ngL(-1)). DGM concentrations were correlated with water temperature (p<0.0001), ORP (p<0.0001), UV intensity (UV-A: p=0.008; UV-B: p=0.003), and DOC concentration (p=0.0107). DGM concentrations varied diurnally with UV intensity. The average summer DGM (109+/-15pgL(-1)) and TM (2.2+/-0.4ngL(-1)) concentrations in Juam Reservoir were higher than the averages for North American lakes (DGM=38+/-16pgL(-1); TM=1.0+/-1.2ngL(-1)), but lower than levels reported for Baihua Reservoir in China.     

Relationship between the loading rate of inorganic mercury to aquatic ecosystems and dissolved gaseous mercury production and evasion

The purpose of our study was to test the hypothesis that dissolved gaseous mercury (DGM) production and evasion is directly proportional to the loading rate of inorganic mercury [Hg(II)] to aquatic ecosystems. We simulated different rates of atmospheric mercury deposition in 10-m diameter mesocosms in a boreal lake by adding multiple additions of Hg(II) enriched with a stable mercury isotope (²⁰²Hg). We measured DGM concentrations in surface waters and estimated evasion rates using the thin-film gas exchange model and mass transfer coefficients derived from sulfur hexafluoride (SF₆) additions. The additions of Hg(II) stimulated DGM production, indicating that newly added Hg(II) was highly reactive. Concentrations of DGM derived from the experimental Hg(II) additions (“spike DGM”) were directly proportional to the rate of Hg(II) loading to the mesocosms. Spike DGM concentrations averaged 0.15, 0.48 and 0.94 ng l⁻¹ in mesocosms loaded at 7.1, 14.2, and 35.5 μg Hg m⁻² yr⁻¹, respectively. The evasion rates of spike DGM from these mesocosms averaged 4.2, 17.2, and 22.3 ng m⁻² h⁻¹, respectively. The percentage of Hg(II) added to the mesocosms that was lost to the atmosphere was substantial (33–59% over 8 weeks) and was unrelated to the rate of Hg(II) loading. We conclude that changes in atmospheric mercury deposition to aquatic ecosystems will not change the relative proportion of mercury recycled to the atmosphere.     

Estimates of air-sea exchange of mercury in the Baltic Sea

The concentrations of total gaseous mercury (TGM) in air over the southern Baltic Sea and dissolved gaseous mercury (DGM) in the surface seawater were measured during summer and winter. The summer expedition was performed on 02–15 July 1997, and the winter expedition on 02–15 March 1998. Average TGM and DGM values obtained were 1.70 and 17.6 ng m⁻³ in the summer and 1.39 and 17.4 ng m⁻³ in the winter, respectively. Based on the TGM and DGM data, surface water saturation and air-water fluxes were calculated. The results indicate that the seawater was supersaturated with gaseous mercury during both seasons, with the highest values occurring in the summer. Flux estimates were made using the thin film gas-exchange model. The average Hg fluxes obtained for the summer and winter measurements were 38 and 20 ng m⁻² d⁻¹, respectively. The annual mercury flux from this area was estimated by a combination of the TGM and DGM data with monthly average water temperatures and wind velocities, resulting in an annual flux of 9.5 μg m⁻² yr⁻¹. This flux is of the same order of magnitude as the average wet deposition input of mercury in this area. This indicates that reemissions from the water surface need to be considered when making mass-balance estimates of mercury in the Baltic Sea as well as modelling calculations of long-range transboundary transport of mercury in northern Europe.     

Gaseous mercury fluxes from the forest floor of the Adirondacks

The flux of gaseous elemental mercury (Hg⁰) from the forest floor of the Adirondack Mountains in New York (USA) was measured numerous times throughout 2005 and 2006 using a polycarbonate dynamic flux chamber (DFC). The Hg flux ranged between −2.5 and 27.2 ng m⁻² h⁻¹ and was positively correlated with temperature and solar radiation. The measured Hg emission flux was highest in spring, and summer, and lowest in winter. During leaf-off periods, the Hg emission flux was highly dependent on solar radiation and less dependent on temperature. During leaf-on periods, the Hg emission flux was fairly constant because the forest canopy was shading the forest floor. Two empirical models were developed to estimate yearly Hg⁰ emissions, one for the leaf-off period and one for the leaf-on period. Using the U.S. EPA's CASTNET meteorological data, the cumulative estimated emission flux was approx. 7.0 μg Hg⁰ m⁻² year⁻¹.     

