Development and application of a regional-scale atmospheric mercury model based on WRF/Chem: a Mediterranean area investigation

The emission, transport, deposition and eventual fate of mercury (Hg) in the Mediterranean area has been studied using a modified version of the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem). This model version has been developed specifically with the aim to simulate the atmospheric processes determining atmospheric Hg emissions, concentrations and deposition online at high spatial resolution. For this purpose, the gas phase chemistry of Hg and a parametrised representation of atmospheric Hg aqueous chemistry have been added to the regional acid deposition model version 2 chemical mechanism in WRF/Chem. Anthropogenic mercury emissions from the Arctic Monitoring and Assessment Programme included in the emissions preprocessor, mercury evasion from the sea surface and Hg released from biomass burning have also been included. Dry and wet deposition processes for Hg have been implemented. The model has been tested for the whole of 2009 using measurements of total gaseous mercury from the European Monitoring and Evaluation Programme monitoring network. Speciated measurement data of atmospheric elemental Hg, gaseous oxidised Hg and Hg associated with particulate matter, from a Mediterranean oceanographic campaign (June 2009), has permitted the model’s ability to simulate the atmospheric redox chemistry of Hg to be assessed. The model results highlight the importance of both the boundary conditions employed and the accuracy of the mercury speciation in the emission database. The model has permitted the reevaluation of the deposition to, and the emission from, the Mediterranean Sea. In light of the well-known high concentrations of methylmercury in a number of Mediterranean fish species, this information is important in establishing the mass balance of Hg for the Mediterranean Sea. The model results support the idea that the Mediterranean Sea is a net source of Hg to the atmosphere and suggest that the net flux is ≈30 Mg year^?1 of elemental Hg.     

A reactive transport model for mercury fate in soil—application to different anthropogenic pollution sources

Soil systems are a common receptor of anthropogenic mercury (Hg) contamination. Soils play an important role in the containment or dispersion of pollution to surface water, groundwater or the atmosphere. A one-dimensional model for simulating Hg fate and transport for variably saturated and transient flow conditions is presented. The model is developed using the HP1 code, which couples HYDRUS-1D for the water flow and solute transport to PHREEQC for geochemical reactions. The main processes included are Hg aqueous speciation and complexation, sorption to soil organic matter, dissolution of cinnabar and liquid Hg, and Hg reduction and volatilization. Processes such as atmospheric wet and dry deposition, vegetation litter fall and uptake are neglected because they are less relevant in the case of high Hg concentrations resulting from anthropogenic activities. A test case is presented, assuming a hypothetical sandy soil profile and a simulation time frame of 50 years of daily atmospheric inputs. Mercury fate and transport are simulated for three different sources of Hg (cinnabar, residual liquid mercury or aqueous mercuric chloride), as well as for combinations of these sources. Results are presented and discussed with focus on Hg volatilization to the atmosphere, Hg leaching at the bottom of the soil profile and the remaining Hg in or below the initially contaminated soil layer. In the test case, Hg volatilization was negligible because the reduction of Hg²⁺ to Hg⁰ was inhibited by the low concentration of dissolved Hg. Hg leaching was mainly caused by complexation of Hg²⁺ with thiol groups of dissolved organic matter, because in the geochemical model used, this reaction only had a higher equilibrium constant than the sorption reactions. Immobilization of Hg in the initially polluted horizon was enhanced by Hg²⁺ sorption onto humic and fulvic acids (which are more abundant than thiols). Potential benefits of the model for risk management and remediation of contaminated sites are discussed.     

Estimating contribution of precipitation scavenging of atmospheric particulate mercury to mercury wet deposition in Japan

