Sources of speciated atmospheric mercury at a residential neighborhood impacted by industrial sources

Speciated measurements of atmospheric mercury plumes were obtained at an industrially impacted residential area of East St. Louis, IL. These plumes were found to result in extremely high mercury concentrations at ground level that were composed of a wide distribution of mercury species. Ground level concentrations as high as 235 ng m(-3) for elemental mercury (Hg0) and 38 300 pg m(-3) for reactive mercury species (reactive gaseous (RGM) plus particulate (PHg) mercury) were measured. The highest mercury concentrations observed during the study were associated with plumes that contained high concentrations of all mercury species (Hg0, RGM, and PHg) and originated from a source located southwest of the sampling site. Variations in proportions of Hg0/RGM/PHg among plumes, with Hg0 dominating some plumes and RGM and/or PHg dominating others, were attributed to differences in emissions from different sources. Correlations between mercury plumes and elevated NO(x) were not observed; however, a correlation between elevated SO2 and mercury plumes was observed during some but not all plume events. Despite the presence of six coal-fired power plants within 60 km of the study site, wind direction data along with Hg/SO2 and Hg/NO(x) ratios suggest that high-concentration mercury plumes impacting the St. Louis-Midwest Particle Matter Supersite are attributable to local point sources within 5 km of the site.     

Significance of RuO2 modified SCR catalyst for elemental mercury oxidation in coal-fired flue gas

Catalytic conversion of elemental mercury (Hg(0)) to its oxidized form has been considered as an effective way to enhance mercury removal from coal-fired power plants. In order to make good use of the existing selective catalytic reduction of NO(x) (SCR) catalysts as a cobenefit of Hg(0) conversion at lower level HCl in flue gas, various catalysts supported on titanium dioxide (TiO(2)) and commercial SCR catalysts were investigated at various cases. Among the tested catalysts, ruthenium oxides (RuO(2)) not only showed rather high catalytic activity on Hg(0) oxidation by itself, but also appeared to be well cooperative with the commercial SCR catalyst for Hg(0) conversion. In addition, the modified SCR catalyst with RuO(2) displayed an excellent tolerance to SO(2) and ammonia without any distinct negative effects on NO(x) reduction and SO(2) conversion. The demanded HCl concentration for Hg(0) oxidation can be reduced dramatically, and Hg(0) oxidation efficiency over RuO(2) doped SCR catalyst was over 90% even at about 5 ppm HCl in the simulated gases. Ru modified SCR catalyst shows a promising prospect for the cobenefit of mercury emission control.     

Mercury speciation in coal-fired power plant plumes observed at three surface sites in the southeastern U.S

Elemental Hg (Hg0), reactive gaseous Hg (RGM) and fine particulate Hg (Hgp) were measured intermittently at three sites in the southeastern U.S. from June 2001 through November 2004. Simultaneous measurements of SO2 and NOy were used to identify plumes from coal fired power plants (CFPPs). Emission signatures and back trajectories were used to identity specific CFPPs, and to compare observed (i.e., at the site) versus expected (i.e., at the stack) Hg speciation. Results for 41 precipitation-free plume events show that observed RGM:SO2 is substantially lower (by a factor of 2-4) than expected RGM:SO2. Hgp represented 2%, or less, of total-Hg in CFPP plumes, in general agreement with emission estimates. Results for 21 events, where both RGM and Hg0 could be estimated, show that total-Hg (i.e., RGM + Hg0) was essentially conserved from the point of emission to the site, and that Hg0 was the dominant form (average 84%). Emission estimates, based on coal analyses and the EPRI-ICR Hg speciation model, indicate that Hg0 should represent about 42% of Hg in the observed plumes. Possible explanations for these differences include, but are not limited to, in-plume reduction of RGM to Hg0, measurement error, errors in emission estimates, and depositional losses. Further work is needed to confirm these results and to determine if they apply to CFPPs in general, or the limited set of observed CFPPs.     

燃煤电厂大气汞排放监测试验分析

为了实现对燃煤电厂汞排放量予以精准的测量,获取大气汞污染管控依据,本文分别分析了Hg-CEMS法、30B法、HJ543-2009法、OHM法这几种目前存在的燃煤电厂大气汞排放监测方法,并总结出了上述几种监测方法的优势和不足,并基于此,对上述监测方法进行了对比试验,实现燃煤电厂大气汞排放监测的试验分析.试验结果显示,使用...     

