Mae Sexauer Gustin

Investigation of the light-enhanced emission of mercury from naturally enriched substrates

Incident radiation has been reported to facilitate mercury release from soils. In this study the influence of light on mercury emissions from substrates amended with pure synthetic mercury species, and from naturally and anthropogenically mercury-enriched substrates were investigated using laboratory experiments and in situ flux measurements. Light-enhanced emissions were found to occur from substrates amended with HgS, and from elemental mercury (Hg 0 ) and HgCl2 amended iron oxide and organic containing substrates. The magnitude of light-enhanced emissions for natural substrates ranged from 1.5 to 116 times that occurring in the dark at the same substrate temperature. Substrates containing corderoite, metacinnabar and ‘‘matrix bound mercury’’ (that bound to organic or inorganic phases) exhibited a higher degree of light-enhanced emissions relative to that containing predominantly cinnabar. Calculated activation energies for both laboratory and field data indicate that photo-reduction is a process associated with the light-enhanced emissions. Activation energies, derived using in situ mercury fluxes and soil temperatures, indicated that photo-reduction was a dominant process facilitating release of Hg from substrates with sunrise. Activation energies, calculated using daytime data, were less than those calculated for sunrise. This is hypothesized to be due to a pool of Hg 0 being developed with photo-reduction at first light that is released as soil temperatures and convective heat transfer increase during the day. This study demonstrated that light energy is the more dominant process controlling mercury emissions from naturally enriched substrates than soil temperature. r 2002 Elsevier Science Ltd. All rights reserved.

Assessing the contribution of natural sources to regional atmospheric mercury budgets

Naturally mercury-enriched substrate is a long-lived source of mercury to the global atmospheric mercury cycle. Field flux chambers, laboratory gas exchange chambers and micrometeorological methods may be applied to estimate emissions from these sources. However, field chamber experimental design may affect the magnitude of the fluxes measured, and the laboratory chamber only provides a minimum estimate of flux. Many factors, such as mercury concentration and speciation in substrate, light, precipitation, and temperature, influence the emission of mercury from the substrate. Mercury concentration in the substrate is a dominant factor controlling emissions and may be used to predict emissions from regions of mercury enrichment. Mercury fluxes measured from three areas of natural enrichment and three areas with low levels of mercury enrichment are 1-5 orders of magnitude greater than the value applied to global belts of natural enrichment. Preliminary scaling of emissions from one of these areas and for western North America indicates that mercury enriched areas may be significant sources of mercury to the atmosphere, and that their contribution to regional and global atmospheric budgets needs to be reassessed.

Are mercury emissions from geologic sources significant? A status report.

Geologic sources of atmospheric mercury include areas of fossil and current geothermal activity, recent volcanic activity, precious and base metal deposits, and organic rich sedimentary rocks. Early estimates of emissions from these sources were not based on measurements of mercury fluxes but implied based on the difference between emissions from anthropogenic point sources and wet/dry deposition estimates. In the past approximately 7 years significant progress has been made in development of methods for the measurement of mercury emissions, definition of those parameters most important in controlling emissions and scaling up emissions from natural source areas. This paper summarizes the work done on scaling of emissions from discrete areas of natural enrichment and from the State of Nevada, which is situated within a global belt of mercury enrichment. Preliminary data indicate that elemental mercury is the predominant (>95%) form of mercury being emitted from these sources. Scaling results suggest that the value used in early models to represent emissions from global mercuriferous belts is too low by at least three times.

Effect of temperature and air movement on the flux of elemental mercury from substrate to the atmosphere

The flux of elemental mercury vapor from intact mill tailings (36 to 1270 μg Hg/g), soil (7 μg Hg/g), and cinnabar ore (934 μg Hg/g) was measured as a function of temperature (20°–60°C) and wind velocity (0.2–0.8 m/s) using a controlled environment, open gas-exchange system. Continuous air movement over core surfaces in the gas-exchange chamber resulted in a logarithmic decline in mercury flux with time. Measurement of the effect of environmental parameters on mercury flux was done after attainment of a quasi steady state of flux. Prior to attainment of this state the activation energy for mercury flux was less than the molar heat of vaporization of element mercury (14 kcal/mol). At steady state the substrate-to-air flux of mercury vapor increased logarithmically with temperature, mimicking the elements vapor pressure curve; and activation energies (16.4 to 25.7 kcal/mol) for mercury flux were higher than the molar heat of vaporization of elemental mercury due to physicochemical properties of the soil (e.g., porosity, organic matter, clay content) that affect gas-phase mercury transport and fate. A change in wind velocity from 0.2 to 0.8 m/s resulted in an increase in mercury flux by a factor of 2 for a core with >150 μg Hg/g and no significant response from two cores with <150 μg Hg/g. Using data from gas-exchange experiments, equations were derived for predicting the response of mercury flux to a range of temperatures and wind velocities for a variety of substrate mercury concentrations. The equations and the results of this study are used to predict the flux of mercury to the atmosphere from substrate enriched in mercury.

