Hg0 Re-emission Research Articles

Mercury in the marine boundary layer and seawater of the South China Sea: Concentrations, sea/air flux, and implication for land outflow

Using R/V Shiyan 3 as a sampling platform, measurements of gaseous elemental mercury (GEM), surface seawater total mercury (THg), methyl mercury (MeHg), and dissolved gaseous mercury (DGM) were carried out above and in the South China Sea (SCS). Measurements were collected for 2 weeks (10 to 28 August 2007) during an oceanographic expedition, which circumnavigated the northern SCS from Guangzhou (Canton), Hainan Inland, the Philippines, and back to Guangzhou. GEM concentrations over the northern SCS ranged from 1.04 to 6.75 ng m−3 (mean: 2.62 ng m−3, median: 2.24 ng m−3). The spatial distribution of GEM was characterized by elevated concentrations near the coastal sites adjacent to mainland China and lower concentrations at stations in the open sea. Trajectory analysis revealed that high concentrations of GEM were generally related to air masses from south China and the Indochina peninsula, while lower concentrations of GEM were related to air masses from the open sea area, reflecting great Hg emissions from south China and Indochina peninsula. The mean concentrations of THg, MeHg, and DGM in surface seawater were 1.2 ± 0.3 ng L−1, 0.12 ± 0.05 ng L−1, and 36.5 ± 14.9 pg L−1, respectively. In general, THg and MeHg levels in the northern SCS were higher compared to results reported from most other oceans/seas. Elevated THg levels in the study area were likely attributed to significant Hg delivery from surrounding areas of the SCS primarily via atmospheric deposition and riverine input, whereas other sources like in situ production by various biotic and abiotic processes may be important for MeHg. Average sea/air flux of Hg in the study area was estimated using a gas exchange method (4.5 ± 3.4 ng m−2 h−1). This value was comparable to those from other coastal areas and generally higher than those from open sea environments, which may be attributed to the reemission of Hg previously transported to this area.

Decline in atmospheric mercury deposition in London

Bulk atmospheric deposition samples were continuously collected using a standard IVL-type mercury (Hg) bulk deposition collector from January 1999 to December 2005 in order to monitor Hg deposition in London. The volume-weighted annual Hg concentrations in deposition gradually declined from 76.0 ng L(-1) in 1999 to 43.8 ng L(-1) in 2005. Correspondingly, Hg fluxes in deposition declined from 45.3 microg m(-2) yr(-1) in 1999 to 15.0 microg m(-2) yr(-1) in 2005. However, this decline in Hg deposition does not agree with trends in UK Hg emissions which are relatively stable over the sampling period. Comparison with contemporaneous data collected at Lochnagar, a remote site in Scotland, suggests that the high Hg concentrations in London deposition are likely to be due to local or regional sources. Surface sediments taken from lakes across London show that the environment has been heavily contaminated by Hg and suggest that Hg re-emission from depositional sinks (e.g. soils, water bodies) may be an important source to London's atmosphere, thereby delaying response to the major reductions in direct emissions to the atmosphere since the 1970s.

Determination of the Potential for Release of Mercury from Combustion Product Amended Soils: Part 1—Simulations of Beneficial Use

This paper describes a project that assessed the potential for mercury (Hg) release to air and water from soil amended with combustion products to simulate beneficial use. Combustion products (ash) derived from wood, sewage sludge, subbituminous coal, and a subbituminous coal-petroleum coke mixture were added to soil as agricultural supplements, soil stabilizers, and to develop lowpermeability surfaces. Hg release was measured from the latter when intact and after it was broken up and mixed into the soil. Air-substrate Hg exchange was measured for all materials six times over 24 hr, providing data that reflected winter, spring, summer, and fall meteorological conditions. Dry deposition of atmospheric Hg and emission of Hg to the atmosphere were both found to be important fluxes. Measured differences in seasonal and diel (24 hr) fluxes demonstrated that to establish an annual estimate of air-substrate flux from these materials data on both of these time steps should be collected. Air-substrate exchange was highly correlated with soil and air temperature, as well as incident light. Hg releases to the atmosphere from coal and wood combustion product-amended soils to simulate an agricultural application were similar to that measured for the unamended soil, whereas releases to the air for the sludge-amended materials were higher. Hg released to soil solutions during the Synthetic Precipitation Leaching Procedure for ash-amended materials was higher than that released from soil alone. On the basis of estimates of annual releases of Hg to the air from the materials used, emissions from coal and wood ash-amended soil to simulate an agricultural application could simply be re-emission of Hg deposited by wet processes from the atmosphere; however, releases from sludge-amended materials and those generated to simulate soil stabilization and disturbed low-permeability pads include Hg indigenous to the material.

