Many ultrafine atmospheric aerosol particles have aggregate structures. Thernmorphological properties of atmospheric aggregates are important for understanding theirrnsources, transport, and health risk. Atmospheric aggregates can be characterized in partrnusing fractal concepts. In this study, particles were collected on the transmission electronrnmicroscope grids placed on the lower stages (7 and 8) of a low-pressure Hering impactor.rnMost of the particles deposited on these stages were fractal-like. The particles arernanalyzed under the transmission electron microscope. Computer programs were used tornfind fractal dimension from the primary particle sizes and location. In the analysis ofrnparticles composed of 10 to 180 primary particles, the number of primary particles andrnfractal dimension (D_f) have an approximately linear relationship for Df values rangingrnfrom near unity to above 2. The atmospheric particles were also compared to soot andrnsimulated aggregates, in terms of their fractal dimension and primary particle sizerndistribution. Although the basic structures are similar, the primary particles making uprnthe aggregates vary between the soot and atmospheric particles.
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