JOURNALOF GEOPHYSICALRESEARCH,VOL. 101,NO. D1, PAGES1489-1504,
JANUARY20, 1996
Heterogeneous atmospheric bromine chemistry
D. J. Lary and M. P. Chipperfield
Centrefor AtmosphericScience,CambridgeUniversity,Cambridge,England
R. Toumi
Department,of Physics,Imperial College,London, England
T. Len•on
Centre for AtmosphericScience,CambridgeUniversity,Cambridge,England
Abstract.
This paper considersthe effect of heterogeneous
brominereactionson
stratospheric photochemistry. We have consideredreactions on both sulfate aerosols
andonpolar stratospheric
clouds(PSCs). It is shownthat the hydrolysisof BrONO2
on sulfate aerosolsenhancesthe HOBr concentration,which in turn enhancesthe
OH and HO2 concentrations,thereby reducingthe HC1 lifetime and concentration.
The hydrolysis
of BrONO2leadsto a nighttimeproductionof HOBr, makingHOBr
a major nighttimebrominereservoir.The photolysisof HOBr givesa rapid increase
in the OH and HO2 concentrationat dawn, as was recentlyobservedby $alawitch
et al. [1994].The increasein the OH and HO2 concentration,
and the decrease
in
the HC1 concentration,leadsto additional ozonedepletionat all latitudes and for
all season.At temperaturesbelow210 K the bulk phasereactionof HOBr with HC1
in sulfateaerosols
becomes
important.The mostimportantheterogeneous
bromine
reactionson polar stratosphericcloudsare the mixedhalogenreactionsof HC1with
HOBr and BrONO2 and of HBr with HOC1 and C1ONO2.
Introduction
phase
chemistry,
fortherapidconversion
ofHC1into
C1Ox(=Cl+C10+2C1202). Sincethe timescalefor HC1
TherecentWorldMeteorological
Organization
(WMO) productionis severaldaysit is only recycledslowly.
assessment
[1992]reportedthat for the first time there Therefore,the heterogeneous
reactionsconsiderably
perare statistically significantdecreasesin ozonein all sea- turb the chlorine partitioning. In contrast, because
sonsin both the northern and southernhemispheres
at bromine speciesare short-livedheterogeneous
bromine
middle and high latitudes during the 1980sand that reactions are important as they allow the formation of
most of this decreaseis occurringin the lower strato- catalytic cycles for the conversionof H20 into HOx
sphere.This findinghas alsobeensupportedby trends (=OH+HO2), HC1 into C10 and NOx (=NO+NO2)
derivedfrom ozonesondes
[Logan,1994]. This paper into HNO3, as well as for the rapid recyclingof the
showsthat at leastpart of this ozonelossis likely to be brominereservoirspeciesBrONO2 and HBr. This leads
dueto in situ heterogeneous
brominereactions.
to ozonelossat all latitudesand for all seasons,
particThe atmospheric
chemistryofreactivebrominespeciesu!arly whenhighloadingsof sulfateaerosolare present
is characterized
by their shortlifetimes.The longest- in the atmosphere.
lived reactive bromine speciesis HBr, which has a lifetime of up to a day but constitutesonly a smallfraction Calculation of Heterogeneous Reaction
of the total reactivebromine(BrOy) presentin the at- Rates
mosphere
[Lary,1995]. In contrast,the longest-lived
Table 1 showsthe heterogeneousbromine reactions
reactive chlorinespeciesis HC! which typically has a
lifetime of over a weekin the lowerstratosphereand of- used in this study. This set of reactions was constructed
chlorinereten constitutesthe largest fraction of the total reactive by analogywith the known heterogeneous
chlorine(ClOy) presentin the atmosphere.
•aions. a•c•ly,
,'•ction• (1) •nd (2)in T•bl• • h•v•
Papernumber95JD02839.
0148-0227/96/95JD-0283
9505.00
as bulk phase reactions on sulfate aerosolsbecause of
"7
Heterogeneous
chlorinereactionsare important be- beenstudiedby Abbatt[1994]on ice. The measured
causethey providea mechanism,not providedby gas valuesare very similar to the value of 0.3 for the reaction
HOC1with HC1onwaterice[HansonandRavishankara,
Copyright
1996bytheAmerican
Geophysical
Union.
1991;Abbattand Molina, 1992].
Reactions(1) to (3)in Table 1 have been treated
1489
1490
Table
LARY ET AL.: HETEROGENEOUS
1.
ATMOSPHERIC
CHEMISTRY
AH as K
Surface
kJ/Mole
Type
A
A,I,II
A,I,II
A,I,II
I,II
I,II
I,II
A,I,II
I,II
B
B
B
HeterogeneousBromine ReactionsUsed in This Study
Reaction
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
(R7)
(R8)
BROMINE
*
*
*
*
*
HBr
HC1
HBr
HBr
HC1
HBr
H•.O
HBr
4+
4+
+
+
4+
HOBr
)
HOBr
)
HOC1
)
BrONO•.
>
BrONO•
>
C1ONO•.
)
BrONO•.
•
N•.O5
--4
Br•.
BrC1
BrC1
Br2'
BrC1
BrC1
HOBr
BrONO
4+
4+
+
+
4+
H•.O
H•.O
H20
HNO3
HNO3
HNO•
HNOa
HNOa
-95.3
-55.6
-115.8
-118.2
-78.5
-107.3
-22.9
-5.6
7
I
II
0.1
0.1
0.1
0.3
0.3
0.3
0.45 0.006
0.005õ
0.12•
0.25t
0.3
0.3
0.3
0.3
0.3
0.005õ
The assumed
radiiusedin thisstudyfor sulfateaerosol
(A), PSCI (I) andPSCII (II) are0.1pro,0.1pro,10pm
respectively.B, a bulk phasereaction. ,, a key reaction.
õFromHansonandRavishankara
[1992].
tFromAbbatt[1994].
*FromHansonandRavishankara
[1995].
their diffusoreactivelength. For example, as mentioned
Heterogeneous Bromine Catalytic
by Danilin and McConnell[1995],the diffusoreactive Cycles
lengthof reaction(2) exceedsi pm for 60 wt % H2SO4
solution. The rates of the bulk phase reactions are calThe hydrolysisof BrONO2 is the rate-limitingstep
culated as a function of temperature by usinga Henry's
law coefficient and an aqueous phase bimolecular rate
coefficientas describedby Cox et al. [1994].The bulk
phasereactions are fastest at cold temperaturesand for
high water concentrationsand are therefore most important in the region closeto the tropopauseor whet'ever the temperature falls below 210 K.
of a catalytic cyclein which H20 is split and converted
into HO2. Three ozonemoleculesare destroyedfor each
moleculeof BrONO2 hydrolyzed.
Cycle A
+
+
+
H20
hv
03
HOBr
+
hv
a function of temperature and effectivehydrogencon-
OH
Br
+
+
03
03
centrationin the mannerdescribed
by Coxet al. [1994].
BrO
+ NO2 M> BrONO2
In each case the bimolecular
rate coefficient
for the
liquid phasereactionis taken to be 105M -• s-•. The
effective Henry's law coefficient for HC1 is calculated as
The effective Henry's law coefficient for HBr is calcu-
BrONO2
HNO3
OH
> HOBr
> OH
> HO2
>
OH
' > HO2
> BrO
+
+
+
HNO3
NO2
02
+
Br
q+
02
02
+ 303
> 2H02
+ 302
lated after DeMoreet al. [1994](seealsoBrimblecombe H20
and Clegg[1988]).The effectiveHenry'slaw coefficient
CycleA is represented
schematically
in Figure1.
for HOBr is takento be 10ø M/arm in accordance
with
Cycle
A
has
a
long
chain
length
for
enhanced
levels
the resultsof HansonandRavishankara
[1995]at 210 K.
of
sulfate
aerosol
reaching
a
peak
of
over
10
•
between
In marked contrast to the hydrolysis of C1ONO2 on
sulfate aerosols, the hydrolysis of BrONO2 on sulfate
aerosols(reaction(7) in Table 1)is not a strongfunc-
I
Conversionof H20 into OH
tion of temperature[HansonandRavishankara,
1995]
(seeFigure3 later). Hansonand Ravishankara
[•9921
studied the heterogeneousreaction of HBr with N205
(reaction(8) in Table 1). Where7 valueswereunavailable, the 7 values used for the bromine reactions were
taken to be the same as those of their chlorine ana-
logues.
