GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 23, PAGES 3025-3028, DECEMBER 1, 1997
A model study of the potential role of the reaction BrO
OH in the production of stratospheric HBr
M.P. Chipperfield, D.E. Shallcross and D.J. Lary
Department of Chemistry, University of Cambridge, U.K.
Abstract. We have used a constrained one-dimensional
photochemical model to investigate the effect of a potential
minor channel of the fast reaction between BrO -{- OH to
produce HBr. There is no direct evidence for this reaction
but the analogous yield of HC1 from C10 + OH is thought
to be about 5%. With only a 1-2% yield of HBr the mod-
elled HBr mixing ratio between 20-30 km increases from
around 0.5 parts per 10 ' by volume (pptv) to 1-2 pptv.
This brings the model into agreement with recent balloon-
borne observations of stratospheric HBr. Should BrO + OH
produce HBr with around 1-2% yield then this reaction will
dominate HBr production between 20-35 km. As the main
loss of HBr is reaction with OH this will lead to steady state
HBr:BrO partitioning which is independent of other species,
and temperature.
Introduction
Two groups have recently reported observations of strato-
spheric HBr using far infrared emission. Following the initial
observation ofan upper limit of 4 parts per 10 TM by volume
(pptv) by [Traub et al., 1992], the same group derived an av-
erage HBr mixing ratio of 2.0+0.8 parts per 10 TM by volume
(pptv) in the altitude range 22-34 km based on 7 balloon
flights between 1988 and 1994 around 34øN [Johnson et al.,
1995]. [Johnson et al., 1995] also presented the first daytime
and nighttime profiles in this altitude range. From the sec-
ond group, [Carlotti et al., 1995] reported HBr observations
of 1.15+0.46 pptv (using the updated values given by [Nolt
et al., 1997]) between 20-36.5 km at 34øN in May 1993. [Nolt
et al., 1997] reported measurements from May 1994 show-
ing an average HBr mixing ratio of 1.31+0.39 pptv between
20-36.5 km. [Nolt et al., 1997] were also able to derive a
vertical profile of HBr.
These observations of around 1-2 pptv of HBr are larger
than model predictions which consider production due to
(l)  () oy:
Br+CH20 -HBr+CHO (R1)
Br + HO2 - HBr + O2 (R2)
This prompted suggestions that the reaction between BrO
and HO2 may have a minor channel producing HBr.
BrO q-H02 -+ HOBr q-02 (R3a)
-+ HBr q-Oa (R3b)
[Johnson et al., 1995] showed that their results could be con-
Copyright 1997 by the American Geophysical Union.
Paper number 97GL53155.
0094-8534/97/97 GL- 53155 $05.00
sistent with a 0-2% yield for reaction (R3b). Following this,
[Nolt et al., 1997] argued that a yield for reaction (R3b)
of around 1-2% is required to reproduce their observations.
[Larichev et al., 1995] reported an upper limit for the channel
(R3b) of 1.5% from the non-observation of Oa as a product
of this reaction, while [Mellouki et al., 1994] studied the
reverse reaction of (R3b) and predicted that the upper limit
for the yield of (R3b) is in fact less than 0.01%. Hence, it
seems that even the modest 1-2% requirement of [Nolt et
al., 1997] is unlikely. The small branching ratio for (R3b)
mirrors that found for the analogous chlorine reaction:
C10 + HO2 - HOCI+ 02 (R4a)
--+ HC1 + Oa (R4b)
where (R4b) has been shown to be very small at room tem-
perature, i.e. less than 2% [Leck et al., 1980; Leu, 1980;
Burrows and Cox., 1981; Finkbeiner et al., 1995]. At the
lowest emperature studied (220 K), [Finkbeiner et al., 1995]
observe that (R4b) is 52 %, at 700 Torr but less than 1%
at 2 Tort. Hence [Finkbeiner et al., 1995] suggest hat the
rate of reaction (R4b) may be pressure dependent, in which
case, over stratospheric pressures, the yield of (R4b) will be
less than 5 %.
