Paul Ohno, Ph.D.
Schmidt Science Fellows
Environmental Fellow, Harvard University
Charged interface and atmospheric aerosol properties via spectroscopy
Chemical and physical processes at charged interfaces play a key role in environmental, biological, and technological systems. Yet, the relative scarcity of interface-selective experimental techniques has left open important questions regarding molecular-level structure and composition in a range of interfacial systems. Second harmonic generation (SHG) has become a workhorse interface-selective technique to characterize interfacial electrostatics. However, interpretation of SHG experiments has long been complicated by an inability to experimentally separate two terms that contribute to the total SHG signal. Here, I describe how the detection of not only SHG amplitude, but also its optical phase, enables separation and quantification of these two terms. The first term can then be used as an experimental benchmark for atomistic simulations of interfacial structure and composition, while the second term is proportional to interfacial potential and thus represents an “optical voltmeter”. I describe the design of an apparatus capable of measuring this optical phase from buried interfaces and demonstrate the application of this method to characterize oxide:water and lipid bilayer:water interfaces. Atmospheric aerosol particle phase state represents another property that has remained challenging to experimentally characterize due to the small size, low density, and delicate nature of the particles. Aerosol particles can undergo liquid-liquid phase separation (LLPS), impacting atmospheric processes including gas-particle partitioning, solar radiation scattering, and cloud nucleation. Here, I describe the first realization of an experimental technique capable of directly probing LLPS in particles of atmospherically-realistic sizes while they remain in suspension. Solvatochromic probe molecules are incorporated into the particles and their fluorescence emission is used to determine particle phase state. Differences in separation relative humidity (SRH) values measured here and previously reported SRH values from optical microscopy of much larger particles on substrates underscore the utility and importance of the technique and motivate future studies into the size dependence of LLPS in the atmosphere. Finally, I comment on the necessity of direct experimental characterizations of aerosol particle surfaces and interfaces to gain a comprehensive understanding of particle processes, properties, and impacts on air quality and climate.
Paul’s overall scientific research interests revolve around the environment, sustainability, and chemistry. He is inspired by his childhood in the beautiful state of Maine and observations of the impacts of human activities on the natural environment around him.
Paul Ohno is a physical chemist studying the physical and chemical properties of secondary organic aerosol particles and the implications of these properties for the climate system.
Paul earned his AB in Chemistry from Princeton University in 2014 and his PhD in Chemistry from Northwestern University in 2019. During his PhD studies, he used laser spectroscopy to measure fundamental properties of aqueous interfaces so as to better understand, predict, and control chemical processes that occur there, like groundwater pollutant capture at the mineral/water interface.
As an Environmental Fellow, Paul works with Professor Scot Martin of the John A. Paulson School of Engineering and Applied Sciences and the Department of Earth and Planetary Sciences. Their work focuses on developing and applying spectroscopic techniques to directly determine physical and chemical properties, such as viscosity and diffusivity, of secondary organic aerosol particles while they remain in suspension. Paul is also a 2019 Schmidt Science Fellow.