Particulate-bound Mercury Research Articles

Evidence for short-range transport of atmospheric mercury to a rural, inland site

Atmospheric mercury (Hg) species, including gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (Hg p), were monitored near three sites, including a cement plant (monitored in 2007 and 2008), an urban site and a rural site (both monitored in 2005 and 2008). Although the cement plant was a significant source of Hg emissions (for 2008, GEM: 2.20 ± 1.39 ng m −3, RGM: 25.2 ± 52.8 pg m −3, Hg p 80.8 ± 283 pg m −3), average GEM levels and daytime average dry depositional RGM flux were highest at the rural site, when all three sites were monitored sequentially in 2008 (rural site, GEM: 2.37 ± 1.26 ng m −3, daytime RGM flux: 29 ± 40 ng m −2 day −1). Photochemical conversion of GEM was not the primary RGM source, as highest net RGM gains (75.9 pg m −3, 99.0 pg m −3, 149 m −3) occurred within 3.0–5.3 h, while the theoretical time required was 14–23 h. Instead, simultaneous peaks in RGM, Hg p, ozone (O 3), nitrogen oxides, and sulfur dioxide in the late afternoon suggested short-range transport of RGM from the urban center to the rural site. The rural site was located more inland, where the average water vapor mixing ratio was lower compared to the other two sites (in 2008, rural: 5.6 ± 1.4 g kg −1, urban: 9.0 ± 1.1 g kg −1, cement plant: 8.3 ± 2.2 g kg −1). Together, these findings suggested short-range transport of O 3 from an urban area contributed to higher RGM deposition at the rural site, while drier conditions helped sustain elevated RGM levels. Results suggested less urbanized environments may be equally or perhaps more impacted by industrial atmospheric Hg emissions, compared to the urban areas from where Hg emissions originated.

Determinants of atmospheric mercury concentrations in Reno, Nevada, U.S.A.

Concentrations of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM) were measured along with ancillary variables 9 km east of downtown Reno, Nevada, U.S.A. from November 2006 through March 2009. Mean two-year (February 2007 through January 2009) GEM, GOM, and PBM concentrations were 2.0 ± 0.7 ng m − 3 (± standard deviation), 18 ± 22 pg m − 3 , and 7 ± 7 pg m − 3 , respectively. Data collected were compared with observations made at another location just north of the city at 169 m higher elevation. At both locations higher concentrations of GEM and PBM occurred in periods with little atmospheric mixing, indicating that local sources were important for enhancing GEM and PBM concentrations in Reno above that considered continental background. Concentrations of GOM were higher (maximum of 177 pg m − 3 ) during periods with higher temperature and lower dew point. Higher GOM concentrations at the higher elevation site with less urban impact relative to the valley site, along with other data trends, support the hypothesis that in northern Nevada subsiding dry air from the free troposphere is a source of GOM to the surface.

Multi-method mercury specification from lignite-fired power plants

Mercury concentration and speciation partitioning, including total mercury, elemental mercury and oxidized mercury from a lignite-fired power plant under different operating conditions, was studied by Ontario hydro method (OHM), two kinds of continuous mercury monitors (semi-continuous emission monitor (SCEM) and continuous mercury monitor (CMM)), and the sorbent trap method. The effects of boiler load, fuel blending ratio, electrostatic precipitator, flue gas desulphurization, flue gas bypassing the FGD ratio, and mercury measuring methods on mercury emission were analyzed. The results indicated that mercury data from OHM, SCEM and CMM presented a good consistency throughout the entire testing period within ±20% acceptable range; however, the results from Appendix K provided bigger discrepancies than the results of OHM and SCEM due to the interferences of higher selenium content in the flue gas. The particulate-bound mercury removal efficiencies of ESP were determined to be 16–35%. The percentages of elemental mercury emitted from two lignite-fired power plants were in the higher ranges of 43.9–74.2%.

