10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Optical Properties and Radiative Forcing of Fractal-like Aggregates of Tar Balls from Wildfire Smoke Plumes
JANARJAN BHANDARI, Swarup China, Giulia Girotto, Barbara Scarnato, Kyle Gorkowski, Allison Aiken, Manvendra Dubey, Claudio Mazzoleni, Michigan Technological University
Abstract Number: 1455 Working Group: Carbonaceous Aerosol
Abstract Tar balls (TBs) are carbonaceous spherical particles present in biomass burning smoke. In most studies, based on electron microscopy, TBs are typically reported as individual spheres (mostly ~100-300 nm in diameter). TBs absorb radiation in the UV-visible part of the solar spectrum, and therefore, they are considered a component of brown carbon aerosol. The TB optical properties reported in the literature are highly variable which in turn, make their radiative effects highly uncertain. TBs are typically studied as individual spheres. However, in a recent study, we report an abundance of fractal-like aggregates of TBs from the White-water Baldy complex fire. The optical properties of an aggregate of spheres depend on various factors such as the sphere size, the number of spheres, the refractive index, and the wavelength of the incident light. Therefore, we numerically simulated the optical properties of these aggregates using T-matrix. To determine the significance of the aggregation, we compare the T-matrix results with Lorenz-Mie simulations in two scenarios: 1) Each TB in an aggregate act as an independent sphere, so the cross sections are equivalent to the cross-section of one TB times the total number of TBs present in the aggregate (Mie-N-spheres simulation); this is equivalent to compare an aggregate with its independent components. 2) The entire aggregate is approximated as a volume equivalent single sphere of the same volume as the aggregate (Mie-VER simulation).
We calculated absorption and scattering cross-sections, single scattering albedo and hemispheric upscatter fraction of TBs for each simulation at 550 nm for several different values of indices of refraction including extreme values reported in the literature. To study the impact of number and size of spheres, and wavelength, we performed sensitivity analyses by varying the number of spheres (N = 8, 16 and 32), at five different wavelengths (350 nm, 550 nm, 750 nm, 950 nm and 1150 nm) and for three radii of 50 nm, 75.25 nm and 100 nm. Finally, we estimated the direct radiative forcing (DRF) for each simulation at 550 nm and for high and low surface albedo of 0.8 and 0.06, respectively; assuming an arbitrary TBs optical depth of 0.1.
Taking the T-Matrix as a reference, our simulations show that for single scattering albedo and upscatter fraction, the percent differences between the T-Matrix and Mie-N-spheres are large, up to ~40% and 120%, respectively. Similarly, the differences between the T-Matrix and Mie-VER are up to ~20% and 40%, respectively. The DRF from the T-Matrix differs by more than -1 Wm-2 from the Mie-N-spheres, while the DRF for T-Matrix exceeds that of Mie-VER by up to ~ +5 Wm-2, depending on the surface albedo.
Our simulations show that the optical properties and radiative forcing of TB aggregates are considerably different from those of individual TBs and aggregation should be accounted for in radiative transfer calculations.