JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D1, PAGES 1505-1516,JANUARY 20, 1996
Gas phase atmospheric bromine photochemistry
D. J. Lary
Centrefor Atmospheric
Science,
CambridgeUniversity,Cambridge,
England
Abstract.
This paper reviewsthe current knowledgeof gas phasebromine
photochemistry
andpresents
a budgetstudyof atmospheric
brominespecies.
The
effectiveness
of the ozonecatalytic losscyclesinvolvingbromineis quantifiedby
considering
their chainlengthand effectiveness.
The chaineffectiveness
is a new
variabledefinedas the chainlengthmultipliedby the rate of the cycle'srate-limiting
step. The chaineffectiveness
enablesa fair comparison
of differentcatalyticcycles
involvingspecieswhichhavevery differentconcentrations.
This analysisclearly
showsthat below 25 km the BrO/C10 and BrO/HO2 cyclesare amongthe most
important ozonedestructioncycles.
The first study of atmospheric bromine chemistry
Introduction
was by Yung et al. [1980], who pointedto the genThis paper is a review of gas phasebromine photo- eral importance of atmosphericbromine chemistryand
chemistryand is intendedto complementthe compan- to the catalytic destructionof ozoneby the C10/BrO
ionpaper[Laryet al. thisissue]
whichconsiders
hetero- cycle. Bromine has been shown to play a significant
geneousbrominephotochemistry.Bromineentersthe role (m20%)in the formationof the ozoneholein polar
regions[McElroyet al., 1986].The contriatmosphereby a variety of natural and anthropogenic stratospheric
processes.The three main brominesourcegasesthat butions to ozone lossfrom bromine reactions are largest
can reachthe stratosphere(i.e. are not removedfrom belowabout20 km [e.g.,Pouletet al., 1992;Garciaand
the troposphereby rainout, reactionwith OH or photol- Solomon,1994]. Bromineplays an important role in
ysis)are CH3Br, CBrC1F2and CBrF3. The mostabun- stratosphericozone depletion despite being much less
Organidant of thesesourcegasesis methyl bromide(CH3Br) abundantthan chlorine[World Meteorological
whosenatural sourceis mainly due to oceanicbiologi- sation( WMO), 1992].
When atmospheric bromine chemistry is compared
cal processes.In these,mainly algal, processesCH3Br
is formed together with other speciessuchas CH2Br2, to chlorine chemistry, it can be seen that much more
CHBr3, CH2BrC1 and CHBrC12. The oceansare a sig- bromine is present in the active forms Br and BrO
nificantnatural sourceof CH3Br [Singhet al., 1983]. than chlorine is present in their counterparts C1 and
Measurements of larger tropospheric northern henri- C10. Consequently, bromine has a greater potential
ozonethan doeschlorine[e.g.
spheremixing ratios suggesta large land based north- to destroystratospheric
ern hemispheresource of CH3Br which could well be WMO, 1990,1992].
Section 2 describes the photochemical model used
anthropogenic
[Penkettet al., 1985; Reevesand Penkett, 1993]. The main industrialuseof CH3Br is as a in this study, which contains a detailed photochemfumigant, particularly for the treatment of soils. CH3Br istry scheme. Section 3 gives a budget study of atlllOis also used in quarantine treatments and in insect and sphericbromine speciesbasedon the current knowledge
rodent control.
of gasphasebromine chemistry. Se,ction 4 considersthe
The wide variety of CH3Br measurementsmade over effectivenessof the various ozone-destroyingcatalytic
the last decadein differentparts of the world [Berget bromine cycles. Section 5 summarizesthe main conclual., 1984; Rasmussen and Khalil, 1984; Penkett et al.,
1985; Cicerone et al., 1988; Fabian et al., 1994; Kaye et
sions.
al., 1994]suggestthat the natural background
concen- Model Description
tration of CH3Br in the troposphereis approximately
10 pptv. CH3Br concentrations
of up to 15 pptv have
The column model used in this study is a version of
also been observed;these are likely to reflect the effect a new suite of models called AuTOCHEM.
The model
of anthropogenicsources.
included a total of 53 species. No family or photochemical equilibrium assumptions are made. Of the
53 species, 51 species are integrated separa•e!y with
Copyright 1996 by the American GeophysicalUnion.
Paper number 95JD02463.
a 15minutetime step, namely: O(•D), O(3p), 03,
N, NO, NO2, NO3, N2Os, HONO, HNO3, HNO3(s),
0148-0227/ 96/ 95JD-02463505.00
HO2NO2, C1, C12, C10, C1OO, OC10, C1202, C1NO2,
1505
1506
LARY: GAS PHASEATMOSPHERICBROMINE PHOTOCHEMISTRY
C1ONO2,HC1,HCI(s),HOC1,H, OH, HO2,H202, CH•, erogeneousnitrogen and chlorine reactions are considbromine
CH302, CH302NO2, CH•OOH, CH•O, HCHO, HCO, eredin thesecalculationsbut not heterogeneous
Br, Br2, BrO, BrONO2, BrONO, HBr, HOBr, BrC1, reactions. Heterogeneousbromine reactions are the
paperLaryet al. [thisissue].
