Metastable Phases in Polar Stratospheric Aerosols

Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid (H$_2$SO$_4$) in the presence of nitric acid (HNO$_3$) and water vapor. A sequence of solid phases was observed to form that followed Ostwald's rule for phase nucleation. For stratospheric partial pressur...

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Bibliographic Details
Published in:Science (American Association for the Advancement of Science) 1995-01, Vol.267 (5196), p.351-355
Main Authors: Fox, Lewis E., Worsnop, Douglas R., Zahniser, Mark S., Wofsy, Steven C.
Format: Article
Language:English
Subjects:
Aerosols
Atmosphere
Atmospheric composition. Chemical and photochemical reactions
Chemistry
Cooling
Crystals
Earth, ocean, space
Exact sciences and technology
External geophysics
Hydrates
Ice
Liquids
Metastable atoms
Nucleation
Observation
Physics of the high neutral atmosphere
Stoichiometry
Stratosphere
Thermodynamics
Vapors
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Summary:Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid (H$_2$SO$_4$) in the presence of nitric acid (HNO$_3$) 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 H$_2$SO$_4$-HNO$_3$ hydrate, H$_2$SO$_4$ · HNO$_3$ · 5H$_2$O, formed in coexistence with binary H$_2$SO$_4$ · kH$_2$O hydrates (k = 2, 3, and 4) and then transformed to nitric acid dihydrate, HNO$_3$ · 2H$_2$O, within a few hours. Metastable HNO$_3$ · 2H$_2$O always formed before stable nitric acid trihydrate, HNO$_3$·3H$_2$O, under stratospheric conditions and persisted for long periods. The formation of metastable phases provides a mechanism for differential particle growth and sedimentation of HNO$_3$ from the polar winter stratosphere.
ISSN:0036-8075
1095-9203
DOI:10.1126/science.267.5196.351