Mercury Re-emission Research Articles

The effect of selenite on mercury re-emission in smelting flue gas scrubbing system

Recently, the mercury re-emission from the scrubbing solution has become a hot topic in wet flue gas desulfurization process (WFGD). Selenite is one of the most important matters in the solution. It is discovered that selenite have great effect on the mercury re-emission in the WFGD process in this paper. Several important parameters, such as selenite concentration, pH value, temperature and chloride concentration, and reaction mechanisms were studied on the Hg0 re-emission. The experimental results indicate that mercury reduction and re-emission can be effectively inhibited with high selenite and chloride concentrations, low temperature and pH value. The mechanisms for inhibition of mercury reduction and re-emission are proved to be different under acid and alkaline conditions respectively. Under acid condition, it is the formation of stable HgSe that decrease Hg0 re-emission. However, the formation of more stable HgSeO3SO32- or Hg(SeO3)22- than Hg(SO3)22- contributes to avoid the oxidized mercury reduction by sulfite ions and then the mercury re-emission under the alkaline condition. In addition, it is also found that Cl- ions can further enhance the inhibition effect and suppress Hg0 re-emission by the formation of stable HgSeO3Cl- and HgSeO3Cl22- complexes.

Influence of Flue-Gas Components on Mercury Removal and Retention in Dual-Loop Flue-Gas Desulfurization

Due to the system, the dual-loop wet limestone flue-gas desulfurization process is assumed to be well-suited for mercury retention. As the behavior of mercury in the dual-loop process has been insufficiently dealt with in literature so far, the influence of sulfur dioxide and halides on mercury removal and retention was investigated. The process conditions in the loops are highly relevant for the removal and retention of mercury. The circulating slurries there differ in pH-value and oxidation–reduction conditions as well as in halides and sulfite concentrations and solid compositions. This work presents investigations that were conducted at a laboratory-scale test rig treating the flue gas of a pulverized coal combustion facility. The share of the removal of mercury in both loops was investigated as well as the removal of oxidized mercury compounds, the mercury re-emission, and the total mercury removal efficiency. Both loops showed comparable removal rates of oxidized mercury compounds. In the quencher, the removal and retention of mercury is assumed to be improved by the formation of stable mercury complexes with chloride. The decomposition of mercury complexes formed with sulfite and redox reactions are supposed to decrease mercury retention in the absorber. A higher sulfite concentration in the quencher induced by higher SO2 concentration in the flue gas can increase mercury removal by the enhanced formation of mercury complexes with sulfite. At the same time it can promote re-emissions due to a higher share of sulfite on total S(IV) at the resulting higher pH-value of the quencher. Furthermore, high S(IV) concentrations might result in an increased mercury adsorption on particles. Increased chloride concentrations in the absorber can lead to higher removal rates of sulfur dioxide and improved mercury retention as well as to relatively high shares of mercury in wastewater, whereas the latter constitutes the desired mercury pathway.

Mercury deposition and re-emission pathways in boreal forest soils investigated with Hg isotope signatures.

Soils comprise the largest terrestrial mercury (Hg) pool in exchange with the atmosphere. To predict how anthropogenic emissions affect global Hg cycling and eventually human Hg exposure, it is crucial to understand Hg deposition and re-emission of legacy Hg from soils. However, assessing Hg deposition and re-emission pathways remains difficult because of an insufficient understanding of the governing processes. We measured Hg stable isotope signatures of radiocarbon-dated boreal forest soils and modeled atmospheric Hg deposition and re-emission pathways and fluxes using a combined source and process tracing approach. Our results suggest that Hg in the soils was dominantly derived from deposition of litter (∼90% on average). The remaining fraction was attributed to precipitation-derived Hg, which showed increasing contributions in older, deeper soil horizons (up to 27%) indicative of an accumulation over decades. We provide evidence for significant Hg re-emission from organic soil horizons most likely caused by nonphotochemical abiotic reduction by natural organic matter, a process previously not observed unambiguously in nature. Our data suggest that Histosols (peat soils), which exhibit at least seasonally water-saturated conditions, have re-emitted up to one-third of previously deposited Hg back to the atmosphere. Re-emission of legacy Hg following reduction by natural organic matter may therefore be an important pathway to be considered in global models, further supporting the need for a process-based assessment of land/atmosphere Hg exchange.

