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Research
My research focuses primarily on the oxidation mechanisms of organic gases in the atmosphere, and quantifying the production of harmful pollutants (mainly ozone and particulate matter) from those oxidation processes. I've focused in particular on the atmospheric chemistry of isoprene, a hydrocarbon emitted in large quantities by trees and other vegetation. The oxidation of isoprene in the air has profound impacts upon the trace chemical constituents of the atmosphere around the world, and contributes substantially to the formation of photochemical smog in forested areas like the Southeast United States.
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Gas-phase oxidation mechanisms
One of my primary focuses has been elucidating the complex processes by which isoprene and other organic gases are oxidized in the atmosphere to form highly functionalized products. While isoprene’s gas-phase reactions have been the subjects of intense study for many years, some of the second- and later-generation steps in its oxidation process are still poorly understood, and better constraints on the rates and products of these reactions would improve our ability to model and predict the impact of isoprene on the atmosphere - for example, how it contributes to ozone and particle formation. Using chemical ionization mass spectrometry and organic synthesis to access otherwise difficult-to-isolate oxidative intermediates, I have measured the rates and products of the OH-initiated oxidation of isoprene epoxydiols (second-generation oxidation products of isoprene), small dihydroxycarbonyl compounds (formed from the oxidation of isoprene epoxydiols), dihydroxy isoprene, and other compounds.
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Organic Aerosol Formation
The oxidation of isoprene and other volatile organic compounds can sometimes produce molecules that are sufficiently water-soluble, reactive, or non-volatile to condense into particles in the atmosphere. This particulate matter is known as secondary organic aerosol (SOA), and can contribute to the adverse health effects, climate forcing, and poor visibility that come with smog. To better understand the chemical pathways that lead to SOA formation, I conduct experiments in an environmental chamber - basically a large Teflon bag in which we can control the temperature, humidity, light flux, and chemical constituents in the air. We measure particle formation and composition with a suite of specialized instruments, including an aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS). We have recently ventured beyond isoprene to study nighttime aerosol formation from monoterpene oxidation using the same techniques.
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Chemical Transport Modeling
To investigate the global effects of the gas- and particle-phase chemistry that I study, I run GEOS-Chem, a global chemical transport model. GEOS-Chem uses meteorological data from the Goddard Earth Observing System (GEOS), compilations of data on chemical emissions to the atmosphere, and a complex set of chemical reactions to simulate the chemistry of the atmosphere. My work with GEOS-Chem initially focused on updating the isoprene oxidation mechanism, allowing me to estimate the quantities and distributions of important oxidation products and intermediates (such as SOA precursors, ozone, and nitrogen oxides) in the atmosphere. Recently, I have also explored model methods of quantifying the production and loss of tropospheric ozone, an important pollutant and greenhouse gas, to better understand its sources and sinks, and used GEOS-Chem to quantify the global budget of atmospheric methanol and the atmospheric implications of aromatic emissions.
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Organic Synthesis
In our study of the complex oxidation mechanism of isoprene, we often find a need for chemical standards that are not commercially available. I worked with Professor Brian Stoltz at Caltech to synthesize and purify some of these atmospherically relevant compounds - such as isoprene epoxydiols, hydroperoxides, and methacryloyl peroxyacetyl nitrate - from commercial precursors. We then inject these compounds into our environmental chambers to study their oxidation mechanisms and their potential to form SOA, or inject them directly into our instruments for calibrations.
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Airborne Field Observations
Beyond my study of isoprene, I have also participated in airborne in-situ sampling of atmospheric trace gases aboard Navy Twin Otter and NASA DC-8 aircraft. In three field campaigns between 2015 and 2018, we partnered with the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) based out of Marina, CA (on Monterey Bay), where we loaded an aerosol mass spectrometer and a suite of other gas- and particle-phase instruments into a Navy Twin Otter and flew around coastal California measuring particle loading and composition. In the summer of 2023, I helped to run two time-of-flight mass spectrometers aboard the NASA DC-8 as part of the AEROMMA campaign, which investigated sulfur chemistry in marine environments and summertime smog formation in urban areas. These highly collaborative projects are excellent opportunities to see how the chemistry we study in the lab and simulate in models plays out in the real atmosphere, and to identify new targets of study.
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