H2, H20, H20(s), CO, CO2, CH4, N20, CH3Brand subjectof the companion
CF2C12. 02 and N2 are included but not time inte- The reactive bromine speciesincluded in AUTOCHEM
grated. The version of AUTOCHEM used in this study are Br, Br2, BrO, BrONO, BrONO2, HBr, HOBr and
contains a total of 236 reactions, 129 bimolecular reac- BrC1.
Figure 2 showsthe calculatedmidlatitudephototions, 27 trimolecular reactions, 42 photochemicalreactionsand 38 heterogeneous
reactions. The rate con- chemical lifetimes of these speciesfor local noon at
stants for the reactions were taken from A tkinson ½t al. equinox, and Figure 3 showsthe calculatedmidlatitude
partitioning of reactive bromine speciesfor local noon
[1992]andDeMote½tal. [1994].
The time integration schemeused is an adaptive at equinox.
timestepBurlisch-$toer
scheme
[$toerandBurlisch,1980' It canbe seenfrom Figure2 that in markedcontrast
specificallydesignedfor integrationof stiff systemsaf- to their chlorinecounterparts
all of the brominespecies
ter Presset al. [1992].The time integration
package are short-lived.Figure 3 showsthat typicallythe most
is as accurateas the oftenusedGear[1971]package abundantinorganicbrominespeciesin the lowerstratobut faster.Photolysisratesare calculated
by usingfull sphereare BrO, BrONO•. and HOBr. Each of the reacsphericalgeometryand multiplescatteringasdescribed tive brominespecieswill now be consideredin turn.
by Lary andPyle[1992a,b]
afterMeieret al. [1982], BrONO•
Nicoletet al. [1982]andAnderson
[1983].Theaverage
photolysisrate overa modeltime stepis calculatedusBrONO• has a photochemicallifetime of a few mining 10 point gaussianquadratureas describedby Press utesthroughout
thesunlitmiddleatmosphere
(thesolid
et al. [1992].
squares
in the right-handpanelof Figure2). The relaAUTOCHEMhas also been usedto performfor the tivelyshortlifetimeof BrONO• is dueto thephotolysis
first timefour-dimensional
variationalanalysisof chem- of BrONO2, which occursin the visible region of the
ical species[Fisherand œary,1995]and to examine spectrum. Consequently,BrONO• is very closeto phothe effectof the reactionOH+C10• HCl+O9.on polar tochemical steady state in the sunlit atmosphere. The
ozonephotochemistry
[Laryet al., 1995].
recentstudyof Burkholderet al. [1995]measured
the
Gas Phase Bromine Chemistry
This sectionexaminesthe budgetsof reactivebromine
speciesaspredictedby our currentunderstandingof the
gas phase kinetics of bromine. Figure 1 is a reaction
schemeof atmospheric bromine photochemistry. Her-
absorption crosssection of BrONO•. in the important
tail region between about 390 nm and 500 nm extending the previousmeasurementsof Spencerand Rowland
[1978]. As pointedout by Burkholderet al. [1995],if
the products are BrO and NO• then BrONO• will not
be involved in catalytic ozone destruction. Photolysis
would merely reverse the formation of BrONO•.. How-
O(3p), OH
NO, 0 3
HO2
HCHO
/
(3p) •
NO2
h•,,O(3P)
Figure 1. Schematicof atmospheric
brominephotochemistry.
LARY: GAS PHASE ATMOSPHERIC
---*---
10o
10"
I I tillIll
Br
BROMINE PHOTOCHEMISTRY
-
- •' - Br2
BrO
,•,
10'
I I IIIIII]
--•---
BrONO
102
I I IIIIIII
HBr
HOBr
BrCI
100
103
I IIIIIIII
I I IIIII1[
/
BrONO2 -•',•,
102
IIIIIIIII
-
I IIIIIII
1507
104
I IIIIIIII
I IIIIIIll
I IIIII111 IIII
•
60
-
ß
40-
:•,.
, • • 40
•
-
_
30_
_
_
20_
11111111]
I llllllll III =
10"
100
10'
102
103
lO0
11
I1'111111]
I IIII1•1
lO•
Lifetime(s)
lO"
Lifetime(s)
LocalNoon, Mid-latitudeat Equinox.No heterogeneousBrominereactions
The reaction BrO + HO2--) HBr + O3 is not included
Figure 2. The calculatedmidlatitudephotochemical
lifetimesof reactivebrominespecies
for localnoonat equinox.
Heterogeneousbromine reactions were not included in these calculations. These simulations do not include the
reaction HOe + BrO
) HBr +03.
ever, if the products are Br and NO3, then an important
catalytic destruction of ozone can occur. This catalytic
destruction
of ozone will be considered
in the next sec-
tion.
