JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D13, PAGES 16,005-16,010,JULY 20, 1997
Measurementsof photolyzablechlorine and bromine during the
Polar Sunrise Experiment 1995
G.A.Impey,
P.B.Shepson,
• D.R.Hastie,
L.A.Barrie
2
Departmentof ChemistryandCentrefor AtmosphericChemistry,York University,Ontario,Canada
K.G. Anlauf
AtmosphericEnvironmentService,Toronto,Ontario,Canada
Abstract.We reportmeasurements
of rapidlyphotolyzable
chlorine(Clp;e.g.,C12andHOC1)
andbromine(Brp;e.g.,Br2andHOBOin thehighArcticusinga newlydeveloped
photoactive
halogendetector(PHD). Groundlevelambientair wassampleddailyfrommid-Februarythrough
mid-Aprilin the CanadianArcticat Alert, NorthwestTerritories(82.5øN,62.3øW),aspartof the
Polar SunriseExperiment(PSE) 1995. Concentrations
of "totalphotolyzablechlorine"varied
from <9 to 100 pptv asC12andthatof "totalphotolyzablebromine"from <4 to 38 pptv asBr2.
High concentration
episodes
of chlorinewereobservedonlypriorto sunrise(March21), while
highconcentration
episodes
of bromineweremeasured
throughout
the study.The high
concentrations
of photolyzablechlorineandbrominepriorto sunrisesuggesta "dark"production
mechanismthat we assumeyieldsC12andBr2.An inversecorrelationof brominewith ozoneis
clearlypresentin onemajorozonedepletionepisodeat the endof March.A trajectoryanalysis,
takenwith the differencesin measuredlevelsof photolyzablechlorineandbromineafter sunrise,
imply differentproductionmechanisms
for thesetwo typesof species.A steadystateanalysisof
the datafor oneozonedepletionepisodesuggests
a [Br]/[C1]ratioin therange100-300.The high
concentrations
of photolyzablebromineaftersunriseimply the existenceof a precursorotherthan
aerosol bromide.
1. Introduction
The dramaticloss of surfacelayer ozone at sunrisehas been
observedin the high Arctic for severalyears. The episodesoccur
with the onset of polar sunriseand are associatedwith large
increasesin the amount of filterable bromine [Bartie et al.,
1988]. Bromine and chlorine atom chemistry has been
implicatedas the causeof the 03 destruction
episodes.Evidence
for halogenatom chemistryhas come from measurements
of
non-methane-hydrocarbons
(NMHC) [Jobsonet al., 1994], BrO
[Hausmannand Platt, 1994], organicnitrates[Muthuramuet al.,
1994] aswell asorganicandinorganicbrominecompounds
[Li et
al., 1994], made during the Polar SunriseExperiment 1992.
Similarobservations
havebeenreportedby Solberget al. [1996],
who find that ozone at Spitsbergencan be depletedfrom the
surfaceto ashigh as 2 km.
Modelingstudies[Bartie et al., 1988;McConnellet al., 1992;
Fan and Jacob, 1992; Tang and McConnell, 1996] have shown
that for the observed ozone depletions,the CI and Br atoms
presentcould originate from a photochemicallyactive "seed"
(such as CHBr3) and that rapid recyclingprocessescan then
maintainthe catalyticcycle.
To improveour understanding
of the halogenatom chemistry
occurring in the troposphere, reliable measurementsof
photolytically
activehalogenspeciessuchasCl2,Br2,HOCI, and
HOBr are clearly needed. Recentmeasurements
of inorganic
chlorine, which include C12and HOCI, have been made in the
marineboundarylayer at Virginia Key, Florida [Pszennyet al.,
1993], and over easternNorth America [Maben et al., 1995]
usinga tandemmist chamberdescribedby Keeneet al., [1993].
However,therehaveasyet beenno measurements
of Br2(or C12)
in the Arctic, largely as a resultof the lack of suitableanalytical
methodologies. Here we report the first measurementsof
photolyticallyactivebromine(Br2 and HOBr) and chlorine(Cl2
and HOCI) in surfaceair at Alert, NorthwestTerritoriesCanada
(82.5øN, 62.3øW), using a newly developed method, the
photoactivehalogendetector(PHD). This methodis describedin
detailby lmpeyet al. [thisissue].
2. Experimental Procedures
The Polar SunriseExperiment1995 (PSE 95) was conducted
at Alert, Canada,on northernEllesmereIsland (82.5øN, 63.2øW)
from February 20 to April 15, 1995. Measurementsof
1_ _.i._
1 ....
