The photochemistry of bromine in the stratosphere has become an important issue since the halons and methyl bromide have been suspected as ozone-depleting substances more powerful than the CFCs. Halons are used as fire extinguishers, especially in the aviation and electronics industries. Methyl bromide, which represents the major source of bromine to the stratosphere, is used as an agricultural fumigant, and is an interesting compound in the context of stratospheric ozone depletion because it has significant natural sources and also has a very short lifetime in the troposphere. Thus, unlike the CFCs, a complete phase-out of which will not purge these compounds from the atmosphere for hundreds of years, atmospheric methyl bromide abundances will drop to background levels in just a few years following a phase-out, and the potential recovery of the ozone will be almost immediate. Consequently, methyl bromide has been the recent target of agressive regulation. In response, a number of studies have focussed on the atmospheric chemistry of this compound.
Bromine photochemistry is believed to be similar to that of chlorine, except
that the reactive forms Br and BrO represent a larger fraction of the budget
due to the relative photochemical instability of the reservoirs HBr and
BrONO
. Laboratory studies [ Poulet et al., 1992 and
Bridier et al., 1993] have shown that the rate constant for the reaction BrO
+ HO
HOBr + O
at room temperature is considerably faster
than previously believed. If the temperature dependence is small or slightly
negative (as for the ClO + HO
reaction) then this reaction accounts for roughly
half of the ozone depletion due to bromine compounds in the lower stratosphere.
The other half is due to reactions of BrO with ClO and NO
.
Photochemical models [ Avallone et al., 1993 and Garcia and
Solomon, 1994] show that the bromine contribution to ozone loss is
approximately equal to that of chlorine in the lower stratosphere at
mid-latitudes. Ozone destruction rates can increase substantially in the polar
regions in the presence of enhanced ClO [ Anderson et al., 1991
and Murphy, 1991]. However, the contribution of bromine to total ozone
loss becomes negligible above 25 km. Garcia and Solomon [1994]
have calculated that bromine is about 50 times more efficient at destroying
ozone than a comparable amount of chlorine. Thus, the ozone-depletion potential
(ODP) for bromine compounds, even methyl bromide with a short tropospheric lifetime,
can be quite high.
Relative to chlorine species, there have been few measurements of bromine
compounds in the stratosphere. Schauffler et al. [1993] have
found abundances of organic bromine of about 20 pptv near the tropical tropopause;
based on similar observations of CFCs, these are roughly the values that would
be expected to enter the stratosphere, where photolysis and reaction with OH
would release bromine atoms. Arpag et al. [1994] have measured
BrO columns at mid-latitudes that are consistent with previous aircraft
observations. Measurements in the arctic polar vortex [ Wahner
and Schiller, 1992] have shown similar abundances there, implying that
the major remaining bromine reservoir shifts from BrONO
at
mid-latitudes to BrCl in the perturbed polar regions. Recent extensive
searches for HBr emission in the far-infrared from balloons [
Traub et al., 1992] have provided an upper limit of a few pptv for this
species, supporting inferences from aircraft studies of BrO and models
[ Garcia and Solomon, 1994] and laboratory studies
that find the HBr yield from the BrO +
HO
reaction to be negligible
[ Poulet et
al., 1992 and Mellouki et al., 1994].
The most recent in situ results from the NASA ER-2 aircraft indicate
that models do a reasonable job of reproducing the
latitude and solar zenith behavior of BrO, but that they appear to overestimate
its abundance by about 25% [ Avallone et al., 1995]. This
might call into question the adequacy of the photochemical treatment of
inorganic bromine upon which the ODPs of bromine source gases are based.
Measurements of HOBr, BrONO
, and BrCl
could help to resolve this issue, but to date there
are no reported observations of these species in the stratosphere where they are
difficult to measure because their abundances are small.
It is safe to say that the stratospheric photochemistry of inorganic bromine is
not as well understood as is that of chlorine.