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Discussion papers
https://doi.org/10.5194/acp-2019-568
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-568
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Jul 2019

Submitted as: research article | 11 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Detection of tar brown carbon with the single particle soot photometer (SP2)

Joel C. Corbin1 and Martin Gysel-Beer2 Joel C. Corbin and Martin Gysel-Beer
  • 1Metrology Research Centre, 1200 Montreal Road, National Research Council Canada, Ottawa, ON K1A 0R6, Canada
  • 2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

Abstract. We investigate the possibility that the refractory, infrared-light-absorbing carbon particulate material known as tar balls or tar brown carbon (tar brC) generates a unique signal in the scattering and incandescent detectors of the single particle soot photometer (SP2). As recent studies have defined tar brC in different ways, we begin by reviewing the literature and proposing a material-based definition of tar. We then show that tar brC results in unique SP2 signals due to a combination of complete or partial evaporation, with no or very little incandescence. Approximately 70 % of tar particles incandesced. At the time of incandescence the ratio of light scattering to incandescence from these particles was up to twofold greater than from soot black carbon (BC). In our sample, where the mass of tar was threefold greater than the mass of soot, this led to a bias of < 5 % in SP2-measured soot mass, which is negligible relative to calibration uncertainties. The enhanced light scattering of tar is interpreted as due to its being more amorphous and less graphitic than soot BC. The fraction of the tar particle which does incandesce was likely formed by thermal annealing during laser heating.

These results indicate that laser-induced incandescence, as implemented in the SP2, is the only BC measurement technique which can quantify soot BC concentrations separately from tar, while also potentially providing real-time evidence for the presence of tar. In contrast, BC measurement techniques based on thermal–optical (EC) and absorption (eBC) measurements cannot provide such distinctions. The optical properties of our tar particles indicate a material similarity to the tar particles previously reported in the literature. However, more- and less-graphitized tar samples have also been reported, which may show stronger and weaker SP2 responses, respectively.

Joel C. Corbin and Martin Gysel-Beer
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Joel C. Corbin and Martin Gysel-Beer
Joel C. Corbin and Martin Gysel-Beer
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Short summary
We review the literature to refine the definition of tar balls (or tar particles). Then, using a marine-engine data set, we show that a standard SP2 can identify tar particles in two ways: as evaporating and non-incandescing (30 % of tar particles by number), or incandescing particles which scatter more light than soot at incandescence (70 % of tar particles by number). To our knowledge, no other technique can provide in-situ, real-time evidence for the presence of tar particles in an aerosol.
We review the literature to refine the definition of tar balls (or tar particles). Then, using a...
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