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 nm, were divided into three groups: fractal (D < 2), possibly nonfractal (D > 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 > 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 > 2 surrounded by outer regions with D < 2.

Carbonaceous aggregates collected from the aerosol of Phoenix, Arizona have an irregular branched morphology. The aggregates resemble combustion soots and were probably emitted by motor vehicles. Fractal analysis provides a means of quantifying morphologic variations among aggregates and relating these variations to mechanisms of formation. Bright-field transmission electron microscope (TEM) images of 38 individual aggregates were recorded on negatives at magnifications of 15,000 to 200,000. The aggregates have maximum lengths ranging from 0.21 to 2.61 μm and are composed of interconnected spherules, each averaging 26 nm in diameter. The number of spherules in each aggregate ranges from 32 to 1842; the average number is 551. “The nesting squares” method of fractal analysis was applied to digital binary images to calculate the fractal dimension (D) of each aggregate.