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).
Highlights
CHBr3 is a short-lived atmospheric trace gas primarily emitted from natural sources such as marine phytoplankton and coastal macrophytes, and represents a source of reactive bromine (Bry; Br + BrO) in the troposphere as well as the stratosphere
The atmospheric abundance of CHBr3 in the marine boundary layer (MBL) has been measured to be in the range 0.5 to 2.4 ppt and the abundance in the upper tropical–tropopause layer to be in the range 0.01 to 0.29 ppt (WMO, 2011, and references cited within)
Hossaini et al (2010) and Aschmann and Sinnhuber (2013) estimate that CHBr3 accounts for ∼ 0.7 ppt of stratospheric bromine, ∼ 50 % from the CHBr3 source gas, and the remainder from product gas injection of bromine containing degradation products
Summary
CHBr3 (bromoform) is a short-lived atmospheric trace gas primarily emitted from natural sources such as marine phytoplankton and coastal macrophytes (seaweeds), and represents a source of reactive bromine (Bry; Br + BrO) in the troposphere as well as the stratosphere. They reported a decrease in the Br atom quantum yield at the longer wavelengths included in their study and suggested that this may be a consequence of a systematic error in the σ (λ, 298 K) values used in their data analysis, which were taken from Moortgat et al (1993); the Br atom quantum yield was expected to be unity at the wavelengths most critical to atmospheric photolysis, < 350 nm (Peterson and Francisco, 2002) Due to this discrepancy, as well as the desire for accurate calculations of the CHBr3 atmospheric photolysis rate (lifetime) and its impact on stratospheric ozone, additional measurements of the CHBr3 UV absorption spectrum at wavelengths ≥300 nm are warranted. The overall impact of the revised σ (λ, T ) values was further quantified using calculations of the CHBr3 atmospheric photolysis rate (local lifetime) and its semi-empirical ODP
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