Effects of mixing state on black carbon measurements by laser-induced incandescence

Laboratory experiments and theoretical calculations were made to characterize the performance of a Single Particle Soot Photometer (SP2) manufactured by Droplet Measurement Technologies (DMT), which was designed to measure the mass and the mixing state of individual black carbon (BC) or elemental carbon (EC) particles, based on the laser-induced incandescence (LIT) technique. In this study, graphite was used as a surrogate of EC. Graphite particles with mass equivalent diameters of 110-200 nm were layered with organic liquids (glycerol and oleic acid) to produce coated graphite with diameters up to 650-800 nm. These were sampled by the SP2 to measure the waveforms (i.e., time development) of the LIT and scattering signals. The peak temperature and the peak LIT signal of graphite particles were independent of the coating thickness or the coating material to within experimental errors. These results indicate that the mass of EC can be measured by using peak LIT signal without interference by the coating conditions. It was also shown that the difference between the times of the scattering and LIT peaks can be used as an indicator of coating on EC with thicknesses larger than about 100-200 nm. LIT and scattering waveforms were calculated using a newly developed theoretical model that takes into account the physical processes controlling the temperature and evaporation rate of the coated graphite particle in the laser beam. The calculations reproduced the general features of observed waveforms of LIT and scattering signals, providing a firm theoretical basis for the interpretation of the SP2 data.