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| Titel |
The absolute absorption cross section of crystalline αg and βg HNO33H2O (NAT) and HNO32H2O (NAD) in the range 180 - 200 K in the mid-IR (4000 to 600 cm-1) |
| VerfasserIn |
Michel J. Rossi, Riccardo Iannarelli |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250087805
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| Publikation (Nr.) |
 EGU/EGU2014-1863.pdf |
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| Zusammenfassung |
| Heterogeneous processing in the polar atmosphere requires the presence of polar
stratospheric cloud particles (PSC’s) that are the seat of interfacial chlorine and NOx
chemistry. A subgroup of PSC’s, namely PSC Ia, are known to consist of hydrates of nitric
acid, mostly nitric acid trihydrate (NAT) as two polymorphs, α- and β-HNO33H2O occurring
in the range 185 to 200 K under prevailing stratospheric partial pressure conditions of 10 ppb
HNO3 or so. Despite the fact that reference IR spectra in the mid-IR range have been
obtained some time ago (Ritzhaupt and Devlin (1991), Koehler et al. (1992)), no absolute
absorption cross section of these important ice particles exist to date except a study of its
refractive indices (Middlebrook et al. (1994), Berland et al. (1994)). Knowledge of
optical cross sections would enable remote sensing of PSC’s in the IR region using
satellite and/or LIDAR platforms. We have embarked on a multidiagnostic research
program aiming at studying the kinetics, thermodynamics and spectroscopy of
PSC’s using a stirred flow reactor equipped with FTIR absorption spectroscopy in
transmission. The gas phase was monitored using electron-impact residual gas mass
spectroscopy together with pulsed and steady-state gas admission and thorough
characterization of the adsorption of HNO3, H2O and HCl onto the stainless-steel
vessel walls under mass balance conditions using measured Langmuir adsorption
isotherms. We have grown α- and β-NAT by doping thin (1 μm thick) ice films
with metered amounts of HNO3. According to known phase diagrams we have
obtained mixtures of pure ice with NAT whose IR spectrum was obtained after
spectral subtraction of the pure ice phase. The concentration of HNO3 deposited
on the ice film was determined by measuring the inflow and taking into account
adsorption of HNO3 on the reactor walls as well as effusive loss out the reactor. We also
independently checked the H2O concentration of α-NAT from the decrease of the
pure H2O ice symmetric stretch vibration at 3233 cm-1 (ν1) upon formation of
α-NAT. For β-NAT mass balance considerations during the transition from the
metastable α-isomer to the stable β-form enables the determination of the HNO3
concentration in the condensed phase. These results will be discussed in terms of
characteristic IR absorptions across the mid-IR range together with corresponding absolute
optical absorption cross sections for pure ice and for crystalline HCl6H2O recently
published (Chiesa and Rossi, (2013)). It turns out that of the three HNO3 hydrates
investigated in this work each has at least one characteristic IR absorption peak enabling
unambiguous identification in the presence of the other two and pure ice. Some
representative results for the symmetric stretch vibration of H3O+ in NAT and NAD are the
following: Ïă(α-NAT, 1760 cm-1) = (6.5 ± 0.3) 10(-19), Ïă(β-NAT, 1846 cm-1) =
(5.6 ± 0.1) 10(-19), Ïă(NAD, 1676 cm-1) = (1.13 ± 0.08) 10(-18) cm2molec-1. |
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