Ammonium Bisulfate/Water Equilibrium and Metastability Phase Diagrams

is monitored continuously using a capacitance manometer. The change in the particle’s composition as the temperature or relative humidity (RH) is varied is determined by measuring the mass-to-charge ratio (with an accuracy of1%). A vertically polarized HeNe laser is focused on the particle. The intensity of the scattered light is continuously monitored at a9 0 °angle from the incident laser, and the phase (solid or liquid) is determined from the Mie scattering. To assure uniform temperature, the electrodynamic trap and the interaction chamber are constructed of copper. The chamber is insulated with a vacuum shroud and is in contact with a liquid nitrogen reservoir via heat transfer straps. An automated feedback heating loop is used to control the temperature to better than 0.2 K. The temperature is monitored at two points, above and below the particle, with the difference kept to less than 0.3 K. The reported temperature is the average between the two points. The system can be used to perform experiments at constant temperature or to scan temperature at a prescribed rate. Both modes were utilized to obtain the data presented here. When the temperature was scanned, the water vapor pressure was maintained at equilibrium with ice, by coating the cell walls with ice. The entire pressure and temperature monitoring system was calibrated against the ice frost line. The results reported here represent observations of over20 particles.