The key heterogeneous
bromine reactionsare marked
with a largestar in Table 1. They involvethe relatively
abundant bromine speciesHOBr and BrONO2. The
net effectof thesereactionsis to convertBrOy from
BrONO2 and HBr into HOBr and BrC1. The HOBr
cansubsequently
be photolyzedor heterogeneously
con- Figure 1. Reactionschemeshowingthe effectsof
into BrC1.
BrONO2 hydrolysison sulphateaerosols.
verted
LARY ET AL.: HETEROGENEOUS
ATMOSPHERIC
BROMINE
BrONO2Hydrolysis
catalyticcycle
1o'•
10ø
10'
102
10•
CHEMISTRY
1491
HOBr+ HCIcatalyticcycle
104
30
10•
30
30
_
10•
, i •l•l
_
10'
• , illrill
100
10'
i
• I J,,,,,I
i iiillll
10•
i
i iiiiii
_
_
AerosolLoading
_
_
_
......
_
25--
•,
-25
_
Background
........... Volcanic
25-
_
-25
_
E
_
..•
_
_
_
_
(i) 20•
,
-
•
-
•
_
(i) 2o-
-20
,,,,
_
_
"o
•
-
•
-
-20
t
'
_
_
• 15--AerosolLoading
...... Background
J
//
/
,,/
/
lO'
-15
. .....
15-
-15
_
_
......
.....Volcanic
_..-_..•:.•.-•-""/'""
lO
_
100
_
lO
lO
o4
10
o3
lO'
10'•
ChainLength
lO0
10'
10•
ChainLength
Figure 2. The chain lengthsof the ozonedestroyingBrONO2 hydrolysisand the HOBr+HC1 catalytic cyclesfor
backgroundand volcanic loadingsof sulphate aerosol.
about 15 km and 20 km (Figure 2). The chain length centration can reduce the HC1 lifetime by up to a facis a measure of how many times the cycle is completed tor of 3. The reduced HC1 lifetime, and the accombefore the chain center is removed.
Because the chain
panying increasein the C10• concentration,alters the
length is a ratio of two rates, it is dimensionless. It is C1ONO2/HC1ratio, enhancingthe effectiveness
of the
discussed
in moredetailin the companion
paper[œary, two gasphaseC10/BrO catalyticcycles.
this issue]whereit is definedasthe rate of propagation Both the production and destruction of HNOa in(the rate of the rate-limitingstep), kr•sdividedby the volve OH. However, owing to the relative rates of these
rate of production or destruction of the source gases. two reactions, the increase in the OH concentration ink dest.
creasesthe production of HNOa more than it enhances
chainlength,;V' =
]Crls
]Cdest
the HNOa
(1)
destruction.
As a result the ratio of the
HNOa production timescaleto the HNO• losstimescale
increases fi'om between
1.5 and 2 in the lower strato-
sphereto approximately 3 when heterogeneousbromine
The rate of the rate-limiting step reaches a peak of
reactions on enhanced loadings of sulfate aerosolsare
approximately1600molecules
cm-3 s-1 closeto 18 km
included for noon at mid-latitudes at equinox. This
for enhancedlevels of sulfate aerosol, and approximately
increase in OH leads to an increase in the HNOa con-
150molecules
cm-3 s-1 for background
levelsof sulfate centration at the expense of the NO2 concentration.
aerosol.
At cold temperatures the HOBr formed by BrONO2
The stickingcoefficientfor hydrolysisof BrONO2 on hydrolysis can react with HC1 within sulfate aerosols
sulfate aerosolsis not temperature dependent. Cycle A
to produce BrC1 instead of being photolyzed. This cycan proceed whenever sunlight is present. It is therecle couplesthe atmosphericchemistryof chlorine and
fore important for ozone loss at all latitudes and for all
bromine, releasingactive chlorinefrom HC1. Each time
seasonsin the lower stratosphere. For example, over the cycle is executed, HC1 is converted into C10 and
a 40-day mid-latitude simulation of a vertical profile three ozone molecules are destroyed.
in a one-dimensionalmodel at the equinox, including
heterogeneousbromine reactions reduced the ozone col- Cycle B
umn by 11.2%for volcanicaerosolloadingsand by 5.4% BrONO2
for background aerosol loadings. The ozone loss due
to heterogeneousbromine reactions took place in the
troposphereand the lower stratosphere. Although this
study did not include the effects of rain out in the troposphere, it is clear that heterogeneousbromine reac-
HNO3
OH
HOBr
+
+
+
+
H20
h•
03
HC1
BrC1
+
Br
tions are alsoimportant in the troposphere[Fan,and C1
Jacob, 1992; Finlayson-Pitts et al., 1990; McConnell et
al., 1992;Toumi,1994].
BrO
)
>
>
HOBr
OH
HO2
BrC1
+
+
+
+
HNO3
NO2
02
H20
h•
>
Br
+
C1
+
q-
03
03
>
>
BrO
C10
+
q-
02
02
+
NO2 M> BrONO2
--•
+
303
•
HO2 q- C10
q- 302
Cycle A enhancesthe OH concentrationand thereby HC1
indirectly couples the atmospheric chemistry of chloThe rate of the bulk phasereactions(1) to (3)in
rine and bromine, because the increase in the OH con- Table 1 are a strong function of both the temperature
1492
LARY ET AL.: HETEROGENEOUS
ATMOSPHERIC
and the water vapor concentration. Therefore cycle B
is only effective in the cold temperatures found in the
very low stratosphere.For enhancedaerosolloadingsit
has a chainlengthapproaching100 (Figure2).
Figure 3 showsthat the hydrolysisof BrONO2 on
sulfate aerosolsis effectiveover a wide range of temperatures.
It can be seen that
in absolute
terms
it has a
relatively low rate, underlining the fact that its importance is due to its being part of a catalytic cycle. It
is interesting that even though the sticking coefficient
is not a function of temperature, the absolute rate of
BrONO2 hydrolysisin units of molecules
cm-3 s-• is
temperaturedependent(Figure3). This is becausethe
BrONO2 concentration is very sensitiveto the amount
of NO2 present, which is in turn a function of the tem-
BROMINE
CHEMISTRY
crease in OH and HO2 is in agreement with the recent SPADE observations of OH and HO2 reported
by $alawitchet al. [1994]. The simulationsshownin
Figure 4 used the initial conditionsgiven in table i of
$alawitchet al. [1994]and assumed
14 pptv of BrOy.
$alawitchet al. [1994]explainedthe suddenincreasein
OH and HO2 at sunriseby the heterogeneous
conversion
of HO2NO2 into HONO. However, Figure 4 showsthat
at least part of the sudden sunriseincreasein OH and
HO2 is due to BrONO2 hydrolysisenhancingthe HOBr
concentration.Hansonand Ravishankara
[1995]mentioned that BrONO2 hydrolysisproducesenoughHOBr
during the night to give a releaseof OH at dawn.
When using the HOBr crosssectionsof Orlando and
Burkholder[1995]the inclusionof heterogeneous
bromine
peratureand ozoneconcentration
[Lary,1991];Lary et reactions leads to a slightly lower BrO concentration
al., 1994]. A similareffectis observedin the rate of for a short period immediately after dawn as HOBr is
N205 hydrolysis(Figure3). By comparingthe ratesof photolyzed slightly more slowly than BrONO2. The
BrONO2, N205 and C1ONO2 hydrolysisshownin Figure 3 it illustrates that the reduction in NO x and en-
hancementin HNO3 that occurswhen BrONO2 hydrolysis is included is not due to the hydrolysis of BrONO2
alone, a relatively slow process, but is also due to the
increasein OH releasedby the photolysisof the HOBr
that
is formed.