Mercury species measured atop the Moody Tower TRAMP site, Houston, Texas

Atmospheric mercury speciation was monitored within Houston, Texas, USA, August 6–October 14, 2006 as part of the TexAQS Radical and Aerosol Measurement Program (TRAMP). On average, all mercury levels were significantly elevated compared to a rural Gulf of Mexico coastal site. Concentrations varied from very clean to very dirty. Multi-day periods of stagnant or low-wind conditions brought elevated concentrations of all mercury species, whereas multi-day periods of strong winds, particularly southerly winds off the Gulf of Mexico, brought very low values of mercury species. Over the entire mercury measurement period, the daily averages of mercury species showed distinct and consistent relationships with the average planetary boundary layer dynamics, with gaseous elemental and particulate-bound mercury near-surface concentrations enhanced by a shallow nocturnal boundary layer, and reactive gaseous mercury concentration enhanced by midday convective boundary layer air entrainment transporting air aloft to the surface. Mercury concentrations were not significantly correlated with known products of combustion, likely indicating non-combustion mercury sources from the Houston area petrochemical complexes. On the morning of August 31, 2006 an observed emission event at a refinery complex on the Houston Ship Channel resulted in extremely high concentrations of aerosol mass and particulate-bound mercury at the TRAMP measurement site 20 km downwind.

Emission of Hg from Coal Consumption in China and Its Summertime Deposition Calculated by CMAQ-Hg

Mercury (Hg) emission from coal combustion in China is estimated in this study and its deposition calculated using the Models-3 Community Multi-scale Air Quality (CMAQ-Hg) modeling system with meteorological fields calculated by MM5. Three kinds of Hg speciation were included in the models: particle-bound mercury (PHg), gaseous elemental mercury (GEM) and gas oxidized mercury (RGM). Much of the Hg was released as RGM from coal combustion in China, accounting for 58.4 - 66.8% of total emissions. The large proportion of RGM in overall Hg emission also led to large Hg deposition. Based on simulations, Hg deposition originating from coal combustion reaches as high as 2 - 6 mu g m(-2) month(-1) in southwestern China (particularly Guizhou Province), the industrialized and populated regions of east central and coastal China, and Liaoning Province in the northeast.

Ambient elemental, reactive gaseous, and particle-bound mercury concentrations in New Jersey, U.S.: measurements and associations with wind direction

Two and a half years of data of ambient concentrations of elemental mercury (Hg(0)), reactive gaseous mercury (RGM), and particle-bound mercury (Hg(p)) were collected at measurement sites at Elizabeth, New Jersey and New Brunswick, New Jersey with Tekran sampling units. The data were processed, summarized, and analyzed from a variety of perspectives. Data quality control and quality assurance procedures are described. Wind direction and wind speed data for each of the sites were also collected. Significant temporal variations in concentrations of all three species were observed. Some significant directional variations were also seen. The sporadic nature of many of the temporal variations is consistent with and could reflect highly variable emissions patterns from anthropogenic mercury sources. Overall mean concentrations of all species were determined. These were, for Hg(0), Hg(p), and RGM respectively; 2.25 +/- 0.04 nanograms per cubic meter (ng/m(3)), 8.21 +/- 0.39 picograms per cubic meter (pg/m(3)), and 8.93 +/- 0.31 pg/m(3) (arithmetic means and 95% confidence intervals) at Elizabeth, and 2.15 +/- 0.02 ng/m(3), 10.73 +/- 0.45 pg/m(3), and 6.04 +/- 0.30 pg/m(3) at New Brunswick. Mean concentrations were determined for 16 different sectors representing wind directions. The impact of one known large source is suggested by these data. Reasons for some directional variations are not apparent and suggest a need for further investigation.

