FRACTAL GEOMETRY OF CARBONACEOUS AGGREGATES FROM AN URBAN AEROSOL

Carbonaceous aerosol aggregates collected in Phoenix, AZ, have an irregular branched morphology. Fractal analysis of transmission electron microscope (TEM) images provides a means of quantifying morphologic variations among aggregates and relating them to mechanisms of formation. The 38 aggregates analyzed, ranging in length from 0.21 to 2.61 mum, were divided into three groups: fractal (D < 2), possibly nonfractal (D greater-than-or-equal-to 2), and mixed geometry. For the 23 fractal aggregates, fractal dimensions (D) range from 1.35 to 1.89 and are interpretable using cluster-cluster and particle-cluster models, which are variations of diffusion-limited aggregation. The 13 aggregates with D greater-than-or-equal-to 2 were divided into two categories: uncoated and coated. The uncoated aggregates have branching shapes and may have formed through particle-cluster aggregation. The coated aggregates have an underlying morphology which may be fractal; the coatings were probably deposited from the ambient atmosphere. The two mixed aggregates have interiors with D greater-than-or-equal-to 2 surrounded by outer regions with D < 2.