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

摘要

CHBr (bromoform) is a short-lived atmospheric trace compound that isprimarily of natural origin and is a source of reactive bromine in both thetroposphere and stratosphere. Estimating the overall atmospheric impact ofCHBr and its transport to the stratosphere requires a thoroughunderstanding of its atmospheric loss processes, which are primarily UVphotolysis and reaction with the OH radical. In this study, UV absorptioncross sections, σ (λ ,), for CHBr were measured atwavelengths between 300 and 345 nm at temperatures between 260 and 330 Kusing cavity ring-down spectroscopy. The present results are compared withcurrently recommended values for use in atmospheric models, and thediscrepancies are discussed. A parameterization of the CHBr UVspectrum for use in atmospheric models is developed, and illustrativephotolysis rate calculations are presented to highlight the impact of therevised σ (λ, ) values on its calculated local lifetimes.For example, the photolysis rate in the tropical region obtained with the presentspectral data is 10–15% lower (longer lifetime) than obtained usingcurrently recommended cross section values. Seasonally dependent ozonedepletion potentials (ODPs) for CHBr emitted in the Indiansub-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 etal. (2010).