Nitric acid trihydrate (NAT) formation at low NAT supersaturations

C. Voigt (1) , H. Schlager (1) , B. P. Luo (2) , A. Dörnbrack (1) , Anke Roiger (1) , P. Stock (1) , J. Curtius (1) , H. Vössing (1) , S. Borrmann (1, 3) , S. Davies (4) , P. Konopka (5) , C. Schiller (5) , G. Shur (6) , T. Peter (2)

1 IPA - DLR Institut für Physik der Atmosphäre = DLR Institute of Atmospheric Physics
2 IAC - Institute for Atmospheric and Climate Science [Zürich]
3 MPIC - Max-Planck-Institut für Chemie
4 School of Environment
5 FZJ - Forschungszentrum Jülich GmbH | Centre de recherche de Jülich | Jülich Research Centre
6 CAO - Central Aerological Observatory

A polar stratospheric cloud (PSC) was observed on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (~10^?4 cm^?3) of nitric acid (HNO₃) containing particles ? probably NAT ? with diameters up to 6 µm were measured at altitudes between 18 and 20 km. These particles have the potential to grow further and to remove HNO₃ from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures T>T_NAT?3.5 K, just slightly below the NAT equilibrium temperature T_NAT. This unique measurement of PSC formation at extremely low NAT saturation ratios (S_NAT?11) constrains current NAT nucleation theories. In particular, NAT formation on ice can for certain be excluded. Conversely, we suggest that meteoritic particles may be favorable candidates for triggering nucleation of NAT at the observed low number densities.

Domains: Ocean, Atmosphere

C. Voigt, H. Schlager, B. P. Luo, A. Dörnbrack, Anke Roiger, et al.. Nitric acid trihydrate (NAT) formation at low NAT supersaturations. Atmospheric Chemistry and Physics Discussions, 2004, 4 (6), pp.8579-8607. ⟨hal-00301636⟩