AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Detection of Tar Brown Carbon with the Single Particle Soot Photometer (SP2)
JOEL CORBIN, Martin Gysel, National Research Council Canada
Abstract Number: 279 Working Group: Carbonaceous Aerosol
Abstract The single-particle soot photometer (SP2) was developed for the measurement of black carbon particles (or refractory black carbon, rBC), and has recently been applied for the detection of iron oxides in dust and industrial emissions. Here, we investigate the application of the SP2 for the detection of “tar balls” or tar brown carbon (tar brC), which consist of refractory, infrared-light-absorbing carbon. Tar may generate a unique signal in the scattering and incandescent detectors of the single particle soot photometer (SP2), either by evaporating in the laser beam without incandescing (whereas all rBC incandesces) or by displaying a unique scattering-to-incandescence ratio. For the latter diagnostic, it is important to consider that thickly-coated rBC particles may appear to scatter more light due to their volatile coatings.
We explored the above hypotheses using a test data set of tar brC, representing marine-engine tar. We identified tar particles as displaying both evaporating-but-not-incandescing behaviour and evaporating-and-incandescing behaviour. In the latter case, tar was distinguishable from rBC because its scattering cross-section at incandescence was up to a factor of two greater than that of soot black carbon (soot BC), for similar incandescence intensities. The incandescence signal observed for tar particles may correspond to internally mixed rBC or to the thermal annealing of tar during laser heating; the latter possibility is considered far more likely. There is therefore a potential positive artifact in rBC mass concentrations due to tar incandescence; we found that this artifact is unlikely to substantially bias SP2 measurements of rBC even in tar-dominated samples like ours, where the mass of tar was three-fold greater than that of soot BC. We predict that that the more- and less-graphitized tar samples reported in the literature from different sources may show stronger and weaker SP2 responses, respectively.