How does the size of crystalline NaCl relate to the depolarization ratio? – First laboratory results compared to model calculations

Sodium Chloride (NaCl) is a major component of sea salt in the atmosphere. In remote marine environments, sea salt aerosol dominates the formation of clouds. Under humid conditions often present in marine environments, sea salt exhibits a spherical shape. However, as relative humidity drops below the efflorescence point (around 45% RH), it exhibits a cubic-like shape, which leads to an enhanced depolarization ratio in lidar observations (Haarig et al., 2017). This transition was simulated by Kahnert & Kanngießer (2024), who created irregularly shaped dry sea salt crystals for single scattering calculations using Discrete Dipole Approximation (DDA) and then applied a brine coating to mimic the transition to wet, spherical sea salt particles.

We want to investigate the relation between particle size and the observed particle linear depolarization ratio of crystalline salt (NaCl) particles. For this purpose, we use the Optical Lab for Lidar Applications (OLALA) established for mineral dust research at TROPOS (see EGU 2026 contribution of Semwal et al.). The scattering laboratory is focused on the exact backscattering direction (180±0.2°) required for lidar applications. The NaCl particles are generated from wet dispersion with subsequent drying. A Differential Mobility Analyzer (DMA) ensures almost mono-modal size distributions for fine mode aerosol. Currently, we investigate NaCl particles in the size range of 250 to 800 nm in diameter at a laser wavelength of 532 nm. The extension to 1064 nm and 355 nm laser wavelengths is under construction. An increase in the depolarization ratio was observed with increasing size, reaching the maximum values of 0.16±0.04 for 800 nm dry NaCl particles.

The size-resolved laboratory results are compared to model calculations with perfect cubes and stacked cubes (different realizations) using DDA. Perfect cubes lead to lower depolarization ratios than observed in the laboratory. This finding indicates that the dry NaCl particles are not perfect cubes. Additionally, we intend to apply further particle shape models such as convex polyhedra (Kanngießer & Kahnert, 2021a), Gaussian random cubes (Kahnert & Kanngießer, 2021b) or super ellipsoids (Bi et al., 2018).

At the end, a better representation of cubic sea salt in the atmosphere will be achieved by constraining the optical particle shape models with the size-resolved laboratory results.

References:

Bi, L. et al., Optical Modeling of Sea Salt Aerosols: The Effects of Nonsphericity and Inhomogeneity, JGR, Vol. 123, No. 1, p. 543-558 (2018).

Haarig, M., et al., Dry versus wet marine particle optical properties: RH dependence of depolarization ratio, backscatter, and extinction from multiwavelength lidar measurements during SALTRACE, ACP, Vol. 17, No. 23, p. 14199-14217 (2017).

Kahnert, M.  and Kanngießer, F., Optical Characterization of Marine Aerosols Using a Morphologically Realistic Model with Varying Water Content: Implications for Lidar Applications and Passive Polarimetric Remote Sensing, GRL, Vol. 51, No. 5, p. e2023GL107541 (2024).

Kanngießer, Franz and Kahnert, Michael, Modeling Optical Properties of Non-Cubical Sea-Salt Particles, JGR, Vol. 126, No. 4, p. e2020JD033674 (2021a).

Kanngießer, Franz and Kahnert, Michael, Optical properties of water-coated sea salt model particles, OE, Vol. 29, No. 22, p. 34926-34950 (2021b).