Abstract
As the chemical and physical state of the stratosphere evolves, so too will the rates of important ozone-destroying reactions. In this work, we evaluate the chemistry-climate sensitivity of reactions of stratospheric iodine, reporting the iodine alpha factor (the efficiency of ozone loss mediated by a single iodine atom relative to the ozone loss mediated by a single chlorine atom) and the iodine eta factor (the efficiency of ozone loss mediated by a single iodine atom relative to the ozone loss mediated by a single chlorine atom in a benchmark chemistry-climate state) as a function of future greenhouse gas emissions scenario. We find that iodine-mediated ozone loss is much less sensitive to future changes in the state of the stratosphere than chlorine- and bromine-mediated reactions. Additionally, we demonstrate that the inclusion of the heterogeneous reaction of ozone with aqueous iodide in stratospheric aerosol produces substantial enhancements in the iodine alpha and eta factors relative to evaluations that consider gas-phase iodine reactions only. We conclude that the share of halogen-induced ozone loss due to reactions of iodine will likely be greater in the future stratosphere than it is today.
Highlights
Until recently, iodine was thought to partition negligibly to the stratosphere due to the exceptionally short tropospheric lifetimes of the natural iodocarbons (Wennberg et al, 1997; Pundt et al, 1998; Bösch et al, 2003; Butz et al, 2009)
For experiments in which only gas-phase reactions of iodine were considered in the iodine chemical scheme, decadal values of annually-averaged extrapolar columnar αBr and αI were calculated using historical data for scenarios occurring between the years 1980–2010 and Representative Concentration Pathways (RCP) projections for each decade between 2020 and 2100
We find that the temporal evolution of the ozone-destroying power of iodine is observed to decline in all chemistry-climate scenarios evaluated; this effect is much less severe in the case of iodine than it is for most chemistry-climate evaluations of the other halogens
Summary
Iodine was thought to partition negligibly to the stratosphere due to the exceptionally short tropospheric lifetimes of the natural iodocarbons (Wennberg et al, 1997; Pundt et al, 1998; Bösch et al, 2003; Butz et al, 2009). Using a benchmark chemistry-climate state corresponding to = 1980, Klobas et al (2020) demonstrate that while annuallyaveraged extrapolar total column αBr is likely to only slightly decline or stay constant throughout the twenty-first century, depending on the greenhouse gas emissions scenario imposed, the actual extent of ozone loss mediated by bromine and chlorine varies to a much greater extent. This effect was most apparent in the most extreme climate change scenario evaluated by Klobas et al (2020), where αBr hardly changed, increasing by 4% from its year 1980 value of 70–73 in the year 2100, while ηCl and ηBr declined precipitously by 35 and 33% over the same time interval, respectively. We demonstrate that, while rates of chlorine- and bromine-mediated ozone destruction show a clear dependence on future climate state, rates of iodine-mediated ozone destruction are less susceptible to perturbations in the physicochemical environment of the stratosphere, indicating that processes with the potential to rapidly transport short-lived iodine species directly into the stratosphere will be increasingly important as Earth’s atmosphere evolves
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