Abstract
Abstract. The concept of Ozone Depletion Potentials (ODPs) is extensively used in policy considerations related to concerns about the effects of various halocarbons and other gases on stratospheric ozone. Many of the recent candidate replacement compounds have atmospheric lifetimes shorter than one year in order to limit their environmental effects, especially on stratospheric ozone. Using a three-dimensional global chemistry-transport model (CTM) of the troposphere and the stratosphere, the purpose of this study is to evaluate the potential effects of several very short-lived iodinated substances, namely iodotrifluoromethane (CF3I) and methyl iodide (CH3I), on atmospheric ozone. Like other chemicals with extremely short lifetimes, the stratospheric halogen loading and resulting ozone effects from these compounds are strongly dependent on the location of emissions. For CF3I, a possible replacement candidate for bromotrifluoromethane (CF3Br), ODPs derived by the three-dimensional model are 0.008 with chemical lifetime of 5.03 days and 0.016 with a lifetime of 1.13 days for emissions assumed to be evenly distributed over land surfaces at mid-latitudes and the tropics, respectively. While this is the first time the ODPs have been evaluated with a three-dimensional model, these values are in good agreement with those derived previously. The model calculations suggest that tropical convection could deliver a larger portion of the gas and their breakdown products to the upper troposphere and lower stratosphere if emission source is located in the tropics. The resulting ODP for CH3I, emitted from mid-latitudes, is 0.017 with lifetime of 13.59 days. Valid simulations of convective transport, vertical mixing and degradation chemistry of CH3I are shown that have good qualitative agreement between the model derived distribution of background CH3I, based on global source emission fluxes from previous studies, and available observations especially in vertical profiles.
Highlights
A number of analyses indicate that human-related emissions of chlorine-containing and bromine-containing halocarbons, such as chlorofluorocarbons (CFCs), carbon tetrachloride (CCl4), methyl chloroform (CH3CCl3) and bromochlorofluorocarbons (Halons), have been largely responsible for the observed decrease in stratospheric ozone concentrations over recent decades (WMO, 2007)
Those ozone-depleting substances (ODSs) are primarily comprised of the halocarbons that have very long lifetimes in the atmosphere. These human produced ODSs would have continued to have a significant impact on future levels of stratospheric ozone
Ozone Depletion Potentials (ODPs) are subject to fewer uncertainties than estimates of the absolute percentage of ozone depletion caused by different gases
Summary
A number of analyses indicate that human-related emissions of chlorine-containing and bromine-containing halocarbons, such as chlorofluorocarbons (CFCs), carbon tetrachloride (CCl4), methyl chloroform (CH3CCl3) and bromochlorofluorocarbons (Halons), have been largely responsible for the observed decrease in stratospheric ozone concentrations over recent decades (WMO, 2007). 3-D CTMs, which include a much more comprehensive treatment of transport than the zonally-averaged 2-D models, are much more preferable for calculations of ODPs both for the longer-lived gases and for the very short-lived candidate replacement compounds These models cannot only determine the amount of the substance reaching the stratosphere directly, without the simplifying assumptions of the 2-D models, but can more directly follow the processes affecting the reaction products. As the stage for further potential impact analysis of the compounds on ozone beyond the previous studies using 2-D models (Li et al 2006; Solomon et al, 1994), ODPs of the two iodinated VSL replacement compounds CF3I and CH3I are evaluated using the National Center for Atmospheric Research (NCAR) Model for OZone And Related Tracers version 3 (MOZART-3) 3-D chemistry-transport.
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