Harrison Research Group
Spectroscopy and Photochemistry of Atmospheric Aerosols
Focus: Analytical, Physical and Atmospheric Chemistry
Professor: Aaron W. Harrison, Ph.D.
Overview
Aerosols are microscopic solid or liquid particles suspended in the atmosphere, ranging in size from a few nanometers to several microns. The atmosphere contains a diverse mix of aerosols from biogenic and anthropogenic sources, including carbonaceous particles (brown carbon, secondary organic aerosols), sea salt, dust, and bioaerosols like viruses and pollen. Aerosols play central roles in atmospheric chemistry by providing surfaces and condensed phases for chemical reactions and serving as cloud condensation nuclei. However, elevated aerosol concentrations also contribute to poor air quality, with links to respiratory and cardiovascular health issues. Understanding the sources, chemical composition, and oxidative transformations of atmospheric aerosols is critical for addressing challenges at the intersection of climate change, air quality, and human health.
Research focus
Research in the Harrison Group focuses on development and application of advanced spectroscopic techniques and computational analysis to characterize the composition, photochemistry, and optical properties of atmospheric aerosols. A central tool in our work is fluorescence spectroscopy, which enables the identification and classification of aerosol types in both laboratory and field studies. Despite its utility, significant difficulties remain in accurately characterizing chemically complex aerosols, leading to potential misidentifications in environmental settings. To address this challenge, research in our group explores the combined use of energy- and time-resolved fluorescence to more reliably distinguish aerosol types, with applications in atmospheric field studies and remote sensing.
In addition, our research leverages fluorescent probe molecules to directly investigate particle characteristics such as pH and viscosity which are key factors that control chemical reactivity and gas-particle partitioning. These properties are difficult to measure due to the size and variability of atmospheric particles. However, by applying laser-induced fluorescence spectroscopy, this research provides insight into how these properties change with composition and humidity and the microenvironment within aerosol particles.
Photochemistry is another major focus of our research. Many light-absorbing compounds including quinones, aromatic ketones, and oxygenated or nitrated polycyclic aromatic hydrocarbons (PAHs) partition into condensed atmospheric phases such as aerosols and cloud water. Sunlight initiates secondary chemistry in these systems through the production of reactive species, yet the ability of these molecules to act as photosensitizers remains poorly quantified under atmospherically relevant conditions. Our work seeks to close this knowledge gap by directly probing photosensitization processes in aerosol proxy matrices.
Rapf Lab
Photochemistry in Complex Aqueous Environment
Type: Physical Chemistry
Professor: Rebecca Rapf, Ph.D.
Overview
The Sun is the largest source of energy to the planet, and it controls, directly or indirectly, the vast majority of physical, chemical, and biological processes that take place on Earth. Photochemical processing of material drives the engine of atmospheric and environmental chemistry in planetary environments, largely through the formation and subsequent reactions of radical species. The reactivity of these radicals is controlled and mediated by the surrounding environmental conditions under which they were generated. Photochemically-generated organic radicals are particularly interesting because they offer an abiotic pathway to make larger, more complex organic molecules, which has applications to atmospheric chemistry, prebiotic chemistry, and astrobiology. This research is grounded in fundamental physical chemistry but is inherently interdisciplinary, drawing on organic chemistry, environmental chemistry, biophysics, and planetary science.
Research Area
The Rapf lab examines the direct aqueous photochemistry of organic molecules under conditions relevant to planetary environments, including the modern and ancient Earth as well as other potentially habitable worlds. We conduct detailed photochemical experiments that allow us to examine, mechanistically, changes in reactivity that occur as a function of reaction conditions, including photon flux, atmospheric composition, and solution conditions (e.g. pH and salinity). Using model chemical systems, we can systematically increase the complexity of model systems to investigate the origins of emergent behavior.
In tandem with photochemical studies, we also explore how intermolecular interactions mediate chemistry, both through orientation and concentration at interfaces and through the formation of supramolecular assemblies. This is motivated by the repeated observations that in many cases the chemistry of single species in a bulk environment cannot be used to predict the reactivity of those species either in confined environments or in concert with other molecules. Of particular interest are recent literature reports that molecules at aqueous interfaces can undergo photochemistry that is not seen in the bulk. We explore how photochemistry is mediated by surface films composed of insoluble surfactants, such as long-tailed fatty acids like stearic and palmitic acid, using photochemical-initiator species, such as pyruvic acid.
Students will conduct photochemical experiments, which are analyzed using a combination of mass spectrometry, optical spectroscopy, and surface tension measurements. Students will also have opportunities for instrument development, as we build surface sensitive spectroscopic techniques to probe interfacial photochemistry directly.