Methylmercury in arctic Alaskan mosquitoes: implications for impact of atmospheric mercury depletion events

Environmental context. Recent research suggests that gross mercury deposition in the Arctic is increased significantly as a result of springtime Atmospheric Mercury Depletion Events (AMDE). A primary environmental and human health concern is whether mercury deposited with these events leads to enhanced production and uptake of the toxic methylmercury species in polar ecosystems. Here, we present an initial assessment of potential impact from AMDE utilising mosquitoes as bioindicators of methylmercury accumulation in freshwater and terrestrial food webs within 200 km of the Arctic Ocean. Abstract. Atmospheric Mercury Depletion Events (AMDE) – phenomena in which elemental Hg is oxidised and stripped from the atmosphere over an 8–12-week period following polar sunrise – appear to increase Hg deposition to environs near the Arctic Ocean with a lesser impact inland. A key concern is whether such events lead to enhanced production and uptake of the toxic methylmercury (MeHg) species into arctic food webs. Here, we used mosquitoes, which are sensitive and site-specific bioindicators of Hg loadings, to assess the impact of AMDE on ecosystem MeHg contamination along a 200-km transect between the Arctic Ocean coast and foothills of the Brooks Range, where gross atmospheric Hg deposition appears to be ~20-fold less than that near the coast. This preliminary survey revealed little variation and no gradient in mosquito MeHg levels, which suggests comparable ecosystem impact. This may also point to significant cycling and reemission (e.g. via photoreduction) of Hg deposited during AMDE from the snow and ice pack to the atmosphere.

A comparison of winter mercury accumulation at forested and no-canopy sites measured with different snow sampling techniques

Atmospheric mercury (Hg) is delivered to ecosystems via rain, snow, cloud/fog, and dry deposition. The importance of snow, especially snow that has passed through the forest canopy (throughfall), in delivering Hg to terrestrial ecosystems has received little attention in the literature. The snowpack is a dynamic system that links atmospheric deposition and ecosystem cycling through deposition and emission of deposited Hg. To examine the magnitude of Hg delivery via snowfall, and to illuminate processes affecting Hg flux to catchments during winter (cold season), Hg in snow in no-canopy areas and under forest canopies measured with four collection methods were compared: (1) Hg in wet precipitation as measured by the Mercury Deposition Network (MDN) for the site in Acadia National Park, Maine, USA, (2) event throughfall (collected after snowfall cessation for accumulations of >8 cm), (3) season-long throughfall collected using the same apparatus for event sampling but deployed for the entire cold season, and (4) snowpack sampling. Estimates (mean ± SE) of Hg deposition using these methods during the 91-day cold season in 2004–2005 at conifer sites showed that season-long throughfall Hg flux (1.80 μg/m2) < snowpack Hg (2.38 ± 0.68 μg/m2) < event throughfall flux (5.63 ± 0.38 μg/m2). Mercury deposition at the MDN site (0.91 μg/m2) was similar to that measured at other no-canopy sites in the area using the other methods, but was 3.4 times less than was measured under conifer canopies using the event sampling regime. This indicates that snow accumulated under the forest canopy received Hg from the overstory or exhibited less re-emission of Hg deposited in snow relative to open areas. The soil surface of field-scale plots were sprayed with a natural rain water sample that contained an Hg tracer (202Hg) just prior to the first snowfall to explore whether some snowpack Hg might be explained from soil emissions. The appearance of the 202Hg tracer in the snowpack (0–64% of the total Hg mass in the snowpack) suggests that movement of Hg from the soil into the snowpack is possible. However, as with any tracer study the 202Hg tracer may not precisely represent the reactivity and mobility of natural Hg in soils.