BrONO2 is produced by the three-body reaction of
BrO with NO2. Because BrONO2 has a relatively
short lifetime, the BrONO2 concentrationrespondsvery
rapidly to any change in NO2, as occurs, for example, due to denoxification on surfaces. The rate recommended for the formation of BrONO2 by DeMote
eral minutes in the lower stratospheredecreasingto a
few secondsin the upper stratosphere(the solid circlesin the right-handpanelof Figure2). In the sunlit
lower stratosphereHOBr representsbetween approxi-
mately20%and30%of thetotalBrOy=totalinorganic
bromine(the solidcirclesin the right-handpanelof Figure 3). The main sourceof HOBr is the reaction
BrO + HO2
k=6x10_•2e+-•500
et al. [1992]is basedon kineticmeasurements
madeby
Thornet al. [1993]andDaniset al. [1990].Thesemea-
BrO
surementswere made at over 260 K and so quite large
temperature extrapolations are involved when considering lower stratospherictemperatures. It would therefore
+
)
HOBr
+
02
(1)
AHf 2osK : -217.7 kJ/Mole
HO2
> HBr
Possibleminor channel
+
03
(lb)
AHf •.osK = -31.8 kJ/Mole
The units of this, and all subsequent
rate constants,
are molecules
cm-a s-•. Recentlaboratorymeasurereaction at the colder temperatures experiencedin the mentsof reaction(1) weremadeby Pouletet al. [1992]
lower stratosphere.
and of reaction(lb) by Melloukiet al. [1994].The efThe companion
paperœaryet al. [thisissue]shows fect of HBr productionby reaction(lb) is considered
be valuable
to have further
measurement
studies of this
that the catalytic hydrolysis of BrONO2 on sulfate
aerosolsis an important sink of BrONO2 in the lower
stratosphere.
below in the subsection
HOBr
sphere, the heterogeneousproduction of HOBr on sulfate aerosolscan play an important role:
on HBr.
As can be seen in the companionpaper œary et
al. [thisissue],in the troposphere
and lowerstrato-
In the sunlit atmosphere, HOBr is almost in immediate photochemicalsteady state with a lifetime of sev- H20
+
BrONO2
> HOBr +
HNO3 (2)
1508
LARY: GAS PHASE ATMOSPHERIC BROMINE PHOTOCHEMISTRY
-
Br/BrO•
- •-- BqJBrO•
--e-- BrO/BrO• •
10'4
10'3
-
BrONO/BrO•
10'•
10"
BrONO•JBrO• - •-- HBr/BrO•
--e-- HOBr/BrO,
10•0"
104
--•-- BrCl/BrO•
103
10'•
I
\
10"
10ø
I
60
\
.•.\
.,,..
\
.,,.
__
40-
ø\ !
-40
_
-
m
-
-
-
_
_
30-
-
_
<::
-30
_
_
_
_
_
¾
20-
-
-20
_
_
10'4
10'•
10'•
10'•
1• 0'•
Paitioning
10'4
10•
10'•
10"
10ø
Paitioning
Lo•l Noon, Mid-latitudeat Equinox.No heterogen•us BrominereacUons
The reactionB• + HO• • HBr + O• is not included
Figure 3. The calculatedmidlatitude partitioning of reactivebrominespeciesfor local noon at equinox.
(seealsoFanandJacob[1992]andHansonandRavis- panelof Figure2). Typically,at 20 km, approximately
hankara[1995]).HOBr destruction
is dueto photolysis 40% of BrOy is in the form of BrO risingto a peak
andthereaction
with O(3p).Therateconstant
forthe of about 75% at around40 km (the solidcirclesin the
reactionof HOBr with O(3P) hasrecentlybeendeter- left-handpanelof Figure3). The main sourceof BrO
is the reaction
minedby Nesbittet al. [1995].
HOBr
+
hv
> Br
+
OH
(3)
Br
+ 03
) BrO
k=l.7x10_11e •oo
Orlandoand Burkholder[1995]
HOBr +
O(3P)
> OH +
k= 1.4xlO-1øe
BrO (4)
43O
ß
Below approximately 25 km photolysisis the most
important lossof HOBr, whereasabovethis altitude the
andBurkholder[1995].
(5)
In the sunlit lower stratospherethe main destruction
of BrO is by photolysis(the peak BrO absorptionis at
around325 nm) and reactionwith NO
BrO +
hv
> Br +
O(3P)(6)
DeMoteet al. [1992]
BrO
+
NO
> Br
+
NO•. (7)
k=8.7x10- l•'e-•'•ø
minutes in the sunlit lower stratosphereto a few seconds
in the upper stratosphere.This is basedon calculations
using the HOBr absorption cross section which have
recently been measuredfor the first time by Orlando
02
T
reactionwith O(3P) is the mainlossof HOBr [Nesbitt
et al., 1995]. This confirmsthe findingsof Nesbittet
al. [1995]. The lifetimeof HOBr variesfrom about 15
+
In the upper stratospherethe main lossof BrO is due
to the reaction
BrO +
O(3P)
> O•. +
Br (8)
BrO
The most abundant bromine species in the sunlit
lower stratosphere is normally BrO, which has a life-
In the lower stratosphere the reaction of BrO with
C10 contributesa few percent to the lossrate of BrO.
time of a few seconds(the solidcirclesin the left hand Closeto the groundthe reactionof BrO with HO•., men-
LARY: GAS PHASE ATMOSPHERIC BROMINE PHOTOCHEMISTRY
1509
tionedabove,contributesabout 10% to the total lossof creasethe HBr concentrationat the expenseof HOBr.
