Finding the missing stratospheric Bry: a global modeling study of CHBr3 and CH2Br2
Other literature type
Stolarski, R. S.
Kawa, S. R.
Nielsen, J. E.
Douglass, A. R.
Rodriguez, J. M.
Blake, D. R.
Atlas, E. L.
Ott, L. E.
(issn: 1680-7324, eissn: 1680-7324)
Recent in situ and satellite measurements suggest a contribution of ~5
pptv to stratospheric inorganic bromine from short-lived bromocarbons. We
conduct a modeling study of the two most important short-lived bromocarbons,
bromoform (CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>), with the
Goddard Earth Observing System Chemistry Climate Model (GEOS CCM) to account
for this missing stratospheric bromine. We derive a "top-down" emission
estimate of CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub> using airborne measurements in
the Pacific and North American troposphere and lower stratosphere obtained
during previous NASA aircraft campaigns. Our emission estimate suggests that
to reproduce the observed concentrations in the free troposphere, a global
oceanic emission of 425 Gg Br yr<sup>−1</sup> for CHBr<sub>3</sub> and 57 Gg Br yr<sup>−1</sup>
for CH<sub>2</sub>Br<sub>2</sub> is needed, with 60% of emissions from open
ocean and 40% from coastal regions. Although our simple emission scheme
assumes no seasonal variations, the model reproduces the observed seasonal
variations of the short-lived bromocarbons with high concentrations in
winter and low concentrations in summer. This indicates that the seasonality
of short-lived bromocarbons is largely due to seasonality in their chemical
loss and transport. The inclusion of CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub>
contributes ~5 pptv bromine throughout the stratosphere. Both the
source gases and inorganic bromine produced from source gas degradation
(Br<sub>y</sub><sup>VSLS</sup>) in the troposphere are transported into the
stratosphere, and are equally important. Inorganic bromine accounts for half
(2.5 pptv) of the bromine from the inclusion of CHBr<sub>3</sub> and
CH<sub>2</sub>Br<sub>2</sub> near the tropical tropopause and its contribution rapidly
increases to ~100% as altitude increases. More than 85% of the
wet scavenging of Br<sub>y</sub><sup>VSLS</sup> occurs in large-scale precipitation
below 500 hPa. Our sensitivity study with wet scavenging in convective
updrafts switched off suggests that Br<sub>y</sub><sup>VSLS</sup> in the stratosphere is
not sensitive to convection. Convective scavenging only accounts for ~0.2 pptv (4%) difference in inorganic bromine delivered to the