Coating material-dependent differences in modelled lidar-measurable quantities for heavily coated soot particles

The depolarisation ratio of heavily coated soot particles was previously found to be sensitive to the chemical composition of the coating material, which is reflected by the refractive index. Employing the Discrete Dipole Approximation code ADDA optical calculations were performed with a set of heavily coating soot aggregates with two different coating materials at 355 nm, 532 nm, and 1064 nm. As coating materials sulphate and a toluene-based material were assumed. The soot aggregates were modelled based on results reported from in-situ field measurements and using a coating model, which allows for a tunable transition between film coating and spherical shell coating. The aggregates’ size was varied by increasing the number of soot monomers inside each aggregate from 26 to 1508 in linearly equidistant steps.

Size-averaged lidar-measureable quantities for the coated aggregates, such as the linear depolarisation ratio, the extinction-to-backscatter ratio (lidar ratio), and the Ångström exponents of the extinction coefficient, the backscatter coefficient, and the extinction-to-backscatter ratio were calculated, and the error of the simulations was estimated. With the exception of the linear depolarisation ratio at 1064 nm these observables do not overlap within the estimated error bounds. As the coating materials result in clearly distinguishable lidar observables, information on the chemical composition of coated soot aerosol can potentially be inferred from lidar measurements.