MEASUREMENT OF THE WATER CYCLE IN MIXED AMMONIUM ACID SULFATE PARTICLES

A single ammonium-hydrogen-sulfate particle is levitated in an evacuated quadrupole trap at room temperature and the temperature of an attached tube containing bulk water is slowly cycled introducing and then removing water vapor. With increasing pressure the particle dissolves in stages, and then grows as a solution droplet by water absorption. With decreasing pressure the droplet supersaturates, crystallizes, then dehydrates completely to return to its initial state. Particle mass, and thus composition, is measured continuously with an electrostatic balance. Twenty-six cycles were studied as solute composition ranged from ammonium bisulfate through letovicite to ammonium sulfate in roughly equal steps. New phase diagrams are presented and used to discuss the dynamics of mixed sulfate particles in the atmosphere.