EECE Seminar - Dr. Richard Flagan

About this event

Richard Flagan, The Irma & Ross McCollum/Williiam H. Corcoran Professor

Department of Chemical and Environemtnal Science and Engineering

California Institute of Technology

Dynamics of Atmospheric Fine Particles

Abstract:

In most urban air basins, direct particulate emissions dominate the atmospheric aerosol, but globally, most atmospheric particles are formed by nucleation of vapor phase products of gas phase reactions. Even where emissions account for most of the particles, products of gas phase reactions often dominate the aerosol mass. Knowledge of the photochemistry that leads to that secondary aerosol has grown dramatically since Haagen-Smit discovered that the Los Angeles began to plague the city in the 1940s. Advances in analytic chemistry and new approaches to both atmospheric sampling and laboratory simulations have coupled the atmospheric chemistry and aerosol science, but understanding of how particles form and grow in the atmosphere has, until recently, lagged behind. Most efforts to understand atmospheric particle formation have been built upon the classical theory of homogeneous nucleation wherein a single, supersaturated chemical species forms clusters that grow slowly until they large enough to become thermodynamically stable. Beyond that critical size, growth accelerates. That theory was extended to include a small number of additional species, particularly sulfuric acid, that were observed to accelerate nucleation, but the framework for atmospheric nucleation theory maintained its classical routes. Extending particle size distribution measurements down to near-molecular sizes has revealed nucleation bananas in which newly form clusters grow to become accumulation-mode aerosols. These particle formation and growth events are commonly seen around the world. Novel atmospheric chamber experiments combined with enable unprecedented control in laboratory simulations of the atmospheric physicochemical dynamics of new particle formation, making it possible to follow nucleation molecule-by-molecule, shaking the foundation of classical nucleation theory in real-world applications. At the same time, ambient aerosol measurements reveal the need for new approaches to modeling atmospheric aerosol dynamics, and for further advances in measurement technology.

Bio:

Richard Flagan studied Mechanical Engineering, receiving his BS from the University of Michigan, and his SM and Ph.D. from MIT. He became Assistant Professor in Environmental Engineering Science in 1975, becoming Professor in 1986, before moving to Chemical Engineering in 1990. He is now the McCollum/Corcoran Professor of Chemical Engineering and Environmental Science and Engineering in 2003. He served as the Executive Officer for Chemical Engineering 2004-2013, and Chair of the Faculty 2014-2018. Among other awards, he received the Fuchs Award from AAAR, GAeF, and JAAST, the highest award in aerosol science, in 2004, was elected to the National Academy of Engineering in 2010. He is a member of the Board of Directors of the California Council on Science and Technology, a nonprofit founded at the behest of the California Legislature to provide science and technology guidance to the California legislature and administrative agencies, with a key component of embedding recent science and engineering PhDs as postdoctoral fellows in those bodies where they apply their skills to the policy challenges faced by the state.

When Flagan joined the Caltech faculty in smoggy 1975 Pasadena, he switched his research focus from combustion to aerosols. His research over have spanned the field of aerosol science, advancing the state of the art for aerosol measurement. His first aerosol instrument was a low-pressure impactor that enabled size-resolved particle sampling for chemical analysis in the 50 nm-10 µm size range. With the LPI he revealed that combustion-stoichiometry affected enrichment of the smallest particles produced during coal combustion with species that vaporize during coal combustion and later condense, sometimes undergoing homogeneous nucleation. Thus began his long-standing interest in nucleation from the vapor phase, and the subsequent dynamics of the aerosol particles.

At the same time he began his long collaboration with a Chemical Engineering modeler, John Seinfeld, developing his first of many aerosol reactors, a large, outdoor, Teflon-balloon smog chamber that was clean enough to enable fundamental studies of both the gas phase chemistry leading to smog, and the aerosol that either grew by as low-volatility products of oxidized VOCs condensed on existing particles, or nucleated new ones. Flagan’s initial focus was on the physical evolution of the aerosol, but the available instruments were unable to capture its rapid dynamics. He invented the scanning electrical mobility spectrometer (SEMS or SMPS) that has become the primary instrument worldwide for aerosol size distribution measurements. Flagan continues to advance aerosol measurement science, identifying, and resolving measurement challenges that accompany the fast measurements, extend the measurement range, and creating new detectors, instruments, and experimental methods in order answer previously unresolvable questions. His measurements in the smog chamber have determined aerosol yields atmospheric reactions of wide range of anthropogenic and biogenic molecules, advancing models of both the chemical kinetics and the aerosol dynamics. He adapted the SEMS for variable-pressure airborne measurements from the boundary layer to the tropopause, characterized atmospheric aerosols around the world, and in the atmosphere from Earth’s surface into the stratosphere. He developed several field-deployable mobility analyzers that enable size distribution measurements down to one nanometer. These instruments overlap with mass spectrometric measurements to probe the mechanism of atmospheric particle nucleation. Applying them in the cleanest atmospheric simulation chamber for the CLOUD collaborative experiment at CERN, he has helped reveal a new mechanism that differs markedly from classical nucleation theory.

Flagan’s aerosol research extends into other fields as well. He developed a multistage aerosol reactors for materials synthesis, including core shell nanoparticles that have been processed through an industrial development fab into novel memory devices. He has elucidated the mechanisms whereby allergens associated with pollen tens of microns in size enter the atmosphere in particles that are small enough to trigger the wheezing in asthma attacks. He has recently helped design a JPL facility that will soon simulate cloud formation in the atmospheres of other worlds, among others.