Calculation of optical properties of light-absorbing carbon with weakly absorbing coating: A model with tunable transition from film-coating to spherical-shell coating

Optical properties of particles consisting of light-absorbing carbon (or soot) and a weakly absorbing coating material are computed at a wavelength of 355 nm and 532 nm. A morphological particle model is used, in which small amounts of coating are applied as a thin film to the surface of the aggregate, while heavily coated aggregates are enclosed in a spherical shell. As the amount of coating material is increased, a gradual transition from film-coating to spherical-shell coating is accounted for. The speed of this transition can be varied by specifying a single parameter. Two different choices of this parameter, corresponding to a slow and a rapid transition from film-coating to spherical-shell coating, respectively, are investigated. For low soot volume fractions the impact of this transition on the linear depolarisation ratio δl is most pronounced. The model that describes a rapid transition to a spherical coating yields results for δl that are more consistent with existing lidar field measurements than the slow-transition model. At 532 nm the relative uncertainty in modelled δl for a rapid transition values due to uncertainties in the aggregate's geometry and chemical composition are estimated to range from 109 to 243%, depending on the soot volume fraction. At 355 nm the relative uncertainties were estimated to range from 90.9 to 200%.