Reactive and particulate mercury in the Asian marine boundary layer

The variability of atmospheric mercury in elemental, reactive, and particulate forms has been studied at a remote site (Cape Hedo Observatory, CHO) at Okinawa Island (Japan) March 23 to May 2, 2004, downwind of the major Asian source regions. Under prevailing meteorological conditions, episodes of higher levels of atmospheric mercury and other atmospheric species are observed at CHO. The mean (±1 σ) concentrations of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (PHg) are 2.04 ± 0.38 ng m −3, 4.5 ± 5.4 pg m −3 and 3.0 ± 2.5 pg m −3, respectively. In Asian outflow the combined contribution of RGM and PHg constitutes less than 1% of the GEM in the boundary layer, which indicates that most mercury export in the marine boundary layer is due to the GEM form, and direct outflow of RGM and PHg is very low. While the data from Okinawa suggest minimal export of RGM and PHg, this does not preclude greater export of these species at higher elevations. Based on the correlations of PHg and submicron aerosol mass (SAM), we found a ΔPHg/ΔSAM ratio of 0.20 μg g −1 ( R = 0.58, p < 0.01), which we believe to be characteristic of East Asian industrial aerosols during outflow. A diurnal variation is observed in RGM with a peak near noon. Using a rate constant of 9 × 10 −14 cm 3 molecule −1 s −1 for the OH oxidation rate (Sommar, J., Gardfeldt, K., Stromberg, D., Feng, X., 2001. A kinetic study of the gas phase reaction between the hydroxyl radical and atomic mercury. Atmospheric Environment 35, 3049–3054; Pal, B., Ariya, P.A., 2004a. Gas-phase HO center dot-initiated reactions of elemental mercury: kinetics, product studies, and atmospheric implications. Environmental Science and Technology 38 (21), 5555–5566.) and a typical OH concentration of 1–5 × 10 6 cm −3 would result in RGM production rates of 0.6–3.0 pg m −3 h −1. Although OH may not be the sole oxidant, this is consistent with the observed change in concentration during daytime of 1.4 ± 1.5 pg m −3 h −1. A significant correlation is found between GEM and CO; GEM and SAM; and PHg and SAM. Lower ΔSAM/ΔCO and ΔGEM/ΔCO are observed for transport events with rainfall and for air parcels remaining in the mixed layer. Back trajectory analysis along with the correlation study suggests that the air from China has a higher GEM concentration compared to the air coming from southern Japan.

Distribution and wet deposition fluxes of total and methyl mercury in Wujiang River Basin, Guizhou, China

Guizhou is one of the most seriously contaminated regions with mercury in China, but mercury deposition fluxes in this region have not been well studied. Concentrations and wet deposition fluxes of total mercury (THg) and methyl mercury (MeHg) were measured from January to December 2006 at 5 sites in Wujiang River Basin, Guizhou, China. Monthly-integrated samples were collected using bulk precipitation samplers. Samples were treated rigorously according to our trace metal protocol, and THg was analyzed by cold vapor atomic fluorescence spectrometry (CVAFS) and MeHg by aqueous phase ethylation, GC separation, and CVAFS detection. The annual volume-weighted mean concentration and wet deposition fluxes of THg and MeHg for 5 sites were 36.0 ng L −1 and 34.7 μg m −2 yr −1, and 0.19 ng L −1 and 0.18 μg m −2 yr −1 in 2006, respectively. The proportion of MeHg to THg ranged from 0.2% to 2.3%, with the mean value of 0.7%. The results also showed that each site had a similar seasonal trend for mercury concentration and wet deposition flux. The largest wet deposition fluxes of THg and MeHg occurred during the rainy season (May–October) when the rainfall was greater, whereas the highest concentrations of THg and MeHg appeared during winter months (December–February). The monthly concentrations of THg and MeHg negatively correlated well with the monthly precipitation depth ( r 2 = 0.17 and 0.12), suggesting that scavenging of particle-bound mercury from the atmospheric is an important mechanism contribution to mercury in rain.

Impact of the Addition of Chicken Litter on Mercury Speciation and Emissions from Coal Combustion in a Laboratory-Scale Fluidized Bed Combustor

Co-combustion of chicken litter with coal was performed in a laboratory-scale fluidized bed combustor to investigate the effect of chicken litter addition on the partitioning behavior of mercury. Gaseous total and elemental mercury concentrations in the flue gas were measured online, and ash was analyzed for particle-bound mercury along with other elemental and surface properties. The mercury mass balance was between 85 and 105%. The experimental results show that co-combustion of chicken litter decreases the amount of elemental and total mercury in the gas phase. Mercury content in fly ash increases with an increasing chicken litter share.