Pilot Plant Testing of Elemental Mercury Reemission from a Wet Scrubber

This paper is to discuss the recent observations of elemental mercury (Hg0) reemissions from a pilot-scale limestone wet scrubber. Simulated flue gas was generated by burning natural gas in a down-fired furnace and doped with 2000 ppm of sulfur dioxide (SO2). Mercuric chloride (HgCl2) solution was delivered to the scrubber at a controlled rate to simulate the absorption of ionized mercury (Hg2+). Testing results have shown that, after Hg2+ was injected, elevated Hg0 concentrations were soon detected both in the scrubber effluent flue gas and the hold tank air, which reflected the occurrence of Hg0 reemissions in both places. When the HgCl2 feed was stopped, the Hg0 reemission continued for more than 2 h. In addition, a significant Hg0 reemission was also detected outside the scrubber loop. In an attempt to understand the Hg0 reemission increase across the wet scrubber system under transient and steady states and to understand the underlying relationship with the mercury complexes retained in the wet scrub...

Laboratory Investigation of the Potential for Re-emission of Atmospherically Derived Hg from Soils

This paper presents data from controlled laboratory experiments focused on investigating the effect of moisture and visible and ultraviolet light on the emission and re-emission of mercury (Hg) from two soils, one with low or background Hg concentrations (14 ng g(-1)) and a soil naturally enriched in Hg (4800 ng g(-1)). Water addition was found to increase emissions from dry soils by an amount greater than that occurring during exposure to PAR or UV-A radiation, whereas UV-B and UV-C exposures facilitated the greatest release. Over all exposures, only a small percentage of Hg(ll) added in a wet spike simulating a precipitation input was released immediately after addition (< 3%). The majority of the Hg being released during all exposures was indigenous and either an original component of the soil or derived from past wet and dry deposition. Under dark and light conditions, elemental Hg was deposited to the dry low Hg-containing soil. On the basis of experimental results, it is hypothesized that dry deposition of gaseous elemental Hg is an important input to low Hg soils and that light, water, and UV-A exposure promote desorption and re-emission of elemental Hg. UV-B exposure is hypothesized to promote indirect photoreduction of Hg(II) existing in the soil. The available pool and the form of Hg in the soil, as well as the chemistry of the soil, will determine the overall flux response to environmental stimulation of emissions.

Evasion of added isotopic mercury from a northern temperate lake

Isotopically enriched Hg (90% 202Hg) was added to a small lake in Ontario, Canada, at a rate equivalent to approximately threefold the annual direct atmospheric deposition rate that is typical of the northeastern United States. The Hg spike was thoroughly mixed into the epilimnion in nine separate events at two-week intervals throughout the summer growing season for three consecutive years. We measured concentrations of spike and ambient dissolved gaseous Hg (DGM) concentrations in surface water and the rate of volatilization of Hg from the lake on four separate, week-long sampling periods using floating dynamic flux chambers. The relationship between empirically measured rates of spike-Hg evasion were evaluated as functions of DGM concentration, wind velocity, and solar illumination. No individual environmental variable proved to be a strong predictor of the evasion flux. The DGM-normalized flux (expressed as the mass transfer coefficient, k) varied with wind velocity in a manner consistent with existing models of evasion of volatile solutes from natural waters but was higher than model estimates at low wind velocity. The empirical data were used to construct a description of evasion flux as a function of total dissolved Hg, wind, and solar illumination. That model was then applied to data for three summers for the experiment to generate estimates of Hg re-emission from the lake surface to the atmosphere. Based on ratios of spike Hg to ambient Hg in DGM and dissolved total Hg pools, ratios of DGM to total Hg in spike and ambient Hg pools, and flux estimates of spike and ambient Hg, we concluded that the added Hg spike was chemically indistinguishable from the ambient Hg in its behavior. Approximately 45% of Hg added to the lake over the summer was lost via volatilization.

Mercury exchange between the atmosphere and low mercury containing substrates

Mercury is emitted to the air from Hg-enriched and low Hg-containing (natural background) substrates. Emitted Hg can be geogenic, or can be derived from the re-emission of Hg that was previously deposited to the soil from the atmosphere. Atmospheric Hg can be derived from natural and/or anthropogenic sources and can be deposited by wet or dry processes. It is important to understand the relative magnitude of emission, deposition, and re-emission of Hg associated with terrestrial ecosystems with natural background soil Hg concentrations because these landscapes cover large terrestrial surface areas. This information is also important for developing biogeochemical mass balances, assessing the impacts of atmospheric Hg sources, and predicting the effectiveness of regulatory controls at local, regional, and global scales. The major focus of this paper is to discuss air–substrate Hg exchange for low Hg-containing soils (<0.1 μg Hg g −1) from two areas in Nevada and one in Oklahoma, USA. Data collected with field and laboratory gas exchange systems are presented. Results indicate that in order to adequately characterize substrate–air Hg exchange, diel and seasonal data must be collected under a variety of environmental conditions. Field and laboratory data showed that dry deposition of gaseous Hg to substrates with low Hg concentrations is an important process. Environmental parameters important in influencing emissions include soil water content, incident light, temperature, atmospheric oxidants, and air Hg concentrations. There are synergistic and antagonistic effects between these parameters complicating prediction of flux.

