Sensitivity of stratospheric inorganic chlorine to differences in transport

Abstract. Correctly modeling stratospheric inorganic chlorine (Cl_y) is crucial for modeling the past and future evolution of stratospheric ozone. However, comparisons of the chemistry climate models used in the latest international assessment of stratospheric ozone depletion have shown large differences in the modeled Cl_y, with these differences explaining many of the differences in the simulated evolution of ozone over the next century. Here in, we examine the role of transport in determining the simulated Cl_y using three simulations from the same off-line chemical transport model that have the same lower tropospheric boundary conditions and the same chemical solver, but differing resolution and/or meteorological fields. These simulations show that transport plays a key role in determining the Cl_y distribution, and that Cl_y depends on both the time scales and pathways of transport. The time air spends in the stratosphere (e.g., the mean age) is an important transport factor determining stratospheric Cl_y, but the relationship between mean age and Cl_y is not simple. Lower stratospheric Cl_y depends on the fraction of air that has been in the upper stratosphere, and transport differences between models having the same mean age can result in differences in the fraction of organic chlorine converted into Cl_y. Differences in transport pathways result in differences in vertical profiles of CFCs, and comparisons of observed and modeled CFC profiles provide a stringent test of transport pathways in models.