2002 Annual

Wednesday, 16 January 2002
Uptake of Nitric Acid on Ice Representative of Cirrus Clouds at Tropspheric Temperatures
Margret Tolbert, CIRES/University of Colorado, Boulder, CO; and P. Hudson, J. Shilling, and O. B. Toon
The uptake of nitric acid by thin ice films was studied using a Knudsen cell flow reactor and FTIR-reflection absorption spectroscopy (FTIR-RAS). Equilibrium coverages and initial uptake efficiencies of HNO3 on ice were determined over the temperature range T=210 220 K as a function of nitric acid partial pressure.

For pressures of nitric acid near 8x10-7 Torr, the uptake of HNO3 saturated at around 1 monolayer (ML) at T=213 K. Lower coverages were observed at temperatures above 213 K and multiple layers of HNO3 were taken up at temperatures below 213 K. Similar behavior was observed at higher nitric acid pressures, but the transition from monolayer to multilayer uptake occurred at a higher temperature. A multilayer Frenkel-Halsey-Hill (FHH) model was applied to our pressure and temperature dependent uptake data. We used this model to compare our data with other laboratory and field data obtained under different temperature and pressure conditions. Our coverage values at T > 213 K are somewhat lower than previous laboratory results by Zondlo et al. [1997] and Abbatt [1997], but agree within a factor of two with laboratory data by Arora et al. [1999]. We have also compared our model to field observations by Weinheimer et al. [1998] and Meilinger et al. [1999] of HNO3 uptake on lee-wave and cirrus clouds, respectively, with quite good agreement.

In addition to measurements of the coverage, we also present measurements of the initial uptake efficiency for HNO3 on ice. We find a negative temperature dependence for the initial uptake efficiency, independent of pressure. The initial efficiencies for uptake of HNO3 on ice range from=7 x 10-3 at T=209 K to=3 x 10-3 at T=220 K. The coverage and uptake efficiency data can be used to predict the expected HNO3 coverage on cirrus cloud particles under a variety of conditions. In general, we expect cirrus clouds to contain 0.01 to 1 ML of HNO3, depending on the temperature, nitric acid pressure and lifetime of the cloud particles.

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