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Department of Atmospheric Science

Seminar: Thurs., Nov. 7, 3:10 pm, EN6085 

Δ-ice: Nitric Acid Increases Vapor Pressure of Ice

Ru-Shan Gao


Boulder, Colorado

In situ measurements of the relative humidity with respect to ice (RHi) and nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere during the CRYSTAL-FACE campaign.  At temperatures lower than 202K, RHi values showed a sharp increase to average values over 130% in both cloud types.  These enhanced RHi values were attributed to the presence of a new class of HNO3-containing ice particles (D-ice).  We proposed that surface HNO3 molecules prevent the ice-vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. 

Recentlty, a laboratory frostpoint hygrometer exposed to an airstream containing nitric acid (HNO3) and water vapor (H2O) was used to demonstrate the existence of a metastable water condensate with saturation H2O pressure greater than hexagonal ice (Ih).  The condensate resembles the hypothized D-ice.  The condensate can be routinely formed with H2O mixing ratios as low as 19 ppm at pressure of 128 hPa and temperatures 202-216K.  The water saturation temperatures of the condensate were below Ih on a reference mirror by as much as 3K, corresponding to a 63% supersaturation with respect to Ih.  The condensate persists for hours in the laboratory.  Analysis of the condensate confirms the existence of HNO3, but the exact structure or stoichiometery of the condensate is not known.  The existence of this condensate may have implications for H2O supersaturation in the tropical tropopause region.

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