Field controlled experiments of mercury accumulation in crops from air and soil

Field open top chambers (OTCs) and soil mercury (Hg) enriched experiments were employed to study the influence of Hg concentrations in air and soil on the Hg accumulation in the organs of maize (Zea mays L.) and wheat (Triticum aestivum L.). Results showed that Hg concentrations in foliages were correlated significantly (p < 0.05) with air Hg concentrations but insignificantly correlated with soil Hg concentrations, indicating that Hg in crop foliages was mainly from air. Hg concentrations in roots were generally correlated with soil Hg concentrations (p < 0.05) but insignificantly correlated with air Hg concentrations, indicating that Hg in crop roots was mainly from soil. No significant correlations were found between Hg concentrations in stems and those in air and soil. However, Hg concentrations in upper stems were usually higher than those in bottom stems, implying air Hg might have stronger influence than soil Hg on stem Hg accumulation.     

Characteristics of mercury speciation in Minnesota rivers and streams

Patterns of mercury (Hg) speciation were examined in four Minnesota streams ranging from the main-stem Mississippi River to small tributaries in the basin. Filtered phase concentrations of methylmercury (MeHg), inorganic Hg (IHg), and dissolved organic carbon (DOC) were higher in all streams during a major summertime runoff event, and DOC was enriched with MeHg but not with IHg. Particulate-phase MeHg and IHg concentrations generally increased with total suspended solids (TSS) concentrations but the event data did not diverge greatly from the non-event data, suggesting that sources of suspended sediments in these streams did not vary significantly between event and non-event samplings. The dissolved fractions (filtered concentration/unfiltered concentration) of both MeHg and IHg increased with increasing DOC concentrations, but varied inversely with TSS concentrations. While MeHg typically constitutes only a minor portion of the total Hg (THg) in these streams, this contribution is not constant and can vary greatly over time in response to watershed inputs.     

Temporal variability of mercury speciation in urban air

Semi-continuous measurements of ambient mercury (Hg) species were performed in Detroit, MI, USA for the calendar year 2003. The mean (±standard deviation) concentrations for gaseous elemental mercury (GEM), particulate mercury (HgP), and reactive gaseous mercury (RGM) were 2.2±1.3 ng m⁻³, 20.8±30.0, and 17.7±28.9 pg m⁻³, respectively. A clear seasonality in Hg speciation was observed with GEM and RGM concentrations significantly (p<0.001) greater in warm seasons, while HgP concentrations were greater in cold seasons. The three measured Hg species also exhibited clear diurnal trends which were particularly evident during the summer months. Higher RGM concentrations were observed during the day than at night. Hourly HgP and GEM concentrations exhibited a similar diurnal pattern with both being inversely correlated with RGM. Multivariate analysis coupled with conditional probability function analysis revealed the conditions associated with high Hg concentration episodes, and identified the inter-correlations between speciated Hg concentrations, three common urban air pollutants (sulfur dioxide, ozone, and nitric oxides), and meteorological parameters. This analysis suggests that both local and regional sources were major factors contributing to the observed temporal variations in Hg speciation. Boundary layer dynamics and the seasonal meteorological conditions, including temperature and moisture content, were also important factors affecting Hg variability.     