BrO, as does the formation of BrONO•.. As will be To illustratethis, Figure4 showsthat an HBr yieldof
at 20 km
discussedlater, the reaction of BrO with C10 is also only0.1%woulddoubletheHBr concentration
and a yieldof 1% wouldgivea tenfoldincrease
in HBr
important for catalytic ozone destruction.
at 20 km. If it wereassumed
that reaction(lb) had a
HBr
yieldof 0.1%,thenbetween
60%and70%of theHBr
produced
at
around
20
km
would
be dueto reaction
AlthoughHBr is the longestlivedBrO• reservoir,it
80%and90%of
still has a lifetime of only about a day in the sunlit (lb); if theyieldis 1%thenbetween
midlatitude lower stratosphere,decreasingto an hour
in the upper stratosphere(the trianglesin the righthand panel of Figure 2). The lifetimedoesincreaseat
high latitudes duringwinter wherethere is lesssunlight.
Accordingto our currentunderstanding,generallyonly
a smallpercentof BrO• in the lowerstratosphereis in
the form of HBr (the trianglesin the right-handpanel
the HBr producedat around20 km is due to reaction
be in the form of HBr if heterogeneousbromine reac-
suggeststhat the yield must be substantially lessthan
(lb). The partitioning
of reactivebromineis affected
by the assumedyield of HBr from reaction(1) up to
about 50 km, above this altitude the effect on the HBr
concentration is relatively small.
Garcia and Solomon[1994]have recentlyexamined
the effect of assumingdifferent branching ratios for re-
of Figure3). However,in the troposphere,
HBr is gen- action (1). They found that the BrO abundancewas
erally a larger fraction of the total BrO•. Typically critically dependent on the yield of HBr. Their combetween
about10%and40%of tropospheric
BrO• can parison between model calculations and observations
tions are not considered
and if it is assumed that there
5%.
Laboratorymeasurements
of reaction(1) weremade
by
Poulet
et
al.
[1992].
They
pointedout that HBr
However, as will be seenin the companionpaper œary
is a possible product of this reaction but stated that
et al. [this issue],if heterogeneous
brominereactions
is negligibleproductionof HBr by reaction(ls) above.
the only product they observed at 298 K was HOBr
total BrO• presentboth in the troposphereand lower suggestinga negligibleyield for HBr. They did not preclude the formation of HOBr at the lower temperatures
stratosphere.
do occur HBr may be only a very small fraction of the
of the stratosphere.Reaction (lb) is a four-centered
HBr is producedmainly by the reactions
Br
+
HO•
•
k=l.4x10-1•e-W
Br
+
HCHO
HBr
+
O•
(9)
on the yield of HBr from reaction(lb) hasrecentlybeen
determinedby Mellouki et al. [1994]by measuringan
590
-
> HBr
reaction and becauseof the required reaction geometry
these reactions are generally very slow. An upper limit
+
HCO
(10)
upper limit for the reverse reaction. The limits were
measuredat 301 K and 441 K and were extrapolated
k=1.7x10-•ewto low temperatures.They foundthat the yield of HBr
from
reaction(lb) is negligiblethroughoutthe stratoAt around20 km at equinoxin the sunlitlowerstratosphere.
It is likelythat lessthan0.01%of reaction(1)
spherereactions(9) and (10) make a significantcon800
tribution to HBr production. However, as shall be
seen later, the reaction of Br with HCHO can some-
yields HBr as a product.
Sincethe yieldof HBr fromreaction(1) wouldaf-
fect the fractionof BrOy in the form of BrO it would
alsoaffectthe OC10concentration,
sinceOC10is produced
mainly
by
reaction
(15).
Increasing
the yieldof
portanceof reaction(10) decreases
and reaction(9) is
the main sourceof HBr, whereasthe main sourceof HBr HBr from reaction(1) from 0% to 1% wouldreduce
in the modeltroposphere
is reaction(10). In the very the largenighttimeOC10 columnby just over16%at
times becomethe most important sourceof HBr in the
lower stratosphere. In the upper stratospherethe im-
midlatitudes.
low stratosphereand tropospherethe higheraldehydes
Our currentunderstanding
of brominephotochemprobably also play a role in producingHBr.
