New methodology for Ozone Depletion Potentials of short-lived compounds: n-propyl bromide as an example

A number of the compounds proposed as replacements for substances controlle d under the Montreal Protocol have extremely short atmospheric lifetimes, o n the order of days to a few months. An important example is n-propyl bromi de (also referred to as 1-bromopropane, CH2BrCH2CH3 or simplified as 1-C3H7 Br or nPB). This compound, useful as a solvent, has an atmospheric lifetime of less than 20 days due to its reaction with hydroxyl. Because nPB contai ns bromine, any amount reaching the stratosphere has the potential to affec t concentrations of stratospheric ozone. The definition of Ozone Depletion Potentials (ODP) needs to be modified for such short-lived compounds to acc ount for the location and timing of emissions. It is not adequate to treat these chemicals as if they were uniformly emitted at all latitudes and long itudes as normally done for longer-lived gases. Thus, for short-lived compo unds, policymakers will need a table of ODP values instead of the single va lue generally provided in past studies. This study uses the MOZART2 three-d imensional chemical-transport model in combination with studies with our le ss computationally expensive two-dimensional model to examine potential eff ects of nPB on stratospheric ozone. Multiple facets of this study examine k ey questions regarding the amount of bromine reaching the stratosphere foll owing emission of nPB. Our most significant findings from this study for th e purposes of short-lived replacement compound ozone effects are summarized as follows. The degradation of nPB produces a significant quantity of brom oacetone which increases the amount of bromine transported to the stratosph ere due to nPB. However, much of that effect is not due to bromoacetone its elf, but instead to inorganic bromine which is produced from tropospheric o xidation of nPB, bromoacetone, and other degradation products and is transp orted above the dry and wet deposition processes of the model. The MOZART2 nPB results indicate a minimal correction of the two-dimensional results in order to derive our final results: an nPB chemical lifetime of 19 days and an Ozone Depletion Potential range of 0.033 to 0.040 for assumed global em issions over landmasses, 19 days and 0.021 to 0.028, respectively, for assu med emissions in the industrialized regions of the Northern Hemisphere, and 9 days and 0.087 to 0.105, respectively, for assumed emission in tropical Southeast Asia.