Establishing Water's Role in the Mechanism of Atmospheric New Particle Formation

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The Environmental Chemical Sciences Program of the NSF Division of Chemistry funds Professor Christopher Johnson at Stony Brook University to study the formation of new particles, called aerosols, directly from gases in the Earth's atmosphere. This is an important process, but no complete framework exists to predict particle formation and growth rates. In particular, the role of water in these processes is unknown, because it is difficult to detect its presence and structure in new particles. This project focuses on isolating "seed" particles with precisely known composition and growing them in inside a mass spectrometer under precisely controlled conditions. This allows the exact changes in composition to be followed one molecule at a time. Water vapor pressure is controlled to determine whether it speeds up growth. Molecular clusters containing water are analyzed to determine the cause of the rate increase. This work establishes how water impacts particle formation and provides critical comparisons for theoretical efforts to predict particle formation rates. This will ultimately help to reduce uncertainties in global climate models. The project provides training and research opportunities for graduate students. In addition, Professor Johnson and his students engage in outreach to the broader public through demonstrations and visualizations of particles forming clouds. This project directly addresses the mechanistic impacts of water on new particle formation. Using variable-temperature ion trap mass spectrometry techniques, the kinetics and energetics of growth are determined for precisely specified seed clusters consisting of sulfuric acid and a set of organic acids and amine bases, in the presence or absence of water. Three hypotheses are being tested: (1) Water serves as a "sacrificial ligand" that enhances the sticking of incoming vapors. (2) Water stabilizes surface-bound molecules through cooperative hydrogen bonding. (3) Water catalyzes growth by lowering the barrier to structural rearrangement of the existing cluster to accommodate incoming vapors. The critical structural features of seed and product clusters are characterized by in situ vibrational spectroscopy to determine the most active sites for growth. This work is carried out in collaboration with theoreticians modeling particle growth. The experiments can validate the theoretical approaches and the calculations provide additional insights into experimental results. This project is expected to yield enhanced predictions of growth rates, explicitly including water, that can be adapted to regional climate models. In addition to providing training and research opportunities to students, Professor Johnson also plans to interact with the Alan Alda Center at Stonybrook on promoting science communication.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.