Mercury Re-emission Behaviors in Magnesium-Based Wet Flue Gas Desulfurization Process: The Effects of Oxidation Inhibitors

The effects of four oxidation inhibitors (including ascorbic acid, formaldehyde, hydroquinone, and sodium thiosulfate) on mercury re-emission in simulated magnesium-based wet flue gas desulfurization solution were evaluated. Experimental results demonstrated that the addition of ascorbic acid significantly increased the mercury re-emission, whereas the formaldehyde and hydroquinone could slightly enhance this process at pH 6. Sodium thiosulfate was found to somewhat inhibit the bivalent mercury reduction owing to the strong binding between mercury and thiosulfate ions. pH variation showed dramatic effects on elemental mercury re-emission in the solution containing ascorbic acid or hydroquinone, and the mercury emission amount in 30 min increased about seven- and two-fold, respectively, when pH was adjusted from 6 to 8. These findings indicated that the ascorbic acid was an important reducer of mercury re-emission under acidic condition besides the sulfite ions, whereas it was the main reducer of mercury reduction under alkaline condition. In addition, the investigation on the effects of temperature and the presence of Cl– ions suggested that the mercury re-emission would be accelerated at elevated temperatures and could be suppressed in the presence of Cl– ions.

Correlation slopes of GEM / CO, GEM / CO<sub>2</sub>, and GEM / CH<sub>4</sub> and estimated mercury emissions in China, South Asia, the Indochinese Peninsula, and Central Asia derived from observations in northwestern and southwestern China

Abstract. Correlation analyses between atmospheric mercury (Hg) and other trace gases are useful for identification of sources and constraining regional Hg emissions. Emissions of Hg in Asia contribute significantly to the global budget of atmospheric Hg. However, due to the lack of reliable data on the source strength, large uncertainties remain in the emission inventories of Hg in Asia. In the present study, we calculated the correlation slopes of GEM / CO, GEM / CO2, and GEM / CH4 for mainland China, South Asia, the Indochinese Peninsula, and Central Asia using the ground-based observations at three remote sites in northwestern and southwestern China, and applied these values to estimate GEM emissions in the four source regions. The geometric mean GEM / CO correlation slopes for mainland China, South Asia, the Indochinese Peninsula, and Central Asia were 7.3 ± 4.3, 7.8 ± 6.4, 7.8 ± 5.0, and 13.4 ± 9.5 pg m−3 ppb−1, respectively, and values in the same source regions were 33.3 ± 30.4, 27.4 ± 31.0, 23.5 ± 15.3, and 20.5 ± 10.0 pg m−3 ppb−1 for the GEM / CH4 correlation slopes, respectively. The geometric means of GEM / CO2 correlation slopes for mainland China, South Asia, and Central Asia were 240 ± 119, 278 ± 164, 315 ± 289 pg m−3 ppm−1, respectively. These values were the first reported correlation slopes of GEM / CO, GEM / CO2, and GEM / CH4 in four important source regions of Asia, not including the GEM / CO ratios in mainland China. The correlation slopes of GEM / CO, GEM / CO2, and GEM / CH4 in Asia were relatively higher than those observed in Europe, North America, and South Africa, which may highlight GEM emissions from non-ferrous smelting, large-scale and artisanal mercury and gold production, natural sources, and historically deposited mercury (re-emission) in Asia. Using the observed GEM / CO and GEM / CO2 slopes, and the recently reported emission inventories of CO and CO2, the annual GEM emissions in mainland China, South Asia, the Indochinese Peninsula, and Central Asia were estimated to be in the ranges of 1071–1187, 340–470, 125, and 54–90 t, respectively. The estimated quantity of GEM emissions from the GEM / CH4 correlation slopes is significantly larger, which may be due to the larger uncertainties in CH4 emissions in Asia as well as insufficient observations of GEM / CH4 correlation slopes, therefore leading to an overestimate of GEM emissions. Our estimates of GEM emissions in the four Asian regions were significantly higher (3–4 times) than the anthropogenic GEM emissions reported in recent studies. This discrepancy could come from a combination of reasons including underestimates of anthropogenic and natural GEM emissions; large uncertainties related to CO, CO2, and CH4 emission inventories; and inherent limitations of the correlation slope method.

Mercury re-emission in flue gas multipollutants simultaneous absorption system.