In agreementwith the findingsof Hanson and Ravis-
decreasein BrO in the short period just after dawn
would not occur if HOBr photolysis was faster than
BrONO2 photolysis. The bulk phasereaction of HOBr
with HC1 proceedsat night causinga slow,but steady,
increaseof BrC1duringthe night (Figure4). At high
latitudes in the cold stratosphereunder volcanic conditions the reaction of HOBr with HC1 proceedsat a nmch
higher rate, leading to a couplingof bromine and chlo-
(seealsoDanilinandMcConnell[1995]).
hankara[1995]it wasfoundthat at temperatures
below rinechemistry
approximately 210 K the fastest stratosphericheterogeneousbromine reaction is the bulk phasereaction of
HOBr with HC1. At the tropopauscandbelow,the high Partitioning
water concentrationsenable the bulk phase reaction of
HOBr with HC1 to proceedvery rapidly.
of Reactive Species
The inclusionof heterogeneousbromine reactionsal-
terstheBr/BrO, C1/C10,NO/NO2 andOH/HO2 ratios
as well as altering the absoluteconcentrationsof these
species. There is an increasein the BrOx, C1Ox and
Diurnal
Cycles
HO• concentrations and a decrease in the NOx con-
centration. The Br/BrO ratio is reducedwhen hetThe hydrolysis of BrONO2 has a marked effect on the
shape of the BrONO2 and HOBr diurnal cycles (Fig-
erogeneousbromine reactions are included due to the
decrease in the NO concentration, and hence the rate
ure 4). Duringthe dayBrONO2is rapidlyproduced
by of the reactionof BrO with NO. The C1/C10 ratio is
the three-body reaction of BrO with NO2, and the hydrolysisof BrONO2 is not fast enoughto competewith
the photolysis of BrONO2. Therefore at mid-latitudes
BrONO2 hydrolysis has a relatively small effect on the
daytime BrONO2 concentration. However, after sunset,
BrONO2 production ceasesand any BrONO2 presentis
converted into HOBr on the timescale of a few hours,
so that at the end of the night little BrONO2 remains
if enhancedaerosolloadingsare present(Figure 4).
BrONO2 would otherwisebe a major BrOy reservoir
reducedwhen heterogeneousbromine reactionsare included due to the decrease in the NO concentration,
and hence the rate of the reaction
of C10 with NO. The
NO/NO2 ratio is reducedwhenheterogeneous
bromine
reactions are included due to the increase in the C10,
BrO, HO2 and CH302 concentrations,and hence, the
rate of their reactionswith NO. The OH/HO2 ratio is
reduced when heterogeneousbromine reactions are included due to the decrease in the N O concentration
the increase
in the C10
concentration.
This
and
decreases
as the reaction of BrO with NO2 goes to completion. the rate of the reaction of HO2 with NO and increases
Consequently,the largest differencein the calculated the rate of the reaction of C10 with OH.
Duringperiodsof enhancedaerosolloadingthe HC1/
BrONO2 concentrationdue to heterogeneousbromine
reactionsoccursjust beforedawn(Figure4).
ClOy ratio is reducedowingto the increasein OH by the
The increase in HOBr is due to the hydrolysis of
BrONO2 during the night causesa suddenincreasein
catalytic hydrolysisof BrONO2 and a correspondingde-
creasein the HC1lifetimeandconcentration
(Figure5).
the OH and HO2 concentrations at dawn as HOBr is
The effect is most pronouncedin the lower stratosphere.
rapidly photolyzed(Figure 4). This suddendawn in-
The decreasein the HC1/C1Oyratio is accompanied
LARY ET AL.: HETEROGENEOUS
ATMOSPHE
,RIC BROMINE
BrONO• + H•O-+ HOBr + HNO•
0
I
2
3
4
CHEMISTRY
1493
HOBr + HCI-+ BrCI + H•O
5
0
240 I , , , , • , , , , I , , ,_, • , , , ;---!--•_-L•_-'•,
_-_'....-'_...
240
I
2
I ....
,,
240 I , , ,,
3
.,.
4
5
......
•240
_
230
230
230
230
:::::::::::::::::::::::::::::::
160 .................
•::•
..........
220
....
•i:•ii220
•
::::::::::::::::::::::::::::::
,•,
700"'"'"'"'"'"'"':':•::'•:•:.
............
:•:•
•' 220
220
600,--:.,,,...
80':::%:
:<>,7
210
.210
210
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
"-''"'-•"-•'"'""••:'"'"•'•"•••:;:
210300',•:'",
"•
:::::;::::::::":•'•"...\•':•
-.=========:=====:==,=====
200
O0 ..::::..
................
.:
:::::::::::::::::::::::::::::::
-":••••••:::':':'"':•:•:::'•':':::-:-:'-.
.......................................
•":-:••
200
::::::::::::::"'::•"'
:•'"•'"?:4<'"•:•-•':•<•:
.........
ß.•,....','•.•....:<,
:•2<•::::' :::?:::
..%:::!::::::.-.:'.'.
...
•oo0
1
.......
II
2
3
200
4
5
0
1
Surface Area
I
2
3
3
4
5
Surface Area
CIONO• + H•O -> HOCI + HNO•
0
2
4
N2Os+ H20 -> + HNO• + HNO•
5
0
I
2
3
4
5
240-I .................................
!.................
..:.!
...............
i...............
...............
i..............
'.....'.......'.....!.....L...!..
!. ' • 240
::::::::::::::::::::::::::::::::::::::
========================================
.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:
::::::::::::::::::::::::::::::
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
::::::::::::::::::::::::::'•/S'•:-•:•
'
-
•
400
i-
300
250 :::•::•
....................
::::::::::::::::::::
....
'- 2oo
'"'••
'"'-'"
:"•"
2•o•:•:?:•:•:•::•:;:A•:'•
•'" •:•:•:•%•:•=::•::•:•*•::•5•:•:•;;•*•:•:•::•:
2•o300--
o .....
•:,.'-,-'.•
-
150
lOO
.• ..................
200
•00
0
I
2
3
4
-
200 -
5
0
Sudace Area
50
200
1
2
3
4
5
Surface Area
Figure 3. The absoluterate of heterogeneous
reactionon sulphateaerosolsat noon in units of moleculescm-3
s-1 asa functionof temperature
in K andaerosol
surfaceareain unitsof pm2/cm3. The resultsshownarefor the
end of a 2-weeksimulationwherean air parcelat 150 mb, 45øN at equinoxwas kept at a giventemperaturewith
a given aerosolsurfacearea present throughout the simulation. Please note that each plot has a different contour
interval.
by an increasein the C10/C10• and HOC1/C10• ra- ing of reactive bromine speciesat cold temperatures.
tios (Figure 5). It wouldbe usefulto havesimultane- For a 2-week simulation, at the equinox at 45øN, inousmeasurements
of HC1, C1ONO2and HOC1 against cluding heterogeneousbromine reactionsincreasedthe
7 value for the hydrolysisof BrONO2 is not tempera-
midnight OC10 column by a factor of just over 2 for
backgroundlevels of sulfate aerosoland by a factor of
turedependent,
the reduction
in theHC1/C10•ratioby
almost
this mechanism is primarily dependent on the aerosol
loading and is not critically dependenton temperature,
latitude or solar zenith angle.
Since including heterogeneousbromine reactions af-
trast, at high latitudes the increasein the OC10 column
which to critically test the photochemistry. Becausethe
3 for enhanced
levels of sulfate aerosol.
In con-
is of the order of 10%. This reinforcesthe findingof
$essleret al. [1995]that the OC10 concentration
is not
a simple function of both C10 and BrO.
fects the fraction of BrO• in the form of BrO it also
affects the
OC10
OC10
concentration.
column at warmer
The
mid-latitudes
increase
in the
due to the inclu-
Gas Phase Catalytic
Ozone Loss
sion of heterogeneousbromine reactionsis much greater
In addition to forming the catalytic cycles A and B
than the increase at colder high latitudes. As will be already described,heterogeneousbromine reactions enseenlater, this is becausethe inclusionof heterogeneous hance the couplingbetween chlorineand bromine chembromine reactions has a smaller effect on the partition- istry whichoccursdue to the two gasphaseC10/BrO
1494
LARY
ET AL.' HETEROGENEOUS
ATMOSPHERIC
BROMINE
CHEMISTRY
1.0
0.005 -
-0.75
_
_
-
rn
_
•
-o.5
-
(.•
0
0.0025-
Cl3
z
o
-
-0.25
_
_
_
_
B,
r,C,
lIB,
,r•,,,
I I I I I ................
