Final Report: Laser Based Studies of Atmospheric Mercury Transformation: Laboratory Kinetics and Ultrasensitive Detection of Elemental and Reactive Gaseous Mercury

EPA Grant Number: R829795
Title: Laser Based Studies of Atmospheric Mercury Transformation: Laboratory Kinetics and Ultrasensitive Detection of Elemental and Reactive Gaseous Mercury
Investigators: Hynes, Anthony J.
Institution: University of Miami
EPA Project Officer: Chung, Serena
Project Period: January 1, 2003 through December 31, 2006
Project Amount: $559,363
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text |  Recipients Lists
Research Category: Mercury , Air Quality and Air Toxics , Safer Chemicals , Air

Objective:

The objectives of this research program were a series of measurements and technique developments that will allow the chemical reactivity, the atmospheric concentrations, and the rates of emission and deposition of both elemental and reactive gaseous mercury, to be better defined. Laboratory studies used the pulsed laser photolysis – pulsed laser induced fluorescence technique to perform direct measurements of the rate coefficients for the reactions of Hg(0) with the chlorine and bromine atoms.

Hg + Cl + M -> HgCl + M (1)
Hg + Br + M -> HgBr + M (2)

The halogen recombination rate coefficients for reactions 3 and 4 were also measured to confirm that the experimental approach was sound.

Cl + Cl + M -> Cl2 + M (3)
Br + Br + M -> Br2 + M (4)

The reaction products, HgCl and HgBr have been identified spectroscopically. The reactions were studied under conditions that are representative of the arctic, the upper troposphere and the global marine boundary layer. In addition we have investigated the feasibility of laser based excitation schemes for the rapid, ultrasensitive detection of gas phase elemental mercury and reactive gaseous mercury and the speciation of reactive gaseous mercury.

Summary/Accomplishments (Outputs/Outcomes):

The three body recombination reactions of mercury, Hg(0), with chlorine atoms, Cl, and bromine atoms, Br, have been studied as a function of temperature and pressure under realistic atmospheric conditions.

Hg + Cl + M -> HgCl + M (1)
Hg + Br + M -> HgBr + M (2)

Direct determination of rate coefficients for the reactions of gaseous elemental mercury presents a significant experimental challenge due to the low vapor pressure of mercury. This low vapor pressure makes it difficult to study the kinetics of this system using a traditional approach with the stable reactant in pseudo-first order excess for anything other than reactions with very fast rate coefficients. To overcome this difficulty we made kinetic measurements under conditions in which chlorine atoms were the reactant in pseudo-first order excess whilst simultaneously monitoring the concentration of both reactants using laser induced fluorescence (LIF). Experiments were performed at 200, 400 and 600 Torr and at temperatures of 245 K, 261 K and 293 K in N2 buffer gas. In addition a series of measurements were performed in He buffer at 200, 400 and 600 Torr and at 293 K. At the halogen atom concentrations required to observe a significant loss of mercury atoms, the halogen atom self recombination reactions resulted in a significant decrease in halogen atom concentration on the timescale of the mercury atom decays and a simple pseudo-first order decay, i.e. an exponential decay, of the mercury atoms was not observed. Instead, the mercury temporal profiles were fit by numerical integration, and the observed halogen atom temporal profiles were analyzed assuming simple second order kinetics. In each case the effective second order rate coefficients showed a linear dependence on pressure indicating that the reaction was in the low pressure, third order regime as might be expected for an atom-atom recombination. The rate coefficients increased with decrease in temperature and the rate coefficients in N2 were faster than those measured in He. Both observations are consistent with the expected behavior for a simple three body recombination. The Arrhenius expression for reaction (1) in N2 is given by equation (I) reported with 2σ errors of precision only.

Equation 1. (I)

The Arrhenius expression for reaction 2 in N2 is given by equation II reported with 2σ errors of precision only.

Equation 2. (II)

Due to uncertainty in the calculation of absolute halogen atom concentrations and other unidentified systematic error we conservatively estimate the error in the rate coefficient to be ± 50%. In both cases the observed behavior is consistent with a three-body recombination, demonstrating a positive pressure dependence, an inverse temperature dependence, and a slower rate coefficient in helium than in nitrogen.

To ensure that our observations were consistent we measured the rate coefficients at room temperature with mercury in large excess over the halogen atom concentration. These results were consistent with the results obtained with the halogen atoms in excess. We also measured the halogen atom self recombination rate coefficients, reactions 3 and 4.

