The hypothesis to be tested in this project is that atmospheric heterogeneous photocatalysis of isoprene may play an important role in secondary organic aerosol (SOA) formation. The atmospheric relevance of transition metal-based particulate semiconductors in the photochemical oxidation of isoprene to low volatility products will be investigated. Photocatalysts will include atmospherically-relevant transition metal oxide particles, desert sand, and volcanic ash in the 0.1 to 10 micrometer size range. Liquid-solid photocatalytic reactions will be studied in terms of the organic substrate (isoprene and the primary products of isoprene gas-phase photochemical oxidation, methacrolein and methyl vinyl ketone), the presence of a sacrificial electron donor (e.g., acetate), oxygen concentration, and pH. The potential for photochemical Diels-Alder reactions to occur between isoprene (a diene) and a range of relevant dienophiles will be investigated, followed by analysis for oligomeric products. The research will be overseen by senior investigators from CalTech and the Oak Crest Institute of Science.
These studies will potentially advance the field of aerosol research by guiding future smog chamber studies; identifying chemical markers characteristic of heterogeneous photooxidation pathways; and investigating the polymerization products, which may account for a significant portion of the SOA mass. The research will be carried out by a post-doctoral scholar, and a graduate student at Caltech, as well as community college students and high school teachers working in teams with high school students via Oak Crest. These efforts will help to broaden the participation of underrepresented groups in scientific research.
Isoprene is a hydrocarbon emitted to the atmosphere primarily by broadleaf trees such as those found in the dense forests of the Amazon and the deciduous forests of the southeastern United States. Aside from methane, isoprene is the most abundant hydrocarbon emitted to the Earth’s atmosphere, where it undergoes a series of chemical transformations that recently have been shown to generate significant amounts of sub-micrometer-sized aerosols. This so-called "Secondary Organic Aerosol" (SOA) has been implicated in local air quality, public health, and potentially climate change. There remain significant gaps in our understanding of the mechanisms that underpin the formation of SOA from isoprene in the atmosphere. This three-year project constitutes a collaborative effort between researchers at the California Institute of Technology and the Oak Crest Institute of Science (Oak Crest) and aims to study photochemical transformations of isoprene in the laboratory to determine if these processes could play a role in atmospheric SOA formation. The Oak Crest contingent developed a novel high-throughput screening approach to efficiently examine hundreds of photochemical reaction systems with an atmospheric relevance. Three successive, increasingly intensive rounds of screening identified a subset of systems that readily formed isoprene oxidation products in the presence of photocatalysts and photochemical mediators using laboratory-generated radiation and natural sunlight. Some of the observed photooxidation products were oligomeric (i.e., small polymers made up of multiple isoprene-derived units) and have been observed previously in other, known atmospheric isoprene oxidation pathways. Other systems gave new, nonvolatile oxidation products. These outcomes suggest that the project has successfully explored a novel area of isoprene chemistry that needs to be studied further to more completely assess the potential atmospheric relevance of the processes. The Oak Crest team consisted almost exclusively of community college students (nine), high school students (nine), and high school teachers (three). Of these twenty-one participants, ten (48%) were from minority groups and persons with disabilities. The environmental relevance and rigor of the research provided a highly enriching experience to these future scientists.
