Organic Aerosol Dynamics: Molecular Interactions, Single Particles and Modest Driving Forces

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Fine particles play a key role in atmospheric chemistry, and effect human health and weather. The physical and chemical properties of fine particles are critical in controlling their behavior. The properties of these particles are significantly different from liquids or solids. Typical particles are about 300 nanometers in diameter and contain roughly 10 million individual molecules. Well over half of those molecules are organic (carbon containing) compounds, and even a single particle often contains hundreds of thousands of different organic molecules, most of which have never been isolated or synthesized by chemists. These organic molecules stick to the tiny particles rather than letting go of the particle and floating (volatilizing) into the air. Some of the organic compounds resemble asphalt or complex sugars. This research advances fundamental understanding of how combustion processes and products related to energy production may influence human health. Professor Neil Donahue and his graduate students at Carnegie Mellon University study the behavior of a number of model systems using sophisticated mass spectrometers that can measure the composition of individual 300 nm particles along with complementary chemical ionization mass spectrometers that can measure individual molecules in the gas and particle phases. They generate organic particles mimicking those found in the atmosphere with progressively higher degrees of oxidation to see if the particles become more viscous in a way that matters - by directly testing if semi-volatile organics can diffuse into the particles and whether two initially distinct populations of particles with semi-volatile constituents evolve toward each other because of mixing. The researchers conduct these experiments over a wide range of relative humidity conditions as humidity varies widely in the atmosphere and because wet particles are known to be much less viscous and more diffusive. Via a collaboration with colleagues at Northwestern University, they use synthesized molecules containing the stable carbon isotope, 13C, to cleanly separate molecules from different particle populations using mass spectrometry. The project prepares doctoral students for a research career. Professor Donahue is an ambassador in the Science and Engineering Ambassadors Program founded by the National Academies. He brings a focus on the coupling between energy use and environmental health issues.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.