Recently, simultaneous removal of SO2, NOx and oxidized mercury in wet flue gas desulfurization (WFGD) scrubber has become a research focus. Mercury re-emission in traditional WFGD system has been widely reported due to the reduction of oxidized mercury by sulfite ions. However, in multipollutants simultaneous absorption system, the formation of a large quantity of nitrate and nitrite ions as NOx absorption might also affect the reduction of oxidized mercury in the aqueous absorbent. As such, this paper studied the effects of nitrate and nitrite ions on mercury re-emission and its related mechanism. Experimental results revealed that the nitrate ions had neglected effect on mercury re-emission while the nitrite ions could greatly change the mercury re-emission behaviors. The nitrite ions could initially improve the Hg(0)-emission through the decomposition of HgSO3NO2(-), but with a further increase in the concentration, they would then inhibit the reduction of bivalent mercury owing to the formation of Hg-nitrite complex [Hg(NO2)x(2-x)]. In addition, the subsequent addition of Cl(-) could further suppress the Hg(0) emission, where the formation of a stable Hg-SO3-NO2-Cl complex was assumed to be the main reason for such strong inhibition effect.

Study of mercury re-emission in a simulated WFGD solution containing thiocyanate and sulfide ions

In this paper, elemental mercury (Hg0) re-emissions from simulated wet flue gas desulfurization (WFGD) solutions were evaluated in the presence of thiocyanate ions (SCN−) or sulfide ions (S2− and HS−). Experimental results revealed that both thiocyanate ions and sulfide ions could inhibit the reduction of oxidized mercury to Hg0. The inhibition effect of SCN− was mainly attributed to the formation of HgSO3SCN− and HgSCNX−(X–2) (X=2–4). It was found that the mercury reduction process in the presence of sulfide ions followed different mechanism under acidic or alkaline condition, though the formation of stable HgS could be ascribed to the main inhibition reason. Under acidic condition, the bivalent mercury ions may not directly combine with sulfide ions to form HgS. This had led to a weaker inhibition effect under the acidic condition compared to that under alkaline condition.

A comprehensive evaluation of the influence of air combustion and oxy-fuel combustion flue gas constituents on Hg0 re-emission in WFGD systems

This paper evaluates the influence of the main constituents of flue gases from coal combustion (CO2, O2, N2 and water vapor), in air and oxy-fuel combustion conditions on the re-emission of Hg0 in wet scrubbers. It was observed that the concentration of water vapor does not affect the re-emission of mercury, whereas O2 and CO2 have a notable influence. High concentrations of O2 in the flue gas prevent the re-emission of Hg0 due to the reaction of oxygen with the metals present in low oxidation states. High concentrations of CO2, which cause a decrease in the pH and the redox potential of gypsum slurries, reduce the amount of Hg0 that is re-emitted. As a consequence, the high content of CO2 in oxy-fuel combustion may decrease the re-emission of Hg0 due to the solubility of CO2 in the suspension and the decrease in the pH. It was also found that O2 affects the stabilization of Hg2+ species in gypsum slurries. The results of this study confirm that the amount of metals present in limestone as well as the redox potential and pH of the slurries in wet desulphurization plants need to be strictly controlled to reduce Hg0 re-emissions from power plants operating under oxy-fuel combustion conditions.

Controlled Re-emission of Mercury in Waste Water Treatment

This paper introduces a novel process for the controlled re-emission and immobilization of Mercury (Hg) in waste water treatment plants of wet flue gas desulfurization (FGD). The aim is to create a highly concentrated sink for Hg for further processing. Experiments are carried out with a continuously operated lab-scale wet FGD system and a batch-wise operated waste water treatment unit. By aeration of the liquid phase, the controlled desorption of Hg during the alkalization process of the waste water treatment is enhanced. It is demonstrated, that Hg is emitted in its elemental form (Hg0). Thus, a chemical reduction of Hg2+ takes place prior to its desorption. Linear dependency of Hg0 formation on Sulfite- (SO32-) and Hydroxide-ion (OH-) concentration indicates first order dependency of reaction kinetics. Decreasing concentration of Hg0 in the exhaust air for increasing Cl--concentration is observed. The results show exponential dependence of Hg0 mass transfer on stirring speed and almost constant re-emissions for the aeration flow rates investigated. Thus, the application of low air flux is beneficial in terms of energy demand of the fan and also for the purpose of creating a highly concentrated sink for Hg in the process. The concentration decrease of Hg2+ in the waste water is proportional to the savings in terms of precipitating agent consumption of the further processing steps.

