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

Abstract

Abstract. A number of the compounds proposed as replacements for substances controlled under the Montreal Protocol have extremely short atmospheric lifetimes, on the order of days to a few months. An important example is n-propyl bromide (also referred to as 1-bromopropane, CH2BrCH2CH 3 or simplified as 1-C3H7Br 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 contains bromine, any amount reaching the stratosphere has the potential to affect concentrations of stratospheric ozone. The definition of Ozone Depletion Potentials (ODP) needs to be modified for such short-lived compounds to account 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 longitudes as normally done for longer-lived gases. Thus, for short-lived compounds, policymakers will need a table of ODP values instead of the single value generally provided in past studies. This study uses the MOZART2 three-dimensional chemical-transport model in combination with studies with our less computationally expensive two-dimensional model to examine potential effects of nPB on stratospheric ozone. Multiple facets of this study examine key questions regarding the amount of bromine reaching the stratosphere following emission of nPB. Our most significant findings from this study for the purposes of short-lived replacement compound ozone effects are summarized as follows. The degradation of nPB produces a significant quantity of bromoacetone which increases the amount of bromine transported to the stratosphere due to nPB. However, much of that effect is not due to bromoacetone itself, but instead to inorganic bromine which is produced from tropospheric oxidation of nPB, bromoacetone, and other degradation products and is transported above the dry and wet deposition processes of the model. The

Rate Coefficient and Product Branching Measurements for the Reaction OH + Bromopropane from 230 to 360 K

“…The detailed mechanism for formation of these products is discussed elsewhere. 8,14,15 Because unique products are formed for each site where H atom abstraction occurs, measurement of the above products provides information on the branching ratios in reaction 1. Therefore, we have measured the degradation products formed following reaction 1 in 620 Torr of synthetic air containing NO x (NO x ) NO + NO 2 ) as a part of this work.…”

“…The photolysis laser fluence was varied over the range (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) mJ pulse -1 cm -2 . The OH probe laser fluence ranged from 0.2 to 0.4 mJ pulse -1 cm -2 .…”

“…Previous laboratory studies [1][2][3][4][5] show that reaction with OH is the primary loss process for CH 3 CH 2 CH 2 Br and its atmospheric lifetime is ∼10 days. [4][5][6][7][8] Recent model studies 6,7 have shown that the ODP of bromopropane depends on the season, location of the emissions, atmospheric transport, and the lifetimes of the tropospheric end products of the OH + bromopropane reaction. Hence, the atmospheric impact of this short-lived compound on stratospheric ozone also depends on the products of the OH + bromopropane reaction.…”

“…To date, products have not been identified and branching ratios have not been measured. In the absence of experimental data, modeling studies 7,8 have assumed hydrogen abstraction from various sites using the empirical structurereactivity relationships of Atkinson. 9,10 In this paper, we report the rate coefficients, k 1 (T)-k 5 (T), for the reaction of various isotopomers of bromopropane with OH:…”

A perspective on time: loss frequencies, time scales and lifetimes

“…In the former case the steady-state lifetime for any surface emissions is very close to the e-fold time scale for decay of a perturbation. This simplification fails for short-lived species, [16][17][18][19] but the lifetime remains the correct integrating factor if one scales the pulse to the steady-state pattern. A correction from lifetime to perturbation lifetime is correctly applied when calculating the environmental effect of additional emissions, such as the global warming potentials for gases with chemical feedbacks like CH 4 and N 2 O, [4,8,15,20,21] but the time scale of those effects cannot always be represented by a simple e-fold as in most assessments.…”