Metastable Phases in Polar Stratospheric Aerosols
Lewis E. Fox 1,
Steven C. Wofsy 1,
Douglas R. Worsnop 2, and
Mark S. Zahniser 2
1 Division of Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA.
2 Center for Chemical and Environmental Physics, Aerodyne Research, 45 Manning Road, Billerica, MA 01821, USA.
Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid (H2SO4) in the presence of nitric acid (HNO3) and water vapor. A sequence of solid phases was observed to form that followed Ostwald's rule for phase nucleation. For stratospheric partial pressures at temperatures between 193 and 195 kelvin, a metastable ternary H2SO4-HNO3 hydrate, H2SO4 · HNO3 · 5H2O, formed in coexistence with binary H2SO4 · kH2O hydrates (k = 2, 3, and 4) and then transformed to nitric acid dihydrate, HNO3 · 2H2O, within a few hours. Metastable HNO3 · 2H2O always formed before stable nitric acid trihydrate, HNO3·3H2O, under stratospheric conditions and persisted for long periods. The formation of metastable phases provides a mechanism for differential particle growth and sedimentation of HNO3 from the polar winter stratosphere.
Submitted on July 19, 1994
Accepted on October 5, 1994