Role of Moisture in Adsorption, Photocatalytic Oxidation, and Reemission of Elemental Mercury on a SiO2−TiO2 Nanocomposite

A novel silica-titania (SiO2-TiO2) nanocomposite has been developed to effectively capture elemental mercury (Hg0) under UV irradiation. Moisture has been reported to have an important impact on this nanocomposite's performance. In this work, the role of moisture on Hg0 removal and reemission as well as the corresponding mechanisms was investigated. Hg0 removal experiments were carried out in a fixed-bed reactor at 65 degrees C using air as the carrier gas. Without UV irradiation, Hg0 adsorption was found to be insignificant, but it could be enhanced by the photocatalytic oxidation product, mercuric oxide (HgO), possibly due to the high affinity between HgO and Hg0. Under dry conditions 95% of Hg0 can be removed; however, increased humidity levels remarkably suppress both Hg0 adsorption and photocatalytic oxidation. Introducing water vapor can also result in significant reemission of captured Hg0 from the nanocomposite, which may be ascribed to the repellant effect of water vapor adsorbed on the superhydrophilic TiO2 surface. Exposure to UV light was found either to prohibit Hg0 reemission when photocatalytic oxidation of reemitted Hg0 prevailed or to promote Hg0 reemission when photocatalytic reduction of HgO to Hg0 dominated later on. The results indicate that minimization of Hg0 reemission can be achieved by appropriate application of UV irradiation.

Foliar Mercury Accumulation and Exchange for Three Tree Species

The goals of this study were to (1) investigate plant mercury (Hg) uptake using different air and soil Hg concentrations near natural background values for three tree species, and (2) test if measured foliar Hg fluxes could explain observed foliar Hg concentrations. Plants were exposed to three soil treatments (<0.01, 0.09 +/- 0.02, and 0.92 +/- 0.27 microg Hg g(-1)), and to three atmospheric exposure concentrations (5.9 +/- 2.3, 14.3 +/- 2.7, and 30.1 +/- 3.5 ng Hg m(-3)). Foliar Hg concentrations were found to be influenced primarily by atmospheric Hg concentrations and to a lesser extent by soil Hg exposures. Data indicated that deciduous species might play a more active role in ecosystem Hg cycling than evergreen trees. Foliar mercury fluxes quantified using a dynamic single-plant gas-exchange chamber for two species were variable and accumulation rates were lower than those predicted based on foliar Hg concentrations. A hypothesis to explain this discrepancy is that the plant gas-exchange chamber measures net flux which includes emission, deposition, adsorption, and reemission of Hg at the leaf surface, while total foliar accumulation represents only deposition and assimilation.

Investigation of Mercury Exchange between Forest Canopy Vegetation and the Atmosphere Using a New Dynamic Chamber

This paper presents the design of a dynamic chamber system that allows full transmission of PAR and UV radiation and permits enclosed intact foliage to maintain normal physiological function while Hg(0) flux rates are quantified in the field. Black spruce and jack pine foliage both emitted and absorbed Hg(0), exhibiting compensation points near atmospheric Hg(0) concentrations of approximately 2-3 ng m(-3). Using enriched stable Hg isotope spikes, patterns of spike Hg(ll) retention on foliage were investigated. Hg(0) evasion rates from foliage were simultaneously measured using the chamber to determine if the decline of foliar spike Hg(II) concentrations over time could be explained by the photoreduction and re-emission of spike Hg to the atmosphere. This mass balance approach suggested that spike Hg(0) fluxes alone could not account for the measured decrease in spike Hg(II) on foliage following application, implying that eitherthe chamber underestimates the true photoreduction of Hg(ll) to Hg(0) on foliage, or other mechanisms of Hg(II) loss from foliage, such as cuticle weathering, are in effect. The radiation spectrum responsible for the photoreduction of newly deposited Hg(II) on foliage was also investigated. Our spike experiments suggest that some of the Hg(ll) in wet deposition retained by the forest canopy may be rapidly photoreduced to Hg(0) and re-emitted back to the atmosphere, while another portion may be retained by foliage at the end of the growing season, with some being deposited in litterfall. This finding has implications for the estimation of Hg dry deposition based on throughfall and litterfall fluxes.

