Experimental Investigation of Radical Propagation Pathways in the OH-Initiated Oxidation of Isoprene Under NOx-Free Conditions

EPA Grant Number: FP917301
Title: Experimental Investigation of Radical Propagation Pathways in the OH-Initiated Oxidation of Isoprene Under NOx-Free Conditions
Investigators: Liljegren, Jennifer
Institution: Indiana University
EPA Project Officer: Michaud, Jayne
Project Period: August 1, 2011 through July 31, 2014
Project Amount: $126,000
RFA: STAR Graduate Fellowships (2011) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Fellowship - Clean Air


Hydroxyl (OH) radicals play a central role in the chemistry of the atmosphere. In addition to controlling the atmospheric lifetime of greenhouse gases such as methane and the alternative chlorofluorocarbons that are important to issues of global climate change, the OH-initiated oxidation of volatile organic compounds (VOCs) leads to the production of ozone and secondary organic aerosols (SOA), the primary components of photochemical smog. Isoprene, a VOC emitted by vegetation, contributes about one third of the combined natural and anthropogenic sources of VOCs worldwide. Recent measurements of OH radicals in forest environments show serious discrepancies with modeled concentrations, suggesting that the current understanding of OH + isoprene chemistry is incomplete. This project will investigate the OH-initiated oxidation of isoprene and directly test whether reaction pathways therein maintain, rather than diminish, the oxidizing capacity of the atmosphere.


Experiments will use a turbulent flow tube reactor operating at atmospheric pressure and room temperature. Hydroxyl radicals will be generated in the reactor by UV photolysis of hydrogen peroxide, and isoprene will be introduced to react with the OH radicals. Any OH radicals produced under the conditions of the experiment will be measured using laser-induced fluorescence (LIF) in a low pressure sampling cell based on the Fluorescence Assay by Gas Expansion (FAGE) technique.

Expected Results:

It has been suggested based on theoretical studies that isomerization reactions in the isoprene oxidation mechanism produce OH radicals. The yield of OH radicals from the isoprene oxidation mechanism will be measured directly under a variety of experimental conditions. The measured yields will be compared to theoretical predictions and used to model ambient OH concentrations to see if the experimental measurements improve the discrepancies with current atmospheric chemistry models.

Potential to Further Environmental / Human Health Protection

Improving the understanding of the impact of biogenic emissions such as isoprene on the chemistry of ozone and SOA production in the atmosphere has important implications for regional air quality control strategies in areas with high isoprene emissions, such as the eastern United States. Because isoprene emissions are so globally abundant, it also is important to investigate how changes in isoprene emissions as a result of global climate change may affect ambient OH concentrations and thus the oxidizing capacity of the atmosphere in the future. Therefore, in addition to improving the scientific understanding of the chemistry of tropospheric ozone and SOA production, the broader environmental applications of this work include an improved ability to predict how changes in isoprene emissions arising from factors such as land use changes, forest succession and climate change affect the composition of the atmosphere and the OH radical concentrations necessary for greenhouse gas removal.

Supplemental Keywords:

VOCs, isoprene, NOx, OH radicals, ground-level ozone, SOA, climate change, greenhouse gases, atmospheric chemistry,

Progress and Final Reports:

  • 2012
  • 2013
  • Final