Liquid interfaces are present everywhere in the environment and in the atmosphere around us. Chemical reactions on cloud droplets, atmospheric aerosols, and ocean surfaces can play large roles in the cycling of molecules in the biosphere. Understanding exactly how chemistry proceeds at these liquid interfaces poses an enormous challenge. This is due in large part to the difficulty of isolating surface chemistry from the large background of reactions occurring in the adjacent bulk gas and liquid phases.
The 2012 Early Career Award (ECA) allowed me to tackle some of these challenging questions. My team developed novel instrumentation to probe the interfaces of small organic nanoparticles.
My group and I then used this instrumentation in our laboratory and at the Advanced Light Source, an Office of Science scientific user facility. We examined how the chemistry of atmospheric trace gases proceeded at organic aerosol surfaces, which are models for combustion emissions in urban environments.
We focused on the gas-surface reactions of ozone and hydroxyl radicals. They can help us better understand the key reaction pathways involved in the oxidative decomposition of organic molecules in the atmosphere.
In the closing years of the ECA, my group became interested in how cloud droplets form and the possible role that the droplet interface might play in this process. At first glance, this didn’t seem like an interesting question since weather forecasting is quite good at predicting when cloudy days occur.
However, we realized that there were fundamental uncertainties when one looked closely at how individual cloud droplets nucleate on existing aerosol particles.
We found, contrary to conventional wisdom, that molecules at the droplet surface played a large role in the eventual formation of a cloud droplet. The insights my group obtained would not have been possible without the longer duration ECA funding and the willingness of the program to support creative high risk but high payoff research ideas.
Kevin Wilson is a Senior Scientist in the Chemical Sciences Division at Lawrence Berkeley National Laboratory.
The Early Career Research Program provides financial support that is foundational to early career investigators, enabling them to define and direct independent research in areas important to DOE missions. The development of outstanding scientists and research leaders is of paramount importance to the Department of Energy Office of Science. By investing in the next generation of researchers, the Office of Science champions lifelong careers in discovery science.
For more information, please go to the Early Career Research Program.
Title: Free Radical Reactions of Hydrocarbons at Aqueous Interfaces
Abstract
Chemical reactions that occur at hydrocarbon/water and electrolyte interfaces govern a wide array of environmentally and technologically important processes, including electrochemistry, aerosol photo‐oxidation, cloud chemistry, corrosion, and heterogeneous catalysis. Hydrocarbon free radicals, formed at these interfaces, play important roles in the chemistry as initiators or propagators of surface reactions or as reactive intermediates. Two experimental techniques will be used in new ways to examine the surface chemistry of hydrocarbon free radicals at gas/liquid interfaces. The atomic and molecular changes at the surface of micron‐sized droplets will be measured by ambient pressure X‐ray photoelectron spectroscopy. A surface sensitive mass spectrometer will be used to make kinetic measurements of reaction rates and product distributions.
The objective of this research is to provide a molecular description of the reaction pathways that lead to either bulk solvation of an organic molecule or its removal from the interface through decomposition into gas phase products. These interfacial processes are important for understanding and eventually predicting the environmental fate of hydrocarbon byproducts of energy use and consumption.
CR Ruehl, JF Davies and KR Wilson, “An interfacial mechanism for cloud droplet formation on organic aerosols.” Science 351, 1447 (2016). [DOI: 10.1126/science.aad4889]
CR Ruehl and KR Wilson, “Surface Organic Monolayers Control the Hygroscopic Growth of Submicrometer Particles at High Relative Humidity.” The Journal of Physical Chemistry A 118 (22), 3952 (2014). [DOI: 10.1021/jp502844g]
JF Davies and KR Wilson, “Nanoscale interfacial gradients formed by the reactive uptake of OH radicals onto viscous aerosol surfaces.” Chem. Sci. 6, 7020 (2015). [DOI: 10.1039/C5SC02326B]
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