Showing papers by "James B. Burkholder published in 2014"

Ozone-Depleting Substances (ODSs) and other gases of interest to the Montreal Protocol

Abstract: Changes in the global atmospheric abundance of a substance are determined by the balance between its emissionand removal. Declines observed for ozone-depleting substances (ODSs) controlled under the Montreal Protocol are due to global emission reductions that have made emissions smaller than removals. Mot ODSs are potent greenhouse gases. As the majority of ODSs have been phased out, demand for hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) substitutes for the substances controlled under the Montreal Protocol have increased; these are also greenhouse gases. HCFCs deplete much less ozone per kilogram emitted than chlorofluorocarbons (CFCs), while HFCs essentially deplete no ozone.

OH + (E)- and (Z)-1-chloro-3,3,3-trifluoropropene-1 (CF3CH═CHCl) reaction rate coefficients: stereoisomer-dependent reactivity.

Abstract: Rate coefficients for the gas-phase reaction of the OH radical with (E)- and (Z)-CF3CH═CHCl (1-chloro-3,3,3-trifluoropropene-1, HFO-1233zd) (k1(T) and k2(T), respectively) were measured under pseudo-first-order conditions in OH over the temperature range 213–376 K. OH was produced by pulsed laser photolysis, and its temporal profile was measured using laser-induced fluorescence. The obtained rate coefficients were independent of pressure between 25 and 100 Torr (He, N2) with k1(296 K) = (3.76 ± 0.35) × 10–13 cm3 molecule–1 s–1 and k2(296 K) = (9.46 ± 0.85) × 10–13 cm3 molecule–1 s–1 (quoted uncertainties are 2σ and include estimated systematic errors). k2(T) showed a weak non-Arrhenius behavior over this temperature range. The (E)- and (Z)- stereoisomer rate coefficients were found to have opposite temperature dependencies that are well represented by k1(T) = (1.14 ± 0.15) × 10–12 exp[(−330 ± 10)/T] cm3 molecule–1 s–1 and k2(T) = (7.22 ± 0.65) × 10–19 × T2 × exp[(800 ± 20)/T] cm3 molecule–1 s–1. The prese...

The very short-lived ozone depleting substance CHBr3 (bromoform): Revised UV absorption spectrum, atmospheric lifetime and ozone depletion potential

Abstract: Abstract. CHBr3 (bromoform) is a short-lived atmospheric trace compound that is primarily of natural origin and is a source of reactive bromine in both the troposphere and stratosphere. Estimating the overall atmospheric impact of CHBr3 and its transport to the stratosphere requires a thorough understanding of its atmospheric loss processes, which are primarily UV photolysis and reaction with the OH radical. In this study, UV absorption cross sections, σ (λ ,T), for CHBr3 were measured at wavelengths between 300 and 345 nm at temperatures between 260 and 330 K using cavity ring-down spectroscopy. The present results are compared with currently recommended values for use in atmospheric models, and the discrepancies are discussed. A parameterization of the CHBr3 UV spectrum for use in atmospheric models is developed, and illustrative photolysis rate calculations are presented to highlight the impact of the revised σ (λ, T) values on its calculated local lifetimes. For example, the photolysis rate in the tropical region obtained with the present spectral data is 10–15% lower (longer lifetime) than obtained using currently recommended cross section values. Seasonally dependent ozone depletion potentials (ODPs) for CHBr3 emitted in the Indian sub-continent were calculated to be 0.10, 0.34, 0.72, and 0.23 (winter, spring, summer, fall) using the semi-empirical relationship of Brioude et al. (2010).