Modelling the Response of Tar Brown Carbon (Tarballs) to Pulsed Laser-induced Incandesce (P-LII)

FENGSHAN LIU, Joel Corbin, Timothy Sipkens, Gregory Smallwood, National Research Council Canada

     Abstract Number: 10
     Working Group: Carbonaceous Aerosol

Abstract
Tar brown carbon (TB) is an important class of light-absorbing carbon produced in biomass burning and heavy fuel oil combustion. TB particles display an amorphous-carbon molecular structure and have diameters from about 50 to 500 nm. The reported imaginary part of refractive index for TBs from all sources varies by two orders of magnitude from 0.002 to about 0.27 in the visible spectrum, which are significantly lower than mature soot at visible and near infrared wavelengths. Correspondingly, it has been believed that TB cannot be detected using pulsed laser-induced incandescence (P-LII). However, it has been demonstrated that certain TB particles can indeed incandesce in a single particle soot photometer (SP2), a continuous-wave LII (CW-LII) instrument. That observation was explained by laser-induced carbonization of TB during laser heating in SP2.

In this study, an LII model was built to account for laser-induced carbonization of TB particles during P-LII. The particle absorption cross section was calculated as a weighted average between TB and mature soot. Heat conduction was modelled using the Fuchs method valid over the entire Knudsen regime from free-molecular to continuum. The carbonization of TB was modelled as an annealing process.

The results show that the peak TB particle temperature during P-LII increases nearly linearly with the laser fluence. Using a 1064 nm excitation laser, a fluence of 12.6 mJ/mm2 is required to heat the peak TB particle (200 nm in diameter) to a temperature of 2500 K. This fluence is about a factor of 6 higher than the typical P-LII fluence used for soot measurements. This study explains why TB particles have not been detected using P-LII in previous studies and confirms that TB particles can potentially be detected using P-LII at sufficiently high laser fluences, opening the door for future diagnostics for these kinds of particles.