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| Titel |
The very short-lived ozone depleting substance CHBr3 (bromoform): Revised UV absorption spectrum, atmospheric lifetime and ozone depletion potential |
| VerfasserIn |
Dimitrios K. Papanastasiou, Stuart A. McKeen, James B. Burkholder |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095381
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| Publikation (Nr.) |
 EGU/EGU2014-13982.pdf |
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| Zusammenfassung |
| CHBr3 (bromoform) is a short-lived atmospheric trace gas primarily of natural origin that
represents a source of reactive bromine (Bry; Br + BrO) in the troposphere as well as the
stratosphere. The transport of short-lived brominated species, and their brominated
degradation products, to the stratosphere is known to be particularly impactful to
stratospheric ozone due to the high efficiency of ozone destruction cycles involving
bromine. Evaluating the impact of CHBr3 on stratospheric ozone requires not only a
thorough understanding of its emissions, but also its atmospheric loss processes,
which are primarily UV photolysis and reaction with the OH radical. The total
global lifetime of CHBr3 is ~24 days and is mostly governed by its photolytic loss.
Therefore, accurate CHBr3 UV absorption cross section data for wavelengths (λ) in
the actinic region, greater than 290 nm, are needed to calculate its photolysis loss
rate.
Currently, there is a single study (Moortgat et al., Springer-Verlag Berlin Heidelberg,
1993; Vol. 17) that reports CHBr3 UV absorption cross sections and their temperature
dependence in a wavelength and temperature range applicable for atmospheric photolysis rate
calculations. However, there are indications that the reported longer wavelength cross section
data, in the Moortgrat et al. study, might be subject to systematic errors which possibly lead
to erroneous CHBr3 atmospheric photolysis rate calculations and a misleading picture of its
impact on stratospheric ozone.
In this study, UV absorption cross sections, Ïă(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. A thorough investigation of possible sources of systematic error in
the measurements is presented. The present UV absorption cross sections at longer
wavelength (>310 nm) are systematically lower compared to currently recommended values
for use in atmospheric models, with the deviation being more pronounced as wavelength
increases and temperature decreases. The source of this discrepancy is further 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
Ïă(λ,T) values on its calculated local lifetimes. For instance, CHBr3 atmospheric photolysis
rate in the tropical region obtained with the present spectral data was found to be
10-15% lower (longer lifetime) than that obtained using the currently recommended
values. Moreover, seasonally dependent ozone depletion potentials (ODPs) for
CHBr3 emitted in the Indian sub-continent were calculated using the semi-empirical
relationship of Brioude et al. (Brioude et al., Geophys. Res. Lett., 37, L19804, doi:
10.1029/2010GL044856, 2010) to evaluate the impact of the present results on stratospheric
ozone.
In conclusion, the present study reports improved UV absorption cross section data for
the short-lived ozone depleting substance CHBr3, which are a result of high quality
measurements and a thorough investigation of possible sources of systematic error. The
CHBr3 UV cross section data, from this study, combined with OH kinetic data enables more
accurate model predictions of stratospheric bromine loading and its impact on stratospheric
ozone. |
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