Fractal-like Tar Ball Aggregates from Wildfire Smoke

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Fractal-like Tar Ball Aggregates from Wildfire Smoke Giulia Girotto, Swarup China, Janarjan Bhandari, Kyle Gorkowski, Barbara Scarnato, Tyler Capek, Angela Marinoni, Daniel Veghte, Gourihar Kulkarni, Allison Aiken, Manvendra K. Dubey, and Claudio Mazzoleni Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00229 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018

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Environmental Science & Technology Letters

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Fractal-like Tar Ball Aggregates from Wildfire Smoke

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Giulia Girotto†,ǂ, Swarup China †,‡*, Janarjan Bhandari†, Kyle Gorkowski†,§,ǁ, Barbara V. Scarnato#, Tyler Capek†, Angela Marinoni˄, Daniel P. Veghte‡, Gourihar Kulkarni◊, Allison C. Aiken§, Manvendra Dubey§, Claudio Mazzoleni†*

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†

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Houghton, Michigan 49931, USA

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‡

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Richland, Washington 99354, USA

Physics Department and Atmospheric Sciences Program, Michigan Technological University,

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

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ǂ

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38122, Italy

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Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento

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Earth Systems Observations, Los Alamos National Laboratory, Los Alamos, New Mexico

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87545, USA

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ǁ

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Present address: Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada

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DNV GL, 1363 Høvik, Norway

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˄

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Bologna 40129, Italy

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◊

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Richland, Washington 99354, USA

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*Correspondence:

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Email: [email protected], and Phone: 509-371-7329 and

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Email: [email protected], phone: 906-487-1226

Institute of atmospheric sciences and climate (ISAC)-Consiglio Nazionale delle Ricerche,

Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory,

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Abstract

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Tar balls are atmospheric particles abundant in slightly aged biomass burning smoke and have a

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significant, but highly uncertain, role on Earth’s radiative balance. Tar balls are typically

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detected using electron microscopy; they are resistant to the electron beam, and generally, they

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are observed as individual spheres. Here, we report new observations of a significant fraction of

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tar ball aggregates (~27% by number) from samples collected in a plume of the Whitewater-

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Baldy Complex fire in New Mexico. The structure of these aggregates is fractal-like and follows

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a scale invariant power law similar to that of soot particles, despite the considerably larger size

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and smaller number of monomers. We also present observations of tar ball aggregates from four

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other geographical locations, including from a remote high elevation site in the North Atlantic

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Ocean. Aggregation affects the particle optical properties and therefore, their climatic impact.

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We performed numerical simulations based on the observed morphology and estimated the

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effects of aggregation on the tar balls optical properties. Based on single particle numerical

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simulations, we find that aggregates had a single scattering albedo up to 41% higher than

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individual tar balls at 550 nm.

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Introduction

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Biomass burning (BB), including residential wood combustion for heating or cooking,

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wildfires, and prescribed burns, is one of the largest sources of carbonaceous particles in the

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atmosphere1 and is the typical source of tar balls (TBs) that are the subject of this study. For

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example, open burnings contribute to 42% in mass of the soot and to 74% in mass of the organic

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carbon aerosol in the atmosphere2. BB particles significantly impact Earth’s climate by scattering

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and absorbing solar radiation and by interacting with clouds1 through different processes that

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depend also on the mixing state and the morphology of the aerosol3. The radiative forcing of BB

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aerosol is highly uncertain with an estimated net positive direct radiative forcing of +0.20 Wm-2

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(90% uncertainty bounds from -0.50 to +1.08 W m-2)1. The large uncertainty range is partly due

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to the balance between the positive forcing due to absorbing aerosols (i.e., black and brown

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carbon) and the negative forcing by some of the organic carbon aerosol that are weakly or non-

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absorbing.

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One of the most abundant kinds of particle in BB smoke is TBs, especially in the smoldering

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phase, where generally a small fraction of soot is produced. TBs are operationally identified

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using electron microscopy by their spherical shape, their average diameter typically in the ~100-

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300 nm range, their elemental composition (mostly carbon, oxygen and minor traces of

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potassium), their amorphous nanostructure, and the fact that they are resistant under the electron

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beam4. A recent study found that TBs are thermally stable with a volume fraction of up to 30%

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retained even when heated to 600°C5. The authors also suggest that because of their thermal

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stability, TBs may not be detected by traditional aerosol mass spectrometers (such as the

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Aerodyne aerosol mass spectrometer), thus the TBs fraction in BB smoke might be



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underestimated5. Soot particles are also emitted during BB and they exhibit a lacy, fractal-like

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structure, made up of spherical monomers composed mostly of carbon6. However, TBs are

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clearly distinguishable from soot monomers, as the size of TBs is larger with respect to the soot

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monomers (20-50 nm), and soot monomers have typically a more graphitic nanostructure4,7.

