Bromine from short-lived source gases in the extratropical northern hemispheric upper troposphere and lower stratosphere (UTLS)
Authors: Keber Timo , Bönisch Harald , Hartick Carl , Hauck Marius , Lefrancois Fides , Obersteiner Florian , Ringsdorf Akima , Schohl Nils , Schuck Tanja , Hossaini Ryan , Graf Phoebe , Jöckel Patrick , Engel Andreas 
Published: April 2020

Abstract. We present novel measurements of five short-lived brominated source gases
(CH2Br2, CHBr3, CH2ClBr, CHCl2Br and CHClBr2).
These rather short-lived gases are an important source of bromine to the
stratosphere, where they can lead to depletion of ozone. The measurements
have been obtained using an in situ gas chromatography and mass spectrometry
(GC–MS) system on board the High Altitude and Long Range Research Aircraft
(HALO). The instrument is extremely sensitive due to the use of chemical
ionization, allowing detection limits in the lower parts per quadrillion
(ppq, 10−15) range. Data from three campaigns using HALO are
presented, where the upper troposphere and lower stratosphere (UTLS) of the
northern hemispheric mid-to-high latitudes were sampled during winter and
during late summer to early fall. We show that an observed decrease with
altitude in the stratosphere is consistent with the relative lifetimes of
the different compounds. Distributions of the five source gases and total
organic bromine just below the tropopause show an increase in mixing ratio
with latitude, in particular during polar winter. This increase in mixing
ratio is explained by increasing lifetimes at higher latitudes during
winter. As the mixing ratios at the extratropical tropopause are generally
higher than those derived for the tropical tropopause, extratropical
troposphere-to-stratosphere transport will result in elevated levels of
organic bromine in comparison to air transported over the tropical
tropopause. The observations are compared to model estimates using different
emission scenarios. A scenario with emissions mainly confined to low
latitudes cannot reproduce the observed latitudinal distributions and will
tend to overestimate organic bromine input through the tropical tropopause
from CH2Br2 and CHBr3. Consequently, the scenario also
overestimates the amount of brominated organic gases in the stratosphere.
The two scenarios with the highest overall emissions of CH2Br2
tend to overestimate mixing ratios at the tropical tropopause, but they are in
much better agreement with extratropical tropopause mixing ratios. This
shows that not only total emissions but also latitudinal distributions in
the emissions are of importance. While an increase in tropopause mixing
ratios with latitude is reproduced with all emission scenarios during
winter, the simulated extratropical tropopause mixing ratios are on average
lower than the observations during late summer to fall. We show that a good
knowledge of the latitudinal distribution of tropopause mixing ratios and of
the fractional contributions of tropical and extratropical air is needed to
derive stratospheric inorganic bromine in the lowermost stratosphere from
observations. In a sensitivity study we find maximum differences of a factor
2 in inorganic bromine in the lowermost stratosphere from source gas
injection derived from observations and model outputs. The discrepancies
depend on the emission scenarios and the assumed contributions from
different source regions. Using better emission scenarios and reasonable
assumptions on fractional contribution from the different source regions,
the differences in inorganic bromine from source gas injection between model
and observations is usually on the order of 1 ppt or less. We conclude that
a good representation of the contributions of different source regions is
required in models for a robust assessment of the role of short-lived
halogen source gases on ozone depletion in the UTLS.