1_1
_
_1_1
_ __'
_
1_
_
ß
_
cn•onne
p•!otmyzao•c
and orom•ne
wereconductedeachday at
the Special StudiesLaboratory(SSL), situatedon a plateau
approximately
6 km SSW of the CanadianForcesStation,while
ozoneandmeteorological
dataweremeasuredcontinuously
from
the
Baseline
Air
Pollution
Monitoring
Network
(BAPMoN)
1Nowat Department
of Chemistry
andDepartment
of Earthand
laboratorysituatedwithin 1 km of the SSL (seeFigure1). Ozone
AtmosphericSciences,PurdueUniversity,West Lafayette,Indiana.
2Also
atAtmospheric
Environment
Service,
Toronto,
Ontario,
Canada. was measuredcontinuouslyusing a TECO model 49 analyzer.
The photoactive
halogendetector,a new methodfor measuring
Copyright1997by theAmericanGeophysical
Union.
Papernumber97JD00851.
0148-0227/97/97JD-00851509.00
photoactive
halogens,
is described
in detailby Impeyet al. [this
issue]. Briefly, the techniqueinvolvesdrawingthe sampleair
througha-2 L cylindricalpyrexreactionvesselat -•1 L/rain(i.e.,
16,005
16,006
IMPEYET AL.:PHOTOLYZABLE
HALOGENS
IN THEARCTIC
50
Arctic
120
Ocean
ozone
*
chlorine
_
40
100
60
©
40
lO
20
o
50
55
60
65
70
75
80
85
90
95
100 105
Day of Year
Figure 2. Comparison
of groundlevelozoneandphotolyzable
chlorine as C12,measuredat Alert.
0,360 ø
180 ø
Ellesmere
Island
lkm
asBr2at Alert are shownin Figures2 and 3. One hourintegrated
sampleswere collectedfor the periodbetweenFebruary20 and
April 14 (days51-104). This periodrepresents
a goodsampling
for the transitionfrom complete24 hoursof darknessto complete
24 hoursof light. March 4 (day 63) wasthe first day the Sunwas
visible abovethe horizon(for a total time of-15 min). By day
80 the Sun was visible for approximately8 hours,highestin the
sky over Alert at about 1110 LT. Measurementsof photolyzable
chlorinebeyondthis day were eitherlow or below the detection
limit of the instrument. In contrast,and interestingly,significant
concentrationsof photolyzable bromine were present after
sunrise,even thougheither likely measuredbrominecompound
(Br2 or HOBr) hasa substantially
shorterphotolyticlifetimethan
the corresponding
chlorinecompound.
The PHD will respondto a number of atmospherichalogen
speciesas notedabove.However,their individualcontributions
to the system responsecan be very region dependent.As
Special
• O'-'
Military
Transmitter
Sitediscussedby Shepsonet al. [1996]theArcticat sunriseis likely
to be an extremelylow [NOx]environment,
whichcanhelprule
outsomepossibilities.
Specifically,
formation
of C1NO,C1NO2,
C1ONO2,and similar nitrogen-containing
brominespeciesis
Figure 1. Map of the northerntip of EllesmereIsland.
unlikelyto beimportant
in thisparticular
environment.
ThePHD
Samplingtook placeat the SpecialStudiesLaboratory.Ozone will not exhibita significantresponseto HC1 (HBr) or any
Studies
La)/¸Baseline
Observatory
wasmeasuredcontinuously
from the BaselineObservatory.
organohalogen
species.Thus,givenwhatis currently
known
aboutthe atmospheric
chemistryof chlorineand bromine,we
a reactorresidencetime of 2 min). Irradiationof the sampleair in assumethat C12and HOC1, and Br2 and HOBr make up the
ClpandBrp,respectively.
