Laboratory Kinetics

Group Members

• Geoff Tyndall
• John Orlando
• Yongxin Tang

The oceans act as a source of biogenic organic species to the atmosphere. Among the important oceanic emissions are species that contain iodine (e.g., methyl iodide, CH3I, and larger compounds). Because these species tend to be very short-lived and because iodine can act as a catalyst for ozone destruction, these species can be important contributors to the oxidative capacity of the marine boundary layer. ACD scientists, in collaboration with scientists at the Georgia Institute of Technology, Auburn University and the Ford Motor Company, have been studying the atmospheric oxidation pathways of these compounds using a variety of experimental and theoretical techniques. Earlier work has shown that alkenes are significant products of the atmospheric oxidation of these iodinated species. The current focus is on an as-yet unexplained large yield of organic peroxyacids from the oxidation of these species, and on the mechanism of iodinated alkyl radicals with O­2.

Both hydroperoxy radicals and acetylperoxy radicals play important roles in the tropospheric oxidation mechanisms of biologically emitted compounds. HO2 radicals are central to the production of ozone, and the generation of hydroxyl radicals, through their interaction with NO. Acetylperoxy radicals, formed in the oxidation of acetaldehyde, are required to form peroxyacetyl nitrate (PAN). Both these reactions require the presence of nitrogen oxides (NOx). However, at low NOx, acetylperoxy radicals and hydroperoxy radicals react together. Until a few years ago, this reaction was thought to result solely in the formation of acetic and peracetic acids. In 2003, ACD scientists, in collaboration with Alam Hasson of Fresno State University, showed that the reaction has another major pathway, leading to hydroxyl and methyl radicals, with a consequent reduction in the formation of acids. In 2007 the collaboration was continued, in order to investigate the reaction system at other temperatures (250-345 K). Acetic acid and peracetic acid were measured by infrared spectroscopy, while peracetic acid and methyl hydroperoxide were measured by liquid chromatography. No major change in mechanism was found over thee extended temperature range. The experiments at low temperature also allowed the products of the addition of HO2 to acetaldehyde to be studied. This reaction may be important at the colder temperatures of the upper troposphere, and could lead to the formation of formic and acetic acids, the levels of which are larger than can currently be explained.

The most definitive measurements of reaction rate constants are usually made through time-resolved methods. However, this requires the application of a sensitive, specific detection technique for the free radicals. ACD scientists have developed a flash photolysis system with a solid-state diode laser operating at 1.4 micrometer to detect HO2 radicals. The experiment uses partially silver-coated mirrors to couple both a UV photolysis beam and the IR detection beam into the reaction cell. In order to quantify the concentration of HO2 radicals, it was first necessary to derive IR absorption cross sections. This was accomplished using acetylene as a photochemical actinometer. Having derived the absorption cross sections, it was possible to measure the rate of the HO2 self reaction, which is in good agreement with literature measurements. Experiments are now in progress to measure the rate of addition of HO2 to acetaldehyde and to acetone, two atmospherically important carbonyl compounds. The results of these time-resolved experiments will be coupled with the chamber measurements described above, to fully understand the oxidation mechanisms of these compounds.

The figure shows the second order decay of HO2 radicals (plot of 1/[HO2] vs time). The diode laser is swept across an HO2 absorption feature at a rate of 100Hz, giving a discrete measurement of HO2 every 10 ms. The inset shows the HO2 absorptions, corresponding to an initial concentration of 8 x 1012 molecule cm‑3.