A numerical modelling study on regional mercury budget for eastern North America

Abstract. In this study, we have integrated an up-to-date physio-chemical transformation mechanism of Hg into the framework of US EPA's CMAQ model system. In addition, the model adapted detailed calculations of the air-surface exchange for Hg to properly describe Hg re-emissions and dry deposition from and to natural surfaces. The mechanism covers Hg in three categories, elemental Hg (Hg0), reactive gaseous Hg (RGM) and particulate Hg (HgP). With interfacing to MM5 (meteorology processor) and SMOKE (emission processor), we applied the model to a 4-week period in June/July 1995 on a domain covering most of eastern North America. Results indicate that the model simulates reasonably well the levels of total gaseous Hg (TGM) and the specific Hg wet deposition measurements made by the Hg deposition network (MDN). Moreover, results from various scenario runs reveal that the Hg system behaves in a closely linear way in terms of contributions from different source categories, i.e. anthropogenic emissions, natural re-emissions and background. Analyses of the scenario results suggest that 37% of anthropogenically emitted Hg was deposited back in the model domain with 5155 kg of anthropogenic Hg moving out of the domain during the simulation period. Overall, the domain served as a net source, which supplied ~a half ton of Hg to the global background pool over the period. Our model validation and a sensitivity test further rationalized the rate constant for gaseous oxidation of Hg0 by hydroxyl radical OH used in the global scale modelling study by Bergan and Rodhe (2001). A further laboratory determination of the reaction rate constant, including its temperature dependence, stands as one of the important issues critical to improving our knowledge on the budget and cycling of Hg.

Updating global Hg emissions from small-scale gold mining and assessing its environmental impacts

Small-scale gold mining is a significant source of Hg to the environment and may reach an annual input of about 450 t of Hg, mostly in South America, Russia and Asia. Countries of the Amazon Basin and Russia are the major contributors. On a regional scale, this source can contribute to more than 50% of the total Hg anthropogenic emission. Because of amalgam burning and bullion smelting under generally uncontrolled conditions, the majority of the emission is to the atmosphere. Therefore, on a global scale small-scale gold mining contributes about 20% of the total Hg atmospheric emissions from human activities. Most emissions occur in tropical developing countries. Preliminary results from research in the Amazon region suggest the Hg cycling in tropical rainforest environments is probably more dynamic and critical than in previously studied temperate environments, being strongly influenced by the existing ecological cycles and affected by land-use changes. At least two components of the Hg cycle can be identified. A short-term component, involving methylation of deposited Hg2+ in aquatic systems and a long-term component, involving the re-emission of Hg deposited in soils particularly because of changes in soil use.

Development of an automated micrometeorological method for measuring the emission of mercury vapor from wetland vegetation

The ability of green plants to act as conduits to enhance the transport ofHg from soils to the atmosphere is now established, but the data base isseverely limited. The potential role of this process in mobilizing Hg inglobal and regional cycles makes it imperative that automated methods bedeveloped to increase our capability to measure and understand the processin a variety of ecosystems. We previously published a tower-basedmicrometeorological gradient method for measuring gas-phase Hg°fluxes in terrestrial systems based on the Modified Bowen ratio (MBR)approach. The method relied on demanding and time-consuming manualsampling of Hg gradients. Automated Hg sampling methods now exist, andwe describe here applications of the Tekran Hg analyzer to automatednear-real-time measurements of Hg gradients over wetland vegetation. Weuse these data with MBR to compute fluxes of Hg from those of othertrace gases. From 1996 to 1998 we sampled Hg fluxes over emergentmacrophytes in the Florida Everglades, beginning with manual methods, butlater deploying automated methods for most of the study to collect morethan 500 30-min fluxes over 2 y. The limitations of the manual methodresulted in considerable uncertainly in our earlier observations, even to theextent that we initially doubted that vegetation emissions were real. However, the automated method allowed us to quantify the actualdevelopment of Hg emission gradients over wetland vegetation. Followingsunrise Hg fluxes show diel patterns similar to those of CO2 andH2O, providing information on the possible mechanisms of Hgemission. Our data suggest mean daytime emission rates of Hg over thesewetlands on the order of 30 ng m-2 h-1. Fluxes wereinfluenced by temperature, solar radiation, and atmospheric turbulence. There exists a significant biotic re-emission of Hg° from the oceans,and our data provide the first direct evidence of a similar process insubtropical wetlands.