','
I .....
_
, , , i IiI i l,.......
0.0
1.o-
.........
I
.....
..•"_
I .....
I .....
' ....
I .....
.............
'i HOBr/BrO
_
0.0
-0.5
_
o
:;
rn
-0.25
0.5-
O
-
T
-
0.0
0.0
0
6
Without
12
18
24
LocalTime (hours)
6
12
18
24
LocalTime (hours)
........... With
37.9øN,214.5K,66.9mb,5 May,Aerosol
area= 6/•m2cm'3.
--
No Het. Br
With Het. Br
--
37.9"N,214.5K,66.9mb,5 May,Aerosol
area= 6/•m2cm•.
I
,
,
I
•
•
I
I
,
,
I
•
,
I
.....
With Het. Br
6
-0.75
0.75 .//?'"%
No Het. Br
37.9øN,214.5K,66.9mb,5 May,Aerosol
area= 6/•m• cm•.
6
_
_
--
'\
5-
_
_
_
4-
0.5•
/
/
/
\\
_
/
-
•
_
O
-0.25
•32
2-
_
_
1
_
_
ß
'
90
'
I
60
'
'
0.0
I
30
30
60
90
Solar ZenithAngle(degrees)
0
0
90
60
30
30
60
90
SolarZenithAngle(degrees)
Figure 4. The effect of heterogeneousbromine reactions on sulfate aerosolson the shapesof diurnal cycles.
The solid line is for a simulation which includesno heterogeneousbromine reactions,and the dashedline is for a
simulation which does include heterogeneousbromine reactions. The initial conditionswere taken from Table i of
$alawitchet al. [1994].
catalytic ozone destructioncycles. The chain length lightedby Burkholderet al. [1995]and is alsoexamof the
of thesetwo gasphaseClO/BrO cyclesis increasedby inedby Lary [1995]togetherwith a description
nearlyan orderof magnitudefrom 105to 10o whenhet- gasphasecatalytic brominecycles.The BrO/NO2 cyerogeneousbromine reactions are included.
cle only leads to ozone lossif the products of BrONO2
The chainlengthof the BrO/H02 catalyticcycleis photolysis are Br and NO3.
To examine the effect of heterogeneousbromine reapproximately
104and is not significantly
affectedby
heterogeneousbromine reactions. However, there is a
actions on lower stratosphere,mid-latitude, ozoneloss,
decreasein the importanceof the BrO/N02 ozonede- a set of idealized model simulationswere performed.
structioncatalytic cycledueto the hydrolysisof BrON02 The numerical model is called AUTOCHEM and is deon sulfate aerosols. The BrO/N02 cycle was high- scribedin Lary et al. [1995]and Lary [thisissue].In
LARYETAL.:HETEROGENEOUS
ATMOSPHERIC
BROMINECHEMISTRY
No H©tBr - Background
o
I
I
WithH©tBr - Background
No H©t Br - Voloanio
0.5
I
- • - With Het Br - Volcanic
0.75
I
I
I
I
I
1.0
I
1495
I
I
I
0.01
I IIIIII
I
I
0.1
I IIIIII
I
I
I II
'•'_ 30.0
-27.5
27.5-_
_
_
-25.0
25.0-_
_
-
-22.5
20.0--
-20.0
_
17.5--
-17.5
_
_
_
15.0--
115.0
_
_
_
12.5-
I
I
I
0.5
I ! !tll
0.75
I" I I I IIII I
1.0
HCI/CIO,
I
I I I
0.01
- 12.5
0.1
HOC!/CIO,
Local Noon, Mid-latitudeat Equinox.
No Het Br- Background
o
0.07
WithHet Br - Background
No Het Br - Volcanic
0.1
0.2
- • - With Het Br - Volcanic
0.4
0.01
0.1
30.0
30.0
_
_
_
_
_
_
_
27.5-
-27.5
_
_
_
_
_
_
25.0-
--25.0
_
_
_
•,
_
-
-
-
-20.0
a=
20.0-.'•
_
<•
_
_
_
_
_
17.5-
-17.5
_
_
_
o
_
_
_
_
_
15.0--
-15.0
x
x
_
_
x
_
_
,,
_
_
_
12.50.04
I
0.07
0.1
0.2
CIONO•/CIO,
0.4
I
0.01
I I II II I
12.5
0.1
CIO/CIO•
LocalNoon, Mid-latitudeat Equinox.
Figure5. Theeffect
ofheterogeneous
bromine
reactions
onsulfate
aerosols
onthecalculated
chlorine
partitioning.
The
solidlineisfornoheterogeneous
bromine
reactions,
andthedashed
lineisforwithheterogeneous
bromine
reactions.
Thecirclesarefor a volcanicaerosolloading.
thesesimulations
the temperatureof a stationarybox
When heterogeneousbromine reactionsare not conwasvariedbetween210K and 198K (Figure6). This sidered,the ozoneconcentration
has decreased
by 283
range of temperatures was chosenso that chlorine and ppbvfor background
levelsof aerosolandby 1430ppbv
bromineactivationmight occuron cold stratosphericfor volcaniclevelsof aerosolby the end of this idealaerosols
withoutPSCsforming. For the first 10 days ized 100-daysimulation(Figure 6). When heterogeof the simulationthe temperaturewaskept at 205 K. neousbromine reactionsare considered,the extra chloForthenextten daysof the simulation
the temperature rine and bromine activation which occurs leads to an
waskept at 200K. Then for 60 daysthe temperature additionalozonedepletionof approximately60 ppbv
waskeptat 198K. The partitioning
of brominespecies for background
levelsof aerosol(an increase
of 21%)
duringthesesimulations
are shownin Figure6.
and an additional 62 ppbv for volcaniclevelsof aerosol
1496
LARY
ET AL.: HETEROGENEOUS
ATMOSPHERIC
BROMINE
CHEMISTRY
', I ', ', ', ', ', ', ', ', I ', ', ', ', I ', ', I
82.5-
•9o
--o.?•
,-
0
Z
.'•
•'
II• '77'.5,
0.•, 9
N
ffi
•.o,
Zonith ^nglo
................... iii
0.0
I
• I
• I
• I
20
30
• • • I
'
I
'
I .•
I
• I
80
90
•
0.0
0.•,
0.0
0
10
20
30
40
50
60
70
80
90
100
10
Day of the run
--
No Het Br. - Background
....... WithHet Br. - Background
40
50
60
70
1 00
Day of the run
o
No Het Br. - Volcanic
......
With Het Br. - Volcanic
Onlythe noonvaluesare shownfor the sake of clarity.
l*igure 6. The effectof heterogeneous
brominereactionson the calculatednoonpartitioningof BrOy for idealized
box model simulationsfor an air parcel at 60øN at 55 mb with backgroundand volcanicaerosolloadings. The
simulationsstart in autumn and go through to spring. The solidline in eachcaseis for no heterogeneous
bromine
reactions, and the dashed line is for with heterogeneousbromine reactions. The circlesare for a volcanic aerosol
loading.Note, that only the noonpartitioningis shownbecausethe amplitudeof the diurnalcyclesis solargethat
it hampersa comparison
of the two simulations
oversucha long(100days)period.
(an increase
of 4.3%). Heterogeneous
brominereactions PSC is encounteredalso leads to a correspondingdrop
therefore contribute to mid-latitude, indeed all latitude,
ozone loss in the lower stratosphere. Because the hy-
in the BrONO2
and HOBr concentrations.
When a
model simulation is performed with an air parcel in a
drolysis of BrONO2 is not temperature dependent, it PSC for extended periods of time, the calculated HBr
enhancesozonelossthroughout the year and at all lat- and BrONO2 concentrationsare virtually zero, and the
itudes.