Cl + Cl + M -> Cl2 + M (3)
Br + Br + M -> Br2 + M (4)

This provides a measure of the accuracy of our calculation of the halogen atom concentrations, a critical component of the rate coefficient calculation. In both cases our results are consistent with literature values suggesting the calculated atom concentrations are correct. The rate coefficients obtained in this work are considerably slower than values obtained in recent competitive rate studies. We believe that there are problems associated with heterogeneous chemistry that make the use of relative rate studies problematic for mercury-halogen reactions. To evaluate the importance of the recombination of elemental mercury and bromine atoms, an effective second order rate coefficient of 4.6 × 10-13 cm3 molecules-1 s-1 was calculated from the reported Arrhenius expression (II), for Arctic conditions, 260 K and 760 Torr. Assuming a peak concentration of bromine atoms of 107 – 108 cm-3 the lifetime of mercury due to bromine is between 2.5 days and 6 hours. This means reaction 2 could play a significant role in Arctic mercury depletion events. However, the importance of the recombination of mercury and bromine atoms, reaction 1, will depend on the stability and reactivity of the HgBr species. Our results suggest that the recombination reaction of mercury with chlorine atoms does not contribute significantly to the chemistry of mercury depletion events.

We have been examining routes to the identification and monitoring of both total and speciated RGM. Absolutely specific identification of both HgCl2 and HgBr2 can be achieved using photofragment LIF. We have found that after excitation of the of D 2Π3/2- X 2+ transition in the uv we see efficient energy transfer from the D 2Π3/2 state to the B 2+ state followed by fluorescence on the B 2+- X 2+ transition at ~ 500 nm. Because the B-X emission is spectrally shifted from the excitation wavelength it is possible to monitor HgBr with much greater sensitivity at this wavelength.

By coupling sequential two-photon LIF and KCl denuder sampling it is possible to make in-situ measurements of RGM concentrations. RGM is collected by flushing ambient air through a KCl denuder at 10 L min_1. The loaded KCl denuder is then flushed with nitrogen at a flow rate of 200 sccm and heated to 500 C. As the denuder is heated the RGM decomposes to Hg(0) and is detected by the LIF system. Currently, the most significant limitation for RGM detection is the 3.6 pg blank, which we believe is mostly due to low level contamination in our sampling lines. RGM samples were collected from known sources of HgO, HgBr2, or HgCl2 onto a KCl denuder. The denuder was then heated in 25°C steps from 100°C to 250°C and then rapidly ramped to 500°C. HgO demonstrates a significantly different profile than HgBr2 or HgCl2. The desorption profiles of HgCl2 and HgBr2 on both coated and uncoated denuders are sufficiently similar to preclude their use for speciation. However we a distinct difference in their desorption profiles from an uncoated Pyrex tube. This is the first speciation study with sufficient sensitivity to observe these variations and could be a viable method of speciating atmospheric RGM. It shows the feasibility of using a novel sensor, sequential two-photon LIF, to detect Hg(0) and RGM at ambient levels and potentially speciate RGM.

Conclusions:

We have reported recombination rate coefficients for the reactions of mercury with chlorine and bromine atoms, together with the self-reactions of bromine and chlorine atoms. In all cases the rate coefficients show pressure and temperature dependencies, as well as, third body deactivation efficiencies, which are consistent with a three-body recombination. For the mercury halogen recombinations, we obtain rate coefficients that are slower than previously reported rate coefficients. The discrepancy observed between this work and the relative rate studies together with the variability in those studies questions the viability of using the relative rate method to determine kinetic rate coefficients for mercury halogen reactions. By coupling sequential two-photon LIF and KCl denuder sampling it is possible to make in-situ measurements of RGM concentrations. Our results suggest that measurement of the thermal desorption profiles of species captured on denuders may offer a route to the speciation of reactive gaseous mercury (RGM).


Journal Articles on this Report : 2 Displayed | Download in RIS Format

Other project views: All 12 publications 4 publications in selected types All 4 journal articles
Type Citation Project Document Sources
Journal Article Donohoue DL, Bauer D, Hynes AJ. Temperature and pressure dependent rate coefficients for the reaction of Hg with Cl and the reaction of Cl with Cl: a pulsed laser photolysis-pulsed laser induced fluorescence study. Journal of Physical Chemistry A 2005;109(34):7732-7741. R829795 (2004)
R829795 (2005)
R829795 (Final)
  • Abstract from PubMed
  • Journal Article Donohoue DL, Bauer D, Cossairt B, Hynes AJ. Temperature and pressure dependent rate coefficients for the reaction of Hg with Br and the reaction of Br with Br: a pulsed laser photolysis-pulsed laser induced fluorescence study. Journal of Physical Chemistry A 2006;110(21):6623-6632. R829795 (2005)
    R829795 (Final)
  • Abstract from PubMed
  • Supplemental Keywords:

    Scientific Discipline, Air, INTERNATIONAL COOPERATION, Waste, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Air Quality, air toxics, Environmental Chemistry, Chemicals, Fate & Transport, Environmental Monitoring, Chemistry and Materials Science, Atmospheric Sciences, fate and transport, air pollutants, Hg, mercury, mercury emissions, modeling, mercury cycling, chemical kinetics, atmospheric mercury chemistry, mercury chemistry, atmospheric chemistry, atmospheric mercury cycling, atmospheric deposition, contaminant transport models, heavy metals, mercury vapor, atmospheric mercury, gaseous mercury

    Progress and Final Reports:

    Original Abstract
  • 2003 Progress Report
  • 2004 Progress Report
  • 2005 Progress Report