A regional scale modeling study of atmospheric transport and transformation of mercury. II. Simulation results for the northeast United States

This paper presents the results of a simulation study of the transport, transformation, and deposition of atmospheric mercury (Hg) in the northeastern United States for 5 consecutive days in summer 1997, using the newly developed regional scale air quality model described in part I. Hourly ambient concentrations, in-cloud transformations, and deposition fluxes were predicted for each of the three mercury species: Hg(0), Hg(II), and Hg(p). The simulation results showed that surface level Hg concentrations over the land varied diurnally with the boundary layer structure. The Hg concentrations over the coastal ocean water were strongly influenced by the concentrations over land because of atmospheric advection by the prevailing westerly winds in the region. Over 90% of the ambient air concentration of Hg was in the form of Hg(0) vapor. Approximately half of the total Hg in wet deposition originated from ambient Hg(0), while the predominant species in dry deposition was Hg(II). The simulation predicted more dry deposition ( 31 kg day −1 ) than wet deposition ( 18 kg day −1 ) over the region for the simulation period. The re-emission of Hg(0) from land and water surfaces ( 46 kg day −1 ) was of the same magnitude as the total anthropogenic emission ( 55 kg day −1 ) within the region. Both precipitating and non-precipitating clouds were shown to be important media for in-cloud transformation of Hg(0). It was predicted that transformation and redistribution caused a net gain of Hg(p) at the upper troposphere during rain events. Over the region of simulation, the amount of Hg(0) transformation in non-precipitating clouds was 30% higher than in precipitating clouds. In co-existing non-precipitating clouds, the amount of Hg(0) transformation in the lower and upper troposphere was approximately equal.

99/03689 Manufacture of artificial aggregate from fly ash: Mori, M. et al. Jpn. Kokai Tokkyo Koho JP 11 116,300 [99 116,300] (Cl. C04B18/08), 27 Apr 1999, Appl. 97/282,290, 15 Oct 1997, 4 pp. (In Japanese)

During combustion of pulverized coals, most of the mercury (Hg) in coal volatilizes. With the wide use of dry and wet scrubber systems for flue gas desulfurization (FGD), various amounts of Hg are captured by coal combustion byproducts, such as, FGD gypsum. The specification of Hg in FGD gypsum is essential not only to determine the risk when the wastes are recycled or disposed but also to understand the behavior of Hg during coal combustion and the mechanisms of Hg oxidation along the flue gas path. In this study, a temperature-programmed decomposition technique was used in order to acquire an understanding of the Hg species associated with FGD gypsum. A series of Hg reference compounds were used to obtain the characteristic temperature of decomposition for each Hg compound. The decomposition temperature of the Hg species is in an increasing order as Hg2SO4 < Hg2Cl2 < HgCl2 < black HgS < HgO < red HgS < HgSO4. The results also indicate that HgCl2 and HgS are the primary Hg compounds in FGD gypsum samples. Hg stability during the reutilization of FGD gypsum is important due to its health and environmental impact, and its value in improving the Hg emission inventory. The industrial calcining process of wallboard production is simulated in order to determine the Hg emission percentage during the reutilization of FGD gypsum. Results indicate that 12.1–55.1% of total Hg would be emitted from FGD gypsum in this process. A larger percentage of Hg will be emitted when the proportion of Hg2Cl2 and HgCl2 is higher. The Hg re-emissions from FGD gypsum during wallboard production in China were estimated to be 4.7 tons in 2008.

