Effects of Aggregate Morphology and Size on SP2 Measurements of Black Carbon

Abstract:

We have used a Single-Particle Soot Photometer (SP2) to measure time-resolved laser-induced incandescence (LII) and laser scatter from combustion-generated mature soot with a fractal dimension of 1.88 extracted from a burner. We have also made measurements on restructured mature-soot particles with a fractal dimension of 2.4. The soot samples were size selected using a differential mobility analyzer and characterized with a scanning mobility particle sizer and centrifugal particle mass analyzer. We reproduced the LII and scattering temporal profiles with an energy- and mass-balance model, which accounted for heating of particles passed through a CW-laser beam over laser-particle interaction times of ~10 microseconds. The results demonstrate a strong influence of aggregate size and morphology on LII and scattering signals. Conductive cooling competes with absorptive heating on these time scales; the effects are reduced with increasing aggregate size and fractal dimension. These effects can lead to a significant delay in the onset of the LII signal, which could be mistaken for a coating effect. These effects may also explain an apparent low bias in the SP2 measurements for small particle sizes, particularly for fresh, mature soot. The results additionally reveal significant perturbations to the measured scattering signal from LII interference and suggest swelling or popping of the aggregates during sublimation. We are characterizing black carbon measurement techniques prior to deployment of instrumentation in Barrow, Alaska for a project focused on measurements and modeling of black carbon in the Arctic.