1. 2-dimensional model simulations

The model runs were performed using the 2-dimensional model described
in Rinsland et al. [2003], with a resolution of 9.5 deg in latitude
and 1.2 km in altitude. The 2-D residual circulation and eddy diffusion
coefficients were calculated from observed climatological values of temperature,
H2O, zonal wind, and ozone [Fleming et al., 1999]. Rate constants for chemical
reactions were taken from the JPL Evaluation 2002 [Sander et al., 2003],
photolysis cross sections were taken from the JPL Evaluation 2000
[Sander et al., 2000].

The CH3CN chemistry included reactions of CH3CN with OH and Cl with rates
based on Sander et al. [2003]

CH3CN + OH -> products   (A = 7.8e-13 cm3/molec.s,  Ea/R = 1050 K),      (1)

CH3CN + Cl -> products   (A = 1.6e-11 cm3/molec.s,  Ea/R = 2140 K),      (2)

where the rate consant is expressed by k = A exp(-Ea/RT), and T is temperature.
Furthermore reactions with atomic oxygen were included

CH3CN + O(3P) -> products   (A = 7.3e-13 cm3/molec.s,  Ea/R = 2385 K),   (3)

CH3CN + O(1D) -> products   (k = 1e-10 cm3/molec.s).                     (4)

The rate coefficients of reaction (3) were taken from Bonnano et al. [1977].
The uniform reaction rate constant for reaction (4) is based on an assumption
by Arjis and Brasseur [1986]. No CH3CN photolysis was included in the model
as solar photodissociation is supposed to be unimportant compared to the
reaction with OH radicals [Sander et al., 2003].

The model was initialized with a uniform CH3CN volume mixing ratio of
150 ppt at the surface, leading to CH3CN mixing ratios slightly lower
than 150 ppt at the tropopause.


2. References

Arjis, E., and G. Brasseur, Acetonitrile in the stratosphere and implications
for positive ion composition, J. Geophys. Res., 91, 4003-4015, 1986.

Bonanno, R. J., R. B. Timmons, L. J. Stief, and R. B. Klemm, The kinetics and
mechanisms of the reactions of O(3P) atoms with CH3CN and CF3CN,
J. Chem. Phys., 66, 92-98, 1977.

Fleming, E. L., C. H. Jackman, R. S. Stolarski, and D. B. Considine,
Simulation of stratospheric tracers using an improved empirically-based
two-dimensional model transport formulation,  J. Geophys. Res., 104,
23,911-23,934, 1999.

Rinsland, C. P., D. K. Weisenstein, M. K. W. Ko, C. J. Scott, L. S. Chiou, E. 
Mahieu, R. Zander, and P. Demoulin, Post-Mount Pinatubo eruption 
ground-based infrared stratospheric column measurements of HNO3, NO, and 
NO2 and their comparison with model calculations, J. Geophys. Res., 108, 
4437, doi:10.1029/2002JD002965, 2003.

Sander, S. P., R. R. Friedl, W. B. DeMore, A. R.Ravishankara,
D. M. Golden, C. E. Kolb, M. J. Kurylo, R. F. Hampson, R. E. Huie,
M. J. Molina, and G. K., Moortgat, Chemical Kinetics and Photochemical Data
for Use in Stratospheric Modeling, Evaluation 13, Supplement to Evaluation 12:
Update of Key Reactions, NASA JPL Pub. 00-3, 2000.

Sander, S. P., D. M. Golden, M. J. Kurylo, R. E. Huie, V. L. Orkin,
G. K. Moortgat, A. R. Ravishankara, C. E. Kolb, M. J. Molina, and
B. J. Finlayson-Pitts, Chemical Kinetics and Photochemical Data for
Use in Atmospheric Studies: Evaluation Number 14,
NASA JPL Pub. 02-25, 2003.