99/00039 Experimental and theoretical studies of catalytic ignition of coal

The behavior of mercury across the flue gas path was investigated in order to obtain a comprehensive understanding of the reactions regarding mercury. An improved high-dust honeycomb selective catalytic reduction (SCR) catalyst was compared with a conventional one regarding its mercury oxidation at different gas hydrogen chloride concentrations and ammonia dosing levels. The measurements were taken in a lab-scale firing system using coal derived flue gas to check the influence of other gas components and competing reactions. The diminishing effect of ammonia presence due to the compatible reactions could be shown in the measurements of both catalysts. On the other hand presence of ammonia increases mercury removal with fly ash. The correlation of mercury adsorption with the unburned carbon of fly ash was shown. Mercury inventory in the slurry of flue gas desulfurization (FGD) was higher when more oxidized mercury was present in the flue gas, which is shown through the mercury content of the gypsum and filtrate in each measurement. Ammonia containing flue gas resulted in higher Hg re-emission from slurry. It was observed that the mercury oxidation was higher by using the improved catalyst, which enhances the total mercury removal efficiency in FGD downstream of the catalyst.

Air/surface exchange of mercury vapor over forests—the need for a reassessment of continental biogenic emissions

Atmospheric sources are significant in the cycling of Hg in the biosphere, but there are few reliable data on air/surface exchange of Hg in terrestrial systems. We developed a tower-based micrometeorological gradient method for measuring gas-phase Hg° fluxes over soils and vegetation. We describe here results of the modified Bowen ratio approach from three separate flux sampling campaigns: over a mature deciduous forest at the Walker Branch Watershed in Tennessee, over a young pine plantation in Tennessee, and over the boreal forest floor at the Lake Gårdsjön watershed in Sweden. Our data show that Hg° exchange over these surfaces is bidirectional, but is primarily characterized by emissions from plants and soil. Dry deposition (foliar uptake) is less frequent, of generally lower magnitude, and may be enhanced by surface wetness. We measured emissions over tree canopies in Tennessee in the range of ∼ 10–300 ng m −2 h −1, and over the boreal forest floor in Sweden of ∼ 1–4 ng m −2 h −1. Fluxes were influenced by temperature, solar radiation, and atmospheric turbulence. The ability of trees to emit Hg° from soil pools has now been established. Others have proposed a significant biotic re-emission of Hg° from the oceans, and our data provide the first direct evidence of a similar process in terrestrial systems. These data have been combined with results from chamber studies to estimate the overall flux of gas-phase Hg° between the atmosphere and terrestrial systems. Transpiration of Hg° represents a previously unmeasured mobilization of Hg from the continents to the troposphere. Including this new source term could increase current estimates of so-called natural emissions by over 100%.

Modeling atmospheric concentrations of mercury and deposition to the great lakes

Regional patterns of seasonal and annual average air concentrations and cumulative deposition of mercury to the Great Lakes basin are calculated with the ASTRAP model. The model treats the Hg system as three chemical components: elemental (Hg 0), particulate (Hg-p), and gaseous divalent (Hg-II). Primary anthropogenic emission inventories (i.e. emissions resulting from current activities) include surface and elevated sources of each of the three Hg species for eastern North America. Natural and secondary anthropogenic: emissions (i.e. reemission of Hg deposited or released during earlier anthropogenic activity) over the United States and Canada are estimated by defining an emission term for Hg' that varies with latitude and season. Global background concentrations of Hg 0 and Hg-p are specified to average annually 1.0 and 0.01 ng m −3, respectively. Rates of parameterizations of wet and dry removal are very rapid for Hg-II, intermediate for Hg-p, and very slow for Hg 0. Because of the disparate removal efficiencies, estimates of deposition resulting from anthropogenic emissions are critically dependent upon the speciation of emissions and, for the rapidly depositing Hg-II, the assumptions about effective stack heights. Integration of the Hg deposition field over the Great Lakes produces estimates of direct atmospheric loading from primary anthropogenic emissions of 1.44 and 2.46 t Hg yr −1 by wet and dry deposition, respectively. Estimates of direct loading from natural and secondary anthropogenic emissions of Hg 0 over the continent are 0.09 and 0.15 t Hg yr −1 for wet and dry deposition, respectively, while the corresponding contributions from the global background are estimated to be 0.15 and 0.39 t Hg yr −1, respectively. Although they constitute only 16% of estimated anthropogenic emissions, emissions of Hg-II contribute 78% of the current direct anthropogenic deposition to the Lakes, or 65% of the deposition of Hg from all sources. Revolatilization of Hg 0 from the Lakes is estimated to lie between 2.3 and 13.7 t Hg yr −1. Thus, revolatilization may well be greater than the direct atmospheric loading of all Hg species to the Lakes, 4.7 t Hg yr −1.