Here we consider whether the reaction between BrO +
OH
BrO+OH -+Br+HOe (R5a)
-+ HBr +O2 (R5b)
may have a channel which produces HBr and can therefore
resolve the HBr discrepancy. For the analogous chlorine
reaction
C10+OH -+Cl+HO2 (R6a)
-+ HC1+O2 (R6b)
both [Hills and Howard, 1984] and [Burrows et al., 1984]
report that the yield of (R6b) could be as high as 15%. Al-
though [Poulet et al., 1986] have observed a much smaller
value for the yield of (R6b), within the uncertainties of their
measurements, it could still be 10%. Very recently, direct
laboratory measurements of the yield of DC1 from C10 + OD
have indicated a branching ratio of 6+2% at 210 K, with a
similar yield of HC1 from C10 -{- OH [Lipson et al., 1997].
Therefore, since (R3) and (R4) display similar behaviour
with respect to products, it is not unreasonable to expect
(R5) and (R6) to do the same. The only reported study of
(R5) is by [Bogan et al., 1996]. These workers report a rate
constant at 300 K (ks = 7.5 + 4.2 x 10  cm a molecule 1
s ) which is 7.5 times faster than the 1994 NASA Panel
recommendation [DeMote et al., 1994], although the quoted
error is large. [Bogan et al., 1996] suggest hat the reaction
3025

3026 CHIPPERFIELD ET AL.: STRATOSPHERIC HBR PHOTOCHEMISTRY
proceeds via the [HOOBr] intermediate, and that the large
rate constant is due to the promotion of efficient spin-orbit
mixing of singlet and triplet surfaces in this [HOOBr] com-
plex by the heavy Br atom. They also present theoretical
calculations which imply that both channels (5a) and (5b)
can arise from the decomposition of the [HOOBr] interme-
diate, although, as in the chlorine case, that the dominant
product channel is likely to be (R5a), and not (R5b). Such
an assertion is not unreasonable since (R5a) involves simple
HOO-Br bond fission, whereas (R5b) will involve the forma-
tion of a four-centred rearrangement of the [HOOBr] com-
plex. Nevertheless, using some realistic estimates for the
activation energies for the decomposition of the [HOOBr]
complex into the two product channels, [Bogan et al., 1996]
calculate that the yield of (R5b) could conceivably be in
the region of 1%. Since the reported variation of k6 with
temperature is either nil [Burrows et al., 1984], or increas-
ing slightly with decreasing temperature [Hills and Howard,
1984], it is reasonable to assume that k5 is invariant with
temperature. Therefore, given that it is reasonable to as-
sume that both (R5) and (R6) proceed along similar poten-
tial energy surfaces [Bogan et al., 1996] and that the yield
of (R6b) is around 5-15%, we are justified in expecting some
HBr formation from (R5).
In this paper we use a constrained one-dimensional (1D)
photochemical model to investigate the effect of reaction
(R5b) on the abundance of stratospheric HBr. We have
necessarily assumed that reaction (R5) occurs with the rate
determined by [Bogan et al., 1996] and does not vary with
temperature.
One-Dimensional Model Experiments
We have used a 1D column model based on the photo-
chemical scheme from the TOMCAT model [Chipperfield et
al., 1995]. The model contains a comprehensive description
of stratospheric Ox, NOy, Cly, Bry (Br, BrO, HBr, HOBr,
BrONO. and BrC1), HOx and CH4 oxidation chemistry and
photochemical data is generally taken from [DeMote et al.,
1997]. The model has 22 levels from the ground to 0.3 hPa
(around 60 km). Photolysis rates are calculated using full
spherical geometry up to 95 ø SZA. For this study all of the
species in the model were integrated separately, with no as-
sumptions of photochemical equilibrium, using a 5 minute
timestep. The model was integrated for 4 days to obtain a
repeating diurnal cycle.