Impact of additives for enhanced sulfur dioxide removal on re-emissions of mercury in wet flue gas desulfurization

The wet flue gas desulfurization process (FGD) in fossil fired power plants offers the advantage of simultaneously removing SO2 and other water soluble pollutants, such as certain oxidized mercury compounds (Hg2+). In order to maximize SO2 removal efficiency of installed FGD units, organic additives can be utilized. In the context of multi-pollutant control by wet FGD, the effect of formic and adipic acid on redox reactions of dissolved mercury compounds is investigated with a continuously operated lab-scale test-rig. For sulfite (SO32-) concentrations above a certain critical value, their potential as reducing agent leads to rapidly increasing formation and re-emission of elemental mercury (Hg0). Increasing chloride concentration and decreasing pH and slurry temperature have been identified as key factors for depressing Hg0 re-emissions. Both organic additives have a negative impact on Hg-retention and cause increased Hg0 re-emissions in the wet FGD process, with formic acid being the significantly stronger reducing agent. Different pathways of Hg2+ reduction were identified by qualitative interpretation of the pH-dependence and by comparison of activation enthalpies and activation entropies. While the first mechanism proposed identifies SO32- as reducing agent and is therefore relevant for any FGD process, the second mechanism involves the formate anion, thus being exclusively relevant for FGDs utilizing formic acid as additive.

Full Scale Evaluation and Suppression of Mercury Re-Emission in Wet Flue Gas Desulfurization System

The full-scale of PC/Cyclone Boilers with common wet flue gas desulfurization (WFGD) with limestone forced oxidation (LSFO) was studied. Ontario Hydro Method (OHM) recommended by the United States Environmental Protection Agency (USEPA) was used to determine mercury emission and speciation at these two full-scale WFGD systems, and OHM quality assurance/quality control (QA/QC) was followed during the field testing. WFGD re-emission problems were repeatedly observed at this unit. Selective catalytic reduction (SCR) had significant effects on mercury removal and Hg0 re-emission rates across WFGD. Effects of injection of continuous chemicals additive containing HS-, S2- or I- on mercury re-emission control were also conducted at this unit.

Influence of Limestone Characteristics on Mercury Re-emission in WFGD Systems

This work evaluates the influence of the effect of the properties of limestones on their reactivity and the re-emission of mercury under typical wet scrubber conditions. The influence of the composition, particle size, and porosity of limestones on their reactivity and the effect of sorbent concentration, pH, redox potential, and the sulphite and iron content of the slurry on Hg(0) re-emission was assessed. A small particle size, a high porosity and a low magnesium content increased the high reactivity of the limestones. Moreover, it was found that the higher the reactivity of the sample the greater the amount of mercury captured in the scrubber. Although sulphite ions did not cause the re-emission of mercury from the suspensions of the gypsums, the limestones enriched in iron increased Hg(0) re-emission under low oxygen conditions. It was observed that the low pH values of the gypsum suspensions favored the cocapture of mercury because Fe(2+) formation was avoided. The partitioning of the mercury in the byproducts of the scrubber depended on the impurities of the limestones rather than on their particle size. No leaching of mercury from the gypsum samples occurred suggesting that mercury was either tightly bound to the impurities of the limestone or was transformed into insoluble mercury species.

Waste disposal sites as sources of mercury in the atmosphere in the coastal zone of the Gulf of Gdańsk (southern Baltic Sea)

Abstract Elemental mercury re-emission into the air from an old burial ground in Gdańsk Letnica and from a modern landfill in Gdańsk Szadółki resulted in noticeably increased TGM concentrations from the urban background level of 1.9 ng m−3 to a maximum value of 164.4 ng m−3. Hgtot concentrations in the soil of the burial ground ranged from 37.3 to 4817.3 ng g−1 and in the surrounding water: from 22.0 to 55.0 ng dm−3. The highest Hgtot concentrations in the modern landfill (Gdańsk Szadółki) were reported for the unsorted waste: 36.1–972.8 ng g−1. Laboratory experiments on the re-emission of Hg(0) into the air showed that emission from soil was stimulated by solar radiation and from the water — additionally by turbulent mixing.