the reactionvesselusinga 150 W xenonarc lampphotolyzesthe measured
For March 14 and 15 (days 73-74) we conductedseveral
ambient C12 and Br2 (and other photolyticallyactive halogen
over the full diel cycle;the resultsare shownin
molecules,suchas HOC1, HOBr, C1NO, BrNO, C1NO2,BrNO2, measurements
C1ONO2,and BrONO2),to yield free C1and Br atoms. TheseCI
50
50
and Br atomsthen react rapidly in the cell with addedpropene
(C3H6)andnitricoxide(NO) to produce,amongotherproducts,
ozoneß bromige
i /
40
40 •,
chromatography
with electroncapturedetection.The systemis
calibratedthroughthe preparationof low concentration
mixtures
of C12and Br2 in air and samplingthesemixturesat the reactor
20
20
inlet.Themethod
then'
measures
photolytically
activeCI andBr
10
10
chloroacetoneand bromoacetone. These reaction productsare
concentratedfrom 36 L reactor samplesonto small adsorbentfilled tubes, extracted, and then measuredusing capillary gas
sources,"as C12,"and "as Br2." The detectionlimits (for a 36 L
sample)for C12andBr2are9 pptvand4 pptv,respectively.
0
..........
0
50 55 60 65 70 75 80 85 90 95 100105
3. Results and Discussion
Day of Year
Measured ground level ozone and the total photolyzable Figure3. Comparison
of groundlevelozoneandphotolyzable
chlorineconcentrations
as Cl2 andthe total photolyzable
bromine bromineas Br2,measuredat Alert.
IMPEY ET AL.: PHOTOLYZABLE HALOGENS IN THE ARCTIC
360
20
300
25
00
- 20
240• 80
from an oceanicoriginto an originfromthe Baffin Inlet direction
when chlorine and bromineconcentrations
were highest.
However,asdiscussed
below,onlythei:hlorinedataindicatethat
elevatedlevelsappearto be typicallycorrelated
with trajectories
originating
fromtheBaffinInletregion.Overtheentiresampling
period,chlorineandbrominewerenot well Correlated
(slopeof
0.05andR2= 0.026foraregression
ofchlorine
against
brOmine).
180•' 60
.
There are severalpotentialmechanismsfor productionof
photolyticallyactivechlorineand brominespeciesin the dark.
Taube [1942] showedthat Br-reacts readily in solutionwith
120 • 40
60 20
-5
wind direction
73.8
16,007
74.0
74.2
74.4
74.6
Day of Year
Figure4a. Winddirection
anda complete
dielcycleshowing
the
diumaltrendforbothchlorine
andbromine
ondays73-74.
dissolved
Ozone
(similarly
forCI-[Yeatts
andTaube,
1949]),
followedby a rapid protonationto produceHOBr. HOBr can
then oxidizecondensed
phaseBr- to Br2, as shownin (R3),
[EigenandKustin,1962;Fan andJacob,1992]:
(RI) 03
+
Br'
--> OBr'
(R2) OBr-
+
H+
•
+
02
HOBr(aq)
(R3) HOBr(aq)+ Br' + H+ --> Br2
q- H20
Figure4a. Thesedatashowa distinct
andsimilardiumalcycle
•suggested that
for both photolyzablechlorineand bromine,with a maximum Similarly, Mozurkewich [1995]
peroxymonosulfuric
acid, producedthrougha free radicalchain
near1100LT anda minimum
nearmidnight.In Figure4b we
presentthe averagediurnalchlorineand bromineconcentrations
forall thedataobtained
for days63-91. Thisperiodis chosen
as
oxidation
ofdissolved
S(IV)species,
could
alsooxidize
bromide
ionsto HOBr,asshownin (R4). Thiswouldthenbefollowedby
the period after sunrisewhere the chlorineconcentrations
were
Br2productionthrough(R3).
abovethedetection
limit. Theaverage
diurnalprofileis similar
to thatshownin Figure4a, i.e., a latemorningmaximumanda
(R4)HSO5-+ Br'
--> 804'2
+ HOBr(aq)
and high SO2 concentrations
minimumnearmidnight.Thisis in contrast
to whatwe might This requireslow temperatures
existonlyduring
thewinterandearlyspring
ifitheArctic
expectfor rapidlyph0tolyzable
species,i.e., low concentrationswhich
duringthe dayandhighat night. The factthatbothchlorineand region. Zetzschet al. [1988] andZetzschand Behnke[1993]
bromine exhibit similar diumal characteristics with maxima in
have found that 03 can react with HCI on aerbsolsurfacesto
C12in thedarkbutthattheheterogeneous
production
of
the daytime might implies a photo-induced(possibly produce
It hasb•enreported
by Livingsto
n
heterogeneous)
source,e.g., as postulated
by McConnellet al. C12canbe photocatalyzed.
