Accretion in Reactions of Organic Peroxy Radicals

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The Environmental Chemical Sciences Program of the Chemistry Division supports Professors Paul Wennberg and Brian Stoltz of the California Institute of Technology to study how particles form in air. These particles, also known as aerosols, make the air hazy (reduce visibility) causing potential problems navigating on the roads, at sea or in the sky. There is also significant evidence that breathing such particles is harmful to human health. Generally, about half of the mass of these particles is composed of molecules comprised of carbon, oxygen, and hydrogen (oxygenated hydrocarbons). There is recent evidence that some of these oxygenated hydrocarbons form in the air via the combination of two smaller molecules, a process known as accretion. To learn what promotes this type of chemical combination reaction, laboratory and computer calculations are used to examine the efficiency for different chemicals found in the atmosphere. This research is important for developing an underlying understanding of air quality but also for biology, human toxicology, soil chemistry and the preservation of artwork. Students and staff from the Wennberg and Stoltz groups are heavily engaged in bringing science to the public. Atmospheric chemistry provides an opening for conversation and interaction as citizens of Los Angeles have direct experience with poor air quality. These outreach activities are performed both on campus and in the local schools. On campus,for example, the science program is conducted with local elementary schools and pre-K children in an outdoor lab. The research team works closely with the Benjamin Franklin Elementary School in Glendale, CA to collaborate with the teachers in designing experiments that enhance the science- and math-based learning curriculum in which the students are engaged.Reactions of organic peroxy radicals with other peroxy radicals are thought to produce much larger molecules via accretion. These larger molecules have a much lower vapor pressure and can condense to form aerosol. Little is known about what controls the rates at which these accretion products form. This study examines the hypothesis that weak bonding interactions in the initially produced complex of the two peroxy radicals determine the efficiency of this chemistry. The rates of this oxidative reaction are studied for a diverse set of organic substrates. The team synthesizes compounds that are thought to characterize these accretion products to confirm their identity. The comprehensive laboratory studies are complemented with computational chemistry through a collaboration with Professor Kjaergaard at the University of Copenhagen.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.