Investigation of the Relationship between Particulate-Bound Mercury and Properties of Fly Ash in a Full-Scale 100 MWe Pulverized Coal Combustion Boiler

The properties of fly ash in coal-fired boilers influence the emission of mercury from power plants into the environment. In this study, seven different bituminous coals were burned in a full-scale 100 MWe pulverized coal combustion boiler and the derived fly ash samples were collected from a mechanical hopper (MH) and an electrostatic precipitator hopper (ESP). The mercury content, specific surface area (SSA), unburned carbon, and elemental composition of the fly ash samples were analyzed to evaluate the correlation between the concentration of particulate-bound mercury and the properties of coal and fly ash. For a given coal, it was found that the mercury content in the fly ash collected from the ESP was greater than in the fly ash samples collected from the MHP. This phenomenon may be due to a lower temperature of flue gas at the ESP (about 135{sup o}C) compared to the temperature at the air preheater (about 350{sup o}C). Also, a significantly lower SSA observed in MH ash might also contribute to the observation. A comparison of the fly ash samples generated from seven different coals using statistical methods indicates that the mercury adsorbed on ESP fly ashes has a highly positive correlation with the unburnedmore » carbon content, manganese content, and SSA of the fly ash. Sulfur content in coal showed a significant negative correlation with the Hg adsorption. Manganese in fly ash is believed to participate in oxidizing volatile elemental mercury (Hg{sup 0}) to ionic mercury (Hg{sup 2+}). The oxidized mercury in flue gas can form a complex with the fly ash and then get removed before the flue gas leaves the stack of the boiler.« less

Measurement of Atmospheric Mercury Species with Manual Sampling and Analysis Methods in a Case Study in Indiana

Ground-level concentrations of three atmospheric mercury species were measured using manual sampling and analysis to provide data for estimates of mercury dry deposition. Three monitoring stations were operated simultaneously during winter, spring, and summer 2004, adjacent to three mercury wet-deposition monitoring stations in northern, central, and southern Indiana. The monitoring locations differed in land-use setting and annual mercury-emissions level from nearby sources. A timer-controlled air-sampling system that contained a three-part sampling train was used to isolate reactive gaseous mercury, particulate-bound mercury, and elemental mercury. The sampling trains were exchanged every 6 days, and the mercury species were quantified in a laboratory. A quality-assurance study indicated the sampling trains could be held at least 120 h without a significant change in reactive gaseous or particulate-bound mercury concentrations. The manual sampling method was able to provide valid mercury concentrations in 90 to 95% of samples. Statistical differences in mercury concentrations were observed during the project. Concentrations of reactive gaseous and elemental mercury were higher in the daytime samples than in the nighttime samples. Concentrations of reactive gaseous mercury were higher in winter than in summer and were highest at the urban monitoring location. The results of this case study indicated manual sampling and analysis could be a reliable method for measurement of atmospheric mercury species and has the capability for supplying representative concentrations in an effective manner from a long-term deposition-monitoring network.

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.

Modification of boiler operating conditions for mercury emissions reductions in coal-fired utility boilers

US EPA's studies have determined that mercury emissions from coal-fired power plants pose significant hazards to public health and must be reduced. Coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere. Mercury emissions are impacted by factors such as coal type, boiler operation, fly ash characteristics and type of environmental control equipment installed on the unit. Field data show a wide variation in the fraction of mercury that is emitted in the gaseous phase, as elemental and oxidized mercury vapor, and as particulate-bound mercury. Oxidized mercury is a preferable species because is less volatile at stack temperatures, water-soluble, and tends to interact with mineral matter and char, and cold-end air pollution control devices. There is also evidence that boiler-operating conditions could be used to influence mercury behavior in the boiler, since they affect the thermo-chemical conditions for mercury formation and reduction. However, operation of boiler control settings, which result in mercury removal benefits, should also consider the tradeoff between mercury reduction and other emissions (for example, NO x and CO), the level of unburned carbon in the fly ash, unit efficiency and the performance of particulate control equipment. This paper reports fieldwork performed to investigate the feasibility of affecting mercury emissions from coal-fired plants by manipulating boiler control settings. Full-scale testing was performed at two units. One of the units is equipped with a back-end train that includes a rotary air preheater followed by two electrostatic precipitators in series. The other unit is equipped with hot and cold precipitators and a tubular air preheater. A strategy for mercury control by selectively manipulating boiler control settings, if used in combination with other control measures such as sorbent injection, should provide a cost-effective option for mercury control.