Co-effects of sulfur dioxide load and oxidation air on mercury re-emission in forced-oxidation limestone flue gas desulfurization wet scrubber

Re-emission of Hg0 refers to the reduction of oxidized mercury (Hg2+) to the elemental mercury (Hg0) in a wet flue gas desulfurization process (WFGD), resulting Hg0 re-emits to the flue gas and increasing mercury emission for a coal combustion power plant. The correlations between re-emission of Hg0 and operation parameters of WFGD process, i.e., sulfur dioxide removal, oxidation air flow, and pH of FGD slurry, were investigated with mercury field data collected from two coal combustion power plants (Plant C and Plant O). At Units 31/32 of Plant C, Coupling mercury results and the plant information (PI) data suggested that an increase of the oxidation air flow in the WFGD effectively inhibited the re-emission of Hg0, and consequently increased the flue gas mercury removal efficiency from 63% to over 92%. But at Plant O, the increase of the oxidation air flow can sometimes increase Hg0 re-emission. It was further found that the re-emission of Hg0 at Plant O was strongly correlated not only to the flow rate of the oxidation air in the forced-oxidation operation conditions, but also to the amount of sulfur dioxide removed from the flue gas by the FGD scrubber (i.e., the sulfur dioxide load in the scrubber). Possible mechanisms were proposed.

Study of Mercury Re-Emission from Simulated Wet Flue Gas Desulfurization Liquors

Hg2+captured by wet flue gas desulfurization (WFGD) systems can easily be reduced by reducing substances such as S(IV) (SO32-or HSO3-) and results in emissions of elemental mercury (Hg0). The re-emission of Hg0would lead to a damping of the total mercury removal efficiency by WFGD systems. The effects of the operating conditions, which included the pH, temperature, Cl-concentrations and oxygen concentrations, on Hg0re-emission from WFGD liquors was carried out. The experimental results indicated that the Hg0re-emission rate from WFGD liquors increased as the operational temperatures and pH values increased. The Hg0re-emission rates decreased as the O2concentration of flue gas and Cl-concentration of WFGD liquors increased. So the Hg0re-emission from WFGD system can be reduced or slowed by decreasing the temperature and pH, or by using forced oxidation. The present findings could be valuable for industrial application of characterizing and optimizing mercury control in WFGD systems.

Effect of anion concentrations on Hg2+ reduction from simulated desulphurization aqueous solutions

The influence of different anions on the stability of mercury in absorbing solutions containing sulphite ions was investigated. The re-emission of gaseous mercury in the presence of fluoride, chloride, bromide, nitrate, carbonate and sulphate ions from the scrubbing solution was determined. Fluoride, chloride and bromide ions contribute to mercury stabilization, bromide being the most efficient anion for retaining mercury in sulphite aqueous solutions. High carbonate concentrations prevent the reduction of mercury by sulphite ions due to the stabilization of the complexes formed between oxidised mercury, hydroxide and sulphite ions or through the formation of mercury carbonate species. However, nitrate and sulphate ions do not influence the equilibrium between mercury and sulphite ions and, as a consequence, the re-emission of mercury is not modified. The results of this study suggest that the pH is one of the most important parameters for achieving an efficient control of mercury pollution in desulphurization systems. The re-emission of mercury was observed to increase when the pH value of the slurries was higher than six due to the stabilization of the mercury in the form of Hg(SO3)22-, Hg(OH)+, Hg(OH)2, HgHCO3+, HgCO3 or Hg(CO3)22-. Since the pH depends on the concentration of SO2 in the flue gas and the type of limestone, the alkalinity that comes from the sorbent used in the scrubber may play an important role in mercury capture in such systems.

Control of Hg0 re-emission from gypsum slurries by means of additives in typical wet scrubber conditions

The effect of different additives on the re-emission of Hg0 in wet limestone-based flue gas desulfurization (WFGD) conditions is assessed. Because re-emission of elemental mercury from limestone slurries is dependent on the pH of the gypsum slurry, re-emission of elemental mercury was evaluated for different additives using gypsum slurry with a constant pH value of 7. The additives investigated included oxidizing and reducing agents (Fenton reagents and sodium thiosulfate) and also chelating agents (2,4,6 trimercaptotriazine trisodium and sodium hydrosulfide). Fenton reagents were added to prevent the reduction of oxidized mercury species as H2O2 was the precursor used to convert sulfite ions into sulfate or to oxidize Hg0. Thiosulfate were added so that mercury could be retained in the gypsum at low concentrations as insoluble HgS, while mercury is being stabilized in the liquid fraction at higher additive doses. The addition of 2,4,6 trimercaptotriazine trisodium (TMT) and NaHS in low doses prevented the reduction of oxidized mercury species as insoluble HgS and Hg3 (TMT) precipitate onto the gypsum slurry. TMT was found to be most effective additive for Hg2+ capture.