For this study we have performed a series of 1D model ex-
periments for the conditions of the [Nolt et al., 1997] balloon
flight. The model was run at 34øN for May 15. The model
profiles of temperature, H.O, and Oa were constrained by
simultaneous balloon measurements [M. Carlotti, personal
communication, 1996]. The model CI4_4 profile was taken
from Halogen Occultation Experiment (HALOE) observa-
tions for 34øN, 246øE, 4 days later on May 19, 1994. The
other 1D model fields were initialised using output from a
two-dimensional (latitude-height) model. The model total
inorganic bromine (Bry) in the middle/upper stratosphere
was 20 pptv. The yield of HBr from reaction (R5b) was
varied from 0 (run A), 1% (run B), 2% (run C), 3% (run D)
to 5% (run E).
Results and Discussion
It is first useful to consider what the basic 1D model
calculates for the partitioning of the principal Br v species
using the currently recommended kinetics. Figure i shows
the vertical distribution of these bromine species calculated
in run A (0% yield). BrO is the major daytime Brv species
between 20-40 km with a mixing ratio of 10-15 pptv. Be-
tween 20-30 km the mixing ratios of BrONO. and HOBr are
much less: 3-4 pptv and 2-3 pptv respectively. The mixing
ratio of HBr is only around 0.4 pptv in this region. Note
the relatively large mixing ratios of atomic Br between 20-30
km; this is due to the fast photolysis of BrO which forms an
odd-oxygen null cycle with Br + Oa.
Figure 2 shows the modelled HBr profiles for experiments
A-E compared with the retrieved profile of [Nolt et al., 1997].
The model results can also be compared with the average ob-
servations of [Johnson et al., 1995] and [Carlotti et al., 1995]
which were obtained under similar conditions (e.g. latitude
and season). With a 0% yield for reaction (R5b) the model
HBr profile is around 0.4 pptv between 20-35km, which is
much less than the average observation of [Nolt et al., 1997].
With only a 1% channel for reaction (R5b) the HBr mixing
ratio increases to around i pptv. The best agreement with
the average results of [Nolt et al., 1997] would be obtained
for a yield of around 1.5%. Similarly, a yield of around 1-
2% for reaction (R5b) would be sufficient to reproduce the
observations of [Johnson et al., 1995] and [Carlotti et al.,
Figure 3 shows the calculated abundance of HBr at 28 km
for experiments A-E. All experiments show little variation.
Again a yield of 1-2% for (RSb) give best agreement with
the observations.
Figure 4a shows the contributions of the 3 HBr produc-
tion reactions considered in the 1D model for run B. With
a 1% yield for reaction (R5b) this contributes around 60%
of the total production in the range 20-40 km. At lower
6O
5O
E
.. 40
_
,,
/ , /
2.0
BrON02 Bry Br
: I ' ' I ' I
0 5 10 15 20
Volume mixing ratio (pptv)
Figure 1. Vertical profiles of principal model Bry species
at 34øN at 1200 from model run A.

CHIPPERFIELD ET AL.' STRATOSPHERIC HBR PHOTOCHEMISTRY 3027
6O
50-
30-
20-
10
ß ii / I
11 //I
/ / - \"' 0% JPL 1997 ß 1% JPL 1997
\"\"' ' 2% JPL 1997 3% JPL 1997
\ ! 5%JPL 1997 ß Noltetal. [1997]
0 1 2 3 4 5
Volume mixing ratio (pptv)
Figure 2. Vertical profiles of HBr at 34øN at 1200 from
model runs A, B, C, D and E. Also shown are the altitude-
resolved observations of Nolt et al. [1997].