Modeling the photo-oxidation of dissolved organic matter by ultraviolet radiation in freshwater lakes: implications for mercury bioavailability

Uncertainties in projected ultraviolet (UV) radiation may lead to future increases in UV irradiation of freshwater lakes. Because dissolved organic carbon (DOC) is the main binding phase for mercury (Hg) in freshwater lakes, an increase in DOC photo-oxidation may affect Hg speciation and bioavailability. We quantified the effect of DOC concentration on the rate of abiotic DOC photo-oxidation for five lakes (DOC=3.27-12.3 mg L(-1)) in Kejimkujik National Park, Canada. Samples were irradiated with UV-A or UV-B radiation over a 72-h period. UV-B radiation was found to be 2.36 times more efficient at photo-oxidizing DOC than UV-A, with energy-normalized rates of dissolved inorganic carbon (DIC) production ranging from 3.8×10(-5) to 1.1×10(-4) mg L(-1)J(-1) for UV-A, and from 6.0×10(-5) to 3.1×10(-4) mg L(-1)J(-1) for UV-B. Energy normalized rates of DIC production were positively correlated with DOC concentrations. Diffuse integrated attenuation coefficients were quantified in situ (UV-A K(d)=0.056-0.180 J cm(-1); UV-B K(d)=0.015-0.165 J cm(-1)) and a quantitative depth-integrated model for yearly DIC photo-production in each lake was developed. The model predicts that, UV-A produces between 3.2 and 100 times more DIC (1521-2851 mg m(-2) year(-1)) than UV-B radiation (29.17-746.7 mg m(-2) year(-1)). Future increases in UV radiation may increase DIC production and increase Hg bioavailability in low DOC lakes to a greater extent than in high DOC lakes.     

Immobilization of mercury by pyrite (FeS2)

Elemental mercury (Hg⁰) is a metal with a number of atypical properties, which has resulted in its use in myriad anthropogenic processes. However, these same properties have also led to severe local subsurface contamination at many places where it has been used. As such, we studied the influence of various parameters on Hg(II) sorption onto pyrite (pH, time, Hg(II) concentration), a potential subsurface reactive barrier. Batch sorption studies revealed that total Hg(II) removal increases with both pH and time. X-ray absorption spectroscopy analysis showed that a transformation in the coordination environment at low pH occurred during aging over 2 weeks, to form an ordered monolayer of monodentate Hg-Cl complexes on pyrite. In column studies packed with pure quartz sand, the transport of Hg(II) was significantly retarded by the presence of a thin pyrite-sand reactive barrier, although dissolved oxygen inhibited Hg(II) sorption onto pyrite in the column.     

Solving mercury (Hg) speciation in soil samples by synchrotron X-ray microspectroscopic techniques

Direct mercury (Hg) speciation was assessed for soil samples with a Hg concentration ranging from 7 up to 240 mg kg⁻¹. Hg chemical forms were identified and quantified by sequential extractions and bulk- and micro-analytical techniques exploiting synchrotron generated X-rays. In particular, microspectroscopic techniques such as μ-XRF, μ-XRD and μ-XANES were necessary to solve bulk Hg speciation, in both soil fractions <2 mm and <2 μm. The main Hg-species found in the soil samples were metacinnabar (β-HgS), cinnabar (α-HgS), corderoite (Hg₃S₂Cl₂), and an amorphous phase containing Hg bound to chlorine and sulfur. The amount of metacinnabar and amorphous phases increased in the fraction <2 μm. No interaction among Hg-species and soil components was observed. All the observed Hg-species originated from the slow weathering of an inert Hg-containing waste material (K106, U.S. EPA) dumped in the area several years ago, which is changing into a relatively more dangerous source of pollution.     

Seasonal differences in mercury accumulation in Trichiurus lepturus (Cutlassfish) in relation to length and weight in a Northeast Brazilian estuary

