Refractive Index of Engine-Emitted Black Carbon and the Influence of Organic Coatings on Optical Properties

Refractive indices (RI) of particles are important in determining their radiative forcing. We measured optical coefficients of particles classified according to their aerodynamic diameter, allowing retrieval of RI using a Mie model. At 405 nm, the RI of BC from a diesel engine was 1.870 (±0.132) + 0.640 (±0.015) i. The RI of secondary organic aerosol (SOA), using α-pinene and o-cresol as precursors, were 1.584 ± 0.015 and 1.738 (±0.021) + 0.0316 (±0.0018) i, respectively. Neither SOAs demonstrated absorption at 660 nm and their RIs were 1.551 ± 0.011 and 1.586 ± 0.011, the similar value suggesting that a single RI may be sufficient for simulating the radiative forcing of SOA at this wavelength. In addition, organics were condensed onto BC to test optical models for coated particles. For BC particles coated with non-absorbing organics, the extinction is predicted accurately by all models. The absorption is significantly over-estimated by core-shell, volume mixing, and effective medium approximations and under-estimated by external mixing. For BC particles coated with weakly absorbing organics, the extinction and absorption are best described by external mixing when the coating ratio is less than 2.5. When the coating ratio is over 2.5, the difference between the external mixing predictions and measurements increases with the coating ratio. Our results show that the absorption of coated BC particles may not be predicted accurately based solely on the equivalent diameter, coating ratio, and pure component RIs, and considerations of additional factors such as morphology may be necessary.