Dry versus wet marine particle optical properties: RH dependence of depolarization ratio, backscatter, and extinction from multiwavelength lidar measurements during SALTRACE

Abstract

Triple-wavelength lidar observations of the depolarization ratio and the backscatter coefficient of marine aerosol as a function of relative humidity (RH) are presented with a 5ĝ€min time resolution. The measurements were performed at Barbados (13°ĝ€N, 59°ĝ€W) during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment (SALTRACE) winter campaign in February 2014. The phase transition from spherical sea salt particles to cubic-like sea salt crystals was observed with a polarization lidar. The radiosonde and water-vapor Raman lidar observations show a drop in RH below 50ĝ€% in the marine aerosol layer simultaneously with a strong increase in particle linear depolarization ratio, which reaches values up to 0.12ĝ€±ĝ€0.08 (at 355ĝ€nm), 0.15ĝ€±ĝ€0.03 (at 532ĝ€nm), and 0.10ĝ€±ĝ€0.01 (at 1064ĝ€nm). The lidar ratio (extinction-To-backscatter ratio) increased from 19 and 23ĝ€sr for spherical sea salt particles to 27 and 25ĝ€sr (at 355 and 532ĝ€nm, respectively) for cubic-like particle ensembles. Furthermore the scattering enhancement due to hygroscopic growth of the marine aerosol particles under atmospheric conditions was measured. Extinction enhancement factors from 40 to 80ĝ€% RH of 1.94ĝ€±ĝ€0.94 at 355ĝ€nm, 3.70ĝ€±ĝ€1.14 at 532ĝ€nm, and 5.37ĝ€±ĝ€1.66 at 1064ĝ€nm were found. The enhanced depolarization ratios and lidar ratios were compared to modeling studies of cubic sea salt particles.