Reconciling Measured and Modeled Optical Properties of Black Carbon
OGOCHUKWU ENEKWIZU, Egor Demidov, Arthur J. Sedlacek, Ernie R. Lewis, Alexei Khalizov, Brookhaven National Laboratory
Abstract Number: 329
Working Group: Carbonaceous Aerosols
Abstract
The optical properties of black carbon (BC) are fundamental to the radiative forcing estimations of aerosols in climate models. These properties depend on the sizes and morphologies of the individual BC particles. Despite advances in numerical predictions of BC properties, there are still differences between measured and modeled optical parameters of fresh BC particles. The mass absorption cross-section (MAC; m2/g) and single scattering albedo (SSA) of BC particles are underestimated by advanced numerical methods for computing BC optical properties, such as the Multiple Sphere T-Matrix (MSTM) and Discrete Dipole Approximation (DDA), which explicitly consider the morphology of BC particles. This discrepancy results from fundamentally incorrect or oversimplified assumptions about the morphological features of fresh BC particles, which are frequently found as uncoated, branched aggregates that are comprised of 10 to 50 nm diameter carbon spheres (monomers). Optical simulation studies of morphologically realistic BC particles often assume a constant monomer diameter for different BC particle sizes whereas multiple experimental observations have shown a direct correlation between monomer diameter and BC particle mobility diameter. Another assumption is that of point-touch contacts between monomers comprising the BC aggregate. Field observations have shown that BC aggregates are often made of overlapping monomers with necking (smooth transition contact area) between neighboring monomers. We assess the effects of variable monomer diameter and necking on the determination of the MAC and SSA for numerically generated BC particles of different sizes. We use MSTM calculations to account for the different monomer diameters and DDA calculations to account for the necking between monomers. We found that using variable monomer diameters yielded more realistic SSA values, and the inclusion of continuous necking between monomers in the BC particles reduced the difference between measured and modeled MAC values.