HOBr concentration is very small. As a result, after an
Figure 7 showsthat when heterogeneousbromine re- extended period in PSCs, the fastest PSC reaction is
actionsare included there is an increasein C1Oz by day that of HC1 with HOBr. However, this has a rate which
20, as well as an increase in the HOx and BrOx con- is between I and 2 orders of magnitude less than the
centrations. There is a coupling between chlorine and analogousbulk phase reaction in cold sulfate aerosols
bromine chemistry which leads to chlorine as well as shownin Figure 3.
bromine activation. There has also been a drop in the
Consequently,whether or not heterogeneous
bromine
NOx and HC1 concentrations.This is in agreementwith reactions are included, when PSCs are present there is
the findingsof Danilin and McConnell[1995].
relatively little bromine present as BrONO2 and HOBr
The additional chlorine and bromine activation which
to react heterogeneously,and so a large fraction of the
occurs is due to heterogeneousbromine reactions on reactive bromine present is converted into BrC1. HowPSCs is not as significantas that due to heterogeneous ever, a heterogeneousreaction involving BrC1 on PSCs
bromine reactions on sulfate aerosols. This is not a recould be significant. At present the authors know of no
flection of the sticking coefficientsbut underlinesthe heterogeneousBrC1 reactions. In the presenceof sunfact that the bromine species are much shorter-lived light BrC1 is of courserapidly photolyzedto releaseBr
than their chlorine counterparts. Since BrONO2 and and C1, so any heterogeneousBrC1 reaction would be
HOBr have such short lifetimes, the drop in the NO2 most important at night.
and HO2 concentrationswhich occurs immediately a
The mostnoticeableeffectof includingheterogeneous
LARY ET AL.: HETEROGENEOUS ATMOSPHERIC BROMINE CHEMISTRY
0
10
20
30
40
50
60
70
80
90
100
10
20
30
40
50
60
70
80
90
1497
100
1.0'1
7.0'1
4.0'1
CI/CIO•
CIO/CIO•
HCI/CIOy
4
•
CIONO•/CIOy
Z
CIO,
BrO,
NO,
HO,
001
"1"
0
10
20
30
40
50
60
70
80
90
100
10
20
Day of the run
•
No Het Br. - Background
.......
With Het Br. - Background
30
40
50
60
70
80
90
100
Day of the run
o
No Het Br. - Volcanic
......
With Het Br. - Volcanic
Onlythe noonvaluesare shownforthe sakeof clarity.
Figure 7. The effectof heterogeneous
bromine
reactions
on the calculated
noonpartitioning
of ClOyandthe
NOx, HOx, C1Oxand BrOx volumemixingratiosfor idealizedboxmodelsimulations
for an air parcelat 60øNat
55mbwithbackground
andvolcanic
aerosol
loadings.
The simulations
startin autumnandgothroughto spring.
Thesolidlinein eachcaseisfornoheterogeneous
brominereactions,
andthedashed
lineis for withheterogeneous
brominereactions.The circlesarefor a volcanic
aerosol
loading.Note,that onlythe noonpartitioning
is shown
becausethe amplitudeof the diurnalcyclesis so largethat it hampersa comparison
of the two simulationsover
sucha long (100 day) period.
voir. Consequently, simply from concentration considerations, the heterogeneousreactions of BrONO2 and
resentsa smallfractionof BrOy, after prolongedPSC HOBr with HC1 are important. In contrast, the reacprocessingthe enhancedlevelsof C1lead to an enhance- tions of HBr with HOC1 and C1ONO• are key reactions
ment in methane oxidation and consequentlythe con- in the model becausethey keep the HBr concentration
centration of HO•. The enhanced HO• concentration small. It would be valuable to have more laboratory
can lead to an enhancedproduction of HBr, suchthat if studies of these reactions. It is important to note that
heterogeneousbromine reactions are not included, more the model description of bromine partitioning would
bromine reactionsin the model is the dramatic drop in
the HBr concentration.AlthoughHBr usuallyonly rep-
than 20% of BrOy can be in the form of HBr. When changedramatically if there is a significantsourceof
there is rapid removalof HBr by heterogeneous
bromine
reactions,HBr doesnot representa significantfraction
HBr which existsin reality which has not been included
in the model. The model currently predicts that where
aerosols are present the HBr concentration should be
of BrOy.
The relative importanceof the variousheterogeneous small. It would therefore be valuable to have observabromine reactions on PSCs varies with the conditions, tions of HBr with which the model could be compared.
but the most important heterogeneousreactionstend If these observationsshowthat HBr is in fact presentin
to be the mixed halogen reactions, particularly reac- appreciable quantities in regionswhere sulfate aerosols
tions (2), (3) and (5)in Table 1, namely,the hetero- are present, it means that there is an important addigeneousreactions of of HC1 with HOBr and BrONO•.
and HBr with HOC1. This is just a reflection of the
tional
source of HBr.
In the next sectionthe wide range of situations enfact that the most abundantbrominespecieswhichun- counteredon the 475 K isentropicsurfaceduring early
dergo heterogeneousreactionsare generally BrONO•. 1993 are considered. Associatedwith the wide range
and HOBr, and that HC1 is an important chlorinereser- of temperaturespresenton the 475 K isentropicsurface
1498
LARYET AL.:HETEROGENEOUS
ATMOSPHERIC
BROMINECHEMISTRY
T on the
475.0
PV on the 475.0
K isentrope.
ECMWFanalysis - 22 February 1993 - Time'12.00 GMT
Contour
from
190 to 225 by 5
K isentrope.
ECMWFanalysis - 22 February 1993 - Time'12.00 GMT
Contour
from
10 to 60 by .5
Figure8. TheECMWFtemperature
andPV analysis
forthe475K isentropic
surface
at 12GMTforthe
February 22, 1992.
(Table2). Simulations
SC1(without
thereare quitelargevariations
in pressure,
with much wereperformed
heterogeneous
brominereactions)
andSC2(withhetlowerpressures
in the coldregions.
erogeneous
bromine
reactions)
assumed
a 0%yieldfor
Isentropic Model Simulationsof the
Winter
of 1993
HBr fromthe reactionof HO2 with BrO (reaction(lb)
discussed
by Lary[thisissue]
andreferred
to asreaction
(lb) hereafter).
Simulations
SC3andSC4weresimilar
Relative to the interannual variability observed in
the northern hemispherelower stratospherictemperatures, the winter of 1992-1993can be classedaslongand
cold. PSCs werepresentin the lowerstratospherefrom
to experiments
SC1andSC2respectively,
but reaction
(lb) wasassumed
to havea 1%channel.Finally,experiments
SC5andSC6weresimilarto SC1andSC2,
respectively,
but with an enhanced
aerosolloadingof
15/•m2/cma.
earlyDecember1992until late February1993[ChipperBy comparingexperiments
SC1 and SC2it wasseen
field,1994].DuringJanuarythe vortexwascentered
on
brominereactions
the pole, but during February,distortionsto the vor- that inclusionof the heterogeneous
modifiedthe partitioningof BrOyspecies
tex ensuredthat air parcelstraveling around its edge significantly
made excursionsto sunlit latitudes. For example, Fig-
(notshown).As alreadyobserved,
including
heteroge-
ure 8 showsthe potentialvorticity(PV) and tempera- neousbrominereactionsgivesadditionalozonelossat
ture fields at 475 K on February 22, 1993. On this day all latitudes. The peak additionalaccumulatedozone
the polar vortexis distortedand descends
overEurope. losspredictedover the 2-month integrationswas ap21ppbvat northernmidlatitudes
(1.0%,
The lowesttemperaturesof 190 K occurto the east of proximately
Figure9(a)). The additionallossdecreased
at northern
Greenland,permitting PSCs to form in this region.
This section considers the effect of heterogeneous highlatitudesand wasa minimumat the centerof the
brominereactionsby includingthemin a two-dimensionalnorthpolarvortex.Thereforethe mostimportanteffect
brominereactionsin expersinglelevel isentropicmodel, SLIMCAT,describedby of includingheterogeneous
Chipperfield
et al. [1993, 1994, 1995], Chipperfieldiment SC2 is due to the reactions on sulfate aerosols
[1994]and Lary et al. [1995]. This modelwasrun
with a minor effect due to reactions on PSCs.