Below about 50 km, HBr is destroyedmainly by re- istry suggeststhat the main sourcesof HBr are reacaction with OH. Above 50 km the reaction of HBr with
O(3P) alsobecomes
an importantlossof HBr.
tions (9) and (10). For all of thesereactionsboth the
reactantsare presentin the atmospherein relatively
smallconcentrations.
Thisis in contrastto HC1,which
HBr
+
OH
•
H20
+
Br
(11)
k=l.lx10 -l•
HBr •-
O(•P)
•
OH +
is formed mainly by the reaction of C1 with the relatively abundant CS4. The extensiveliterature review
of Baulchet al. [1981]quotesa rate constantfor the
analogous
brominereactionwhichis veryslowat strato-
Br (12) spherictemperatures
k=6.6x10_•2e •o
T
Br +
The partitioning of reactive bromine is very sensi-
tive to the branchingratio of reaction(1) in the lower
stratosphere. Even a very small yield of HBr will in-
CH4 •
k=7.8x10-•e
HBr +
CH3 (13)
9180
•
A reaction
whichmayplaya rolein thetroposphere
1510
LARY: GAS PHASE ATMOSPHERIC
BROMINE
PHOTOCHEMISTRY
Yield of HBr
0%
0.0
70
0.1
0.2
0.3
1%
........... 0.1%
0.4
0.5
• • • • I • • • • I i • • ; I • • • • I ; I I • .
0.03
I
0.06
0.1
0.3
0.6
1.0
I
I
I
I
I
70
,,
30 !
•
',
•
\
',,,
30
\\
\
//
20
0.0
0.1
0.2
0.3
0.4
0.5
0.03
0.•
HBdBrO,
0.1
0.3
0.6
20
1.0
HBr(pptv)
Figure 4. The calculatedmidlatitudeconcentration
andfractionof BrOy presentasHBr for localnoonat equinox
if 0%, 0. 1% and 1% of the reaction of HO2 with BrO yields HBr. Heterogeneous
bromine reactionswere not
included
in these calculations.
and lower stratosphereis
Br
+
H202
. >
HBr
+
HO•. (14)
k< 5110 -x6
in the left-handpanelof Figure2). At 30 km approximately10%of BrOy is in the formof Br risingto over
90% above50 km (the solidsquaresin the left-hand
panel of Figure 3). This is in contrastto C1,whichin
the
lowerstratospheretypicallyconstitutesonlyaround
Therehavebeenfivedifferentstudiesof reaction(14)
chlorine),
inby Leu[1980],Poseyet al. [1981],Heneghan
andBen- 0.001%ofthetotalClOy(-Total inorganic
creasing
to
about
4%
in
the
upper
stratosphere.
As
one
son [19831,Tooheyet al. [1987]and Melloukieta!.
[1994].ApartfromHeneghan
andBenson
[1983]all descendsthroughthe group of halogensfrom fluorine
through to iodine, the partitioningshiftstowardsthe
more
reactivespecies.Throughoutmost of the middle
Melloukiet al. [1994]suggest
that thisdiscrepancy
may
atmosphere
the most important loss of Br is reaction
have been due to somereactiveimpurity from the miwith
O3
and
the two most important sourcesof Br are
crowavedischargesourceusedby Heneghanand Benson
showthat Br has a very low reactivity towardsH202.
the photolysis
of BrO (reaction(6)) andthe reactionof
BrO with NO (reaction(7)). In the upperstratosphere
due to its reaction with HO2. They concludedthat the reactionof BrO with O(3P) is the mostimportant
sourceof Br (reaction(8)). Two channelsof the reacthere was no measurable evidence for this reaction. If
tion
of BrO with C10 contributea few percentto the
thereis an additionalsourceofHBr whichcouldproceed
of Br in the sunlitlowerstratosphere
(reat a rate comparableto reaction(1), it wouldconsid- production
actions
(15)
and
(15)
below),
as
does
the
photolysis
of
erablyaffectour understanding
of the partitioningof
[1983].
Mel!oukiet al. [1994]alsoconsidered
the lossof HBr
reactive
bromine.
Sr
HOBr and BrONO2.
BrC1
Br constitutesabout 1% of BrOy in the sunlitlower
The lifetimeof BrC1in the sunlitlowerstratosphere
stratosphere,where it is in photochemicalsteady state is approximately
a minute (the opensquaresin the
with a lifetimeof abouthalf a second(the solidsquares right-handpanel of Figure 2). During the day BrC1
LARY: GAS PHASE ATMOSPHERIC
BROM1NE PHOTOCHEMISTRY
1511
generallyconstitutesmuchlessthan 1% of BrOy in lived sourcegases,suchas CHaBr, instead of in terms of
the lower stratosphere(Figure 3) when no heteroge- the productionor destructionof a specificchaincentre,
neous reactions have occurred on polar stratospheric suchas Br or BrO. Becausethe chainlength is a ratio
clouds(PSCs) for a long time. When PSCs, or cold
of two rates it is dimensionless.
