Optical Characterization of Marine Aerosols Using a Morphologically Realistic Model With Varying Water Content: Implications for Lidar Applications and Passive Polarimetric Remote Sensing

• Published in: Geophysical research letters 2024-03, Vol.51 (5), p.n/a
• Main Authors: Kahnert, M., Kanngießer, F.
• Format: Article
• Language: English
• Subjects: aerosol optics; Aerosols; Atmospheric models; Backscatter; Backscattering; Brines; Climate system; Coefficients; Color; Crystals; Depolarization; Lidar; Light; Linear polarization; Luminous intensity; Marine aerosols; marione aerosol; Moisture content; Ocean surface; Optical properties; Particle size; Physical properties; Polarimetry; Polarization; Remote sensing; Salts; Satellite observation; Satellites; Scattering cross sections; Sunlight; Thermal radiation; Water; Water content; Wavelength; Wavelengths
• ISSN: 0094-8276; 1944-8007; 1944-8007
• DOI: 10.1029/2023GL107541

Retrieving the physical properties and water content of marine aerosols requires understanding the links between the particles' optical and microphysical properties. By using a morphologically realistic model with varying salt mass fractions fm, describing the transition from irregularly shaped, dry salt crystals to brine‐coated geometries, optical properties relevant to polarimetric remote sensing are computed at wavelengths of 532 and 1,064 nm. The extinction cross section and its color ratio depend on particle size, but are insensitive to changes in fm; thus, measured extinction coefficients at two wavelengths contain information on both particle number and size. The lidar ratio's dependence on both size and wavelength has implications for inverting the lidar equation. The results suggest that active observations of the backscattering cross section's color ratio and the depolarization ratio, as well as, passive observations of the degree of linear polarization offer avenues to obtain the water content of marine aerosols. Plain Language Summary Salt aerosols ejected into air from the ocean surface are the most abundant type of particles in the atmosphere. Clouds form by condensation onto these particles, which, in turn, reflect, absorb, and emit light and thermal radiation, thereby influencing the climate system. To better describe these processes in models, one needs to monitor salt aerosols from satellites. The main difficulty is to obtain information on the particles' concentration, size, and water content from satellite measurements that can only observe light scattered by the aerosols. In this study a model is being employed to study how the shape, size, and composition of marine aerosols impact their light‐scattering properties. The results suggest that the extinction of light measured at two wavelengths provides a robust method for obtaining the concentration and size of the particles. The water content can be obtained from observing the intensity of backscattered laser light at two wavelengths, or the change in polarization of the backscattered light at a single wavelength. One can also observe the polarization of scattered near‐infrared sunlight close to the backscattering direction to obtain information on the water content of the particles. Key Points Modeled extinction coefficient of marine aerosol depends on particle radius and wavelength, but not on water content The depolarization ratio and the color ratio of the backscattering cross secti