Tar Ball Aggregates: Morphological and Optical Properties

 

Authors

Claudio Mazzoleni — Michigan Technological University
Giulia Girotto — MTU

Janarjan Bhandari — Michigan Technological University
Tyler Capek — Michigan Technological University
Barbara Scarnato — Naval Postgraduate School
Kyle Gorkowski — Los Alamos National Laboratory
Angela Marinoni — Institute of Atmospheric Science and Climate-CNR
Daniel Veghte — Pacific Northwest National Laboratory
Gourihar Kulkarni — Pacific Northwest National Laboratory
Allison C Aiken — Los Alamos National Laboratory
Manvendra K. Dubey — Los Alamos National Laboratory

Category

General topics – Aerosols

Description

Atmospheric tar balls are carbonaceous particles abundant in slightly aged biomass burning smoke and are typically identified using electron microscopy. Tar balls (TBs) are almost perfectly spherical, have an amorphous nano-structure, they are composed mostly of carbon and oxygen, they are resistant to the electron beam and they have typical sizes in the range of ~100 to 300 nm. TBs absorb solar radiation, but their optical properties are still highly uncertain with published values of the imaginary part of their index of refraction that span more than two orders of magnitude. Therefore, TBs effects on Earth’s radiative balance are highly uncertain. In 2012, at the Los Alamos National Laboratory, we collected aerosol samples from the Whitewater-Baldy Complex fire that is to date, the largest wildfire in New Mexico’s history. The plume traveled to the sampling site in less than 10 hours. We used scanning electron microscopy to detect the TBs, determine their abundance, and study their morphology and mixing. We observed that TBs were very abundant in the plume. Past studies typically reported TBs as individual externally mixed particles. However, in our samples, we observed a significant fraction of lacy aggregates of several TBs. The morphology of these TB aggregates appears to be fractal-like, and we found that they follow a scale-invariant power-law similar to that of soot particles. We also analyzed samples from other sites affected by biomass burning plumes and found several lines of evidence that these aggregates might be relatively common in the atmosphere. We used T-matrix and Lorenz-Mie simulations to estimate the effect of aggregation on the optical properties of TBs, and then performed calculations on how the changes in optical properties affect the TBs radiative forcing.