The Use of Transmission Electron Microscopy with Scanning Mobility Particle Size Spectrometry for an Enhanced Understanding of the Physical Characteristics of Aerosol Particles Generated with a Flow Tube Reactor

EMMA TACKMAN, Devon Higgins, Murray Johnston, Miriam Freedman, The Pennsylvania State University

     Abstract Number: 286
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Microscopic examination of aerosol particles provides insight into physical characteristics such as specific shape, phase separation if present, and qualitative speciation, all of which have significant implications for particle phase atmospheric chemistry. The scanning mobility particle sizer (SMPS) is a common tool for measuring the size of aerosol particles but must be paired with other techniques for information about particle morphological attributes which are inaccessible to a SMPS alone. Transmission electron microscopy (TEM) is one such complementary technique whereby, after impaction onto a substrate, individual particles can be observed with high throughput. An often-overlooked aspect of aerosol characterization is the influence the substrate which may cause physical deformation to a particle. Deformation must be considered when comparing data from a SMPS, which assesses particles suspended in a gas flow, to the results of a substrate-supported measurement.

Here, we explore the relationship between SMPS- and TEM-derived aerosol measurements; particles containing ammonium sulfate and α-pinene SOA generated in a flow tube under varying conditions were measured by SMPS and subsequently impacted onto a substrate and examined using TEM. Using the same particles for both measurements facilitated the direct comparison of the two techniques as well as the ability to observe the influence of the substrate. We found that impacted ammonium sulfate particle diameters increased by +0% to +30% when compared to the suspended mobility diameters, an unexpectedly wide range for a single component system. SOA-coated particles showed an even wider range of diameter deviations from –34% to +60%. Morphology was also highly variable in coated samples where between 18% and 98% of particles displayed obvious phase separation. The surprisingly high level of diversity in morphology and the deviation between suspended and impacted aerosols implies that great care should be taken when comparing data sets from disparate sources.