ExperimentsSC5 and SC6 were initialized with an
at an altitude of 475 K using winds analysedby the
loadingof 15/•m2/cma, corresponding
to conEuropeanCentrefor Medium-RangeWeatherForecasts aerosol
(ECMWF) from i January,1993,until i March,1993. ditions after a moderate volcanic eruption. Figure 10
shows the distribution of bromine species from the
model also has a detailed chemical schemewith the heterogeneousbromine reactions two-diemsional model on February 22, 1993, from run
The two-dimensional
included
as described
above.
Six model simulations
SC6(withheterogeneous
brominereactions).
Thetotal
LARY ET AL.: HETEROGENEOUS
ATMOSPHERIC
BROMINE
CHEMISTRY
1499
Table 2. Summary of SLIMCAT Model Experiments
Run
Heterogeneous
Aerosol Area
Quantum Yield
pm2/cm•
of reaction(1•)
BromineReactions
SC1
no
1
0
SC2
yes
I
0
SC3
no
I
0.01
SC4
yes
I
0.01
SC5
no
15
0
SC6
yes
15
0
abundance
of BrOyat the centerofthevo•'texis around der volcanic aerosol loading. In early March the dif20 pptv. In the sunlit vortex the main bromine species ference in ozone between run SC6 and SC5 is around
is BrO with a peak mixingratio of 9 pptv. Figure 10(b) 25 ppbv in the northern polar vortex, around 40 ppbv
showsthat the main nighttimereservoirof BrOy in the at northern mid-latitudes, and interestingly, a maxiPSC-processedvortex is BrC1 with peak mixing ratio nmm of 150 ppbv during summerat the high southern
of around 15 pptv. Owing to the rapid hydrolysis of latitudes(Figure9(b)). This is due to the enhanced
BrONO2 on sulfate aerosols,HOBr is the main night- HO• concentration
(FigureiX(e)) whichhasbeenprotime reservoir outside the PSC-processedregion, but ducedby the catalytichydrolysisof BrONO2 (cycleA).
the mixing ratio inside the vortex is lower. The max- This finding emphasizesthat heterogeneous
brominereinmm mixing ratio of HOBr in the sunlit polar vortex actions can catalytically enhance ozone loss at all lati-
is around6 pptv. Figure 10(d) showsthe distribution
tudes
in all seasons.
of HBr from run SC6 with heterogeneousbromine reFigure 11 showsthe differencein somebrominespecies,
actions included. The highest values of only 0.006 pptv C1Oz and HO2 between SC5 and SC6. As was found
occur within the PSC region. Without heterogeneous in the earlier idealized sinrelations,in regions of PSC
brominereactions(runSC5,not shown)theHBr mixing processingthere is an almost completeremoval of HBr
ratio is much higher but still only 0.3-0.5 pptv at mid- (not shown). HBr has also been completelyremoved
latitudes, decreasinginside the polar vortex to around at mid-latitudes. The only region where HBr remains
0.2 pptv (not shown). However,thesemixing ratios is coincident with the PSC region between Iceland and
are still low, and HBr is only a minor component of Scandinavia(Figure 10). The HBr has beenreduced
the total inorganic bromine at this altitude. The two- by up to 0.5 pptv. The bromine releasedfrom the HBr
dimensional model has a simpler CH4 oxidation scheme has been photochemically repartitioned among the mathan the box model and so does not produce as nmch jor bromine species, as would be expected from their
HBr asthe boxmodelin PSC regions(seeabove).The short lifetimes. On February 22 the largest increase
low abundance of HBr and BrONO2 predicted in run
in BrO between runs SC6 and SC5 of 2.5 pptv also
SC5(andalsorun SC1)in the Arcticpolarvortexlimits occurs at high southern latitudes, while in the norththe effect of the heterogeneousbromine reactionswhich
ern hemisphere the increase is similar to that seen be-
occur on PSCs.
tweenthe backgroundaerosolruns SC2 and SC1 (not
shown). The heterogeneous
removalof HBr has led
Comparison of run SC5 and run SC6 shows the effect of including heterogeneousbromine reactions un03
(ppmv).
SLIMCAT CTM
SC2-SC1
L:
03
SLIMCATCTM
1.000
22/ 2/
to a small increasein BrO of around 0.1 pptv inside
93 Time:12.00
(ppmv).
SC6-SC5
L:
1.000
22/ 2/ 93 Time:12.00
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....
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......
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a•'-'---::'---.
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.............
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""""
••'"'•'"
":"
''''•....
' '.,:' .
.:......----..•
..........
...........................
•. ,...•-•.•.:••
Contour from -.018 to -.006 by .002 (X 1)
[
I•i!!i!ii:..:iii!ii!!i•i!iii:..:!iiiii•iiiiii
.......... •"'"'"'•'"'•,:"'""•'1••"-----•:::•'-----•-'-••••-----;-
-0.018
-0.016
-0.014
-0.012
-0.01
-0.008
Contour from-.13
to -.01 by .01 (X 1)
....
-0.006
-0.13
-0.12
-0.11
-0.1
-0.09
-0.08
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
Figure 9. Difference
in the ozonemixingratio (ppmv)nearthe endof the 2-monthsimulations
between(a) SC2
and SC1and (b) SC6and SC5. Differentcontourintervalsare usedin Figures9(a) and 9(b).
1500
LARY
BrO
SLIMCATCTM
:
ET AL.: HETEROGENEOUS
ATMOSPHERIC
(pptv). L:
1.000
EXP.06
22/ 2/ 93 Tirne:12.00
:
.
:
-
.
:
BROMINE
CHEMISTRY
BrCl
(pptv). L:
1.000
SLIMCATCTM EXP.06
22/ 2/ 93 Time:12.00
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93 Time:12.00
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:::::::::::::::::::::::::::::::::::::::
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1
2
3
4
5
6
HOBr
SLIMCATCTM
0.001
7
(pptv).
EXP.06
0.002
0.003
0.004
0.005
0.006
L:
1.000
22/ 2/ 93 Time:12.00
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Contour from 4 to 15 by 1 (X 1)
I
[iiiii•iE;i!i•i•i:iE•ii•i;i!!•iEE•!•i•i;i•
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4
5
6
7
8
9
10
-•-
-•
11
12
13
14
15
Figure 10. Mixingratio (pptv) of (a) BrO, (b) BrC1,(c) BrONO•.,(d) HBr and(e) HOBr at 475K onFebruary22,
1993,fromthe two-dimensional
modelrunwith heterogeneous
brominereactions
andvolcanicaerosol
loading(SC6).
the polarvortex.As wasobserved
in the earlierideal- earlystageof the night. The conversion
of BrONO9.
is
ized experiments,includingheterogeneous
brominere- alsorapid duringthe day and accountsfor the reducactionsslightlyreducesthe BrO concentration
immedi- tion of 7 pptv of BrONO•.at high southernlatitudes.
ately after dawn(Figure11(a)) as HOBr is photolyzed As well as the increasein BrO thereis an increaseof
at a slightly slowerrate than BrON09.. The hetero- 4 pptv in HOBr. Underhighaerosolloadingthe ratesof
geneoushydrolysisof BrONO•. on sulfateaerosolseffi- the heterogeneous
brominereactionscompetewith the
ciently convertsBrONO9. to HOBr. With the higher rapid daytime gas phasereactionswhich normally con-
aerosolloadingthe nighttime conversion
of BrONO•. trol the BrOy partitioning. Figure 11 showsthat this
to HOBr is more rapid and effectivelycompleteat an conversion
is most effectiveat night. Justbeforesunrise
LARY ET AL.' HETEROGENEOUS ATMOSPHERIC BROMINE CHEMISTRY
Br0
(pptv). L:
1.000
SLIMCAT
CTM SC6-SCõ
22/ 2/ 93 Ttme:12.00
Br0N02
SLIMCAT
CTM
Contour from -14 to -2 by 2 (X 1)
Contour from -1.4 to 2.2 by .4 (X 1)
[::::::!:•:i:i:.::•:•:•
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:--'•' --'.-'.::•-'.::
:--:
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-1.4
-1
-0.6
-0.2
0.2
0.6
J
....
