2005 Progress 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 Period Covered by this Report: January 1, 2005 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 objective of the research project is to produce a series of measurements and technique developments that will allow both the chemical reactivity, the atmospheric concentrations, and the rates of emission and deposition of both elemental and reactive gaseous mercury (RGM), to be better defined. Laboratory studies will measure the rate coefficients for the reactions of Hg(0) with the hydroxyl radical, halogen atoms, X, and halogen monoxides, XO, where X = Cl, Br, I. When feasible, the reaction products and their yields will be identified. Reactions will be studied under conditions that are representative of the arctic, the upper troposphere, and the global marine boundary layer. In addition, we will investigate the feasibility of laser based excitation schemes for the rapid, ultrasensitive detection of gas phase elemental mercury, and RGM.

Progress Summary:

In our previous report, we summarized experiments, which reported detailed rate coefficients for the rate coefficients for the reaction of Hg(0) and Br, using photolysis of Br2 and monitoring the decay of Br in an excess of Hg(0).

Hg + Br + M → HgBr + M    (1)

We now have measured the rate coefficient for reaction (1) above at room temperature using the photolysis of CF3Br as a source of Br atoms and with Hg atoms in excess. Our results, which can give only an upper limit to the rate coefficient, are consistent with our studies using Br2 photolysis with Br in excess. To ensure that our kinetics observations are indeed consistent with reaction (1), we have attempted to observe product HgBr from the recombination reaction. The laser excitation spectrum obtained after photolysis of Br2 generating excess Br atoms in the presence of Hg is much more complex and bears no resemblance to the well defined band structure we have obtained for the D-X system of HgBr. Laser excitation spectra using the photolysis of CF3Br, producing Br in the presence of Hg, however, are essentially identical with those produced by photolysis of HgBr2 showing that the chemistry is indeed consistent with reaction (2). We speculate that HgBr may react with Br2 producing HgBr2 and that our observations of the dense but reproducible spectral features may be photofragment emissions from the product HgBr2.

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 laser-induced fluorescence (LIF). We have found that after excitation of the of D 2Π3/2- X 2Σ+ transition in the ultraviolet, we see efficient energy transfer from the D 2Π3/2 state to the B 2Σ+ state followed by fluorescence on the B 2Σ+- X 2Σ+ transition at approximately 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 caused by 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 then was 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. 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.

Future Activities:

We now are within a 1 year no-cost extension and we plan to attempt to complete studies of the Hg + Br at high temperature.


Journal Articles on this Report : 4 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
  • Journal Article Donohoue D, Bauer D, Hynes AJ. Sequential two-photon LIF detection of Hg0 and RGM. RMZ - Materials and Geoenvironment 2004;51(3):1532-1534. R829795 (2005)
    not available
    Journal Article Hynes AJ, Donohoue D, Bauer D. Laser induced fluorescence studies of atmospheric mercury cycling: ultra-sensitive detection and laboratory kinetics. RMZ-Materials and Geoenvironment 2004;51(3):1616-1619. R829795 (2005)
    not available

    Supplemental Keywords:

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

    Progress and Final Reports:

    Original Abstract
  • 2003 Progress Report
  • 2004 Progress Report
  • Final Report