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 (CaSO ₄·2H ₂O) or the metastable hemihydrate (CaSO ₄·0.5H ₂O) 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 μm ³/s. Stable gypsum was formed at 80.7%-82.2% RH, and the apparent crystal growth rate was between 0.70 and 0.81 μm ³/s.