Reformulating the Bromine/Iodine Alpha Factor and EESC: Evolution of Ozone Destruction Rates of Bromine, Iodine, and Chlorine in Future Climate Scenarios

Abstract:

The future rates of bromine-, iodine-, and chlorine-mediated catalytic ozone destruction will be altered via changes in the partitioning of inorganic halogen reservoirs and the underlying temperature structure of the stratosphere as a consequence of the Earth’s changing climate. The current formulation of the bromine/iodine alpha factor, the ozone-destroying power of stratospheric bromine/iodine atoms relative to stratospheric chlorine atoms, is invariant with chemistry/climate state. In this presentation, we refactor the bromine/iodine alpha factor, introducing the eta factor, which provides normalization of the extent of halogen-mediated ozone processing to a benchmark chemistry/climate state. With the eta factor, we show that rates of bromine- and chlorine-mediated ozone destruction are projected to significantly slow under certain climate assumptions while rates of iodine-mediated ozone destruction are less sensitive to changes in the chemistry/climate of the stratosphere. We then use the eta factor to formulate Equivalent Effective Stratospheric Benchmark-normalized Chlorine (EESBnC). This metric reflects changes in the rates of both bromine- and chlorine-mediated ozone loss catalysis with time. We show that the ozone-processing power of the extrapolar stratosphere is significantly perturbed by future climate assumptions. Our EESBnC-based estimate of the extrapolar ozone-recovery date is in closer agreement with extrapolar ozone recovery dates predicted using more sophisticated 3-D chemistry/climate models than predictions made using Equivalent Effective Stratospheric Chlorine (EESC).