Mercury Vapor Pressure of Flue Gas Desulfurization Scrubber Suspensions: Effects of pH Level, Gypsum, and Iron

Calcium-based scrubbers designed to absorb HCl and SO(2) from flue gases can also remove oxidized mercury. Dissolved mercury halides may have an appreciable partial vapor pressure. Chemical reduction of the dissolved mercury may increase the Hg emission, thereby limiting the coremoval of mercury in the wet scrubbing process. In this paper we evaluate the effects of the pH level, different gypsum qualities, and iron in flue gas desulfurization (FGD) scrubber suspensions. The impact of these parameters on mercury vapor pressure was studied under controlled laboratory conditions in model scrubber suspensions. A major influence is exerted by pH values above 7, considerably amplifying the mercury concentration in the vapor phase above the FGD scrubber suspension. Gypsum also increases the mercury re-emission. Fe(III) decreases and Fe(II) increases the vapor pressure significantly. The consequences of the findings for a reliable coremoval of mercury in FGD scrubbers are discussed. It is shown that there is an increased risk of poor mercury capture in lime-based FGD scrubbers in comparison to limestone FGD scrubbers.

Effect of adding aluminum salts to wet FGD systems upon the stabilization of mercury

This paper evaluates the effect of using aluminum salts in wet Flue Gas Desulphurization Plants (FGD) upon mercury reemission. Aluminum sulfate is employed to increase the SO2 retention capacity of low porosity limestone. However, its presence in the FGD system also influences the chemical reactions of the species in the scrubber. This study, performed at lab-scale to assess the reduction of mercury from aqueous solutions and gypsum slurries, has demonstrated that the addition of aluminum sulfate mainly affects the pH of the solution and the formation of sulfite ions, which in turn affects the mechanisms of mercury re-emission.

Regional scale photochemical model evaluation of total mercury wet deposition and speciated ambient mercury

Methylmercury is a known neurotoxin with deleterious health effects on humans and wildlife. Atmospheric deposition is the largest source of mercury loading to most terrestrial and aquatic ecosystems. Regional scale air quality models are needed to quantify mercury deposition resulting from complex emissions sources and physical and chemical processes that govern the fate of mercury in the atmosphere. Total mercury wet deposition estimates from multiple regional photochemical transport models applied at 12 km grid resolution over the continental United States compare well with observations (CAMx fractional error = 45%, CMAQ fractional error = 33%) despite uncertainties in global mercury emissions inventories and certain chemical transformation pathways. In addition, both CMAQ and CAMx well represent observed diel and seasonal patterns of Hg(0) and tend to exaggerate the diel patter of Hg(II) at AMNet monitor locations. The observed fraction of particulate mercury to total oxidized mercury (sum of particulate mercury and Hg(II)) is generally greater in colder months and during overnight hours. The modeling systems tend to capture these patterns but have a systematically lower fraction of particulate mercury to total oxidized mercury, especially in winter months. Annual total mercury deposition from wet and dry processes is 65% greater in CMAQ compared to CAMx over the entire modeling domain. This is largely due to higher wet deposition in CMAQ and higher dry deposition of Hg(0), which is treated as equilibrium with mercury re-emissions and not modeled in CAMx. A sensitivity using CAMx with Hg(0) dry deposition treated similar to CMAQ resulted in more comparable total mercury deposition estimates. Modeled dry deposition velocities for Hg(II) compare well with the limited experimental data, while Hg(0) dry deposition velocities are lower than published experimental data. A mercury bi-directional flux sensitivity application in CMAQ had the overall effect of reducing total mercury dry deposition and slightly improving ambient Hg(0) performance. The range of the domain wide total deposition from all model sensitivities was within 25% of the mean but exhibited larger deviations in the individual wet and dry deposition budgets. The contribution of mercury initial conditions and lateral boundary inflow conditions were tracked separately using CAMx source apportionment. Initial contribution to total mercury deposition for the entire model domain falls below 5% after 2 weeks. Boundary contribution to total mercury deposition vary considerably across the continental United States, but ranges between 20 and 99% at MDN monitor locations.