Mercury wet deposition is dependent on both the scavenging of divalent reactive gaseous mercury (RGM) and atmospheric particulate mercury (Hg(p)) by precipitation. Estimating the contribution of precipitation scavenging of RGM and Hg(p) is important for better understanding the causes of the regional and seasonal variations in mercury wet deposition. In this study, the contribution of Hg(p) scavenging was estimated on the basis of the scavenging ratios of other trace elements (i.e., Cd, Cu, Mn, Ni, Pb and V) existing entirely in particulate form. Their wet deposition fluxes and concentrations in air, which were measured concurrently from April 2004 to March 2005 at 10 sites in Japan, were used in this estimation. The monthly wet deposition flux of mercury at each site correlated with the amount of monthly precipitation, whereas the Hg(p) concentrations in air tended to decrease during summer. There was a significant correlation (P<0.001) among the calculated monthly average scavenging ratios of trace elements, and the values in each month at each site were similar. Therefore, it is assumed the monthly scavenging ratio of Hg(p) is equivalent to the mean value of other trace elements. Using this scavenging ratio (W), the wet deposition flux (F) due to Hg(p) scavenging in each month was calculated by F=WKP, where K and P are the Hg(p) concentration and amount of precipitation, respectively. Relatively large fluxes due to Hg(p) scavenging were observed at a highly industrial site and at sites on the Japan Sea coast, which are strongly affected by the local sources and the long-range transport from the Asian continent, respectively. However, on average, at the 10 sites, the contribution of Hg(p) scavenging to the annual mercury deposition flux was 26%, suggesting that mercury wet deposition in Japan is dominated by RGM scavenging. This RGM should originate mainly from the in situ oxidation of Hg⁰ in the atmosphere.     

A review of current knowledge concerning dry deposition of atmospheric mercury

The status of the current knowledge concerning the dry deposition of atmospheric mercury, including elemental gaseous mercury (Hg⁰), reactive gaseous mercury (RGM), and particulate mercury (Hg_p), is reviewed. The air–surface exchange of Hg⁰ is commonly bi-directional, with daytime emission and nighttime deposition over non-vegetated surfaces and vegetated surfaces with small leaf area indices under low ambient Hg⁰ conditions. However, daytime deposition has also been observed, especially when the ambient Hg⁰ is high. Typical dry deposition velocities (V_d) for Hg⁰ are in the range of 0.1–0.4 cm s^?1 over vegetated surfaces and wetlands, but substantially smaller over non-vegetated surfaces and soils below canopies. Meteorological, biological, and soil conditions, as well as the ambient Hg⁰ concentrations all play important roles in the diurnal and seasonal variations of Hg⁰ air–surface exchange processes. Measurements of RGM deposition are limited and are known to have large uncertainties. Nevertheless, all of the measurements suggest that RGM can deposit very quickly onto any type of surface, with its V_d ranging from 0.5 to 6 cm s^?1. The very limited data for Hg_p suggest that its V_d values are in the range of 0.02–2 cm s^?1.A resistance approach is commonly used in mercury transport models to estimate V_d for RGM and Hg_p; however, there is a wide range of complexities in the dry deposition scheme of Hg⁰. Although resistance-approach based dry deposition schemes seem to be able to produce the typical V_d values for RGM and Hg⁰ over different surface types, more sophisticated air–surface exchange models have been developed to handle the bi-directional exchange processes. Both existing and newly developed dry deposition schemes need further evaluation using field measurements and intercomparisons within different modelling frameworks.     

Atmospheric mercury species in the European Arctic: Measurements and modelling

Concentrations of different species of mercury in arctic air and precipitation have been measured at Ny-Ålesund (Svalbard) and Pallas (Finland) during 1996–1997. Typical concentrations for vapour phase mercury measured at the two stations were in the range of 0.7–2 ng m⁻³ whereas particulate mercury concentrations were below 5 pg m⁻³. Total mercury in precipitation was in the range 3–30 ng l⁻¹. In order to evaluate the transport and deposition of mercury to the arctic from European anthropogenic sources, the Eulerian transport model HMET has been modified and extended to also include mercury species. A scheme for chemical conversion of elemental mercury to other species of mercury and deposition characteristics of different mercury species have been included in the model. European emission inventories for three different forms of Hg (Hg⁰, HgCl₂ and Hg_p) have been implemented in the numerical grid system for the HMET model.     

