 |
| Titel |
Thermochemical Kinetics of H2O and HNO3 on crystalline Nitric Acid Hydrates (alpha-, beta-NAT, NAD) in the range 175-200 K |
| VerfasserIn |
Michel J. Rossi, Riccardo Iannarelli |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104066
|
| Publikation (Nr.) |
 EGU/EGU2015-3487.pdf |
|
|
|
|
|
| Zusammenfassung |
| The growth of NAT (Nitric Acid Trihydrate, HNO3x3H2O) and NAD (Nitric Acid Dihydrate,
HNO3x2H2O) on an ice substrate, the evaporative lifetime of NAT and NAD as well as the
interconversion of alpha- and beta-NAT competing with evaporation and growth
under UT/LS conditions depends on the interfacial kinetics of H2O and HNO3
vapor on the condensed phase. Despite the existence of some literature results we
have embarked on a systematic investigation of the kinetics using a multidiagnostic
experimental approach enabled by the simultaneous observation of both the gas
(residual gas mass spectrometry) as well as the condensed phase (FTIR absorption in
transmission).
We report on thermochemically consistent mass accommodation coefficients alpha and
absolute evaporation rates Rev/molecule s-1cm-3 as a function of temperature which yields
the corresponding vapor pressures of both H2O and HNO3 in equilibrium with the crystalline
phases, hence the term thermochemical kinetics. These results have been obtained using
a stirred flow reactor (SFR) using a macroscopic pure ice film of 1 micron or so
thickness as a starting substrate mimicking atmospheric ice particles and are reported
in a phase diagram specifically addressing UT (Upper Troposphere)/LS (Lower
Stratosphere) conditions as far as temperature and partial pressures are concerned. The
experiments have been performed either at steady-state flow conditions or in transient
supersaturation using a pulsed solenoid valve in order to generate gas pulses whose
decay were subsequently monitored in real time. Special attention has been given to
the effect of the stainless-steel vessel walls in that Langmuir adsorption isotherms
for H2O and HNO3 have been used to correct for wall-adsorption of both probe
gases.
Typically, the accommodation coefficients of H2O and HNO3 are similar throughout the
temperature range whereas the rates of evaporation Rev of H2O are significantly larger than
for HNO3 thus leading to the difference in vapor pressure revealed in the phase diagram. A
noteworthy effect seems to be that the accommodation coefficients obtained in pulsed gas
admission experiments (transient supersaturation) lead to significantly lower values owing to
surface saturation, especially in the case of the thermodynamically stable beta-NAT substrate. |
|
|
|
|
|
|