1
1.4
1.8
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-14
2.2
HOBr
(pptv). L:
1.000
SLIMCAT
CTM SC6-SC5
22/ 2/ 93 Time:12.00
-12
[i!ii•:3::!::i:i:i:•:i::i:i::
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:j:..----------.-.-.--.- -
2
4
6
8
H02
SLIMCAT CTM
12
-8
-6
-4
-2
Contour from .01 to .05 by .01 (X 1)
;:L=-
10
-10
ClOx
(ppbv). L:
1.000
SLIMCAT
CTM SC6-SC5
22/ 2/ 93 Time:12.00
Contour from 2 to 14 by 2 (X 1)
j
1501
-....
J
J•:-:i•ii::3::i:•:i:i:i:•:i:-•i:.
•-':'-'!-'.::i:i•-'Y.-•31:'
................
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(pp[v).
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0.01
L:
0.02
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0.03
0.04
0.05
1.000
22/ 2/ 93 Time:12.00
Contour from .2 to 3 by .4 (X 1)
I
[•!•i•i•i•i•i•!•i•!•i•i•i•i[
...............
0.;2
0.6
•"'">' .......... • ..... ..•'"'•"•'
1
1.4
••••----=•-:---::
1.8
2.2
2.6
..... ---
-
3
Figure 11. Difference
in mixingratio(pptv)between
thetwo-dimensional
modelrunwithheterogeneous
bromine
reactions
minusrunwithout(SC6-SC5)
for (a) BrO, (b) BrONO2,(c) HOBr (d) C1Oxand(e) HO2at 475K on
22 February1993. The simulationsuseda volcanicaerosolloading.
there is 12 pptv more of HOBr in run SC6. During the
daytime, photochemicalreactions rapidly partition the
up to 2.2 pptv just after sunriseand duringpolar day
BrOy species
andproduceBrONO•, andsothe hetero-
The catalyticincreasein OH dueto the hydrolysisof
BrONO• leads to a reduction in the HC1 lifetime and
geneousreactionshave a much smaller effect than the
gas phasereactionswhich normally control the BrOy
partitioning. There is up to 50 pptv more C10• in run
SC6 at high southern latitudes and in PSC regionsin
the north polar vortex. Finally, HO• has increased by
in the southern hemisphere.
the HC1/C10• ratio. This can be seenin Figure 12
whichshowsthe HC1/C10• ratio for simulations
SC2
and SC6. The substantial reduction in the mid-latitude
HC1/C10• ratio is particularlynoticeablein the simu-
1502
LARY ET AL.: HETEROGENEOUS
HC1/ClOy
SLIMCAT CTM
::
'
EXP.02
22/ 2/
ATMOSPHERIC
HC1/ClOy
SLIMCATCTM
93 Time:12.00
• •......:_•j
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---:> : •.•, :.• •
i•_.
BROMINE
'
CHEMISTRY
EXP.06
22/ 2/
93 Time:12.00
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.-.:
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' .:Y.{:':.'.Y::.!:::::':':.'..-.::?--::!':-!-::'::-'!:
:
Contour from .1 to .8 by .1 (X 1)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
:
Contour from .1 to .8 by .1 (X 1)
I
0.1
:
[•i•i•i•i•i•i•½•f•½t'"'"'•:':<'•-::•:•:.:.
:.-:•-:•
'"••-="-:-="-••---:---•::•½---••-•-:-_''-.................
'
t•" • ............
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Figure 12. The HC1/C1Oy
ratiofor simulations
SC2(background
aerosol
loading)andSC6(enhanced
aerosol
loading)at 475 K on February22, 1993.
lation with a volcanicaerosolloading(SC6). In agree- significant effect, although there is no evidencefor this
mentwith the resultspresented
earlier,the HC1/C10• channel from laboratory studies. A review of the kiratio is approximatelybetween0.4 and 0.6 throughout neticmeasurements
of (lb) is givenby Lary[thisissue].
much of the mid-latitudes, as opposedto a value of Althoughthe workof Melloukiet al. [1994]showsthat
between approximately 0.7 and 0.8 in simulationsSC1 reaction(lb) is in fact very slow,a comparison
of SC1
and
SC2
with
SC3
and
SC4
(not
shown)
illustrated
that
and SC2. For simulationSC6 the HC1/C10• ratio is
also low in the summer hemisphere,with valuestypi- an additional sourceof HBr would substantiallyaffect
cally between 0.15and 0.6.
our view of the partitioning of bromine species.
As discussed
by Lary [thisissue],it is unlikelythat
Thesesimulationswith the two-dimensional
isentropic
reaction(lb) proceeds.However,to investigatethe pos- model for early 1993 have confirmed the results from the
sible importance of such a source of HBr simulations
idealized box model runs above. The effect of the het-
SC3 and SC4 were performed. A Colnparisonof ex- erogeneousbromine reactions on PSCs is small. The
perimentsSC1 and SC3 showedthe effectof including most important heterogeneousbromine reaction is the
reaction (lb) with a 1% channel. Run SC3 has less
polar ozonedestructionthan run SC1, the largesCdifferenceat the end of the run being23 ppbv (around1%,
not shown)at the edgeof the polar vortex. This is a
larger effect on polar O3 than that from includingthe
hydrolysis of BrONO2.
Under nonvolcanic conditions
the reactionled to an additionalO• depletionat northern mid-latitudesof up to 18 ppbv during the 2-month
modelrun. With a volcanicaerosolloadingthe northern mid-latitudeozonedepletionincreasedto 40 ppbv,
heterogeneousbromine reactions alone. The inclusion whilein the southernhemisphere,
significantadditional
of reaction(lb) increasedHBr in the polar vortexby ozonedepletionof over 130ppbv was producedalong
around2 pptv (and by up to 4 pptv at mid-latitudes) with changesto C10 and HO2.
at the expenseof BrOx (BrO + BrC1). This decrease
in BrO reducesthe ozonedepletion.
Summary
When reaction (lb) is includedin the model, the
additional inclusionof the heterogeneous
bromine reFigure 13 summarizesthe main gas phase and hetactionshad a larger effect in the polar region. The erogeneousatmosphericphotochemistryof bromine deincreasein HBr due to reaction(lb) enablesthe het- scribedin this paper m•d the compmfionpaper Lary
erogeneousreactions to recycle the additional HBr into [thisissue].
BrOx. The largestdifferencein accumulated03 lossbeThe effectsof heterogeneous
brominereactionson sultweenrunsSC4andSC3is 45 ppbv(around2%) at the fate aerosols
andpolarstratospheric
clouds(PSCs)have
edgeof the polar vortex in early March. The additional been examinedby usinga range of photochemicalmodlossat mid-latitudes is around 20 ppbv, similar to but els under a wide variety of conditions. The most imslightly less than the difference between runs SC2 and portant aerosolreactions were found to be the hydrolSC1above.However,the additionaldepletioninsidethe ysis of BrONO2, and, for temperatures below 210 K
polar vortex is now greater than that at mid-latitudes. and/or high water concentrations,
the bulk phasereThis increaseoffsetsthe reducedozonedepletiondue action of HOBr with HC1. Thesereactionstake part
to the inclusionof reaction(lb) as the levelsof BrO in two catalytic ozonedestructioncycles. Each cycle
were similar betweenruns SC2 and SC4. Therefore, destroysthree ozone moleculesper cycle and enhance
with only a 1% channel,reaction(lb) wouldhavea the HO•, BrOw, C10• and HNO3 concentrationswhile
LARY
ET AL.: HETEROGENEOUS
,
O(aP),
ATMOSPHERIC
BROMINE
CHEMISTRY
1503
OH
NO,
O••
- o(aP)
IiO•
OH
/ \\\
HCHO
/
HC1
C10
h•,
O(3p)
BrC•
Figure 13. A schematicof atmosphericbromine photochemistry.
ment of Chemistry and the Department of Applied Mathematics and Theoretical Physics. This work forms part
the effectiveness
of the two gasphaseC10/BrO cycles of the NERC U.K. UniversitiesGlobal AtmosphericModellingProgramme.M.P. C. thanksPascalSimonof CNRM,
whosechain lengths can increaseby an order of magni- Toulouse,for help with SLIMCAT.
tude for enhanced aerosolloadings due to the hydrolysisof BrONO2. The catalytic BrONO2 hydrolysiscycle References
is shown to be important for lower stratosphere ozone
Abbatt, J.P. D., and M. J. Molina, The heterogeneousrelossat all latitudes, with a chain length of greater than
action of HOCl+HCl-->C12+H20
on ice and nitric-acid
reducingthe HC1 and NOx concentrations.The addi-
tional C1Ox and BrO.• activation that results enhances
103 for enhancedaerosolloadings. The enhancement
in the OH concentration can substantially reduce the
HC1 lifetimeand HC1/C1Oyratio in the lowerstrato-
trihydrate: reaction probabilities and stratosphericimplications, Geophys.Res. Left., 19 (5), 461-464, 1992.