Using bromine gas to enhance mercury removal from flue gas of coal-fired power plants

Bromine gas was evaluated for converting elemental mercury (Hg0) to oxidized mercury, a form that can readily be captured by the existing air pollution control device. The gas-phase oxidation rates of Hg0 by Br2 decreased with increasing temperatures. SO2, CO, HCl, and H2O had insignificant effect, while NO exhibited a reverse course of effect on the Hg0 oxidation: promotion at low NO concentrations and inhibition at high NO concentrations. A reaction mechanism involving the formation of van der Waals clusters is proposed to accountfor NO's reverse effect. The apparent gas-phase oxidation rate constant, obtained under conditions simulating a flue gas without flyash, was 3.61 x 10(-17) cm3 x molecule(-1) x s(-1) at 410 K corresponding to a 50% Hg0 oxidation using 52 ppm Br2 in a reaction time of 15 s. Flyash in flue gas significantly promoted the oxidation of Hg0 by Br2, and the unburned carbon component played a major role in the promotion primarily through the rapid adsorption of Br2 which effectively removed Hg0 from the gas phase. At a typical flue gas temperature, SO2 slightly inhibited the flyash-induced Hg0 removal. Conversely, NO slightly promoted the flyash induced Hg0 removal by Br2. Norit Darco-Hg-LH and Darco-Hg powder activated carbons, which have been demonstrated in field tests, were inferred for estimating the flyash induced Hg0 oxidation by Br2. Approximately 60% of Hg0 is estimated to be oxidized with the addition of 0.4 ppm of gaseous Br2 into full scale power plant flue gas.     

Survey of catalysts for oxidation of mercury in flue gas

Methods for removing mercury from flue gas have received increased attention because of recent limitations placed on mercury emissions from coal-fired utility boilers by the U. S. Environmental Protection Agency and various states. A promising method for mercury removal is catalytic oxidation of elemental mercury (Hg0) to oxidized mercury (Hg2+), followed by wet flue gas desulfurization (FGD). FGD cannot remove Hg0, but easily removes Hg2+ because of its solubility in water. To date, research has focused on three broad catalyst areas: selective catalytic reduction catalysts, carbon-based materials, and metals and metal oxides. We review published results for each type of catalyst and also present a discussion on the possible reaction mechanisms in each case. One of the major sources of uncertainty in understanding catalytic mercury oxidation is a lack of knowledge of the reaction mechanisms and kinetics. Thus, we propose that future research in this area should focus on two major aspects: determining the reaction mechanism and kinetics and searching for more cost-effective catalyst and support materials.     

Should a coal-fired power plant be replaced or retrofitted?

In a cap-and-trade system, a power plant operator can choose to operate while paying for the necessary emissions allowances, retrofit emissions controls to the plant, or replace the unit with a new plant. Allowance prices are uncertain, as are the timing and stringency of requirements for control of mercury and carbon emissions. We model the evolution of allowance prices for SO2, NOx, Hg, and CO2 using geometric Brownian motion with drift, volatility, and jumps, and use an options-based analysis to find the value of the alternatives. In the absence of a carbon price, only if the owners have a planning horizon longer than 30 years would they replace a conventional coal-fired plant with a high-performance unit such as a supercritical plant; otherwise, they would install SO2 and NOx, controls on the existing unit. An expectation that the CO2 price will reach $50/t in 2020 makes the installation of an IGCC with carbon capture and sequestration attractive today, even for planning horizons as short as 20 years. A carbon price below $40/t is unlikely to produce investments in carbon capture for electric power.     

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.     

Atmospheric mercury deposition to Lake Michigan during the Lake Michigan Mass Balance Study