Background, aim, and scope  At tropical latitudes, and especially on the semi-arid coasts of the Brazilian Northeast, the rainfall regime governs the water quality of estuaries due to the pronounced difference between the rainy and dry seasons. These changes may be responsible for seasonal changes in bioavailability of mercury (Hg) and other pollutants to the estuarine and coastal biota. Mercury bioaccumulates along estuarine–marine food chains usually result in higher concentrations in tissues of top predators and posing a risk to both marine mammals and humans alike. The Goiana River Estuary (7.5° S) is a typical estuary of the semi-arid tropical regions and supports traditional communities with fisheries (mollusks, fish, and crustacean). It is also responsible for an important part of the biological production of the adjacent coastal waters. Materials and methods   Trichiurus lepturus (Actinopterygii: Perciformes) is a pscivorous marine straggler. Fish from this species (n = 104) were captured in a trapping barrier used by the local traditional population and using an otter trawl net along the main channel of the low estuary during two dry seasons (D1 = November, December 2005, January 2006; D2 = November, December 2006, January 2007) and the end of a rainy season (R = August, September, October 2006). Fish muscle samples were preserved cold and then freeze-dried prior to analysis of its total mercury (Hg-T) contents. Total mercury was determined by cold vapor atomic absorption spectrometry (CV-AAS) with sodium borohydride as a reducing agent. Results  The studied individuals (n = 104) were sub-adult (30–70 cm, 71 ind.) and adult fish (>70 cm, 33 ind.). Weight (W) (204.1 ± 97.9 g, total biomass = 21,229.7 g) and total length (TL) (63.1 ± 10.1 cm, range 29.5–89.0 cm) presented a significant (p < 0.05) correlation. Two-way ANOVA (n = 81) showed that TL and W had significant differences (p < 0.05) among seasons, being higher in D1 than in D2 and R, respectively. Moreover, season vs. month interaction were detected for the variables length and weight. For the variable weight was detected significant difference for the factor month (p < 0.05). It suggests that the fish enter the estuary at the end of the rainy season and increase in length and weight during the time they spend in the estuary. Fish from this estuary are shown to be fit for human consumption (125.3 ± 61.9 μgHg-T kg^–1 w.wt.; n = 104). Fish mercury contents increased with size and weight. Correlations between TL and Hg-T (r = 0.37286) and between W and Hg-T (r = 0.38212) were significant (p < 0.05). Dryer months showed higher mercury concentrations in fish (D1 773.4 ± 207.5 μgHg-T kg^–1 d.wt., n = 27; D2 370.1 ± 78.8 μgHg-T kg^–1 d.wt., n = 27; R 331.2 ± 138.5 μgHg-T kg^–1 d.wt., n = 27). The variable mercury concentration showed differences in relation to the factor season (p < 0.05), where fish captured during the first dry season showed the highest concentration of mercury. The correlation between Hg-T and rainfall (Rf) showed a negative correlation (r = –0.56; p < 0.05). Discussion  The main likely source of mercury to this estuary is diffuse continental run off, including urban and industrial effluents. Since concentration of mercury in fish tissue is negatively correlated to rainfall, but positively correlated with fish length and weight, it suggests that fish growth in this estuary results in mercury uptake and concentration on the fish tissue. In the dry season of 2005–2006, when rainfall remained below the historic average, fish bioaccumulated significantly more mercury than in the dry season 2006–2007, when rainfall was within the predictable historic average. It is suggested that less rainfall, and consequently less particulate matter and less primary production in the estuary, make mercury more available to the higher levels of the estuarine food chain. In the case of higher rainfall, when river flow increases and water quality in the estuary is reduced, mercury probably is quickly exported associated to the particulate matter to the adjacent coastal waters where it then disperses. This species is a potential routine bioindicator for mercury contamination of the biota, but so far was used only with a limited number of individuals and contexts. Conclusions  Fish from the Goiana River estuary can still be safely consumed by the local population. However, any further contamination of this resource might lead to total mercury levels above the recommended limits for pregnant women and small children. The proposed heavy dependency of total mercury levels in fish on water quality indicates that land use and water quality standards must be more closely watched in order to guarantee that best possible practices are in place to prevent bioaccumulation of mercury and its transfer along the food chain. Human interventions and climatic events which affect river water flow are also playing a role in the mercury cycle at tropical semi-arid estuaries. Recommendations and perspectives   T. lepturus is largely consumed by coastal populations of tropical and sub-tropical countries all over the world. It is also consumed by a number of marine mammals over which we have a strong conservation interest. This species is also a link among different ecosystems along the estuarine ecocline. Therefore, knowledge of its degree of contamination might contribute to public health issues as well as marine conservation actions. Studies on mercury and other contaminants using this species as bioindicator (cosmopolitan, readily available) could help elucidating mechanisms through which pollutants are being transferred not only through the food chain, but also from estuarine–coastal–open waters. In addition, using the same species in marine pollution studies, especially as part of a mosaic of species, allows for wide range comparisons of marine food chain contamination.     

