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Research
Our research combines observational and modeling tools to discover the ways in which organic gases in the atmosphere react to produce harmful air pollutants like ozone and particles, and to quantify the impacts of these pollutants on human health and global climate. By better understanding the chemical processes that control ozone and particle formation, we can identify steps that individuals and institutions can take to mitigate air pollution and its harmful effects.
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Gas-phase Oxidation Mechanisms
Ozone, an air pollutant with harmful effects toward human health, plants, and the climate, is produced when volatile organic compounds (emitted both naturally and anthropogenically) are oxidized in the atmosphere in the presence of nitrogen oxides (emitted in the burning of fossil fuels and other combustion activities). In environmental chambers and benchtop reactors, we perform experiments to characterize the oxidation reactions of organic gases that can lead to ozone formation. These experiments help us to quantify the rates and products of gas-phase reactions, identify novel reaction pathways and organic intermediates, and determine optimal emission reduction strategies to reduce human exposure to ozone.
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Organic Aerosol Formation
The oxidation of volatile organic compounds can also 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, we 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 recently used these techniques to study nighttime aerosol formation from monoterpene oxidation.
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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. As a member of the GEOS-Chem steering committee with a focus on chemistry, I also make sure the model's mechanism is kept up to date.
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Instrument Development & Organic Synthesis
In our study of the complex oxidation mechanisms of organic molecules, we often find a need for chemical standards that are not commercially available. Instead, we can synthesize and purify these atmospherically relevant compounds from common 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. We are also constantly developing, calibrating, characterizing, and improving our instruments (mostly chemical ionization mass spectrometers) to better measure trace organic gases and aerosols in both the laboratory environment and the ambient atmosphere.
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Field Observations
To investigate how the organic gases I study behave in the actual atmosphere, I have also participated in in-situ sampling of atmospheric trace gases aboard Navy Twin Otter and NASA DC-8 aircraft as well as on ground-based platforms. 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. In the summer of 2024 we put some of the same instruments on a van and drove around Salt Lake City to investigate local pollution sources as part of the USOS campaign. 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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