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Chemistry of $\rm SF_6$ in the middle atmosphere

Mesospheric loss of $\rm SF_6$ has been evaluated by different authors: specific global lifetimes of 13 500 and 4200 years respectively were found when assuming either Lyman-$\alpha$ or a dissociative electron attachment to destroy the molecule (Ravishankara et al., 1993). When assuming photodissociation by UV radiation with a wavelength of less than 240 nm, a lifetime of 1000 years was estimated (Ko et al., 1993). In 2-D model calculations even an atmospheric lifetime as low as 800 years was found (Morris et al., 1995). For our study we use a simplified chemistry scheme (Reddmann et al., 2000), but besides electron attachment and photodissociation we also consider reactions of $hexafm$ and evaluate uncertainties of reaction rate constants by comparing several chemical scenarios.

The reaction of $\rm SF_6$ in the mesosphere can be described by the following scheme which extends that given by Odom et al. (1975) by adding the reactions of $\rm SF_6^-$:

\begin{eqnarray*} \textstyle \rm SF_6 \hspace{10mm} \stackrel{e^-} {\stackre... ...yle \rm SF_5 + F \hspace{2mm} \rm SF_5^- + F & & \rm SF_6+ e^- \end{eqnarray*}



Reaction rates used are given in 1, note the variants for reaction R8. Details of the chemistry scheme are given in Reddmann et al. (2000).


Table 1: Reaction rates constants of reactions of $\rm SF_6$ and $\rm SF_6^-$ and expected rates for standard atmospheric conditions at 60 km, included in the chemical scheme.
Id Reaction Total rate constant in $10^{-9} \rm cm^{-3} s^{-1}$ or $s^{-1}$ Rate at 60 km in $\rm s^{-1}$ Remarks
R1 $\rm SF_6+ h\nu \to products$     destructive
R2 $\rm SF_6+ \rm e^- \to \rm (SF_6^-)^*$ 270 $1.2 \times 10^{-4}$  
R2a $\rm SF_6+ \rm e^- \to SF_5^+ + F, ...$     destructive branch of R2, branching fraction $< 0.001 $
R5 $\rm (SF_6^-)^*+ M \to \rm SF_6^-$ 0.19    
R6 $\rm (SF_6^-)^*\to \rm SF_6+ e^-$ $ 1 \times 10^{-6}$    
R3 $\rm SF_6^-+ \rm h \nu \to \rm SF_6+ e^- $   0.3 Photodetachment
R4 $\rm SF_6^-+ H \to SF_5^+ + HF$ 0.21 $1.2 \times 10^{-4}$ destructive
R7 $\rm SF_6^-+ \rm HCl \to products$ 1.5 $0.02$ destructive
R8a $\rm SF_6^-+ \rm O_3 \to \rm SF_6+ O_3^-$ 0.032 $0.18$  
R8b $\rm SF_6^-+ \rm O_3 \to \rm SF_6+ O_3^-$ 1.2 $6.8$  


Figure 1: Electron density profiles used in model runs, midlatitude winter conditions at noon, see text for description of versions.
\includegraphics*[width=85mm, trim=0 0 0 0]{jgr9904.eps}

The result of the chemistry scheme of $\rm SF_6$ depends on the assumptions made for some reaction rates and electron density. To test the influence of various parameters we choose several combinations in the model runs. The electron density profile is represented by four versions to account for the uncertainties especially in the mean electron energy and to test the influence of diurnal and seasonal variation, see Fig. 1. All scenarios tested are given in Table 2.


Table 2: Different scenarios of $\rm SF_6$ chemistry in model runs
Notation Description Electron profile
S1 direct loss by electron attachment (R2) B
S2 S1 + R3 + R4 + R5 B
S3 S2 + R7 + R8a B
S4 as S3, but R8b B
S5 as S3 B1
S6 as S3 + R6 B
S7 as S4 A
S8 as S7 + cosmic ray ionisation A1
See text for versions of electron profiles,
see Table 1 for the identification of reactions.


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