Characteristics of atmospheric speciated mercury concentrations (TGM,Hg(II) and Hg(p)) in Seoul,Korea

Ambient speciated mercury concentrations including total gaseous mercury (TGM), gaseous divalent mercury (Hg(II)), and particulate mercury (Hg(p)) were measured on the roof of the Graduate School of Public Health building in Seoul, Korea from February 2005 to February 2006. The average concentrations were 3.22 ± 2.10 ng m^?3, 27.2 ± 19.3 pg m^?3, and 23.9 ± 19.6 pg m^?3 for TGM, Hg(II), and Hg(p), respectively. Hg(II) and Hg(p) concentrations were higher during the daytime than during the nighttime, probably because of high photochemical activity. Hg⁰ concentrations were not significantly correlated with ozone however a positive correlation between ozone and Hg(II) was found during periods of high humidity. Eighteen days were characterized as pollution events with 24 h average PM_2.5 concentrations >65 μg m^?3. The average concentrations of TGM and Hg(p) during these events were 1.4–2 times higher than those during non-pollution events. In order to identify the contribution of long-range transported mercury to the enhanced mercury concentrations in Korea, an episode was defined as a period with hourly average TGM and CO concentrations higher than the monthly average TGM and CO concentrations and with significant enhancement of both TGM and CO concentrations for at least 10 h. A total of 70 episodes were identified during the sampling period: 36 local episodes and 34 long-range transport episodes. The mean ΔTGM/ΔCO slope for all episodes was 0.0063 ng m^?3 ppbv^?1 which agreed well with the slope (0.0036–0.0074 ng m^?3 ppbv^?1) found in previous studies that identified long-range transport of TGM from China. The mean slope during non-events was 0.0011 ng m^?3 ppbv^?1. Back-trajectory analysis showed that during episodes, air parcels arrived mostly from the major industrial areas in China (n = 25, 73%), followed by Japan (n = 4, 12%), Yellow Sea (n = 3, 9%), and Russia (n = 2, 6%).     

Sediment processes and mercury transport in a frozen freshwater fluvial lake (Lake St. Louis, QC, Canada)

An open-bottom and a closed-bottom mesocosm were developed to investigate the release of mercury from sediments to the water column in a frozen freshwater lake. The mesoscosms were deployed in a hole in the ice and particulate mercury (Hg_P) and total dissolved mercury (TDHg) were measured in sediments and in water column vertical profiles. In addition, dissolved gaseous mercury (DGM) in water and mercury water/airflux were quantified. Concentrations of TDHg, DGM, and mercury flux were all higher in the open-bottom mesocosm than in the closed-bottom mesocosm. In this paper we focus on the molecular diffusion of mercury from the sediment in comparison with the TDHg accumulation in the water column. We conclude that the molecular diffusion and sediment resuspension play a minor role in mercury release from sediments suggesting that solute release during ebullition is an important transport process for mercury in the lake.     

The linear accumulation of atmospheric mercury by vegetable and grass leaves: Potential biomonitors for atmospheric mercury pollution

One question in the use of plants as biomonitors for atmospheric mercury (Hg) is to confirm the linear relationships of Hg concentrations between air and leaves. To explore the origin of Hg in the vegetable and grass leaves, open top chambers (OTCs) experiment was conducted to study the relationships of Hg concentrations between air and leaves of lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), alfalfa (Medicago sativa L.) and ryegrass (Lolium perenne L.). The influence of Hg in soil on Hg accumulation in leaves was studied simultaneously by soil Hg-enriched experiment. Hg concentrations in grass and vegetable leaves and roots were measured in both experiments. Results from OTCs experiment showed that Hg concentrations in leaves of the four species were significantly positively correlated with those in air during the growth time (p?<?0.05), while results from soil Hg-enriched experiment indicated that soil-borne Hg had significant influence on Hg accumulation in the roots of each plant (p?<?0.05), and some influence on vegetable leaves (p?<?0.05), but no significant influence on Hg accumulation in grass leaves (p?>?0.05). Thus, Hg in grass leaves is mainly originated from the atmosphere, and grass leaves are more suitable as potential biomonitors for atmospheric Hg pollution. The effect detection limits (EDLs) for the leaves of alfalfa and ryegrass were 15.1 and 22.2 ng g^–1, respectively, and the biological detection limit (BDL) for alfalfa and ryegrass was 3.4 ng m^–3.     