sulfate aerosols, are encountered, the BrC1 concentraIf a particular radical is involvedin a catalytic cytion rapidly increasesand BrC1 can become a sizable cle which has a very long chain length, but it is only
fractionof the total BrOy. BrC1is typicallythe ma- present in small concentrations,the effectivenessof the
jor nighttimereservoirof BrOy whenPSCsarepresent. cycle will be limited. It is therefore useful to define
The gasphaseproductionof BrC1 is almost entirely due a new variable, which shall be called the chain effec-
to reaction(17):
BrO
+
C10
) Br +
k=l.6x10_• e 430
OC10
(15)
C10
) Br +
k_2.9x10_•2e .o
C1OO (16)
T
tiveness,as the chain length multiplied by the rate of
the cycle's rate-limiting step. The chain effectiveness
enablesa fair comparisonto be made of different catalytic cyclesinvolvingspecieswhichhavevery different
concentrations.
BrO
+
chain effectiveness,
œ- krlsJ•f
T
BrO
+
C10
) BrC1 +
k=5.8x10_x3
e ,•o
O2 (17)
T
The reactionof BrO with C10 is an important reaction as it is the first step in the catalytic lossof ozone
due to the cycle described in the next section. The
lossof BrC1belowabout30 km is almostentirelydue
to photolysis,the peak BrC1 absorptionis at around
375nm ($eeryandBritton[1964],R. A. Uox,private
communication
[1993]). Aboveabout 30 km the main
lossof BrC1is dueto the reactionwith O(3P) . This
reaction is not normally included in numerical models
but its ratewasdetermined
by Ulyneet al. [1976].
BrC1 d- hv ---+ Br d- C1 (18)
$eeryandBritton [1964]
BrC1 +
O(3P) --+
BrO -k C1 (19)
k=2.2xl0 -•l
(21)
This sectionconsidersthe chain length and chain effectivenessof the various atmosphericbromine catalytic
cyclesas a function of altitude comparedwith other catalytic cycleswhich are of importance in the atmosphere.
Becausethis paper focuseson the gas phase chemistry
of atmospheric bromine, the situation chosenwas noon
for midlatitudes at the equinox.
As was seen earlier, all of the atmosphericbromine
species are relatively short lived. Therefore, the termination steps of the gas phase bromine cycleswhich
involve the formation of HBr or BrONO2 are not very
effective since the Br or BrO can easily be liberated
from these species.In fact, BrONO2 is itself involved in
the catalyticheterogeneous
destructionof ozone[Lary
et al., this issue],so formationof BrONO2 is not even
the termination
of a chain. This is in marked contrast
to the C1/C10 and NO/NO2 catalytic cycles,where
the formation of the reservoirs HC1 and HNO3 is an
effective
termination
of the chain
reactions.
Conse-
quently,the brominechainreactionstend to havelonger
chain lengths than their chlorine counterparts. Catalytic ozonelosscan be due to severalcyclesinvolving
bromine (Figure 5), which will now be consideredin
Catalytic Cycles
Thereareseveral
importantchainreactions
involving
atmospheric
brominespecies.Thesechainscanpropagateafteran initiationsteptransforming
reactantsinto
products
by repeated
cycles
ofthechain.Thelengthof
thesecyclesis limitedby terminationstepswhichdestroythe chaincentre,or radical,involvedin the cycle.
The chainlengthis a measureof howmanytimesthe
turn.
cycle is executed before the chain centre is removed.
Thechain
length,
Af,isusually
defined
astherateof
propagation
(th• rateoftherate-limiting
step),krlsdivided by the rate of productionor destructionof the
chain center, kaest.
chainlength,Af- krls/kdest
(20)
In the atmosphere
thereare a verylargenumberof
interacting
andcompeting
cyclesoccurring.
Therefore,
a more useful definition has been used which defines
Figure 5. The brominegasphasecatalyticozone
thechainlengthin termsofthedestruction
ofthelong- destructioncycles.
1512
LARY: GAS PHASE ATMOSPHERIC BROMINE PHOTOCHEMISTRY
o
10'2
NO/NO•
100
102
•
010/HO2
[]
10•
100
104
BrO/HO2
105
10•ø
E
•4o ß
_
_
-30
_
_
_
_
-20
_
10'2
100
102
104
10•
100
105
10•ø
Chain Effectiveness
Chain Length
Local Noon, Mid-latitudeat Equinox.
CIO/NO2
10'2
•
100
BrO/NO2
102
[]
104
10•
BrO/010 (A)
100
-
BrO/010 (B)
105
--
_
.•.
_
_
-30
30-
_
_
_
_
_
-20
_
I
10'2
i
I
100
102
Chain Length
104
10•
100
105
Chain Effectiveness
LocalNoon, Mid-latitudeat Equinox.
Figure 6. The calculatedmidlatitude chainlengthand chaineffectiveness
of variouscatalyticcyclesfor local noon
at equinox.The chaineffectiveness
is the chainlengthmultipliedby the rate of the cycle'srate-limitingstep and
has units of molecules cm -s s-[.
