Surfaces of Secondary Organic Aerosol Particles

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In this project, funded by the Environmental Chemical Science Program of the Chemistry Division, Professors Franz Geiger and Regan Thomson of Northwestern University and Professor Scot Martin of Harvard University are developing new methods to investigate the surfaces of atmospheric aerosol particles formed from plant emissions over the world's tropical and boreal forests. The surface of aerosol particles is the first species encountered by molecules approaching the particles from the gas phase in processes leading to particle growth and subsequent cloud formation. The particle surface is a challenging region of matter to access experimentally because it is thin and very reactive. The team specifically probe aerosol particle surfaces, the particle bulk, and the gas phase and then focuses on identifying chemical compounds residing at the boundary between the gas and particle phases of atmospheric aerosols. The team also determines how these surface compounds promote or disrupt particle growth and their propensity to form clouds. The synthesized compounds are available to the research community, enabling new collaborations that cross traditional disciplinary boundaries. The field of atmospheric chemistry requires a workforce trained in crosscutting disciplines. The collaborative training provided by the project addresses that need through interdisciplinary research activities. The proposed activities are also integrated with a meaningful focus on diversity. Rigorous training and mentoring experiences for graduate students includes research rotations at Northwestern and Harvard Universities, formal training in grant proposal writing, cross-disciplinary training in synthetic chemistry and spectroscopy, aerosol sample collection and analysis, and participation in community outreach programs. The proposed activities are fine-tuned by regular assessment and evaluation.This research project tests the hypotheses that the growth of secondary organic aerosol (SOA) particles and their propensity to act as cloud condensation nuclei (CCN) can be significantly influenced by surface chemistry. The hypotheses is tested by synthesizing both deuterated and unlabeled isoprene and á-pinene and their oxidation products in pure forms, preparing SOA particles from these precursors, and (3) obtaining through measurement surface- and bulk-specific spectroscopic, chemical, and mechanistic information for determining how the surface-localized species contribute to SOA growth and CCN activity. The research includes new synthesis and measurement approaches that reveal unique physical insights of how particle surface chemistry matters for particle growth under varying conditions of relative humidity and particle size. The following overarching five science questions are addressed: Which organic species partition to the surface of SOA particles? What are the residence times of the surface-localized species, and to what extent does diffusion of the interfacial species occur into the bulk particle phase? Do gas phase species adsorb reversibly or irreversibly to the surfaces of SOA particles? Which surface species increase CCN activity by acting as a surfactant pool? Do some surface species present a barrier (attractor) to further particle growth and thereby inhibit (spur) further particle growth?