Scientific uncertainties in atmospheric mercury models II: Sensitivity analysis in the CONUS domain

In this study, we present the response of model results to different scientific treatments in an effort to quantify the uncertainties caused by the incomplete understanding of mercury science and by model assumptions in atmospheric mercury models. Two sets of sensitivity simulations were performed to assess the uncertainties using modified versions of CMAQ-Hg in a 36-km Continental United States domain. From Set 1 Experiments, it is found that the simulated mercury dry deposition is most sensitive to the gaseous elemental mercury (GEM) oxidation product assignment, and to the implemented dry deposition scheme for GEM and reactive gaseous mercury (RGM). The simulated wet deposition is sensitive to the aqueous Hg(II) sorption scheme, and to the GEM oxidation product assignment. The inclusion of natural mercury emission causes a small increase in GEM concentration but has little impact on deposition. From Set 2 Experiments, it is found that both dry and wet depositions are sensitive to mercury chemistry. Change in model mercury chemistry has a greater impact on simulated wet deposition than on dry deposition. The kinetic uncertainty of GEM oxidation by O₃ and mechanistic uncertainty of Hg(II) reduction by aqueous HO₂ pose the greatest impact. Using the upper-limit kinetics of GEM–O₃ reaction or eliminating aqueous Hg(II)–HO₂ reaction results in unreasonably high deposition and depletion of gaseous mercury in the domain. Removing GEM–OH reaction is not sufficient to balance the excessive mercury removal caused by eliminating the HO₂ mechanism. Field measurements of mercury dry deposition, better quantification of mercury air-surface exchange and further investigation of mercury redox chemistry are needed for reducing model uncertainties and for improving the performance of atmospheric mercury models.     

Spatial patterns in wet and dry deposition of atmospheric mercury and trace elements in central Illinois,USA

An intensive 1-month atmospheric sampling campaign was conducted concurrently at eight monitoring sites in central Illinois, USA, from June 9 to July 3, 2011 to assess spatial patterns in wet and dry deposition of mercury and other trace elements. Summed wet deposition of mercury ranged from 3.1 to 5.4 μg/m² across sites for the total study period, while summed dry deposition of reactive mercury (gaseous oxidized mercury plus particulate bound mercury) ranged from 0.7 to 1.6 μg/m², with no statistically significant differences found spatially between northern and southern sites. Ratios of summed wet to summed dry mercury deposition across sites ranged from 2.2 to 4.9 indicating that wet deposition of mercury was dominant during the study period. Volume-weighted mean mercury concentrations in precipitation were found to be significantly higher at northern sites, while precipitation depth was significantly higher at southern sites. These results showed that substantial amounts of mercury deposition, especially wet deposition, occurred during the study period relative to typical annual wet deposition levels. Summed wet deposition of anthropogenic trace elements was much higher, compared to summed dry deposition, for sulfur, selenium, and copper, while at some sites summed dry deposition dominated summed wet deposition for lead and zinc. This study highlights that while wet deposition of Hg was dominant during this spring/summer-season study, Hg dry deposition also contributed an important fraction and should be considered for implementation in future Hg deposition monitoring studies.     

Characteristics of mercury exchange flux between soil and air in the heavily air-polluted area,eastern Guizhou,China

To investigate the characteristics of mercury exchange between soil and air in the heavily air-polluted area, total gaseous mercury (TGM) concentration in air and Hg exchange flux were measured in Wanshan Hg mining area (WMMA) in November, 2002 and July–August, 2004. The results showed that the average TGM concentrations in the ambient air (17.8–1101.8 ng m⁻³), average Hg emission flux (162–27827 ng m⁻² h⁻¹) and average Hg dry deposition flux (0–9434 ng m⁻² h⁻¹) in WMMA were 1–4 orders of magnitude higher than those in the background area. It is said that mercury-enriched soil is a significant Hg source of the atmosphere in WMMA. It was also found that widely distributed roasted cinnabar banks are net Hg sources of the atmosphere in WMMA. Relationships between mercury exchange flux and environmental parameters were investigated. The results indicated that the rate of mercury emission from soil could be accelerated by high total soil mercury concentration and solar irradiation. Whereas, highly elevated TGM concentrations in the ambient air can restrain Hg emission from soil and even lead to strongly atmospheric Hg deposition to soil surface. A great amount of gaseous mercury in the heavily polluted atmosphere may cycle between soil and air quickly and locally. Vegetation can inhibit mercury emission from soil and are important sinks of atmospheric mercury in heavily air-polluted area.     

