Optical properties of marine aerosol: modelling the transition from dry, irregularly shaped crystals to brine-coated, dissolving salt particles

Abstract

An aerosol-optics model for water-coated marine aerosols is introduced that accounts for irregularities in the geometry of dry salt particles, and that mimics the processes of water-adsorption, dissolution of salt, and rearrangement of the liquid mantle following dissolution. The model can be tuned to adjust how rapidly the dry salt particles become spherical as water is being added to them. Size-shape distributions of the model are generated and employed to compute the ensemble-averaged extinction and backscattering cross sections, the lidar ratio, and the linear backscatter depolarisation ratio (LDR). A power law distribution that is frequently used in chemical transport models yields lidar ratios and LDR values that are consistent with field and satellite observations. But the results are found to be quite sensitive to the assumed size distribution. A generic lognormal size distribution tends to produce higher extinction cross sections, backscattering cross sections, and somewhat higher lidar ratios than the power-law distribution, while the depolarisation ratios are of comparable magnitude. We further gauged the model’s performance by comparing it with homogeneous superellipsoids. For a salt mass fraction of 0.97, the cross sections and the lidar ratio of cubic superellipsoids (i.e., those with unit aspect ratios) agree best with the reference model over all effective radii at a superellipsoid roundness parameter of 0.6. The LDR is more challenging to reproduce. For a salt mass fraction of 0.97, cubic homogeneous superellipsoids mostly give lower LDR values than the reference model. However, by increasing one aspect ratio, the superellipsoids can be tuned to yield higher LDR values. For a salt mass fraction of 0.91, the reference model yields LDR values below 0.1. Homogeneous superellipsoids that match the cross sections and lidar ratio of the reference model tend to give LDR values exceeding the reference results, at least for super-micron particles.