LARY: GAS PHASE ATMOSPHERIC
BrO/C10
In polar regionsthe couplingof BrO and C10 chem-
istry via the BrO/C10 catalyticcycleis particularly
important. The couplingis also important at midlati-
tudes.The BrO/C10 cyclecanoperatevia two routes.
One route, here referred to as route A, results in the
formation
of BrCh
BrO
BrC1
Br
C1
+
+
+
+
Net
C10
h•
03
O•
•
•
•
•
BrC1
Br
BrO
C10
203
•
302
+
+
+
+
O•.
C1
O2
O•.
BROMINE
PHOTOCHEMISTRY
1513
HO2
+
BrO
•
HOBr
+
02
HOBr
Br
OH
+
+
+
h•
O3
Os
•
•
•
OH
BrO
HO2
+
+
+
Br
02
02
203
•
30•.
Net
Below about 15 km the rate-limiting step is the pho-
tolysisof HOBr; abovethis the formationof HOBr is
the rate limiting step. In the sunlit lower stratosphere
thiscyclehasa longchainlengthof approximately
103
(Figure6). The chaineffectiveness
of the BrO/HO2
catalyticcycleis greaterthan 106molecules
cm-3 s-•
between 13 and 30 kin. The chain length and effective-
nessdecreaseabove35 km. The BrO/HO•. catalytic
cycle is approximately 2 ordersof magnitude more ef-
Dependingon the physicalconditions,in the low
fectiveat destroyingozoneat 12 km than the NO/NO•.
stratosphere
belowabout 15 km the rate-limitingstep
is the formation of BrC1. However, higher up in the
catalytic cycle, and twice as effective at 20 km.
The analogousC10/HO•. catalytic cyclehas a much
stratosphere
it is the photolysisof BrC1.
shorter chain length of lessthan 50 above 15 km, where
In the sunlit stratospherebetween15 and 35 km this
the rate-limitingstep is the photolysisof HOC1 (Fig-
cyclehasa chainlengthof approximately
103withthe
ure6). TheC10/HO•.chainlengthisapproximately
10•'
chainlengthdecreasing
above35 km (Figure6). The
closeto the tropopause and in the troposphere,where
chaineffectiveness
of this cycleis approximately106
the rate-limiting step is the reaction of OH with 03.
Between15 and 20 km the BrO/HO2 cycleis between
ure 6). This can be comparedto the NO/NO2 cati and 2 orders of magnitude more effective than the
alytic cycle[Crutzen,1970;Johnston,1971],whichis
the most important ozone loss cycle at 38 km where it C10/HO2 cycleat destroyingozone,eventhoughBrO
molecules
cm-3 s-x between
about17and30 km (Fig-
has a chainlengthof 104 and a chaineffectiveness
of is much less abundant than C10. This finding emphasizes the importance of atmospheric bromine for catapproximately101ømolecules
cm-3 s-•, decreasing
to
106 molecules cm -3 s-•
at 17 km and 102 molecules
cm-3 s-1 at 10km (Figure6).
The alternative cycle, here referred to as route B, is
much more effective
marion
than route A and involves the for-
of C1OO:
BrO
+
C10
C1OO
Br
C1
Net
•
Br
M) C1
+
+
O3
O3
•
•
BrO
C10
203
•
30•.
+
C1OO
-]- 02
+
+
O2
O2
alytic ozone destruction.
Br/BrO
The Br/BrO catalytic cycleincreasesin lengthfrom
102in the lowerstratosphere
to 104in the upperstratosphere(Figure 6).
Br
•-
BrO +
O3 +
O3
•
BrO
•-
O•.
O(sp)
O(SP)
•
•
Br
202
+
O2
At all sunlitaltitudesthereactionof BrO with O(sP)
is the rate limiting step. The chain effectivenessin-
The rate-limiting step is the formation of C1OO. creases from 103 molecules cm -3 s-1 at 12 km to 10ø
Throughout the sunlit lower stratospherethis cyclehas molecules
cm-3 s-1 at 38 km (Figure7). The analoa chainlengthof approximately
104 (Figure6). The gousC1/C10 hasa shorterchainlength,whichincreases
chain effectiveness
of this cycleis approximately10s from 10 in the lower stratosphereto l03 in the up-
molecules
cm-3 s-• between
about20and30km (Fig-
per stratosphere and has a chain effectiveness which
ure 6). Route B of the C10/BrO catalyticcycleis ap- increases from 102 molecules cm -3 s-• at 12 km to
at
proximately an order of magnitudemore effectiveat de- 10•ø moleculescm-3 s-1 at 38 kin. Consequently
stroyingozonebetween16and20 km thanthe NO/NO2 38 km the NO/NO2 cycleis the main ozonelosscycle
catalytic cycle.
but abovethis the C1/C10 cycleis the main ozoneloss
The efficiencyof the BrO/C10 cycleis reducedby cycle[$tolarskiand Cicerone,1974;Molina and Rowthe alternate channel of the BrO + C10 reaction which
land, 1974].
yieldsOC10 (reaction(15)). OC10isphotolyzed
to give
BrO/NO2
an oxygenatom, this channelconstitutesa null cycle.