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The number concentration of TBs in BB smoke plumes can vary due to several factors,

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including the age of the plume. For example, the fraction of TBs in one fire was found to be as

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high as ~90% close to the source4,8, while a much lower fraction (~15%) was found far away

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from the source7. However, a recent aircraft study observed a higher fraction of TBs downwind

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(~45%) compared to near source (100 nm,

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the size of the filter pores; this fraction includes the TBs in the aggregate). On average, the

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number fraction of individual TBs was ~54% and TB aggregates was ~27%, while only a minor

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fraction was soot (~6%), and 13% was organic matter and other particles (such as dust), with

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respect to total number of particles. High number fractions of TBs in BB smoke plumes are

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typically observed during the smoldering phase of a fire4,8,12,13. The number fraction that we

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report here, is similar to our previous study (80%)3 where we investigated a relatively fresh

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smoke plume (~1-2 hours aged) and similar (up to 85%) to another study by Pósfai et al.4 who

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investigated a similarly aged (~1hour) BB plume. However, TBs in the study discussed here

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were much more abundant than those found in Mexico City (~15%) in a fresher smoke plume (~

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minutes)7. The size distribution of the TBs is consistent with previous studies, where most of the

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size distribution of TBs was found to lay typically between 100 and 300 nm3,7,8. Over 3000

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individual TBs were used for analysis. In Figure 1c, we report the size distribution of TBs; the

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geometric mean diameter of the TBs was 150.5 nm with geometric standard deviation of 1.4 nm.

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Morphology, elemental composition and abundance of TB aggregates. We characterized 227

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TB aggregates. In Figure 1a-b, we show some examples of these aggregatesWe defined a particle

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to be an aggregate if it contained 8 or more monomers, based on Zangmeister et al.25, who

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suggested that particles containing a number of monomers N between 2 and 7 are in the

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intermediate growth phase, before they form an aggregate. Figure 1c shows the size distribution

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of the monomers within the TB aggregates. We estimated that a large fraction (48%) of the total

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number of TB aggregates consisted of 8-10 monomers. However, we observed a significant 8 ACS Paragon Plus Environment

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fraction (~52%) of particles composed of 11 or more monomers and 13% of the particles

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composed between 20 and 30 monomers. Only a minor fraction of the TB aggregates (~3%) had

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a number of monomers between 80 and 110. Figure 1d shows the frequency of TB aggregates,

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which contain 8 or more monomers. We calculated the roundness of single TBs in the aggregates

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and found that they are nearly spherical (most had roundness > 0.9) and are composed mainly of

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C and O, supporting the assumptions that these are indeed aggregates of TBs. Interestingly, the

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presence of S is prominent in most of the TB aggregates (Figure S1 and S2). The diameter of the

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monomers in the TB aggregates (62 to 458 nm) is not as narrowly distributed as typically found

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for the monomers in soot particles (20-60 nm)3,17.

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We investigated the fractal nature of the TB aggregates following an analysis similar to that

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used for soot particles3,17. We applied the same equation to estimate N and to calculate the fractal

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dimension of soot particles. To this purpose, in Figure 2, we plotted the estimated number of

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monomers N versus (Lmax×Wmax)0.5/dp. For comparison, in the same figure, we also plotted the

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data for the soot aggregates analyzed from the same sample. The fit in Figure 2 suggests that the

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TB aggregates and the soot aggregates have indeed similar fractal dimension and similar scale

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invariance, with a fractal dimension close to 2. We used a range of the kα and α values based on

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different overlap parameters to study the sensitivity of Df and we found it to vary between1.98

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and 2.11 (Table S3). For completeness, in Table S4, we report several other morphological

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descriptors, including aspect ratio, roundness, and convexity.

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TBs are typically externally mixed, but some previous studies reported occasional incidences of

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TB aggregates in the field4,13 and in the laboratory16. We note that the detection of TB aggregates



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and their relative abundances depend on the sampling technique, owing to their rather large size.

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For example, we observed a higher fraction (~29%) of TB aggregates collected onto stage 7

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(0.32-0.56 µm) compared to stage 8 (0.18-0.32 µm). Figure S3 shows representative SEM

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images captured at different field of views and magnifications (1500X to 8000X) of samples

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collected in Richland using a MOUDI cascade impactor. Pósfai et al.4 reported the presence of a

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few TBs attached to other particles in a sample collected from an aged plume from regional haze

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in southern Africa. Hand et al.13 also reported TBs agglomerates during the Yosemite Aerosol

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Characterization Study on samples collected after long-range transport (2 days or more).

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However, they reported the dominance of small aggregates (

Fractal-like Tar Ball Aggregates from Wildfire Smoke

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