Sensitivity of mixing states on optical properties of fresh secondary organic carbon aerosols

- Fixed volume model is used for optical simulations of fresh secondary organic carbon.
- Measured optical properties can be reproduced for cases of large soot volume fraction.
- Enhancements of absorption and scattering can reach to ~15% and ~35% for Fsoot=0.8.
- Morphology of these secondary aerosols may highly influence their scattering (~100%).
- Larger refractive index of organic coating may lead to larger absorption enhancements.

At the beginning of the interaction of black carbon (or soot) and organic particles, fresh secondary organic carbon aerosols are generated by the dominant fractal aggregated soot monomers and the slight organic coatings. The complex morphologies and mixing states of these fresh secondary organic carbon aerosols significantly influence their optical properties. In this study, these heterogeneous particles were reconstructed using the fixed volume fraction model, and their optical properties are calculated using the discrete dipole approximation (DDA) method. For soot particles aged in a short time, the simulated absorption, scattering and single scattering albedo (SSA) showed a good agreement with the measurements. The amplifications of absorption and scattering between the thinly coated states (soot volume fraction equals 0.8, or the shell/core diameter ratio equals 1.08) and the freshly emitted states (bare soot with soot volume fraction equals 1) can reach to ~15% and ~35%, respectively. The simulations with these thinly coated states also indicated that the variations of morphologies may lead to the significant relative deviations on the absorption (up to ~15%) and scattering (up to ~100%) of these secondary aerosols. The effects of soot compactness and size on their optical properties were sensitive to the thickness of organic coatings, and larger organic refractive index may lead to larger absorption enhancements.