Effects of Aggregate Morphology and Size on SP2 Measurements of Black Carbon
Authors
Hope A. Michelsen — Sandia National Laboratories
Ray P. Bambha — Sandia National Laboratories
Category
Absorbing Aerosol
Description
SP2 LII and scattering temporal profiles. (a) Modeled signals are plotted as a function of time (lines) and are compared with average measurements (symbols) for (a) LII and (b) scatter channel signals. Results are shown for fresh and restructured soot. Data were sorted by particle size and morphology. The particle sizes shown correspond to the peaks of the multi-mode size distribution associated with single-, double-, and triple-charged particles passing through the DMA during size selection, as indicated in the legend. Averages of 50 traces per particle size are shown. The laser-particle interaction profile (dotted line in both panels), used as input to the model, is also shown and is scaled to the top of the graph.
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 also reveal significant perturbations to the measured scattering signal from LII interference and suggest swelling of the aggregates during sublimation.