Wet and dry mercury (Hg) deposition were calculated to Lake Michigan using a hybrid receptor modeling framework. The model utilized mercury monitoring data collected during the Lake Michigan Mass Balance Study and the Atmospheric Exchange Over Lakes and Oceans Studytogether with high-resolution over-water meteorological date provided by the National Oceanic and Atmospheric Administration (July, 1994-October, 1995). Atmospheric deposition was determined to be the primary pathway for mercury inputto Lake Michigan, contributing approximately 84% of the estimated 1403 kg total annual input (atmospheric deposition + tributary input). Wet (10.6 microg m(-2)) and dry deposition (9.7 microg m(-2)) contributed almost equally to the annual atmospheric Hg deposition of 20.3 microg m(-2) (1173 kg). Re-emission of dissolved gaseous Hg from the lake was also significant (7.8 microg m(-2)), reducing the net atmospheric deposition to 12.5 microg m(-2) (720 kg). A strong urban influence was observed in the over-water mercury deposition estimates in the southern portion of the lake. The Chicago/Gary urban area was estimated to contribute approximately 20% (127 kg) of the annual atmospheric mercury deposition to Lake Michigan. The magnitude of local anthropogenic mercury sources in the Chicago/Gary urban area suggests that emission reductions could significantly reduce atmospheric mercury deposition into Lake Michigan.     

Concentration and dry deposition of mercury species in arid south central New Mexico (2001-2002)

This research was initiated to characterize atmospheric deposition of reactive gaseous mercury (RGM), particulate mercury (HgP; <2.5 microm), and gaseous elemental mercury (Hg0) in the arid lands of south central New Mexico. Two methods were field-tested to estimate dry deposition of three mercury species. A manual speciation sampling train consisting of a KCl-coated denuder, 2.5 microm quartz fiber filters, and gold-coated quartz traps and an ion-exchange membrane (as a passive surrogate surface) were deployed concurrently over 24-h intervals for an entire year. The mean 24-h atmospheric concentration for RGM was 6.8 pg m(-3) with an estimated deposition of 0.10 ng m(-2) h(-1). The estimated deposition of mercury to the passive surrogate surface was much greater (4.0 ng m(-2) h(-1)) but demonstrated a diurnal pattern with elevated deposition from late afternoon to late evening (1400-2200; 8.0 ng m(-2) h(-1)) and lowest deposition during the night just prior to sunrise (2200-0600; 1.7 ng m(-2) h(-1)). The mean 24-h atmospheric concentrations for HgP and Hg0 were 1.52 pg m(-3) and 1.59 ng m(-3), respectively. Diurnal patterns were observed for RGM with atmospheric levels lowest during the night prior to sunrise (3.8 pg m(-3)) and greater during the afternoon and early evening (8.9 pg m(-3)). Discernible diurnal patterns were not observed for either HgP or Hg0. The total dry deposition of Hg was 5.9 microg m-2 year-' with the contribution from the three species as follows: RGM (0.88 microg m(-2) year(-1)), HgP (0.025 microg m(-2) year(-1)), and Hg0 (5.0 microg m(-2) year(-1)). The annual wet deposition for total mercury throughout the same collection duration was 4.2 microg m(-2) year (-1), resulting in an estimated total deposition of 10.1 microg m(-2) year(-1) for Hg. On one sampling date, enhanced HgP (12 pg m(-3)) was observed due to emissions from a wildfire approximately 250 km to the east.     

Oxidation of gaseous elemental mercury to gaseous divalent mercury during 2003 polar sunrise at Ny-Alesund

The springtime phenomenon, termed as the mercury depletion event (MDE), during which elemental gaseous mercury (Hg0) may be converted to a reactive form that accumulates in polar ecosystems, first noted in the Arctic, has now been observed at both poles and results in an important removal pathway for atmospheric mercury. An intensive international springtime mercury experiment was performed at Ny-Alesund, Spitsbergen, from 19 April to 13 May 2003 to study the atmospheric mercury chemistry in the Arctic environment and, in particular, the MDEs which occurred in the arctic boundary layer after polar sunrise. Automated ambient measurements of Hg0, divalent reactive gaseous mercury (RGM) and fine particulate mercury (<2.5 microm) (Hg(p)) were made at the Zeppelin Mountain Station (ZMS). During the experiment mercury concentrations in the lower atmosphere varied in synchrony with ozone levels throughout the Spring. Hg0 concentrations ranged from background levels (approximately 1.6 ng m(-3)) to undetectable values (<0.1 ng m(-3)) during the first and major MDE, while RGM data showed an opposite trend during the sampling period with concentrations increasing dramatically to a peak of 230 pg m(-3), synchronous with the depletion of Hg0. The results of a meteorological transport analysis indicate the MDEs observed at ZMS were primarily due to air masses being transported in from open water areas in the Arctic Ocean that were already depleted of Hg0 when they arrived and not due to in-situ oxidation mechanisms.     

