Optical properties of black carbon aerosols encapsulated in a shell of sulfate: comparison of the closed cell model with a coated aggregate model.

At 532 nm wavelength, optical properties of black carbon (BC) particles mixed with sulfate are computed by use of two morphological models, a closed cell and a coated aggregate model. For high BC volume fractions f, both models yield comparable results. As more sulfate is added, some of the optical properties diverge. The backscattering depolarization ratio δL is particularly sensitive to the morphology. Comparison with field measurements suggests that the closed cell model underestimates δL; the coated aggregate model yields good results for intermediate and high values of f, but somewhat too high results for low f. This could be improved by taking the collapse of fractal structure with decreasing f into account.