Airborne Emissions of Mercury from Municipal Solid Waste. I: New Measurements from Six Operating Landfills in Florida

Abstract

Mercury-bearing material enters municipal landfills from a wide array of sources, including fluorescent lights, batteries, electrical switches, thermometers, and general waste; however, the fate of mercury (Hg) in landfills has not been widely studied. Using automated flux chambers and downwind atmospheric sampling, we quantified the primary pathways of Hg vapor releases to the atmosphere at six municipal landfill operations in Florida. These pathways included landfill gas (LFG) releases from active vent systems, passive emissions from landfill surface covers, and emissions from daily activities at each working face (WF). We spiked the WF at two sites with known Hg sources; these were readily detected downwind, and were used to test our emission modeling approaches. Gaseous elemental mercury (Hg⁰) was released to the atmosphere at readily detectable rates from all sources measured; rates ranged from ～1–10 ng m^?2 hr^?1 over aged landfill cover, from ～8–20 mg/hr from LFG flares (LFG included Hg⁰ at μg/m³ concentrations), and from ～200–400 mg/hr at the WF. These fluxes exceed our earlier published estimates. Attempts to identify specific Hg sources in excavated and sorted waste indicated few readily identifiable sources; because of effective mixing and diffusion of Hg⁰, the entire waste mass acts as a source. We estimate that atmospheric Hg releases from municipal landfill operations in the state of Florida are on the order of 10–50 kg/yr, substantially larger than our original estimates, but still a small fraction of current overall anthropogenic losses.     

Wet deposition mercury fluxes in the Canadian sub-Arctic and southern Alberta,measured using an automated precipitation collector adapted to cold regions

This paper reports mercury (Hg) concentrations and fluxes in precipitation that was collected from 2006 to 2008 at three sites in Canada: sub-Arctic boreal forest, sub-Arctic coast, and southern Alberta, using cold-adapted precipitation collectors which operated reliably at temperatures below ?30 °C during the study. The southern Alberta site (Crossfield) may be influenced by Calgary urban air, whereas the sub-Arctic coastal (Churchill, Manitoba) and boreal forest (Fort Vermilion, Alberta) sites are in more remote northern areas. Annual mean Hg concentrations in precipitation (5.0–9.2 ng L^?1) at the study sites were in the lower half of the range reported for southern Canada and the USA by the Mercury Deposition Network (MDN). But owing to typically low precipitation rates, gross wet Hg fluxes (0.54–2.0 μg m^?2 yr^?1) were among the lowest reported by MDN, with Crossfield having about twice the flux in 2007 of the other two sites. Flux was significantly correlated with precipitation, and thus was highest in summer (June–August) and lowest during winter, a pattern typical of other temperate continental locations. There was no evidence of higher wet Hg fluxes or concentrations in springtime at Churchill where atmospheric mercury depletion events (AMDEs) occur. Measured gross deposition fluxes at the study locations were ～2–8 times lower than estimated by GEOS-Chem and GRAHM atmospheric models. The largest discrepancy occurred for Churchill, which raises the question of how well Hg deposition from AMDEs is described by current models. Better agreement between measurements and models was obtained from MDN stations in Alberta and Alaska, where wet Hg fluxes were 2–10 times higher than the study sites either because of power plant emissions (Alberta), or because of high precipitation rates (Alaska).     

The influence of ozone on atmospheric emissions of gaseous elemental mercury and reactive gaseous mercury from substrates

Experiments were performed to investigate the effect of ozone (O₃) on mercury (Hg) emission from a variety of Hg-bearing substrates. Substrates with Hg(II) as the dominant Hg phase exhibited a 1.7 to 51-fold increase in elemental Hg (Hg^o) flux and a 1.3 to 8.6-fold increase in reactive gaseous mercury (RGM) flux in the presence of O₃-enriched clean (50 ppb O₃; 8 substrates) and ambient air (up to ∼70 ppb O₃; 6 substrates), relative to clean air (oxidant and Hg free air). In contrast, Hg^o fluxes from two artificially Hg^o-amended substrates decreased by more than 75% during exposure to O₃-enriched clean air relative to clean air. Reactive gaseous mercury emissions from Hg^o-amended substrates increased immediately after exposure to O₃ but then decreased rapidly. These experimental results demonstrate that O₃ is very important in controlling Hg emissions from substrates. The chemical mechanisms that produced these trends are not known but potentially involve heterogenous reactions between O₃, the substrate, and Hg. Our experiments suggest they are not homogenous gas-phase reactions. Comparison of the influence of O₃ versus light on increasing Hg^o emissions from dry Hg(II)-bearing substrates demonstrated that they have a similar amount of influence although O₃ appeared to be slightly more dominant. Experiments using water-saturated substrates showed that the presence of high-substrate moisture content minimizes reactions between atmospheric O₃ and substrate-bound Hg. Using conservative calculations developed in this paper, we conclude that because O₃ concentrations have roughly doubled in the last 100 years, this could have increased Hg^o emissions from terrestrial substrates by 65–72%.     

