Oxidative Mechanisms

The chemistry involved in the oxidation of atmospheric organic compounds is immensely complex, involving hundreds to thousands of different intermediate and product species. Therefore, our laboratory studies this chemistry using a range of analytical techniques (mostly based on mass spectrometry), in order to characterize a large fraction of the organic carbon. Experiments are carried out in our fixed-temperature, fixed-volume 7.5 m³ Teflon chamber (as well as other reactors, such as our smaller “mini-chamber”), with the chemistry initiated either by various oxidants or by the photolysis of precursors of key organic radicals. Measurements of reaction kinetics and products enable an improved understanding of key reaction pathways, and/or the refinement of atmospheric chemical mechanisms. Because implementing full mechanisms in large-scale models is computationally expensive, we focus on critical branching points, especially peroxyl radical (RO2) chemistry, to recommend simplified yet accurate mechanisms for air quality and climate models. We are also developing a framework including a database and automated pipeline to systematically translate laboratory insights into mechanism development. Recently, we’ve expanded our research into non-traditional regimes, including studying organosulfur oxidation and chlorine radical initiated oxidation in systems representative of the remote troposphere.

Aqueous-Phase Oxidation

While the gas-phase oxidation of organics is very well studied, aqueous-phase oxidation has received comparatively less attention, despite water being ubiquitous in the atmosphere. Organic radical chemistry is substantially more complicated than in the gas phase due to a number of different effects (e.g. solvent cage effect, high ionic strength and acidity, etc.) and can result in widely different product distributions. In turn, the chemical composition and properties of aerosol particles can be more accurately described by including this chemistry. To study the oxidation of key organic species both in the bulk aqueous phase and in suspended aqueous aerosols, our laboratory uses several different reactors (bulk aqueous-phase reactors and an environmental chamber) along with a suite of analytical instruments. Our group is exploring the multi-phase oxidation of a variety of water-soluble volatile organic compounds, including biomass burning products and organosulfur species.

Indoor Air Quality

While chemical oxidation and organic aerosol formation occur in the atmosphere, these processes can also occur in the indoor environment. Many indoor sources (including fragrances and cleaning products) emit volatile species that can react with oxidants and form aerosol. Technologies such as air purifiers, humidifiers, and germicidal ultraviolet light (UV) are often advertised to clean the indoor air, but they may emit species of their own and/or have other photochemical effects that can negatively impact human health. We carry out experiments in our environmental chamber, combined with box modeling of real-world indoor environments, to characterize these indoor air technologies and elucidate the photochemistry that they initiate. We also engineer ways to minimize their harmful effects of these technologies, e.g. by using activated charcoal and HEPA filters to remove ozone and other pollutants produced by germicidal UV. Our research applies engineering to indoor air quality to enable a better understanding of air cleaning technologies and their potential impacts on human health.

Low-Cost Sensors

Low-cost air quality sensors (LCS) present the opportunity to dramatically increase the spatial and temporal resolution of existing atmospheric chemistry measurements, assess the effectiveness of air quality interventions, and quantify human exposure to air pollution. We have deployed sensor networks in Boston, India, and Hawai‘i, to better understand the extent to which LCS can quantify the pollutant exposure of affected populations. Though promising, the abilities and limitations of LCS are still not fully understood. Thus our efforts involve not only sensor deployment and use, but also the investigation of methods for calibrating and evaluating these sensors, and more fundamentally the study of how such sensors can be used to better understand the chemistry of the atmosphere.

Aerosol Aging

While many studies of atmospheric organic aerosol chemistry focus on the initial formation of organic particulate matter, there has been relatively little study of the chemical transformations that condensed-phase organic species undergo over their atmospheric lifetime. We use a range of reactors (environmental chambers, flow tubes, and bulk aqueous reactors) to probe how oxidative “aging” affects the amounts and chemical composition of organic aerosol, over timescales of days to weeks. A wide range of systems and phases are studied, including the gas-phase oxidation of semivolatiles, heterogeneous reactions at the gas-particle interface, oxidation within the aqueous phase, and the direct photolysis of condensed-phase organic compounds.