About the Presenter:
Chris Hogan is an associate professor in the department of mechanical engineering at the University of Minnesota. He received his bachelor’s degree in Biological & Environmental Engineering from Cornell University in 2004, and a PhD degree in Energy, Environmental, & Chemical Engineering from Washington University in 2008. After studying as a Postdoctoral Associate at Yale University, he joined the faculty at the University of Minnesota in 2009. He is the recipient of the 2011 Sheldon K. Friedlander Award for “Outstanding PhD dissertation in a field of aerosol science and technology”, and the 2013 Marian Smoluchowski Award for “Outstanding contributions in aerosol science”. Currently, his laboratory group focuses on measurements and theory to explain the growth and transport of nanoparticles and ions in the gas phase (aerosols).
New particle formation in the gas phase occurs when vapor phase precursors collide with and bind to one another without dissociating; vapor molecules binding in succession eventually leads to the formation of nanoparticles. Traditionally, particle formation rate predictions have been based on classical nucleation theory (CNT), in which the energy barrier to growth is based on the assumed validity of the Kelvin effect. However, in realistic chemically inhomogeneous systems, the energy barrier predicted by CNT is typically an upper limit; growth can occur much faster through heterogeneous pathways (i.e. the binding of vapor molecules which are chemically distinct from those already incorporated into the growing particle). This talk will present recent research results on probing heterogeneous vapor binding to clusters using ion mobility spectrometry-mass spectrometry (IMS-MS) with a parallel-plate differential mobility analyzer (DMA). It will be shown that by doping the sheath gas of the DMA with controlled amounts of vapor dopants, the shifts in mobility/collision cross section for chemically well-identified particles/clusters can be monitored, with shifts in mobility <2% (corresponding to the transient binding of a single vapor molecule) observable. Case study results for water binding to salt clusters as well as atmospherically relevant dimethylamine-sulfuric acid clusters will be presented. Direct comparison between measured mobility shifts and predictions based upon CNT derived models will additionally be provided; this comparison shows that such models do not appropriately describe heterogeneous nanoparticle growth.