BrO/HO•.
As was recently pointed out by Burkholder et al.
[1995],if the productsof BrONO2 photolysisare Br
The BrO/HO•. catalyticcycleinvolvesthe formation and NOs, then a very effectiveBrONO2 catalytic cycle
of HOBr.
can exist, namely
1514
LARY: GAS PHASE ATMOSPHERIC
o
10•
CI/CIO
100
10•
I
I
10'•
•
Br/BrO
10•
i
PHOTOCHEMISTRY
[]
OH/HO•
10• 100 10•
10•
10•
10•
10•ø
i
•
100
BROMINE
I
10=
104
10a10ø
Chain Length
I
I
10=
I
I
104
I
I
10a
•
I
I
10a
I
10•ø
ChainEffectiveness
Local Noon, Mid-latitudeat Equinox.
Figure 7. The calculated
midlatitudechainlengthandchaineffectiveness
ofvariouscatalyticcycles
forlocalnoon
at equinox.
BrO
+
NO2 M> BrONO2
BrONO2
NO3
NO
Br
+
-I+
+
h•
hz•
03
>
>
>
O•
)
Br
NO
NO2
BrO
203
>
302
Net
+
+
+
+
NO3
02
02
O2
Depending on the conditions,the rate-limiting step of
the cycleis the photolysisof BrONO2 or of NO3 to NO.
If it is assumedthat the main BrONO2 photolysisprod-
not normallyconsidered
havebeenfoundto be important in the upperstratosphere.They are the reactions
of O(3P)with BrC1andHOBrwhoseratesweremeasuredby Clyneet al. [1976]andNesbittet al. [1995],
respectively.
The effectivenessof the ozone catalytic loss cycles
involvingbrominehas been quantifiedby considering
their chain length and effectiveness.The chaineffectivenessis a new variable defined as the chain length
multipliedby the rate of the cycle'srate-limitingstep.
The chain effectivenessenablesa fair comparisonof dif-
uctsare Br and NOs then the BrO/NO2 cyclebetween ferent catalyticcyclesinvolvingspecieswhichhavevery
about 15 km and 30 km has a chain length of more than
different
concentrations.
103. The chainlength decreases
above30 km. The
It is shownthat in the low stratospherethe most efchaineffectiveness
is approximately
107molecules
cm-3 fectiveozonelosscyclesaretheBrO/HO2 andBrO/C10
s-x betweenabout15 and 30 km (Figure6). This re- cycles.Both catalyticcycleshavelong chainlengthsof
sult can be comparedto the analolgous
C10/NO2 cy- greaterthan 103and a chaineffectiveness
of between
cle [Toumi et al., 1993]whichhas a chainlengthof 106 and 108 molecules cm-3 s-1 in the lower stratoabout 102 at the tropopause
decreasing
to only 3 at sphere.The cyclesare thereforeeffectiveozonedestruc40 km, and a chaineffectiveness
of 105molecules
cm-3 tion cyclesevenwhen no PSCs are present.
s-x closeto the tropopause
decreasing
to 102molecules If it is assumedthat the main photolysisproducts
cm-3 s-1 at 38 km. This findingemphasizes
the point of BrONO2are Br and NO3, thenthe BrO/NO2 cycle
madeby Toumiet aL [1993]that the C10/NO2 cycle between about 15 km and 30 km has a chain length of
is only important for high levelsof C1ONO2. For typi-
at least 103. The chainlengthdecreases
above30 km.
cal levelsof BrONO2and C1ONO2the BrO/NO2 cycle This cyclewill onlybe effectivefor ozonelossif BrONO2
is more effectiveat removing ozonethan the analogous photolysisleadsto the productionof Br and NO3. This
C10/NO2 cycle.
conclusion
agreeswith the recentfindingsof Burkholder
et al. [1995]and can be comparedto the anolgous
C10/NO2cycle[Toumiet al., 1993],whichhasa chain
The current knowledgeof gas phasebrominechem- lengthofabout102at thetropopause
decreasing
to only
Summary
istry has been reviewedand two gas phasereactions
3 at 40 km.
LARY:
GAS PHASE
ATMOSPHERIC
Acknowledgments.
The author wishes to thank R.
A. Cox for very useful conversationsand seminars, J. A.
Pyle for his support, J. J. Orlando and J. B. Burkholder for
making their results availableto us before publication and
D. Shallcross
and J. Sessler for useful conversations.
The
Centre for Atmospheric Science is a joint initiative of the
Department of Chemistry and the Department of Applied
Mathematics and Theoretical Physics. This work forms part
of the NERC U.K. Universities Global Atmospheric Modelling Programme.
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(ReceivedMarch 9, 1995;revisedJune 29, 1995;
acceptedJune 29, 1995.)