Grantee Research Project Results

1999 Progress Report: The Chemical Kinetics and Mechanisms of Hydrocarbons Contributing to Ozone Production in the Atmosphere

EPA Grant Number: R825258
Title: 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, 1998 through November 24, 1999
Project Amount: $534,939
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air

Objective:

The objective of this research project is to 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 unsaturated hydrocarbons in the gas-phase is believed to be an important source of HOx radicals in the troposphere. However, since the reaction mechanism involves vibrationally excited intermediate species, and most yield measurements are indirect, the exact mechanism and product branching ratios remain uncertain. Direct pressure-dependent measurements of radical yields for a number of ozone-alkene reactions have been performed. OH and H radicals are detected using laser induced fluorescence (LIF) and resonance fluorescence (RF), and are measured at steady state, formed from the ozone-alkene reaction and lost to reaction with the alkene. Short reaction times (always < 100 ms, usually 10 ms) ensure minimal interference from secondary reactions. Measurements from 1 torr to hundreds of torr cover a set of simple symmetric alkenes: ethene, trans-2-butene, 2,3-dimethyl-2-butene, 3-hexene, and 3,4-dimethyl-3-hexene. OH yields for the smaller alkenes are pressure independent and consistent with results from previous indirect measurements at 1 atm. However, yields for the larger alkenes decrease rapidly with pressure, resulting in 1 atm yields significantly lower than current recommendations. This pressure dependence is caused by the large number of nonreactive modes of the carbonyl oxide (Criegee) intermediate. Larger intermediates have longer lifetimes with respect to unimolecular reaction and therefore are more susceptible to collisional stabilization; we motivate this effect more quantitatively using statistical theory. Therefore, while radicals are produced directly in the ozone-alkene reaction, yields measured in environmental chambers may be overestimates due to interferences by secondary reactions.

Future Activities:

Further studies of the mechanistic and theoretical basis for the pressure dependence of OH and H yields from a series of ozone-olefin reactions. Application of FTIR analytical techniques in a high-pressure flow system to study the products and mechanistic pathways of the OH-isoprene and OH- -, -pinene reactions.

Journal Articles:

No journal articles submitted with this report: View all 17 publications for this project

Supplemental Keywords:

natural 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, atmosphere

Relevant Websites:

http://www-kinetics.harvard.edu
http://www.asrc.cestm.albany.edu

Progress and Final Reports:

Original Abstract

The 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.