Crystallization kinetics of sea-salt aerosols studied by high-speed photography

Morphological changes with decreasing relative humidity (RH) of supersaturated sea-salt aerosol droplets on a quartz substrate were observed using a high-speed video-camera. Stable gypsum (CaSO₄·2H₂O) or the metastable hemihydrate (CaSO₄·0.5H₂O) were precipitated as the RH decreased. The dynamic process of crystal growth under steady-state humidity was studied by controlling the RH; the metastable hemihydrate was precipitated at 70.5%–77.1% RH, and the apparent crystal growth rate was between 1.42 and 2.33 μm³/s. Stable gypsum was formed at 80.7%–82.2% RH, and the apparent crystal growth rate was between 0.70 and 0.81 μm³/s.