Oslo CTM3 Chemistry : The stratosphere

The stratospheric chemistry module
The stratospheric chemistry scheme is an extension of the scheme used by Stordal et al. (1985) for the Oslo 2D model, which was later updated to include heterogeneous chemistry (Isaksen et al., 1990), before it was included in the 3D Oslo SCTM-1 (Rummukainen et al., 1999) and the Oslo CTM2 before Oslo CTM3.

The stratospheric chemistry module must be run together with tropospheric chemistry, and the main reference for the stratospheric and tropospheric chemistry is Søvde et al. (2008).

55 species and 7 families are treated, and a total of 159 reactions (104 thermal, 47 photolytic and 8 heterogeneous), which are integrated with a numerical time step of 5 minutes. Of theses species, 17 are also treated in the tropospheric scheme. The heterogeneous chemistry scheme is a part of the stratospheric chemistry.

Heterogeneous chemistry
The heterogeneous chemistry scheme is a part of the stratospheric chemistry. The following heterogeneous reactions on aerosols, PSC1/STS and PSC2 are treated in the Oslo CTM2:

Reactions
N2O5 + HCl(s) → ClNO2 + HNO3(s)
ClONO2 + HCl(s) → Cl2 + HNO3(s)
ClONO2 + H2O(s) → HOCl + HNO3(s)
HOCl + HCl(s) → Cl2 + H2O(s)
BrONO2 + H2O(s) → OHBr + HNO3(s)
BrONO2 + H2O(s) → OHBr + HNO3(s)
BrONO2 + HCl(s) → BrCl + HNO3(s)
OHBr + HCl(s) → BrCl + H2O(s)
ClONO2 + HBr(s) -> BrCl + HNO3 (s) (*)

For reactions on PSCs/STS, (s) denotes that the products stay on the
particle (solid state). Otherwise the products are in gaseous form. Reaction marked (*) occurs only on PSC/STS.

The reaction rates are based on uptake coefficients, which are calculated in the microphysics scheme.

Microphysics
The microphysics and heterogeneous chemistry scheme (Smyshlyaev et al., 1998; de Zafra and Smyshlyaev, 2001), represents formation and evolution of PSCs, including denitrification and dehydration through sedimentation.

We distinguish between PSC1 (both nitric acid trihydrate (NAT) and supercooled ternary solutions (STS; liquid H2SO4/HNO3/H2O) and PSC2 (frozen H2O possibly coated by NAT), and the essential part of the microphysics is to calculate the reaction rates for the heterogeneous reactions. The formation of STS and NAT are calculated according to Carslaw et al. (1995) as explained by Considine et al. (2000).

The distribution of particles is given by a log-normal size distribution. This distribution consists of 40 different particle sizes ranging from 0.014μm to 19μm in particle radius. The mass distribution is then calculated according to the size bins, based on the mean radii of PSC1 (0.4μm) and PSC2 (5.0μm), with standard deviations of σ=1.6 and σ=1.8, respectively.

The sedimentation velocity of a particle is taken from look-up tables as a function of particle size and density (Kasten, 1968). Sedimentation of particles containing HNO3 and H2O result in irreversible denitrification and dehydration. Particles sedimented into warmer air will be subject to melting and evaporation if the temperature is high enough. Although the formation of particles and the heterogeneous chemical reactions on them are constrained to the stratosphere, PSCs/STS and existing solid/liquid HNO3 and H2O may be sedimented into the troposphere, where they are allowed to evaporate when the melting temperature is reached.

Components
Listed here are all the stratospheric components in the Oslo CTM3, whether they are transported (T=Y) or not (T=N), and also the chemical domain of interest (C): S is stratosphere.