Aqueous phase methylation as a potential source of methylmercury in wet deposition

The source of monomethylmercury (MMHg) in wet deposition is unknown. Volatilization of gaseous MMHg, evasion and demethylation of dimethylmercury, and methylation of Hg⁰ have been either proposed or tested unsuccessfully as potential sources. Here, we show that MMHg in precipitation, sampled across a wide geographical range in North America, is related positively to an operationally defined and measured reactive Hg species (Hg_R), but connected weakly to total Hg. The mean molar ratio of MMHg:Hg_R measured in continental precipitation (0.025±0.006) is comparable to the MMHg:Hg(II) ratio estimated from first-order rate constants for acetate-mediated Hg methylation and MMHg photolysis (0.025±0.002). This suggests MMHg may be formed in the atmosphere through a reaction between labile Hg(II) complexes and an unknown methylating agent(s), potentially acetate or similar molecules. Availability of Hg(II) appears to limit the reaction, and accordingly, increased atmospheric loadings of Hg could lead to enhanced MMHg in precipitation.     

Mercury chemistry in the MBL: Mediterranean case and sensitivity studies using the AMCOTS (Atmospheric Mercury Chemistry over the Sea) model

The atmospheric oxidation of mercury in the Mediterranean marine boundary layer (MBL) has been studied using the Atmospheric Mercury Chemistry over the Sea (AMCOTS) model. The model results have been compared to measured data obtained during an oceanographic research campaign in 2000, with more success than previous modelling attempts. In light of the often high concentrations of ozone present in the Mediterranean boundary layer, seasonal case studies using typical meteorological conditions and average ozone concentrations have been performed to identify the main oxidants of elemental mercury. The sensitivity of the modelled reactive gaseous mercury (RGM) concentrations to the Hg+O₃ rate constant has been assessed using the two most recent rate determinations. The results using the higher of the two literature values gives results inconsistent with measured values of RGM when the reaction between Hg and O₃ is assumed to give a gas phase product. This does not necessarily indicate that the rate constant is incorrect but possibly that other rate constants in the model are overestimated or indeed that there may be reduction reactions occurring in the atmosphere which have yet to be identified. Alternatively, when the reaction product of Hg and O₃ is assumed to be a solid and therefore not contribute to RGM the modelled and measured results are comparable. The deposition rates calculated by the model when compared with calculated and measured sea surface emission fluxes available in the literature indicate that dry deposition flux of RGM is comparable to the sea surface emission flux. The calculated lifetime of Hg⁰ in the Mediterranean MBL is between one and two weeks.     

Sources and deposition of reactive gaseous mercury in the marine atmosphere

Observations of reactive gaseous mercury (RGM) in marine air show a consistent diurnal cycle with minimum at night, rapid increase at sunrise, maximum at midday, and rapid decline in afternoon. We use a box model for the marine boundary layer (MBL) to interpret these observations in terms of RGM sources and sinks. The morning rise and midday maximum are consistent with oxidation of elemental mercury (Hg⁰) by Br atoms, requiring <2 ppt BrO in most conditions. Oxidation of Hg⁰ by Br accounts for 35–60% of the RGM source in our model MBL, with most of the remainder contributed by oxidation of Hg⁰ by ozone (5–20%) and entrainment of RGM-rich air from the free troposphere (25–40%). Oxidation of Hg⁰ by Cl is minor (3–7%), and oxidation by OH cannot reproduce the observed RGM diurnal cycle, suggesting that it is unimportant. Fitting the RGM observations could be achieved in the model without oxidation of Hg⁰ by ozone (leaving Br as the only significant oxidant) by increasing the entrainment flux from the free troposphere. The large relative diurnal amplitude of RGM concentrations implies rapid loss with a lifetime of only a few hours. We show that this can be quantitatively explained by rapid, mass-transfer-limited uptake of RGM into sea-salt aerosols as HgCl₃^? and HgCl₄^2?. Our results suggest that 80–95% of Hg^II in the MBL should be present in sea-salt aerosol rather than gas-phase, and that deposition of sea-salt aerosols is the major pathway delivering Hg^II to the ocean.     

