Aerosol particles in the presence of a vapor will often change in size and composition due to heterogeneous vapor uptake. The physics and thermodynamics of this phenomenon are not well understood for particles less than ~10 nm where traditional models using bulk properties begin to break down. Further, existing methods for measuring/quantifying vapor uptake by particles are not effective for examining particles below 5 nm in size, and at relatively high vapor saturation ratios. This dissertation presents two new methods for measuring vapor uptake by aerosol particles in this size range. Each system measures the change in electrical mobility (which can be related to size) of aerosol particles when they are introduced to a vapor of known concentration. The first system consists of a tandem High Resolution Differential Mobility Analyzer - Drift Tube Ion Mobility Analyzer (HRDMA-DTIMS) for measuring uptake by particles ranging from ~2nm to >12nm, and the second system is a tandem HRDMA-Mass Spectrometer for measuring uptake by particles ranging from a single molecule to ~2nm. For the HRDMA-DTIMS system a new drift tube ion mobility spectrometer was developed and is described, with the goal of high resolution and fast measurement times. The device is capable of sub second mobility distribution scans and resolving powers similar to DMAs currently used in similar vapor uptake experiments. Measurement of water vapor uptake by hygroscopic salts of lithium iodide and sodium iodide particles compared to theoretical calculations exposes the flaws in existing vapor uptake models. The precision of the growth factor (wet diameter / dry diameter) measured using this system is shown to be ~0.2% for the presented data. For the HRDMA-MS system we are able to identify electrospray generated ions of a specific composition and then measure their change in electrical mobility as a function of relative humidity. Using this system we measured vapor uptake by alkyl halide salt cluster ions ranging from one to 27 molecules. We also describe how structures determined using density functional theory can be used to estimate the change in electrical mobility due to additions of vapor molecules. In addition to describing new instrumentation and systems, a model for estimating mobility changes based on collision mechanics as well as thermodynamics of individual molecule uptake is presented. This model can be applied to any vapor uptake measurement systems.
University of Minnesota Ph.D. dissertation. October 2013. Major: Mechanical Engineering. Advisors: Christopher J. Hogan Jr., Peter H. McMurry. 1 computer file (PDF); ix, 160 pages + 1 MPEG file.
New Instrumentation and Methods for Studying Heterogeneous Vapor Uptake by Aerosol Particles Ranging In Size From One Molecule to 10nm.
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