Light absorption properties of laboratory-generated tar ball particles
Tar balls (TBs) are a specific particle type that is abundant in the global troposphere, in particular in biomass smoke plumes. These particles belong to the family of atmospheric brown carbon (BrC), which can absorb light in the visible range of the solar spectrum. Albeit TBs are typically prese...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/239/2016/acp-16-239-2016.pdf |
| Summary: | Tar balls (TBs) are a specific particle type that is abundant in the global
troposphere, in particular in biomass smoke plumes. These particles belong to
the family of atmospheric brown carbon (BrC), which can absorb light in the
visible range of the solar spectrum. Albeit TBs are typically present as
individual particles in biomass smoke plumes, their absorption properties
have been only indirectly inferred from field observations or calculations
based on their electron energy-loss spectra. This is because in biomass smoke
TBs coexist with various other particle types (e.g., organic particles with
inorganic inclusions and soot, the latter emitted mainly during flaming
conditions) from which they cannot be physically separated; thus, a direct
experimental determination of their absorption properties is not feasible.
Very recently we have demonstrated that TBs can be generated in the
laboratory from droplets of wood tar that resemble atmospheric TBs in all of
their observed properties. As a follow-up study, we have installed on-line
instruments to our laboratory set-up, which generate pure TB particles to measure
the absorption and scattering, as well as the size distribution of the particles.
In addition, samples were collected for transmission electron microscopy
(TEM) and total carbon (TC) analysis. The effects of experimental parameters
were also studied. The mass absorption coefficients of the laboratory-generated TBs were found to be in the range of 0.8–3.0 m<sup>2</sup> g<sup>−1</sup> at
550 nm, with absorption Ångström exponents (AAE) between 2.7 and 3.4
(average 2.9) in the wavelength range 467–652 nm. The refractive index of
TBs as derived from Mie calculations was about 1.84 − 0.21<i>i</i> at 550 nm.
In the brown carbon continuum, these values fall closer to those of soot than
to other light-absorbing species such as humic-like substances (HULIS).
Considering the abundance of TBs in biomass smoke and the global magnitude of
biomass burning emissions, these findings may have substantial influence on
the understanding of global radiative energy fluxes. |
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| ISSN: | 1680-7316 1680-7324 |