Aggregation of Spherical Carbonaceous Particles in Wildfire Smoke

Mazzoleni, C., Michigan Technological University

Aerosol Processes

Aerosol Processes

Girotto G, S China, J Bhandari, K Gorkowski, B Scarnato, T Capek, A Marinoni, D Veghte, G Kulkarni, A Aiken, M Dubey, and C Mazzoleni. 2018. "Fractal-like Tar Ball Aggregates from Wildfire Smoke." Environmental Science & Technology Letters, 5(6), doi:10.1021/acs.estlett.8b00229.


An example of tar ball aggregate.


An example of tar ball aggregate.

Science

Tar balls are submicron carbonaceous spherical particles, commonly found in wildfire smoke. Their formation mechanisms and optical properties are still much debated, owing to contrasting results published in the literature. In addition, they are often analyzed as single particles; however, in our recent study, we found that they can frequently form fractal-like aggregates.

Impact

Fires of natural or human cause are one of the most important sources of aerosol particles in the atmosphere. Among different types of particles, tar balls can be quite abundant in fire smoke, depending on the burning conditions and smoke age. Typically, tar balls are analyzed as individual spheres, but recent studies found that they can form fractal-like aggregates. Even though aggregation does not change the mass loading of tar balls in the atmosphere directly, the process can significantly impact the particles' properties. In particular, aggregation can substantially affect how the particles interact with sunlight and clouds, posing the question of how to best represent these aggregates' properties in global climate models, to accurately estimate the radiative effects of fire smoke.

Summary

While analyzing particles collected during the Las Conchas fire (New Mexico, USA, 2012), we noticed an abundance of aggregates of carbonaceous spheres. The particles were imaged using electron microscopy and their elemental composition was determined using energy dispersive x-ray spectroscopy. The components of these aggregates had all the properties of particles called "tar balls". Tar balls are made mostly of carbon and oxygen with traces of other elements. They have a typical size of 100-300 nm, quite larger than soot monomers--another type of particle that typically appears in aggregated form. Tar balls also resist the electron beam. When they are imaged at high resolution with a transmission electron microscope, they show an amorphous nanostructure. All these distinctive properties ease their unambiguous identification. We observed tar ball aggregates also at several other geographical locations, with variable number fractions. In the past, tar balls have been analyzed individually, and although aggregates of tar balls have been sparsely reported, their properties have not been studied in detail. In this work, we focused on studying their physical properties and found that their structure is fractal-like, similar to soot particles, with a 3D fractal dimension close to 2. Aggregation can affect the particle optical properties, especially the scattering cross-section and the scattering angular distribution (both important for the particle’s climatic impact). However, aggregation could also change their deposition rate, their ability to affect warm and cold cloud formation, and their detectability by real-time instrumentation or deposition efficiency on sampling media, as well as remote-sensing estimates. Results from numerical simulations suggest that tar balls aggregation might need to be accounted for in radiative forcing models.