Grantee Research Project Results
1998 Progress Report: The Chemical Kinetics and Mechanisms of Hydrocarbons Contributing to Ozone Production in the Atmosphere
EPA Grant Number: R825258Title: The Chemical Kinetics and Mechanisms of Hydrocarbons Contributing to Ozone Production in the Atmosphere
Investigators: Demerjian, Kenneth L. , Anderson, James G.
Institution: The State University of New York , Harvard University
EPA Project Officer: Hahn, Intaek
Project Period: November 25, 1996 through November 24, 1999
Project Period Covered by this Report: November 25, 1997 through November 24, 1998
Project Amount: $534,939
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
Perform laboratory chemical kinetic and mechanistic studies of hydrocarbon oxidation by ozone and hydroxyl radical using a high-pressure flow system. Specific studies include: (1) direct measurement of OH formation yields in ozone-olefin reactions; and (2) kinetic rate constant and mechanistic pathway studies of the OH-isoprene and OH- -, -pinene reactions.Progress Summary:
The reaction of ozone with alkenes is believed to be an important source of HOx in the urban and regional troposphere. However, the reaction mechanism and radical yields are highly controversial. Studies have been performed which provide direct measurements of OH and H radicals produced by the reactions of ozone with tetramethylethylene, trans-2-butene and ethylene. Measurements were made over a range of pressures in an attempt to observe any collisional stabilization of highly vibrationally excited reaction products. Using LIF for OH and RF for H, we observe the radicals at steady-state, produced by the ozone-alkene reaction and lost to reaction with the alkene. The measured pressure-dependent radical yields for these three representative alkenes are consistent with the calculated potential energy surface for the reaction, further supporting the conclusion that the radicals are prompt products produced by unimolecular decomposition of the vibrationally excited ozone-alkene reaction products. Isoprene, 2-methyl-1,3-butadiene, is the most abundantly emitted biogenic hydrocarbon, and its dominant loss mechanism in the atmosphere is reaction with OH. This oxidation mechanism has important implications for tropospheric ozone production and potentially impacts NOx chemistry in moderate to high NOx regions. With four sites for OH attack, a total of eight hydroxycarbonyl species, eight alkyl nitrates, methyl vinyl ketone (MVK), methacrolein (MAC), formaldehyde and 3-methylfuran are thought to comprise the first stage of stable products from isoprene's oxidation in the presence of high NO levels. In the present work, isoprene's first stage products were produced, without detectable subsequent reaction, and analyzed in-situ by FTIR spectroscopy. We quantitatively identified methyl vinyl ketone, methacrolein, formaldehyde, reacted isoprene, NO and NO2 levels. While our experimental conditions are somewhat different from previous studies (440 torr, 10% Ar/air mixture), our results are consistent, with 70 +/- 10% of reacted isoprene forming methacrolein (30%) or methyl vinyl ketone (40%). However, we were unable to detect any 3-methyl furan, giving a yield below 1.5%. The FTIR analysis precludes differentiation of the hydroxycarbonyl and alkylnitrate species, but using anominal alkylnitrate cross section, an overall kylnitrate yield of about 7% is also consistent with previous IR product studies. The isoprene oxidation by OH in the presence of high NO was studies in a high pressure flow system. OH is produced by microwave discharge of H2, followed by reaction with )2 in synthetic air carrier gas. While this radical source also produces H20, the high NO levels and short overall reaction time prevent HO2 + RO2 reactions from playing a significant role in production formation. The total reaction time is six seconds, and interaction with the reactor wall is insignificant. The present experimental design allows oxidation products to be studies following just a few seconds of reaction time and without the build-up of significant secondary products. The 3-methyl furan observed in previous product studies may have required a longer reaction time, involved heterogeneous reaction or (most unlikely) required a higher pressure.Future Activities:
Studies of the pressure dependence of OH and H yields from a series of ozone-olefin reactions and th mechanism and theoretical basis for these reactions. Product analysis and mechanistic pathway studies of the OH-isoprene and OH- -, -pinene reactions using FTIR.Journal Articles:
No journal articles submitted with this report: View all 17 publications for this projectSupplemental Keywords:
naturally hydrocarbons, oxidation, reactivity., RFA, Scientific Discipline, Air, tropospheric ozone, Atmospheric Sciences, rate constant determinations, high pressure flow system, fate and transport, ozone occurrence, Reaction Modulation Spectroscopy, spectroscopic studies, ambient air, ozone formation, hydrocarbon oxidation, chemical kinetics, atmosphereRelevant Websites:
http://www-kinetics.harvard.edu
http://www.asrc.cestm.albany.edu
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.