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.

We have used transmission electron microscopy with electron energy-loss spectroscopy (EELS) to characterize the structures of carbonaceous aerosol particles from Phoenix, AZ. The particles are <2 jum in diameter and consist of tens to hundreds of aggregated spherules. EELS reveals a range of carbon structures, from amorphous to graphitic, within aggregates. Structural variations occur among individual spherules, on a scale of approximately 0.05 fim. The energy-loss data suggest combinations of single, double, and triple carbon bonds, implying the presence of both organic and elemental carbon. We interpret the graphitic domains as part of the primary spherules and the amorphous areas as condensed hydrocarbons. Surface coatings occur on carbonaceous aggregates collected in the summer months. The coatings contain oxygen, nitrogen, and sulfur. Coated aggregates have carbon structural variations similar to uncoated aggregates. The coatings are interpreted as sulfates and nitrates deposited from the atmosphere as end products of photochemical reactions.

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