'What are: ozone depletion potentials (ODP) and how have they helped in understanding the current global situation on atmospheric ozone?'

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6. (i) ‘What are: ozone depletion potentials (ODP) and how have they helped in understanding the current global situation on atmospheric ozone?’

        It is generally accepted by the scientific community that chlorofluorocarbons and halons are contributing the most severe threat to the ozone layer, as they can breakdown Ozone (O3) (Reiss et al,2000). This puts the global populous at risk from increasing chances of skin cancer, suppression of the immune system & a decline in crop productivity – resulting from the fall in O3 and the subsequent rise in Ultraviolet (UV) radiation reaching earths surface. Ozone Depletion Potential (ODP) has been formulated to help us gauge the effect of synthetic chemicals on depleting stratospheric ozone (Parker,1996). This work will discuss what ODPs are and what they have done to halt ozone depletion.

        ODP is a number gauged by a mathematical calculation (Appendix 1A) (Person,1997), given to a substance that refers to the amount of ozone destruction it can cause, over its entire lifetime relative to that of a ‘referenced substance’ which is CFC-11. So for example CFC-115 has an ODP of 0.6 (Appendix 2A) (EPA,2004), which destroys 3/5 as much ozone as CFC-11. ODP is often used in conjunction with Chlorine Loading Potential (CLP) which gives the total amount of chlorine in any substance reaching the atmosphere, relative to a similar release of CFC-11. ODP focuses on ozone destruction for an entire lifetime of an Ozone Depleting Substance (ODS). CFCs can survive for nearly 200 years and so ODP is key in showing their full impact. So far the “steady-state” ODP (Parker,1996) has been focused upon as it most clearly represents the effects of long-term ozone depleting substances that survive as long as or longer than CFC-11 (50 years). But more short lived substances may still have a high impact on ozone which is not reflected in this “steady-state” ODP but the “time-dependant” ODP portraits ozone loss over a set timescale e.g. 12 months and thus is more representative of the short lived substances.

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        Appendix one shows that the Clean Air Act (CAA) (EPA,2004) has used ODP to rank many ODS’s e.g. methyl bromine into 8 classes. Thanks to the use of ODPs, alternatives like hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) (Pool,1989) are being implemented. HCFCs contain ozone destroying chlorine but also contain hydrogen; making them prone to reaction with hydroxyl radicals in the troposphere, thus limiting them, but they do have an ODP of 0.6 (Gumprecht,2003). HFCs have an ODP of 0 but like HCFCs they are more expensive than CFCs.

        Ozone Depletion Potential has allowed for the quantification and ranking of ODS (appendix ...

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