A passive sampler for ambient gaseous oxidized mercury concentrations

This paper reports on the development of a passive sampler for estimating gaseous oxidized mercury concentrations. Atmospheric gaseous oxidized mercury concentrations calculated from passive sampler data were correlated with those obtained using an automated analyzer (r² = 0.71, p < 0.01, n = 110 for one-week deployments; r² = 0.89, p < 0.01, n = 22 for two-week deployments). Sampler uptake was not significantly affected by changes in temperature, humidity, or ozone concentration, but it was slightly dependent on wind speed. As such, an equation for correcting data due to this factor was developed based on wind tunnel and field data. The detection limit for a two-week sampler deployment was ～5 pg m^?3. Field data collected in Nevada and the southeastern United States showed these samplers are useful for investigating spatial and temporal variability in gaseous oxidized mercury concentrations.     

Seasonal and diel variation of atmospheric mercury concentrations in the Reno (Nevada,USA) airshed

This paper describes total gaseous mercury (TGM) concentrations measured in Reno, Nevada from 2002 to 2005. The 3-year mean and median air Hg concentrations were 2.3 and 2.1 ng m⁻³, respectively. Mercury concentrations exhibited seasonality, with the highest concentrations in winter, and the lowest in summer and fall. A well-defined diel pattern in TGM concentration was observed, with maximum daily concentrations observed in the morning and minimum in the afternoon. A gradual increase of TGM concentration was observed in the evening and over night. The early morning increase in TGM was likely due to activation of local surface emission sources by rising solar irradiance and air temperature. The subsequent decline and afternoon minimum in TGM were likely related to increased vertical mixing and the buildup of atmospheric oxidants during the day resulting in increased conversion to oxidized species that are quickly deposited, coupled with weakening of the surface emissions processes. The described diel pattern was seasonally modulated with the greatest amplitude in variation of TGM concentrations occurring in the summer. It is suggested based on the comparison of diel TGM pattern with other gaseous pollutants that natural source surface emissions are a dominant source of TGM in the study area.     

Field controlled experiments on the physiological responses of maize (Zea mays L.) leaves to low-level air and soil mercury exposures

Thousands of tons of mercury (Hg) are released from anthropogenic and natural sources to the atmosphere in a gaseous elemental form per year, yet little is known regarding the influence of airborne Hg on the physiological activities of plant leaves. In the present study, the effects of low-level air and soil Hg exposures on the gas exchange parameters of maize (Zea mays L.) leaves and their accumulation of Hg, proline, and malondialdehyde (MDA) were examined via field open-top chamber and Hg-enriched soil experiments, respectively. Low-level air Hg exposures (<50 ng m^?3) had little effects on the gas exchange parameters of maize leaves during most of the daytime (p?>?0.05). However, both the net photosynthesis rate and carboxylation efficiency of maize leaves exposed to 50 ng m^?3 air Hg were significantly lower than those exposed to 2 ng m^?3 air Hg in late morning (p?<?0.05). Additionally, the Hg, proline, and MDA concentrations in maize leaves exposed to 20 and 50 ng m^?3 air Hg were significantly higher than those exposed to 2 ng m^?3 air Hg (p?<?0.05). These results indicated that the increase in airborne Hg potentially damaged functional photosynthetic apparatus in plant leaves, inducing free proline accumulation and membrane lipid peroxidation. Due to minor translocation of soil Hg to the leaves, low-level soil Hg exposures (<1,000 ng g^?1) had no significant influences on the gas exchange parameters, or the Hg, proline, and MDA concentrations in maize leaves (p?>?0.05). Compared to soil Hg, airborne Hg easily caused physiological stress to plant leaves. The effects of increasing atmospheric Hg concentration on plant physiology should be of concern.     

