Laboratory Studies of Contact Efflorescence

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As atmospheric relative humidity cycles, saline aerosols exist either as ionic salts, or as aqueous ionic solution. Supersaturated and undersaturated phase change typically demonstrate hysteresis behavior as a function of relative humidity. The (increasing) RH at which the phase transition abruptly changes from solid particle to a saturated aqueous solution is the deliquescence relative humidity (DRH). The decreasing RH that drying aqueous droplets remain as supersaturated sols until crystallization, is the efflorescence relative humidity (ERH). Efflorescence may differ based on the nucleation mechanism, either by contact with a seed, or immersion of that seed into the supersaturated droplet. The objective of a series of laboratory investigations is to further study contact efflorescence compared to immersion efflorescence on aerosol types and sizes representative of atmospheric particles. Aerosol phase changes in a single droplet optical trap will be used to systematically follow efflorescence mechanisms. The importance of accounting for aerosol particle phase changes includes effects on heterogeneous chemical processes, light scattering, and cloud formation and precipitation. Direct effects on homogeneous (gas phase) chemistry and climate also remain uncertain.