Updated evaluation of ozone depletion potentials for chlorobromomethane (CH2ClBr) and 1-bromo-propane (CH2BrCH2CH3)

We have updated the analysis of the atmospheric lifetimes and ozone depleti on potentials (ODPs) of chlorobromomethane (CH2ClBr) and l-bromo-propane (C H2BrCH2CH3 or simplified as 1-C3H7Br) based on a new version of our two-dim ensional chemical-transport model of the global atmosphere. in the previous analysis of these compounds, now published in Atmospheric Environment, Wue bbles et al. (1997) had determined an ODP of 0.11-0.13 for CH2ClBr and an O DP of 0.006 for 1-C3H7Br. For CH2ClBr, the range in the ODP value is due to uncertainties in the magnitude of its loss to the oceans, Since the time of the prior analyses, the two-dimensional model has been ex tensively revised and updated. The most important changes include: -the chemistry and kinetic rates updated to the latest NASA recommendations (DeMore et al., 1997); -the treatment of advective and eddy transport processes revised to better account for tropospheric and stratospheric transport compared to atmospheri c tracers; -tropospheric chemistry improved by improved representation of hydrocarbon chemistry; -parameterization of tropospheric convective transport processes now includ ed; improved representation of polar stratospheric clouds and ozone hole pr ocesses; -improved treatment of latent heat effects on radiative derivation of atmos pheric dynamics. These changes have a significant impact on the derived ODPs for CH2ClBr and 1-C3H7Br. Of particular importance are the many changes made to the tropos pheric chemistry and transport processes in the model. As mentioned in the previous paper, ODPs are generally normalized to a CFC- 11 lifetime of 50 years, and to an OH-driven methyl chloroform (CH3CCl3) pa rtial lifetime of 5.9 years based on WMO (1995). The CFC-11 and CH3CCl3 sca ling accounts for uncertainties associated with determining the Cl and OH d istributions in the model. However, as shown in Wuebbles et at (1998), the measurements by Prinn et al, (1995) suggest that the best partial lifetime for reaction of CH3CCl3 with tropospheric OH is now thought to be 5.7 years . In this paper, we will present the ODPs based on both normalizations but will recommend that 5.7 years be considered as the standard. The new version of the model now gives an atmospheric lifetime of CFC-11 of 49.3 years (as compared to 57 years in the prior model) and a partial life time for reaction of CH3CCl3 with tropospheric OH of 4.8 years (as compared to 8.1 years in the prior version). There are several reasons for the bett er comparison, including the improvements in transport processes, faster pr oduction of excited oxygen from ozone photolysis, and inclusion of hydrocar bon chemistry. As a result there is much less scaling of the final lifetime s and ODPs for CH2ClBr and 1-C3H7Br than in the previous version of the mod el.