Fractal-like Tar Ball Aggregates from Wildfire Smoke

Document Type


Publication Date



© 2018 American Chemical Society. Tar balls are atmospheric particles that are abundant in slightly aged biomass burning smoke and have a significant, but highly uncertain, role in Earth's radiative balance. Tar balls are typically detected using electron microscopy and generally, they are observed as individual spheres. Here, we report new observations of a significant fraction of tar ball aggregates (∼27% by number) from samples collected in a plume of the Whitewater-Baldy Complex fire in New Mexico. The structure of these aggregates is fractal-like and follows a scale invariant power law similar to that of soot particles, despite the considerably larger size and smaller number of monomers. We also present observations of tar ball aggregates from four other geographical locations, including a remote high-elevation site in the North Atlantic Ocean. Aggregation affects the particle optical properties and, therefore, their climatic impact. We performed numerical simulations based on the observed morphology and estimated the effects of aggregation on the optical properties of the tar balls. On the basis of single-particle numerical simulations, we find that aggregates had a single scattering albedo up to 41% and 23% higher than that of individual tar balls at 550 nm and 350 nm, respectively.

Publication Title

Environmental Science and Technology Letters