Do We Understand What the Mercury Speciation Instruments Are Actually Measuring? Results of RAMIX

From August 22 to September 16, 2012, atmospheric mercury (Hg) was measured from a common manifold in the field during the Reno Atmospheric Mercury Intercomparison eXperiment. Data were collected using Tekran systems, laser induced fluorescence, and evolving new methods. The latter included the University of Washington-Detector for Oxidized Mercury, the University of Houston Mercury instrument, and a filter-based system under development by the University of Nevada-Reno. Good transmission of total Hg was found for the manifold. However, despite application of standard protocols and rigorous quality control, systematic differences in operationally defined forms of Hg were measured by the sampling systems. Concentrations of reactive Hg (RM) measured with new methods were at times 2-to-3-fold higher than that measured by Tekran system. The low RM recovery by the latter can be attributed to lack of collection as the system is currently configured. Concentrations measured by all instruments were influenced by their sampling location in-the-manifold and the instrument analytical configuration. On the basis of collective assessment of the data, we hypothesize that reactions forming RM were occurring in the manifold. Results provide a new framework for improved understanding of the atmospheric chemistry of Hg.

Dynamic flux chamber measurement of gaseous mercury emission fluxes over soils: Part 2—effect of flushing flow rate and verification of a two-resistance exchange interface simulation model

Both field and laboratory tests demonstrated that soil Hg emission fluxes measured by dynamic flux chamber (DFC) operations strongly depend on the flushing air flow rates used. The general trend is an increase in the fluxes with increasing flushing flow rates followed by an asymptotic approach to flux maximum at sufficiently high (optimum) flushing flow rates. This study indicates that the DFC measurements performed at low flushing flow rates can underestimate Hg emission fluxes over soils, especially Hg-enriched soils. High flushing flow rates therefore are recommended for accurate estimation of soil Hg emission fluxes by DFC operations. The dependence of DFC-measured soil Hg emission fluxes on flushing flow rate is a physical phenomenon inherent in DFC operations, regardless of DFC design and soil physical characteristics. Laboratory tests using DFCs over different soils confirmed the predictions of a two-resistance exchange interface model and demonstrated the capability of this model in quantitatively simulating Hg emissions from soils measured by DFC operations.

Reducing the Uncertainty in Measurement and Understanding of Mercury in the Atmosphere

Elucidating the extent of mercury in the atmosphere requires deployment of robust and sensitive instruments.

Scaling of atmospheric mercury emissions from three naturally enriched areas: Flowery Peak, Nevada; Peavine Peak, Nevada; and Long Valley Caldera, California

With the development of analytical capabilities that allow for almost real time measurement of mercury concentrations in air, the fluxes of mercury between environment compartments is being more carefully scrutinized. Recent advances have demonstrated that the mercury cycle is much more complicated than previously realized. This study quantified the mercury emissions from three areas with low levels of mercury enrichment associated with precious and base metal mineralization and recent volcanic/geothermal activity. Area emissions were calculated using Geographic Information System technology, and in situ derived mercury fluxes and those parameters found to statistically be dominant in controlling emissions. The most important controls on emission strengths were found to be geologic while environmental parameters such as light and temperature were found to drive the diel pattern typically observed for mercury emissions. Calculated area averaged emissions were 18.5, 10.0, and 13.6 ng/m2 h for the Flowery Peak, NV, Peavine Peak, NV, and Long Valley Caldera, CA areas, respectively. These emissions are an order of magnitude higher than values applied in global models for natural sources. This study, along with other recent work, demonstrates that natural sources may contribute more mercury than previously recognized to the atmospheric mercury pool.

Comparison of Gaseous Oxidized Hg Measured by KCl-Coated Denuders, and Nylon and Cation Exchange Membranes

The chemical compounds that make up gaseous oxidized mercury (GOM) in the atmosphere, and the reactions responsible for their formation, are not well understood. The limitations and uncertainties associated with the current method applied to measure these compounds, the KCl-coated denuder, are not known due to lack of calibration and testing. This study systematically compared the uptake of specific GOM compounds by KCl-coated denuders with that collected using nylon and cation exchange membranes in the laboratory and field. In addition, a new method for identifying different GOM compounds using thermal desorption is presented. Different GOM compounds (HgCl2, HgBr2, and HgO) were found to have different affinities for the denuder surface and the denuder underestimated each of these compounds. Membranes measured 1.3 to 3.7 times higher GOM than denuders in laboratory and field experiments. Cation exchange membranes had the highest collection efficiency. Thermodesorption profiles for the release of GOM compounds from the nylon membrane were different for HgO versus HgBr2 and HgCl2. Application of the new field method for collection and identification of GOM compounds demonstrated these vary as a function of location and time of year. Understanding the chemistry of GOM across space and time has important implications for those developing policy regarding this environmental contaminant.

Empirical Models for Estimating Mercury Flux from Soils

Multiple parameters have been suggested to influence the exchange of mercury (Hg) between the atmosphere and soils. However, models applied for estimating soil Hg flux are simple and do not consider the potential synergistic and antagonist relationships between factors controlling the exchange. This study applied a two-level factorial experimental design in a gas exchange chamber (GEC) to investigate the individual and combined effects of three environmental factors (temperature, light, and soil moisture) on soil Hg flux. It was shown that individually irradiation, soil moisture, and air temperature all significantly enhance Hg evasive flux (by 90-140%). Synergistic effects (20-30% of additional flux enhancement) were observed for all two-factor interactions, with air temperature/soil moisture and air temperature/irradiation being the most significant. Results from the factorial experiments suggest that a model incorporating the second-order interactions can appropriately explain the flux response to the changes of the studied factors. Based on the factorial experiment results and using the flux data for twelve soil materials measured with a dynamic flux chamber (DFC) at various temperatures, soil moisture contents, solar radiation exposures, and soil Hg contents, two empirical models for estimating Hg flux from soils were developed. Model verification with ambient flux data not used to develop the models suggested that the models were capable of estimating dry soil Hg flux with a high degree of predictability (r ∼ 0.9).