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. 2013;4:2122.
doi: 10.1038/ncomms3122.

Morphology and Mixing State of Individual Freshly Emitted Wildfire Carbonaceous Particles

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Free PMC article

Morphology and Mixing State of Individual Freshly Emitted Wildfire Carbonaceous Particles

Swarup China et al. Nat Commun. .
Free PMC article

Abstract

Biomass burning is one of the largest sources of carbonaceous aerosols in the atmosphere, significantly affecting earth's radiation budget and climate. Tar balls, abundant in biomass burning smoke, absorb sunlight and have highly variable optical properties, typically not accounted for in climate models. Here we analyse single biomass burning particles from the Las Conchas fire (New Mexico, 2011) using electron microscopy. We show that the relative abundance of tar balls (80%) is 10 times greater than soot particles (8%). We also report two distinct types of tar balls; one less oxidized than the other. Furthermore, the mixing of soot particles with other material affects their optical, chemical and physical properties. We quantify the morphology of soot particles and classify them into four categories: ~50% are embedded (heavily coated), ~34% are partly coated, ~12% have inclusions and~4% are bare. Inclusion of these observations should improve climate model performances.

Figures

Figure 1
Figure 1. Identification of electronically dark and bright spherical TB particles.
Field-emission scanning electron microscopy micrographs of ambient particles collected from the Las Conchas fire. (a) Image at low working distance (3.4 mm) and low accelerating voltage (1 kV). (b) Image of the same particles, but at higher working distance (13 mm) and higher accelerating voltage (10 kV). Electronically bright and dark TBs are evident at the low accelerating voltage, but not at the high accelerating voltage. Scale bars, 1 μm. Black circles are pores in the nucleopore filter.
Figure 2
Figure 2. Size distribution of ambient- and denuded-bright and -dark TBs.
Size distribution and lognormal fits of: (a) ambient particles (435 dark and 930 bright particles); (b) denuded particles (415 dark and 1,086 bright particles). The number of particles in each size bin is normalized by the bin width in logarithmic space, and the mode of the distribution is normalized to 1. The light grey lines represent bright TBs and the dark lines represent dark TBs. The difference between the count median diameter for ambient-dark and denuded dark TBs is 33 nm (209−176 nm), whereas the difference between ambient-bright and denuded-bright TBs is 61 nm (173−112 nm). Dark TBs display smaller reduction in size on denuding, consistent with being more refractory.
Figure 3
Figure 3. Mixing and classification of soot particles.
Field-emission scanning electron microscope images of four different categories of soot particles: (a) embedded, (b) partly coated, (c) bare and (d) with inclusions. Approximately 50% of the ambient soot particles are embedded, 34% are partly coated and 12% have inclusions. Only 4% of the particles are bare soot (not coated or very thinly coated). Scale bars, 500 nm.
Figure 4
Figure 4. Fractal dimension of soot particles.
Fractal dimension of ambient-1 (in black) and denuded-1 (in grey) soot particles. The fractal dimension of ambient and denuded soot is 1.85±0.05 (n=176) and 1.53±0.07 (n=209), respectively. s.e. was calculated from the uncertainty in the mean‐square fit considering the uncertainty in N and dp. The insets provide examples of ambient-1 and denuded-1 soot particles. Scale bars, 500 nm.

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