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 go 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, pHCl is assumed to be much greater than
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 ClONO2 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 gos 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 gos for the ClONO 2 and HOCl with HCl reactions, are displayed in Figure 3a, below. The plots for the N2O5 and BrONO2 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.  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.
The expression in the middle column of the last row of Table A-4 on page 62 contains a typographical error:FHOCl should be FHCl.
Reference Typographical Error
The Robinson et al. references  and  are the same work. The year of publication listed in  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 ClONO2 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 N2O5 and BrONO2 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 N2O5 and BrONO2 is
set to be zero).
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