AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Prediction of Black Carbon Mass Absorption Cross Section: Effects of Particle Morphology and Refractive Index
FENGSHAN LIU, Jerome Yon, Andrés Fuentes, Joel Corbin, Prem Lobo, Gregory Smallwood, National Research Council Canada
Abstract Number: 281 Working Group: Carbonaceous Aerosol
Abstract Mass absorption cross section (MAC) is an important parameter in optically based methods to convert the measured black carbon (BC) absorption coefficient to BC mass concentration. The commonly used MAC values in instrumentation are based on limited measurements from different BC emission sources of different particle morphology and particle size. It is known that the measured MAC values display a fairly large deviations from one measurement to another and it is not entirely clear if such deviations are due to the different microstructure (maturity) of BC particles, different measurement methods, or different BC particle morphology and size. Several experimental measurements have showed that the measured MAC exhibits fairly large size dependence.
In this study, MAC of BC aggregates of different morphology (primary particle diameter, fractal dimension, primary particle overlapping and necking) was numerically investigated using GMM and DDA by assuming different refractive indices in the visible and near infrared. The results show that the predicted MAC values of typical BC aggregates emitted from combustion systems display deviations on the order of 20% due to variation in morphology. The refractive index (microstructure of BC) has a stronger influence on the predicted MAC. Moreover, the effect of BC particle morphology on the predicted MAC is dependent on the assumed refractive index. The results can be interpreted by the multiple scattering effects among primary particles within a BC aggregate. The results suggest that the uncertainty of MAC of BC particles emitted from different sources is likely to be higher than 10%. This uncertainty in MAC will propagate to the measured BC mass concentration inferred from the measured BC absorption coefficient.