|Anthony Hynes presents two seminars Aug. 24|
On Friday, Aug. 24, Anthony Hynes will visit UND and speak at two events. He will give a noon seminar at chemical engineering, as well as a talk at the physics department Friday colloquium at 3:30 p.m.
Hynes is a professor at the Division of Marine and Atmospheric Chemistry at the Rosenstiel School of Marine and Atmospheric Science (RSMAS), University of Miami. His research focuses on the kinetics and spectroscopy of small radicals and molecules which are important in atmospheric and combustion chemistry. In addition, he is developing laser-based techniques for the detection of elemental and oxidized mercury species. He will discuss "Atmospheric Chemistry of Mercury: Kinetics, Spectroscopy and Ultrasensitive Detection."
Measurements of the rapid depletion of Hg(0) in the arctic have demonstrated that, at least under some circumstances, mercury can undergo fast chemical cycling. Correlation with the depletion of tropospheric ozone suggests that the mercury transformation mechanism is also driven by halogen chemistry. However the paucity of laboratory kinetic data on reactions of Hg(0) with free radical halogen species makes it difficult to identify and assess the importance of this chemistry in atmospheric transformation. We have made direct measurements of the absolute rate coefficients for the gas phase recombination reactions of Hg(0) with chlorine atoms, Cl, and bromine atoms, Br, using the pulsed laser photolysis-pulsed laser induced fluorescence (PLP-PLIF) technique. Laser photolysis is used to produce the radical of interest followed by LIF detection with a second, tunable, laser. We have studied the reactions of Hg(0) with Cl and Br using photolysis of Cl2 and Br2 while simultaneously monitoring the concentrations of both reactants using LIF. Kinetic measurements were performed with each of the reactants in excess concentration. Rate coefficients for both reactions were determined as a function of pressure from 200 to 600 Torr. Experiments were performed at three temperatures, 243, 261 and 293 K in N2 bath gas. In addition experiments were also performed in He bath gas at 293 K. In both cases the recombination rate coefficients show a linear dependence on temperature, a negative activation energy and an increased deactivation efficiency for N2 as compared to He. These observations are consistent with the expected behavior of a three body recombination reaction but we obtain rate coefficients which are significantly slower than those obtained in relative rate studies. In addition we have monitored the product HgCl and HgBr molecules using LIF. We have shown that sequential two photon laser induced fluorescence is a sensitive, and specific approach for the in-situ detection of gaseous elemental mercury at ambient levels. The detection scheme involves an initial laser excitation of the 63P1-61S0 transition at 253.7 nm, followed by excitation of the 71S0- 63P1 transition at 407.8 nm. Fluorescence is observed on the 61P1-61S0 transition at 184.9 nm using a solar blind photomultiplier tube. We have now applied this approach to the detection of reactive gaseous mercury (RGM) using preconcentration on KCl denuders followed by thermal decomposition / desorption to generate Hg(0). Because of the high sensitivity of sequential two photon-laser induced fluorescence there is no need to reabsorb the Hg(0) on gold, making it possible to monitor the evolution of Hg(0) in real time and use programmable thermal decomposition / desorption to speciate RGM. Preliminary results suggest that it is indeed possible to distinguish between HgCl2 and HgBr2 using this approach. -- Blaise Mibeck, EERC, 777-5077, email@example.com