2003 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, 2003 through December 31, 2004
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
Recent observations have suggested that gas phase chemical reaction may play a role in the atmospheric transformation of mercury. The overall objectives of this research project are a series of measurements and technique developments that will allow both the chemical reactivity, atmospheric concentrations, and 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 Hg(0) with the hydroxyl radical and with halogen atoms, X, and halogen monoxides, XO, where X = 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 mercury and reactive gaseous mercury.
We have studied the reactions of Hg(0) with Cl and Br using photolysis of Cl2 and Br2 monitoring the decay of Cl and Br in an excess of Hg(0):
Hg + Cl + M -> HgCl + M (1)
Hg + Br + M -> HgBr + M (2)
We performed measurements at 100 and 200 Torr in N2. We saw no evidence for any reaction within the precision of our measurements. To ensure that our experimental approach was reasonable, we monitored the decay of Cl atoms in a known concentration of C2H6, and we obtained good agreement with the literature rate coefficient for this reaction. Our results suggest that the rate coefficients for reactions (1) and (2) are slower than 5 x 10-13 cm3 molecule-1 s-1 at 200 Torr. For reaction (1), this would give a limit of 2 x 10-12 cm3 molecule-1 s-1 at 1 atmosphere pressure. We have photolyzed HgCl2 at 266 nm and directly monitored HgCl on the 23/2- 2 transition. This is the first observation of this molecule using the laser-induced fluorescence (LIF) technique. We have obtained LIF spectra of the (0-0) and (1-0) bands. We have obtained dispersed fluorescence spectra, pumping the (0-0) and (1-0) bandheads. In addition, this demonstrates that we can use photofragment LIF to unambiguously monitor HgCl2. We have examined the use of sequential two photon LIF in conjunction with preconcentration for the detection of ambient Hg(0) and reactive gaseous mercury.
We will monitor the decay of Hg(0) in an excess of Cl and Br atoms, monitoring both species by LIF. To avoid problems with secondary chemistry, we will produce both Hg(0) and our free radical photolytically. It required considerable effort to detect BrO using 2-photon LIF excitation on the C 2 -X 2 transition at 353 nm, and we have not been able to reproduce the results of Delmdahl, et al., and observe BrO fluorescence. One potential problem may have been the insensitivity of our photomultiplier tube (PMT) at the detection wavelength. We have purchased a PMT, which is similar to that used by Delmdahl, et al., and we plan to repeat these experiments. We will attempt to detect both HgBr and HgI using LIF on the 23/2- 2 transition, using photolysis of HgBr2 and HgI2 to produce the molecules. We also will attempt to calibrate the sensitivity of photofragment LIF for the direct detection of HgCl2, HgBr2, and HgI2, using KCl denuders to characterize the HgX2 source concentrations. Work on ambient detection approaches to Hg(0) and reactive gaseous mercury will focus on improving our sampling approach to reduce our blank signals.