A diagnostic evaluation of modeled mercury wet depositions in Europe using atmospheric speciated high-resolution observations

This study is part of the Global Mercury Observation System (GMOS), a European FP7 project dedicated to the improvement and validation of mercury models to assist in establishing a global monitoring network and to support political decisions. One key question about the global mercury cycle is the efficiency of its removal out of the atmosphere into other environmental compartments. So far, the evaluation of modeled wet deposition of mercury was difficult because of a lack of long-term measurements of oxidized and elemental mercury. The oxidized mercury species gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) which are found in the atmosphere in typical concentrations of a few to a few tens pg/m³ are the relevant components for the wet deposition of mercury. In this study, the first European long-term dataset of speciated mercury taken at Waldhof/Germany was used to evaluate deposition fields modeled with the chemistry transport model (CTM) Community Multiscale Air Quality (CMAQ) and to analyze the influence of the governing parameters. The influence of the parameters precipitation and atmospheric concentration was evaluated using different input datasets for a variety of CMAQ simulations for the year 2009. It was found that on the basis of daily and weekly measurement data, the bias of modeled depositions could be explained by the bias of precipitation fields and atmospheric concentrations of GOM and PBM. A correction of the modeled wet deposition using observed daily precipitation increased the correlation, on average, from 0.17 to 0.78. An additional correction based on the daily average GOM and PBM concentration lead to a 50 % decrease of the model error for all CMAQ scenarios. Monthly deposition measurements were found to have a too low temporal resolution to adequately analyze model deficiencies in wet deposition processes due to the nonlinear nature of the scavenging process. Moreover, the general overestimation of atmospheric GOM by the CTM in combination with an underestimation of low precipitation events in the meteorological models lead to a good agreement of total annual wet deposition besides the large error in weekly deposition estimates. Moreover, it was found that the current speciation profiles for GOM emissions are the main factor for the overestimation of atmospheric GOM concentrations and might need to be revised in the future. The assumption of zero emissions of GOM lead to an improvement of the mean normalized bias for three-hourly observations of atmospheric GOM from 9.7 to 0.5, Furthermore, the diurnal correlation between model and observation increased from 0.01 to 0.64. This is a strong indicator that GOM is not directly emitted from primary sources but is mainly created by oxidation of GEM.     

Global atmospheric cycle of mercury: a model study on the impact of oxidation mechanisms

Mercury (Hg) is a global pollutant since its predominant atmospheric form, elemental Hg, reacts relatively slowly with the more abundant atmospheric oxidants. Comprehensive knowledge on the details of the atmospheric Hg cycle is still lacking, and in particular, there is some uncertainty regarding the atmospherically relevant reduction-oxidation reactions of mercury and its compounds. ECHMERIT is a global online chemical transport model, based on the ECHAM5 global circulation model, with a highly customisable chemistry mechanism designed to facilitate the investigation of both aqueous- and gas-phase atmospheric mercury chemistry. An improved version of the model which includes a new oceanic emission routine has been developed. Results of multiyear model simulations with full atmospheric chemistry have been used to examine the how changes to chemical mechanisms influence the model’s ability to reproduce measured Hg concentrations and deposition flux patterns. The results have also been compared to simple fixed-lifetime tracer simulations to constrain the possible range of atmospheric mercury redox rates. The model provides a new and unique picture of the global cycle of mercury, in that it is online and includes a full atmospheric chemistry module.     

