2004 Progress Report: Laser Based Studies of Atmospheric Mercury Transformation: Laboratory Kinetics and Ultrasensitive Detection of Elemental and Reactive Gaseous MercuryEPA 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, 2004 through December 31, 2005
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
The overall objectives of the research project are to obtain a series of measurements and develop techniques 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 will measure the rate coefficients for the reactions of elemental mercury (Hg(0)) with the hydroxyl radical, halogen atoms (X), and halogen monoxides (XO), where X is Cl, Br, or 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 Hg and reactive gaseous Hg.
We have completed detailed studies of reactions (1) and (2) under conditions in which we observe the decay of Hg(0) in an excess of Cl and Br atoms monitoring both species by laser- induced fluorescence (LIF).
Hg + Cl + M HgCl + M (1)
Hg + Br + M HgBr + M (2)
Under these conditions, the excess halogen atoms also decay by self reaction, and accurate rate coefficients for these reactions are required to interpret the Hgmercury-halogen data. As a consequence, we also measured the rate coefficients for the halogen atom recombination reactions (3) and (4).
Cl + Cl + M Cl2 + M (3)
Br + Br + M Br2 + M (4)
Rate coefficients were determined as a function of pressure (200-600 Torr) and temperature (243 K-293 K) in N2 buffer gas and as a function of pressure (200-600 Torr) in He buffer gas at room temperature. For reaction (1), an effective second order rate coefficient of 7.6 H 1013 cm3 molecules-1 s-1 was calculated from the reported Arrhenius expression for AaArctic conditions, 260 K and 760 Torr. Assuming a peak concentration of chlorine atoms of 104-105 cm-3, the lifetime of Hgmercury as a result ofdue to reaction with Clchlorine atoms is between 4.2 years and- 152 days. This suggests that the recombination reaction of Hgmercury with Clchlorine atoms does not contribute significantly to the chemistry of arctic Hgmercury depletion events. For reaction (2), an effective second order rate coefficient of 4.5 H 1013 cm3 molecules-1 s-1 was calculated from the reported Arrhenius expression for AaArctic conditions, 260 K and 760 Torr. Assuming a peak concentration of Brbromine atoms of 107 - 108 cm-3 , the lifetime of Hgmercury as a result of reaction with because ofdue to Brbromine is between 2.5 days -to 6 hours. This means the reaction (1) could play a significant role in the arctic Hgmercury depletion events. However, tThe importance of the recombination of Hgmercury and Brbromine atoms, reaction (2), will depend on the stability and reactivity of the HgBr species. Further studies into the reactivity of the HgBr are ongoing.
Reaction Product Detection
We have photolyzed both HgCl2 and HgBr2 and directly monitored HgCl and HgBr on the 23/2 - 2∑ transition. This is the first observation of these molecules using the LIF laser induced fluorescence technique. We have been using these transitions to attempt to monitor the HgX (X = Cl, Br) product of the recombination reactions in order to attempt to derive rates for the reaction of HgX with X. However t The spectra we have obtained, however, are extremely complex and this work continues.
We are attempting to measure the rate coefficients for the reactions of HgCl with Cl and HgBr with+ Br. The rate of the reaction of HgBr with Br is critical in estimating the role of Br atoms in arctic Hgmercury depletion events. We also plan to measure the rate of the reaction of Hg(0) with OH, with OH as the excess reactant. We will also will attempt to calibrate the sensitivity of photofragment LIF for the direct detection of HgCl2, and HgBr2 and useing KCl denuders to characterize the HgX2 source concentrations. Work on ambient detection approaches to Hg(0) and RGM will continue to focus on improving our sampling approach to reduce our blank signals.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
|Other project views:||All 12 publications||4 publications in selected types||All 4 journal articles|
||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.||