Variability of the gaseous elemental mercury sea-air flux of the Baltic Sea

The importance of the sea as a sink for atmospheric mercury has been established quantitatively through models based on wet and dry deposition data, but little is known about the release of mercury from sea areas. The concentration of elemental mercury (Hg0) in sea surface water and in the marine atmosphere of the Baltic Sea was measured at high spatial resolution in February, April, July, and November 2006. Wind-speed records and the gas-exchange transfer velocity were then used to calculate Hg0 sea-air fluxes on the basis of Hg0 sea-air concentration differences. Our results show that the spatial resolution of the surface water Hg0 data can be significantly improved by continuous measurements of Hg0 in air equilibrated with water instead of quantitative extraction of Hg0 from seawater samples. A spatial and highly seasonal variability of the Hg0 sea-air flux was thus determined. In winter, the flux was low and changed in direction. In summer, a strong emission flux of up to 150 ng m(-2) day(-1) in the central Baltic Sea was recorded. The total emission of Hg0 from the studied area (235000 km2) was 4300 +/- 1600 kg in 2006 and exceeded deposition estimates.     

Methylmercury in freshwater fish linked to atmospheric mercury deposition

A connection between accumulation of methylmercury (MeHg) in wild fish populations and atmospheric mercury deposition has not been made. Large databases for both MeHg in fish and atmospheric mercury deposition have been assimilated from monitoring efforts spanning the contiguous United States. Here, we compare results of these data sets and show that state-wide average concentrations of MeHg in a cosmopolitan freshwater fish, the largemouth bass Micropterus salmoides, are related positively to wet atmospheric Hg fluxes among most of the 25 states that are analyzed, which span a 5-fold range in Hg deposition. Differences in largemouth bass MeHg concentrations among states are unrelated to average precipitation depth, wet atmospheric acid deposition, or interstate variations in the type of water body (river, lake, reservoir) from which the fish were sampled. There are modest correlations between MeHg in bass and surface water pH, temperature, and wet atmospheric deposition of sulfate. However, when fish and atmospheric mercury results are combined at the state level, wet atmospheric Hg deposition accounts for about two-thirds of the variation in bass MeHg among most states, and stepwise multiple regression analysis reveals thatthese variables do not improve the linear model significantly. This suggests the accumulation of MeHg in wild fish populations is linked to atmospheric Hg loadings, two-thirds of which are estimated to be from anthropogenic sources.     

Ambient mercury sources in Rochester, NY: results from Principle Components Analysis (PCA) of Mercury Monitoring Network Data

Continuous atmospheric measurements of speciated mercury (Hg) (elemental mercury (Hg?), reactive gaseous mercury (RGM), and particulate mercury (Hgp)) were made in Rochester, NY from Dec 2007 to May 2009. Continuous measurements of ozone (O?), sulfur dioxide (SO?), carbonmonoxide (CO), particulate matter (PM?.?), and meteorological data were also available. A principle components analysis (PCA) of 3886 observations of 13 variables for the period identified six major factors. Melting snow was observed to be a source of Hg?in winters. Positive correlations between RGM and O? in the spring and summer may be indicative of Hg? oxidation. RGM concentrations increased simultaneously with SO? suggesting the influence of coal fired power plants (CFPP). The ?fth factor (F5) containing O? (high negative loading), CO (positive loading), Hg? and Hg(p) (positive), and/or RGM (negative) was identified as a mobile source which was usually observed during morning rush hours (6:00-9:00 a.m.). The concentrations of the three mercury species from the direction of the CFPP were significantly reduced following the shutdown of this source.     

Impacts of halogen additions on mercury oxidation, in a slipstream selective catalyst reduction (SCR), reactor when burning sub-bituminous coal

