Metastable Phases in Polar Stratospheric Aerosols

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Science  20 Jan 1995:
Vol. 267, Issue 5196, pp. 351-355
DOI: 10.1126/science.267.5196.351


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.