6O
5O
' 4O
 30
2O
10
BrO+OH//
0 20 40 60 80
A
100
contribution (%)
B
total
k7[OH]
i
5 10 15
HBr lifetime (hours)
Figure 4. a) Percentage contribution of model reactions
(al), (a2) and ((a5b) to total HBr production rate at noon
from model run B with a 1% yield for reaction (R5b). Con-
tribution is proportional to area of each region. b) Photo-
chemical lifetime of HBr (hours) due to reaction only with
OH (1/(k?[OH])), and the overall lifetime due to reaction
with OH, O(3p)and O('D) from run B.
altitudes production from Br + CH20 dominates while at
higher altitudes Br + HO2 is most important. The main
sink of HBr in the lower-mid stratosphere is:
HBr+OH -+Br+HO (R7)
The other HBr sinks treated in the model are reaction with
O(ZD) and O(3p). The lifetime of HBr, and the loss rate due
to only reaction (R7), is shown in Figure 4b. The lifetime
of HBr is therefore around a few hours between 20-30 km,
and this explains the small diurnal cycle exhibited in Figure
3. Therefore, if BrO + OH is indeed the major source of
stratospheric HBr, the steady state concentration of HBr
0 4 8 12 16 20 24
Local time
-- 0% JPL 1997
1% JPL 1997
...... :- 2% JPL 1997
.... 3% JPL 1997
5% JPL 1997
[] Johnson et al.
'*' NoR et al.
O Cadotti et al.
Figure 3. Mixing ratio of HBr at 28 km, 34øN from model
runs A, B, C, D and E. Also shown are the average observa-
tions of Johnson et al., [1995], Carlotti et al., [1995] and Nolt
et al. [1997]. These time-averaged observations are plotted
near 12 local time for convenience only.
will be, to a first approximation, independent of OH:
[HBr] ksb
[BrO]
Both ksb and k? are currently believed to be independent
of temperature [DeMore et al., 1997] and so this ratio can
be calculated to be 6.8 x Y, where Y is the fractional yield
of HBr from reaction (R5). Obviously, simultaneous obser-
vations of BrO and HBr would be useful for verifying this
partitioning. This simple ratio for the partitioning of HBr
to BrO justifies the extension of our constrained 1D model
runs for May 1994 to HBr observations made at other times.
Although this study shows that production of HBr from
BrO + OH may resolve the discrepancy between the obser-
vation and models there may be another, unknown, source
of HBr. However, this is unlikely to be production of HBr
from the photolysis of HOBr (which would be energetically
possible at wavelengths less than about 440 nm). [Benter
et al., 1995] observed a quantum yield of Br of > 0.95, and
observed no evidence of HBr formation despite a high sen-
sitivity.
Conclusions
Our model results clearly indicate that only a very small
yield of HBr from (R5) is required to resolve the discrepancy
between modelled and measured HBr, if k5 is indeed as fast
as recently measured. The large error on the recommended
value of ks, and our assumption of temperature invariance
should be noted, however. Although there is no direct ex-
perimental evidence for such a yield, the fact that the chan-
nel forming HC1 is evident (around 5%) for the analogous
chlorine system (R6) strongly suggests that HBr should be
formed from (R5). In addition, the theoretical calculations
of [Bogan et al., 1996] are not inconsistent with a yield of

3028 CHIPPERFIELD ET AL.: STRATOSPHERIC HBR PHOTOCHEMISTRY
(R5b) of around 1%. Direct laboratory measurements of the
yield of HBr from (R5), and an accurate determination of
ks at stratospheric temperatures, are evidently necessary to
resolve this issue.
Acknowledgments. We thank B. Carli and M. Carlotti
for the observations used to constrain the 1D model and L.J.
Stief for useful discussions about (R5). MPC thanks NERC for a
Fellowship.
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M.P. Chipperfield, D.E. Shallcross, D.J. Lary, Department of
Chemistry, Lensfield Rd., Cambridge, UK., CB2 !EW. (e-mail:
martyn@atm.ch.cam.ac.uk)
(Received June 4, 1997; revised October 27, 1997;
accepted October 3 I, 1997.)