This paper presents a comparison of impacts of halogen species on the elemental mercury (Hg(0)) oxidation in a real coal-derived flue gas atmosphere. It is reported there is a higher percentage of Hg(0) in the flue gas when burning sub-bituminous coal (herein Powder River Basin (PRB) coal) and lignite, even with the use of selective catalytic reduction (SCR). The higher Hg(0)concentration in the flue gas makes it difficult to use the wet-FGD process for the mercury emission control in coal-fired utility boilers. Investigation of enhanced Hg(0) oxidation by addition of hydrogen halogens (HF, HCl, HBr, and HI) was conducted in a slipstream reactor with and without SCR catalysts when burning PRB coal. Two commercial SCR catalysts were evaluated. SCR catalyst no. 1 showed higher efficiencies of both NO reduction and Hg(0) oxidation than those of SCR catalyst no. 2. NH3 addition seemed to inhibit the Hg(0) oxidation, which indicated competitive processes between NH3 reduction and Hg(0) oxidation on the surface of SCR catalysts. The hydrogen halogens, in the order of impact on Hg(0) oxidation, were HBr, HI, and HCl or HF. Addition of HBr at approximately 3 ppm could achieve 80% Hg(0) oxidation. Addition of HI at approximately 5 ppm could achieve 40% Hg(0) oxidation. In comparison to the empty reactor, 40% Hg(0) oxidation could be achieved when HCl addition was up to 300 ppm. The enhanced Hg(0) oxidation by addition of HBr and HI seemed not to be correlated to the catalytic effects by both evaluated SCR catalysts. The effectiveness of conversion of hydrogen halogens to halogen molecules or interhalogens seemed to be attributed to their impacts on Hg(0) oxidation.     

Development and characterization of an annular denuder methodology for the measurement of divalent inorganic reactive gaseous mercury in ambient air

Atmospheric mercury is predominantly present in the gaseous elemental form (Hg0). However, anthropogenic emissions (e.g., incineration, fossil fuel combustion) emit and natural processes create particulate-phase mercury(Hg(p)) and divalent reactive gas-phase mercury (RGM). RGM species (e.g., HgCl2, HgBr2) are water-soluble and have much shorter residence times in the atmosphere than Hg0 due to their higher removal rates through wet and dry deposition mechanisms. Manual and automated annular denuder methodologies, to provide high-resolution (1-2 h) ambient RGM measurements, were developed and evaluated. Following collection of RGM onto KCl-coated quartz annular denuders, RGM was thermally decomposed and quantified as Hg0. Laboratory and field evaluations of the denuders found the RGM collection efficiency to be >94% and mean collocated precision to be <15%. Method detection limits for sampling durations ranging from 1 to 12 h were 6.2-0.5 pg m(-3), respectively. As part of this research, the authors observed that methods to measure Hg(p) had a significant positive artifact when RGM coexists with Hg(p). This artifact was eliminated if a KCl-coated annular denuder preceded the filter. This new atmospheric mercury speciation methodology has dramatically enhanced our ability to investigate the mechanisms of transformation and deposition of mercury in the atmosphere.     

Millennial-scale records of atmospheric mercury deposition obtained from ombrotrophic and minerotrophic peatlands in the Swiss Jura Mountains

Peat cores from two bogs were used to reconstruct changes in net atmospheric mercury deposition in Switzerland for the past 2-3 millennia. The two records were compared to assess the reliability of peat cores as archives of atmospheric mercury deposition. Net mercury accumulation rates and Hg(ex), an indicator of significant anthropogenic mercury contamination, were calculated for both cores. Both records showed stable background values (0.5-1.9 and 1.0-3.0 microg of Hg m(-2) yr(-1)). In both profiles, mercury accumulation rates began to increase during the 12th century, and Hg(ex) appeared during the 14th century. The late 19th and early 20th centuries have been studied in detail. The profiles match well with the history of local and global mercury emissions. The magnitude of increase from the pre-anthropogenic to anthropogenic period was also very similar in both records. Although the two sites are botanically very similar and lie only 3.5 km apart, accumulation rates at TGE were generally higher than those at EGR. This indicates that, although such records can be used to determine the chronologies of and relative changes in rates of atmospheric mercury deposition, differences in rates of mercury accumulation rates between sites do not necessarily indicate differences in deposition rates of mercury from the atmosphere.     

Some sources and sinks of monomethyl and inorganic mercury on Ellesmere Island in the Canadian High Arctic

