Research

Research

My research has focused primarily 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 to the formation of photochemical smog in forested areas like the Southeast United States. During my ten years in this field of research, I've used a variety of tools to investigate aspects of isoprene's chemistry in the atmosphere, and have branched out into related topics including the atmospheric budgets of other organic gases and oxidants, the formation and fate of atmospheric particulate matter, and fundamental gas-phase chemical reaction processes.

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.

Organic Aerosol Formation

The oxidation of isoprene can sometimes result in molecules that are sufficiently water-soluble, reactive, or non-volatile to condense into existing 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.

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.

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.

Airborne Field Observations

Beyond my study of isoprene, I have also participated in three field campaigns at the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS). Based out of Marina, CA (on Monterey Bay), we load our instruments into a Navy Twin Otter and fly around coastal California measuring particle loading and composition. In the summer of 2015, we teamed up with colleagues at the University of Arizona and Georgia Tech to investigate oceanic aerosol, with a particular focus on biological content in particles. In 2016, we worked with a group at NASA’s Jet Propulsion Lab to look at emissions from wildfires and urban areas. We returned to the same site in 2018 to further characterize oceanic particles and their cloud-nucleating capacity, and to study emissions from wildfires in California. In the summer of 2023, I'll be back in the field for the AEROMMA campaign.