Introduction to Aerosols. I.
Richard Flagan, California Institute of Technology, USA
Abstract: This tutorial is the first of two that introduce the broad field of aerosol science. We begin with the particle size distribution, which provides a concise physical description of the aerosol that determines its size-dependent properties. We then turn our attention to the behavior of individual particles that governs how the aerosol evolves, its impacts, and enables measurements. The drag forces that act on a particle determine its settling velocity and whether it can follow the motions of the surrounding gas. Stokes law describes the drag forces on spherical particles moving at modest velocities, but corrections are needed if the particles are small relative to the mean-free-path of the gas molecules, too large to be in the creeping flow regime, or not spherical. Knowledge of these scaling principles makes it possible to relate particle aerodynamics in seemingly disparate systems. The equations of motion for an aerosol particle reveal a measurable property, the aerodynamic relaxation time, that determines its sedimentation velocity and whether the particle will follow changes in velocity or direction of the gas in which it is entrained. We will examine how these inertial effects lead to deposition of large particles in the respiratory tract, filters, and sampling systems, and how they can lead to biases in aerosol sampling. Inertial effects are also used to determine an inertial size of the aerosol particles. Instruments for measuring this aerodynamic equivalent diameter will be briefly discussed. We will end this first introductory tutorial with a brief look at how inertial effects alter the size distribution of the aerosol released from a cough in a closed, well-mixed room.
Short bio: Richard C. Flagan is the McCollum/Corcoran Professor of Chemical Engineering and Environmental Science and Engineering at the California Institute of Technology. He has served as the President of AAAR, and as Editor-in-Chief of Aerosol Science and Technology. His research spans the field of aerosol science, including atmospheric aerosols, aerosol instrumentation, homogeneous nucleation, aerosol synthesis of nanoparticles and other materials, bioaerosols ranging from pollen to the SARS-CoV-2 virus, and aerosols in the atmospheres of other worlds. His many contributions to the field of aerosol science have been acknowledged with the Sinclair Award of the AAAR, the Smoluchowski Award of the Gesellschäft für Aerosolforschung, and the Fuchs Award. He is a member of the National Academy of Engineering.