We identified some of the sources and sinks of monomethyl mercury (MMHg) and inorganic mercury (HgII) on Ellesmere Island in the Canadian High Arctic. Atmospheric Hg depletion events resulted in the deposition of Hg(II) into the upper layers of snowpacks, where concentrations of total Hg (all forms of Hg) reached over 20 ng/L. However, our data suggest that much of this deposited Hg(II) was rapidly photoreduced to Hg(0) which then evaded back to the atmosphere. As a result, we estimate that net wet and dry deposition of Hg(II) during winter was lower at our sites (0.4-5.9 mg/ha) than wet deposition in more southerly locations in Canada and the United States. We also found quite high concentrations of monomethyl Hg (MMHg) in snowpacks (up to 0.28 ng/L), and at times, most of the Hg in snowpacks was present as MMHg. On the Prince of Wales Icefield nearthe North Water Polynya, we observed a significant correlation between concentrations of Cl and MMHg in snow deposited in the spring, suggesting a marine source of MMHg. We hypothesize that dimethyl Hg fluxes from the ocean to the atmosphere through polynyas and open leads in ice, and is rapidly photolyzed to MMHgCl. We also found that concentrations of MMHg in initial snowmelt on John Evans Glacier (up to 0.24 ng/L) were higher than concentrations of MMHg in the snowpack (up to 0.11 ng/L), likely due to either sublimation of snow or preferential leaching of MMHg from snow during the initial melt phase. This springtime pulse of MMHg to the High Arctic, in conjunction with climate warming and the thinning and melting of sea ice, may be partially responsible for the increase in concentrations of Hg observed in certain Arctic marine mammals in recent decades. Concentrations of MMHg in warm and shallow freshwater ponds on Ellesmere Island were also quite high (up to 3.0 ng/L), leading us to conclude that there are very active regions of microbial Hg(II) methylation in freshwater systems during the short summer season in the High Arctic.     

Gaseous elemental mercury in the marine boundary layer: evidence for rapid removal in anthropogenic pollution

In this study, gas-phase elemental mercury (Hg0) and related species (including inorganic reactive gaseous mercury (RGM) and particulate mercury (PHg)) were measured at Cheeka Peak Observatory (CPO), Washington State, in the marine boundary layer during 2001-2002. Air of continental origin containing anthropogenic pollutants from the urban areas to the east contained on average 5.3% lower Hg0 levels as compared to the marine background. This result is difficult to reconcile since it is known that industrial emissions in our region are sources of Hg0. The rate of removal of Hg0 from a pollution plume necessary to account for our observations is inconsistent with the accepted view of Hg0 as a stable atmospheric pollutant. The largest and most frequent Hg0 loss events occurred in the presence of increased ozone (O3) during the summer. Hg0 and O3 also display diurnal cycles that are out-of-phase with one another. In other seasons Hg0 behavior is less consistent, as we observe weak positive correlations with O3 and occasional Hg0 enhancements in local pollution. RGM and PHg concentrations are enhanced only slightly during Hg0 loss events, comprising a small fraction of the mercury pool (approximately 3%). Long-range transported pollution of Asian origin was also detected at CPO, and this contains both higher and lower levels of Hg0 as compared to the background with maximum changes being <20%. Here, the more photochemically processed the air mass, as determined by propane/ethane ratios, the more likely we are to observe Hg0 loss. Air from the marine background in summer displays a significant diurnal cycle with a phase that matches the diurnal cycles seen in polluted air masses. A Junge lifetime for Hg0 in the clean marine boundary layer is calculated to be 7.1 months, which is on the low end of previous estimates (0.5-2 yr).     

Estimation of dry deposition of atmospheric mercury in Nevada by direct and indirect methods

Atmospheric models and limited measurements indicate that dry deposition of atmospheric mercury is an important process by which mercury is input to ecosystems. To begin to fill the measurement data gap, multiple methods were used simultaneously during seasonal campaigns conducted in 2005 and 2006 to estimate dry deposition of atmospheric mercury at two Mercury Deposition Network (MDN) sites in rural Nevada and in Reno, Nevada. Gaseous elemental mercury (Hg0), reactive gaseous mercury (RGM), and particulate-bound mercury (Hgp) concentrations were measured using Tekran 2537A/1130/ 1135 systems. These speciated measurements were combined with on-site meteorological measurements to estimate depositional fluxes of RGM and Hgp using dry deposition models. Modeled fluxes were compared with more direct measurements obtained using polysulfone cation-exchange membranes and foliar surfaces. Dynamic flux chambers were used to measure soil mercury exchange. RGM concentrations were higher during warmer months at all sites, leading to seasonal variation in the modeled importance of RGM as a component of total depositional load. The ratio of dry to wet deposition was between 10 and 90%, and varied with season and with the methods used for dry deposition approximations. This work illustrates the variability of mercury dry deposition with location and time and highlights the need for direct dry deposition measurements.