Atmospheric oxidation of elemental mercury by ozone in the aqueous phase

The aqueous phase oxidation of elemental mercury by ozone has been investigated in the laboratory using a quartz glass reactor with gas phase concentrations of 400–1800ng m⁻³ and 70–200 ppb for Hg(0) and O₃, respectively. The absorption of Hg in the water phase was increased by three orders of magnitude with O₃ present. If the oxidation were to proceed with the same speed in liquid water in contact with the atmosphere,conversion rales of 1–4% h⁻¹ would be implied. Experiments using ambient urban air with 2–6 ng Hg m⁻³ confirm the process at elevated O₃ concentrations. At ambient O₃ concentrations competitive reactions become important, e.g. O₃ consumption by SO₂, hydrocarbons etc., and even some reduction of Hg²⁺ could occur. The atmospheric oxidation of Hg(0) by O₃ in water is thus considered important at high O₃ levels in regionally polluted or remote areas.     

Airborne Emissions of Mercury from Municipal Solid Waste. II: Potential Losses of Airborne Mercury before Landfill

Abstract

Waste distribution and compaction at the working face of municipal waste landfills releases mercury vapor (Hg⁰) to the atmosphere, as does the flaring of landfill gas. Waste storage and processing before its addition to the landfill also has the potential to release Hg⁰ to the air if it is initially present or formed by chemical reduction of Hg^II to Hg⁰ within collected waste. We measured the release of Hg vapor to the atmosphere during dumpster and transfer station activities and waste storage before landfilling at a municipal landfill operation in central Florida. We also quantified the potential contribution of specific Hg-bearing wastes, including mercury (Hg) thermometers and fluorescent bulbs, and searched for primary Hg sources in sorted wastes at three different landfills. Surprisingly large fluxes were estimated for Hg losses at transfer facilities (～100 mg/hr) and from dumpsters in the field (～30 mg/hr for 1,000 dumpsters), suggesting that Hg emissions occurring before landfilling may constitute a significant fraction of the total emission from the disposal/landfill cycle and a need for more measurements on these sources. Reducing conditions of landfill burial were obviously not needed to generate strong Hg⁰ signals, indicating that much of the Hg was already present in a metallic (Hg⁰) form. Attempts to identify specific Hg sources in excavated and sorted waste indicated few readily identifiable sources; because of effective mixing and diffusion of Hg⁰, the entire waste mass acts as a source. Broken fluorescent bulbs and thermometers in dumpsters emitted Hg⁰ at 10 to >100 μg/hr and continued to act as near constant sources for several days.     

Investigation of Selective Catalytic Reduction Impact on Mercury Speciation under Simulated NOx Emission Control Conditions

Abstract

Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NO_x) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NO_x reduction, promote the formation of oxidized mercury (Hg²⁺).

The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg⁰) oxidation. Bench-scale experiments were conducted to investigate Hg⁰ oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO₂) for simulating the combustion of bituminous coals and subbituminous coals were tested in these experiments. The effects of HCl and SO₂ in the flue gases on Hg⁰ oxidation under SCR reaction conditions were studied. It was observed that HCl is the most critical flue gas component that causes conversion of Hg⁰ to Hg²⁺ under SCR reaction conditions. The importance of HCl for Hg⁰ oxidation found in the present study provides the scientific basis for the apparent coal-type dependence observed for Hg⁰ oxidation occurring across the SCR reactors in the field.     

Reduced mercury deposition in New Hampshire from 1996 to 2002 due to changes in local sources

Changes in deposition of gaseous divalent mercury (Hg(II)) and particulate mercury (Hg(p)) in New Hampshire due to changes in local sources from 1996 to 2002 were assessed using the Industrial Source Complex Short Term (ISCST3) model (regional and global sources and Hg atmospheric reactions were not considered). Mercury (Hg) emissions in New Hampshire and adjacent areas decreased significantly (from 1540 to 880 kg yr⁻¹) during this period, and the average annual modeled deposition of total Hg also declined from 17 to 7.0 μg m⁻² yr⁻¹ for the same period. In 2002, the maximum amount of Hg deposition was modeled to be in southern New Hampshire, while for 1996 the maximum deposition occurred farther north and east. The ISCST3 was also used to evaluate two future scenarios. The average percent difference in deposition across all cells was 5% for the 50% reduction scenario and 9% for the 90% reduction scenario.