Atmospheric mercury concentrations and speciation measured from 2004 to 2007 in Reno, Nevada, USA

Atmospheric elemental, reactive and particulate mercury (Hg) concentrations were measured north of downtown Reno, Nevada, USA from November 2004 to November 2007. Three-year mean and median concentrations for gaseous elemental Hg (Hg⁰) were 1.6 and 1.5 ng m⁻³ (respectively), similar to global mean Hg⁰ concentrations. The three-year mean reactive gaseous Hg (RGM) concentration (26 pg m⁻³) was higher than values reported for rural sites across the western United States. Well defined seasonal and daily patterns in Hg⁰ and RGM concentrations were observed, with the highest Hg⁰ concentrations measured in winter and early morning, and RGM concentrations being greatest in the summer and mid-afternoon. Elevated Hg⁰ concentrations in winter were associated with periods of cold, stagnant air; while a regularly observed early morning increase in concentration was due to local source and surface emissions. The observed afternoon increase and high summer values of RGM can be explained by in situ oxidation of gaseous Hg⁰ or mixing of RGM derived from the free troposphere to the surface. Because both of these processes are correlated with the same environmental conditions it is difficult to assess their overall contribution to the observed trends.     

Evaluation of use of EcoCELL technology for quantifying total gaseous mercury fluxes over background substrates

Total gaseous mercury (Hg) fluxes from large (7.3×5.5×4.5 m, L×W×D) climate-controlled gas exchange mesocosms (Ecologically Controlled Enclosed Lysimeter Laboratories or EcoCELLs) containing tallgrass prairie soil–plant monoliths were measured from 2002 to 2005. EcoCELL Hg fluxes (calculated based on the difference in air Hg concentrations inside mesocosms and in incoming air, soil area of the monoliths, and airflow through the system) indicated a net annual emission of 102 μg m⁻², while soil Hg fluxes measured simultaneously using a dynamic flux chamber were an order of magnitude lower. Since Hg fluxes measured from empty EcoCELLs in winter and when housing the soil–plant monoliths at the same time of year were similar, we hypothesized that the Hg signal generated by the tallgrass prairie soil–plant monoliths was too low to be detected using the EcoCELL technology. Because mesocosm Hg exchange was correlated with solar radiation and temperature, with the largest emissions occurring at midday and in the summer, we also hypothesized that the flux from mesocosm infrastructure would change over time. Limited by the ongoing experiment, the EcoCELLs were manipulated to test the above hypotheses. When monoliths were completely covered and excluded from the exchange with the surrounding air, mesocosm Hg exchange was unaffected. Furthermore, removal of vegetation at the end of each growing season did not affect mesocosm Hg fluxes. Tests with changing mesocosm airflow also indicated that the signal from the tallgrass prairie monoliths was not being measured. These results suggest that, although EcoCELLs performed well in a study using Hg contaminated soils and have been successfully applied to understand processes controlling Hg fluxes, there are limitations of this technology for quantifying Hg exchange from background substrates. Prior to the use of similar systems the detection limit and Hg exchange from an empty system need to be carefully quantified.     

Growing season total gaseous mercury (TGM) flux measurements over an Acer rubrum L. stand

Relaxed eddy accumulation (REA) measurements of the total gaseous mercury (TGM) flux measurements were taken over a deciduous forest predominantly composed of Red Maple (Acer rubrum L.) during the growing season of 2004 and the second half of the growing season of 2005. The magnitudes of the flux estimates were in the range of published results from other micrometeorological mercury fluxes taken above a tall canopy and larger than estimates from flux chambers. The magnitude and direction of the flux were not static during the growing season. There was a significant trend (p < 0.001), from net deposition of TGM in early summer to net evasion in the late summer and early fall before complete senescence. A growing season atmosphere-canopy total mercury (TGM) compensation point during unstable daytime conditions was estimated at background ambient concentrations (1.41 ng m^?3). The trend in the seasonal net TGM flux indicates that long term dry deposition monitoring is needed to accurately estimate mercury loading over a forest ecosystem.