The Environmental Chemical Sciences (ECS) program of the Division of Chemistry (NSF/CHE) and the Atmospheric Chemistry program of the Division of Atmospheric and Geospace sciences (AGS) will support the research project of Prof. Mitchio Okumura of California Institute of Technology (Caltech). Prof. Okumura and his students will employ Cavity Ringdown Spectroscopy (CRDS) to detect free radical intermediates formed during photooxidation of volatile organic compounds (VOCs). The project will contribute to our understanding of the chemical reactions that lead to photochemical smog and aerosol formation. Specific data and mechanisms will be provided for air quality models that form the scientific basis for policy decisions regulating anthropogenic emissions related to air pollution and climate change. The project will provide excellent training opportunities for graduate students in a research area of great societal importance and at the forefront of scientific research.
Air quality has serious impacts on human health. The U.S. Environmental Protection Agency (EPA) lists six "criteria pollutants" that have direct ramifications for the quality of human life, particularly in urban areas. Our research focuses on discovering the underlying chemistry that yields these pollutants, thereby producing information that helps policymakers inhibit the effect of unclean air on humans. While in recent years our understanding of air pollutant formation has vastly improved, there remain many subtle chemical processes that have yet to be characterized. This unknown chemistry produces uncertainty in models of atmospheric composition and, by extension, air quality. The chemistry of the air we breathe is governed by transient, highly reactive intermediates called free radicals. The fate of these radicals is extremely important: depending on which reactions they undergo, the final chemicals produced can be harmless or deleterious to human health. Our research has studied free radicals in the laboratory by flowing atmospheric molecules through a reactor and detecting the resulting intermediates using two highly sensitive techniques: Cavity Ringdown Spectroscopy and Multiplexed Photoionization Mass Spectrometry. In order to initiate the chemistry, we use lasers to simulate the type of radiation experienced by molecules in the atmosphere. In this work, we have characterized important chemistry of free radicals that yield two of the EPA criteria pollutants: surface ozone and particulate matter formed by organic aerosols. These laboratory experiments directly complement field observations and atmospheric modeling, producing a framework for air quality that can guide policymakers as they take action to keep our air clean. Significant Achievements: We investigated the fundamental chemical physics of many free radicals in the atmosphere. Our studies have primarily focused on two atmospheric oxidants: the chlorine radical, which accrues primarily in the costal marine boundary layer, and the nitrate radical, which is the principal nighttime oxidant. We have: Measured the light absorption spectra of peroxy radical intermediates formed by chlorine oxidation of various hydrocarbons emitted into the atmosphere, including isoprene, 2-butene, 3-methyl-1-butene, and 2-methyl-3-buten-2-ol. Characterized the self-reaction rates and nitric oxide reaction rates for peroxy radical intermediates formed by chlorine oxidation of ethene, propene, 2-butene, and isoprene. Obtained the product branching ratios of peroxy radical reactions, specifically focusing on the ethyl peroxy self-reaction and the reaction of acyl peroxy with hydroperoxy. Investigated the Jahn-Teller effect in the nitrate radical. Observed the dynamics of the trans-DOCO radical, a key atmospheric intermediate. Measured the isomerization rate of the n-butoxy radical, a prototypical system for alkoxy radical isomerization. Our work has provided information on the rates of reaction and fundamental structure of many important free radical intermediates. These results represent an important step forward in understanding the chemistry that occurs in the air we breathe, specifically as it relates to the formation of air pollution. Broader Impacts: Our results will contribute directly to our understanding of the chemical reactions that lead to air pollution. The measured reaction and spectral parameters will be incorporated into widely used chemical databases that are publicly accessible to scientists in a variety of disciplines as well as the general public. Our results improve the accuracy of photochemical models that are the basis for state and federal regulations designed to improve air quality. Our studies have benefitted through the establishment of long term partnerships with other universities and government institutions: JILA, Jet Propulsion Labratory, the Combustion Resource Facility at Sandia National Laboratory, the Advanced Light Source at Lawrence Berkeley National Laboratory, The Ohio State University, University of Texas, and Texas A&M. Two undergraduate students, six graduate students, and three postdoctoral scientists have received professional training in scientific research. These researchers developed communication skills through seminars and posters presented at scientific conferences. This project provided significant training opportunities for underrepresented groups including one Hispanic and four female individuals. We hosted a visiting professor, Allan Nishimura, from the undergraduate teaching college, Westmont College. Through a partnership with the Institute for Educational Achievement, a high school student was mentored while working in our laboratories. Our research group also performed outreach activities with elementary school students in the Pasadena Unified School District (PUSD). The demographics of this district consist predominantly of minorities underrepresented in the physical sciences. PUSD demographics for the 2010-2011 school year were 60.6% Hispanic, 16.9% African American, 13.7% White, 3.2% Asian, and 5.6% other. During the past two years, over 100 4th and 5th grade students from PUSD visited the Caltech campus to learn about atmospheric chemistry. We guide the students through various demonstrations focusing on the scientific method and allow them to become junior experimentalists, performing their own chemical reactions. Furthermore, we teach the students about opportunities available to them through a college education. These activities are organized and carried out entirely by our research group.