Abstract

Soot particles positively influence radiative forcing due to their strong absorption. Because of their chain-like structure, aggregated soot particles become more compact with the aging process, and the monomers or particles are always covered by water coatings. In this study, the optical parameters of two typical soot-water mixtures (i. e. an aggregate with core-shell monomers and a soot aggregate inside a water droplet) at 550 nm were investigated using the superposition T-matrix method, with a focus on the impact of the morphology and water coating of soot aggregates. For the soot aggregate with core-shell monomers, a relationship among the fractal dimension, relative humidity (RH) and monomer number was established and used to calculate optical parameters. The intensity of forward scattering declined with the increasing RH. The Cext, Csca, Cabs and SSA are much more insensitive to RH under higher RH conditions (RH>90%) than at a lower RH levels. In addition, hygroscopic shrinkage and the thickness of water coating have a stronger effect on the optical properties of larger aggregated soot at higher RH than lower RH. For another mixing state, the soot aggregate inside a water droplet, the morphology of the soot core plays an important role in the optical properties when the thickness of the water shell is small. When D_ratio changes from 1.2 to 2.8, Cext difference increases from 0.23μm2 to 2.36μm2 for particles with N=100 and 500, whereas the SSA difference decreases from 0.12 to 0.01. If the agglomerated structure of the soot core is not considered, the Cext, Csca and SSA will be underestimated for a relatively small D_ratio of 1.2. Ignoring the soot core in the water droplet could introduce large errors into the calculation of the optical parameters, and ignoring the structure of the aggregated soot core could enlarge the errors.

© 2015 Optical Society of America