Abbatt, J.P. D., Heterogeneousreaction of HOBr with HBr
and HC1 on ice surfaces at 228 K, Geophys. Res. Left., 21
sphere.The nighttime hydrolysisof BrONO2 leadsto a
(8), 665-668, 1994.
nighttimeenhancementof HOBr, sothat at dawnHOBr Brimblecombe, P., and S.L. Clegg, The solubility and bephotolysisleads to a rapid rise in OH and HO2 as has
haviour of Acid gases in the marine aerosol, J. Atmos.
recentlybeenobservedby $alawitchet al. [1994].
Chem., 7, 1-17, 1988.
For a simulation
of the winter
of 1992-1993
on the
475 K isentropicsurfaceconstrainedby ECMWF analyses, under nonvolcanic conditions the hydrolysis of
BrONO2 led to an additional O3 depletion at northern mid-latitudes of up to 18 ppbv during the 2-month
model run. With a volcanic aerosol loading the northern mid-latitude ozone depletion increasedto 40 ppbv,
while in the summer of the southern hemispheresignificant additional ozone depletion of over 130 ppbv was
producedalong with changesto ClO and H02.
On PSCs the most important heterogeneousbromine
reactions are the mixed halogen reactions of HC1 with
HOBr and BrONO2 and HBr with HOC1 and C1ONO2.
Acknowledgments.
The authors wish to thank J. A.
Pyle for his support, R. A. Cox and J. Sesslerfor useful
conversations and D. R. Hanson• A. R. Ravishankara, J.
J. Orlando, J. B. Burkholder and M. Danilin for making
their results available to us before publication. The Centre
for Atmospheric Scienceis a joint initiative of the Depart-
Burkholder, J. B., A. R. Ravishankara and S. Solomon, UV
visible and IR absorption cross-sectionsof BrONO2, J.
Geophys.Res., 100 (D8), 16,793-16,800, 1995.
Chipperfield, M.P., D. Cariolle, P. Simon, R. Ramaroson
and D. J. Lary, A three-dimensional modeling study of
trace speciesin the Arctic lower stratosphere during winter 1989-1990, J. Geophys. Res., 98, 7199-7218, 1993.
Chipperfield, M.P., D. Cariolle, and P. Simon, A 3D chemical transport model study of chlorine activation during
EASOE, Geophys. Res. Left., 21, 1467-1470, 1994.
Chipperfield, M.P, A 3D Model comparisonof PSC process-
ing during the Arctic winters of 1991/92 and 1992/93,
Ann. Geophys., 12, 342-354, 1994.
Chipperfield, M.P, J. A. Pyle, C. E. Blom, N. Glatthor, M.
H;Spfner, T. Guide, C. Piesch and P. Simon, The variability of C1ONO2 and HNOa in the Arctic polar vortex: Comparison of Transall MIPAS Measurements and
3D Model Results, J. Geophys. Res., 100, 9,115-9,129,
1995.
Cox, R. A., A. R. MacKenzie, R. H. Muller, T. Peter and P.
J. Crutzen, Activation of stratospheric chlorine by reac-
tions in liquid sulfuric acid, Geophys.Res. Left., 21 (13),
1,439-1,442, 1994.
1504
LARY ET AL.: HETEROGENEOUS ATMOSPHERIC BROMINE CHEMISTRY
Danilin, M. J., and J. C. McConnell, Stratosphericeffects McConnell, J.C., et al., Photochemicalbromine production
of bromine activation on/in sulfate aerosol,J. Geophys.
implicated in Arctic boundary-layerozonedepletion,NaRes., 100 (D6), 11,237-11,243,1995.
ture, 355 (6356), 150-152,1992.
DeMote, W. B., et al., Chemical kinetics and photochemical Mellouki, A., R. K. Talukdar and C. J. Howard, Kinetics of
data for use in stratosphericmodeling, Evaluation Numthe reactions of HBr with O3 and HOa: the yield of HBr
ber 10, Jet Propul. Lab., Pasedena, Calif., Publ. 9•-26,
from HOa + BrO, J. Geophys.Res., 99 (Dll), 22,9491994.
Fan, S.M., and D. J. Jacob, Surfaceozonedepletionin Arctic
spring sustainedby bromine reactionson aerosols,Nature,
359, 6395, 522-524, 1992.
Hanson, D., and K. Mauersberger,Laboratory studiesof the
nitric acid trihydrate: Implications for the south polar
stratosphere, Geophys.Res. Left., 15, 855-858, 1988.
22,954, 1994.
Orlando, J.J., and J. B. Burkholder, Gas phaseUV/visible
absorption spectra of HOBr and BraO, J. Phys. Chem.,
99 (4), 1,143-1,150,1995.
Salawitch, R. J., et al., The diurnal-variation of hydrogen,
nitrogen, and chlorine radicals: implications for the heterogeneousproduction of HNO=, Oeophys._Res.Lett., œ1
(23), 2,551-2,554,1904.
Hanson,D.R., and A. R. Ravishankara,Investigationof the
reactive and nonreactiveprocesses
involvingC1ONO2and Sessler, J., M.P. Chipperfield, J. A. Pyle and R. Toumi,
HC1 on water and nitric-acid dopedice, J. Phys. Chem.,
StratosphericOC10 measurementsas a poor quantitative
indicator of chlorine activation, Geophys.Res. Lett., œœ
96 (6), 2682-2691,1991.
Hanson, D.R., and A. R. Ravishaukara, Heterogeneous
(6), 687-690, 1995.
chemistryof HBr and HF, J. Phys. Chem.,96 (23), 9441- Toumi, R., BrO as a sink for dimethylsulfide in the marine
9446, 1992.
atmosphere,Geophys.Res. Lett., œ1(2), 117-120, 1994.
Hanson, D.R., and A. R. Ravishankara, Heterogeneous WMO, ScientificAssessmentof StratosphericOzone: 1991,
chemistry of bromine speciesin sulfuric acid under stratoWMO Global Ozone Research and Monitoring Project,
Report œ5,Geneva, Switzerland, 1992.
sphericconditions, Geophys.Res. Lett., 2œ(4), 385-388,
1995.
Lary, D.J., Photochemical studies with a 3D model of the
atmosphere,PhD. thesis, Cambridge Univ., Cambridge,
England, 1991.
D. J. Lary, M.P. Chipperfield and T. Lenton, Centre
Lary, D.J., Gas Phase Atmospheric Bromine Photochemfor Atmospheric Science,Department of Chemistry, Camistry, J. Geophys.Res., this issue.
Lary, D. J., J. A. Pyle and G. Carver, A 3D model study bridge University,Lensfield Road, Cambridge, CB2 1EW,
atm.cm.cam.ac.uk)
of nitrogen-oxidesin the stratosphere, Q. J. R. Meteorol. U.K. (eraall:david@
Soc., 1œ0(516), 453-482, 1994.
R. Toumi, Department of Physics,Imperial College,LonLary, D.J., M.P. Chipperfield and R. Toumi, The potential don, SW7 2BZ, U.K.
impact of the reaction OH+C10-•HCI+O2 on polar ozone
photochemistry,J. Atmos. Chem., œ1(1), 61-79, 1995.
Logan, J.A., Trends in the vertical distribution of ozone: An
analysisof ozonesondedata, J. Geophys.Res., 99 (D12),
(ReceivedMarch 9, 1995; revisedJune 28, 1995;
22,553-25,585, 1994.
acceptedAugust 24, 1995.)