Figure 3 - Recommended Reactive Uptake Coefficients

For ease of reproduction by those implementing the halogen activation model
presented in the Appendix on pages 57-62, the g_{o} values for the
reactions presented in Figure 3 assumed a fresh sulfuric acid aerosol surface
(i.e., as stated on page 45, the reacto-diffusive length,
= 0, for each reaction is assumed to be
small, this is equivalent to setting,
= 0 for the ClONO_{ 2} and HOCl reactions in the model.) Also
as stated on page 45, p_{HCl} is assumed to be much greater than
p_{ClONO2}.

While the assumption of a negligible l for the HOCl + HCl reaction is
chemically unreasonable for stratospheric aerosols, it has the advantage that it
avoids the necessity to specify a representative aerosol radius. The second
assumption avoids the necessity of specifying a ClONO_{2} partial
pressure. Ross Salawitch of JPL has pointed out that those implementing the
heterogeneous model and checking it against the uptake coefficients presented in
Figure 3 could easily miss or misinterpret the assumptions on page 45.
Furthermore, the relative g_{o}s presented in Figure 3 for the reactions of ClONO_{
2} and HOCl with HCl are unrealistic for normal stratospheric conditions.

To address these concerns the model has been exercised for a 50 mbar case
assuming 5ppmv of water vapor and 2 ppbv of HCl, as before, but with 0.1 ppbv of
ClONO_{ 2 }and an aerosol diameter of 10^{-5} cm (0.1
mm). These new results, which
primarily effect the plots for g_{o}s for the ClONO_{ 2} and HOCl with HCl
reactions, are displayed in Figure 3a, below. The plots for the
N_{2}O_{5 }and BrONO_{2 }hydrolysis reactions are
unchanged (i.e. the calculation assumes that
= 0 for these reactions).

Water Activity Parameterizations

The parameterization for calculating the activity of water, aw, in
sulfuric acid aerosols in the chlorine activation model is adopted from
Tabazadeh et al. [307] and presented in Table A-1 on page 58. A simpler
approximation for estimating aw is noted below Table A-2 on page 59; this
approximation is presented for convenience, but is not used in the chlorine
activation model implementation.

F_{HCl}**Typographical Error**

The expression in the middle column of the last row of Table A-4 on page 62 contains a typographical error:

F**Reference Typographical Error**

The Robinson et al. references [270] and [272] are the same work. The year of publication listed in [270] is incorrect, it should be 1997 not 1996. Call out to these two references on pages 49 and 53, 57 will be consolidated in future editions.

**Figure 3a.** Recommended reactive
uptake coefficients as a function of temperature for key stratospheric
heterogeneous processes on sulfuric acid aerosols. For ClONO_{2} and
HOCl species, the aerosol diameter used in the calculation is 10^{-5}
cm, a characteristic value in the stratosphere. Because the current uptake
models for N_{2}O_{5} and BrONO_{2} hydrolysis do not
provide the information about the reacto-diffusive length (
= 0 ), the aerosol diameter used in the calculation is
assumed to be much larger than their reacto-diffusive length (i.e.
= 0 for N_{2}O_{5} and BrONO_{2} is
set to be zero).

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