X-ray photoelectron spectroscopy analysis of mercury sorbent surface chemistry

During coal combustion, mercury completely volatilizes and can remain in the vapor phase throughout the combustion and environmental control system. Mercury is partitioned into elemental, oxidized and particulate phases in the flue gas stream. Air pollution control devices such as wet scrubbers can remove the oxidized and particulate-bound mercury. Elemental mercury cannot be effectively removed by current control technologies. One proposed new technology involves injecting a powdered carbon-based sorbent upstream of the air pollution control device to capture mercury. Previous work has identified lignite-derived activated carbon as a possible sorption material for mercury in flue gas atmospheres. This study is focused on determining the chemical changes taking place on the surface of two activated carbons that may influence mercury sorption and desorption. The carbons tested included the Norit FGD lignite activated carbon and an activated carbon derived from high-sodium lignite. The sorbents were tested for their ability to sorb and retain elemental mercury in a simulated baseline flue gas containing relatively high concentrations of SO 2, NO 2 and HCl, representative of bituminous coal firing. The breakthrough of mercury was determined for each sorbent under different flue gas compositions. The effect of SO 2, NO 2, HCl and H 2O on the carbon's ability to sorb mercury was evaluated for both activated carbons.

Geological sequestration of carbon dioxide: implications for the coal industry

The predominant forms of mercury in coal-fired flue gas are elemental and oxidized. During combustion, the mercury in the coal is transformed into three species that include particle-bound mercury, vapor-phase elemental mercury, and vapor-phase oxidized mercury, primarily in the form of HgCl2. For the best possible removal of mercury from flue gas, a high level of oxidation is helpful. Because HgCl2 is water soluble, it can be removed during the wet flue gas desalifurization process. Particle-bound (Hgp) is easily removed by dust control equipment, such as baghouse filters and electrostatic precipitators. Existing air pollution control devices (APCDs) are capable of removing mercury to some extent. Existing APCDs include electrostatic precipitators (ESPs), fabric filters (FF or baghouse), flue gas desulfurization (FGD), and selective catalytic reduction (SCR). ESPs are designed to reduce fly ash emissions by creating an ionized field that removes charged particles. They have low operating costs, but a limited capacity to remove mercury because it is not generally adsorbed in the fly ash at combustion temperatures. SCR technology is used to reduce emissions of nitrogen oxides by installation of a fixed catalyst bed with reductant injection to reduce NOx to N2. In conjunction with SCR, low NOx burners create a fuel-rich primary combustion zone. This reduces the amount of thermal and fuel NOx created during combustion.

Direct determination of mercury in atmospheric particulate matter by graphite plate filtration–electrothermal atomic absorption spectrometry with Zeeman background correction

A new analytical method has been developed for direct analysis of mercury in atmospheric particulate matter by a graphite plate collection technique coupled with Zeeman electrothermal atomic absorption spectrometry (ETAAS). A small porous graphite plate filter was used initially for collecting particulates and subsequently as a platform in ETAAS. A relatively high ashing temperature of 250°C and a low atomization temperature of 900°C could be achieved by using both a PdCl 2-coated pyrolytic graphite tube and a chemical modifier consisting of a 5% (NH 4) 2S solution. Excellent linearity was maintained over the range of 0–5 ng of Hg. When sampling at a flow rate of 350 ml min −1 for 3 h, this was equivalent to a working concentration range of 0–80 ng m −3 of particle-bound mercury in air. Analysis of the NIST SRM 1648, Urban Particulate Matter, gave a recovery of 101.4%, and a precision of 9.0% R.S.D. The detection limit was estimated to be 0.74 ng m −3. The accuracy was within ±8% when compared with the traditional glass-fiber filtration–acid digestion–cold-vapor atomic absorption spectrometry method.