[1991]andbyZetzsch
andBehnke
[1993]
[1992] and McConnelland Henderson[1993]. However,the andFintayson-Pitts
presence
of substantial
concentrations
of bothspecies
well before that reactionof N2Oswith NaCl(s) can produceNO2CI, which
sunrise(beforeday 63) clearlyindicatesthat theremustbe a dark subsequentlyphotolyzesto yield CI atoms. However, as
by Shepson
et al. [1996],andasshownby Honrathand
mechanism
for theirformation.Several
cases
for whichBrpis discussed
high in the dark (e.g., days 55-60) occur when the back Jaffe [ 1992], the levels of availableNO,. in the Arctic at sunrise
trajectoriesshowthe air arrivingat Alert from an Arctic oceanic shouldbe very low. It hasbeenrecentlydiscussed
by Tangand
origin and thus are not likely to have been sunlit. The wind McCOnnell[1996] that only a small Br atom initiationsource
of CHBr3)is necessary
for•ignificant
ratesofdirectiondatashownfor theonecasein Figure4a implya change (e.g.,photolysis
productionof Br2 after sunrise,if the Fan and Jacob [1992]
30
18
ß
chlorine
mechanism
(reaction
(R3)) is operative,
sincethatmechanism
is
autocatalytic.
I• thusseemspossible
thatthe Fan andJacob
20-
- 12
mechanism could be operative in the dark but that it is
accelerated(at leastfor bromine)after sunrise.
The Fan and Jacob [1992] mechanisminvolvesconversionof
gasphaseXO to HOX, via (R5) and(R6), followedby uptakeof
15-
-10
(R5) X
+
03
•>
-8
(R6) XO
+
HO2
--> HOX
-6
the HOX into the acidic aqueousfine aerosol. Since this
mechanismnot only destroys03 but alsorequiresits presence,it
is instructive to examine the relationship between our
measurements
of the photolyticallyactivehalogenspeciesand
25-
•
bromine
-16
- 14
-4
2
XO
q-
02
+
02
03. Althoughphotolyzable
chlorinewasnot well correlaiedwith
ozoneoverthe entirestudy,the maximumobservedphotolyzable
12'00
0'00
24:00
bromineconcentration
(38 pptv as Br2) occurredduringthe first
major 03 depletionepisodebetweendays 87 and 90 and was
Time of Day
inverselycorrelatedwith 03 during this event. An expanded
Figure 4b. Averagediurnalchlorineandbromineconcentrations view of the comparison between ozone and bromine
for days63-80.
concentrations
betweendays 76 and 92 is shownin Figure 5.
0
-0
16,008
IMPEYET AL.' PHOTOLYZABLE
HALOGENS
IN THEARCTIC
50
50
bromine•
40
40•.
30
:20
20
10
0
76
78
80
82
i
i
84
86
i
88
90
Day of Year
Figure5. An expanded
viewforthecomparison
of ground
level
ozoneandphotolyzable
brominefor days76-92.
Three-dayback trajectoryanalysisindicatesthe sampleair
arrivingfromthe Arcticoceanregionalongthenorthcoastof
Greenland,
in the generaldirectionof Spitsbergen,
implyingan
Arcticsourcefor thephotolytically
activebromine.
To examinein moredetailtheoriginof highconcentrations
of
Figure6a. Average
bromine
concentration
(0-16pptv)bysector
the photolyticallyactive halogens,back trajectories
were region.
obtainedfor the endpointcorresponding
to the time of each
sample.We thenplottedtheresults
aspolarplotsof [halogen] campaign
in Ny-,•lesund,
Spitsbergen
(in April 1995)
versusthe trajectory
originsector.We arbitrarily
dividedthe [(H.Lorenzen-Schtnidt
et al., unpublished
manuscript,
1995)]
back trajectoryoriginsinto six equal 45ø intervalsand duringwhichsignificant
levelsof BrO (asmuchas•-40pptv)
determinedthe averagechlorineand bromineconcentrationswereobserved
on thewingsof the ozonedepletion
episodes.It
withineachsector. The resultsare presented
in Figures6a and shouldbenotedthatthereis onlya smallperiodof timewhenwe
6b, for bromineand chlorine,respectively.On averagethe wouldexpecta goodcorrelation
between
photolyzable
bromine
highestbromineconcentrations
wereobserved
for air parcels and ozone, i.e., only after sunrisewhen ozone depletion
arrivingatAlertfromsector
6 (seeFigure6a). Thisisinterestingchemistry
occurs
butbeforethesolarfluxis solargethatthe
in lightof theMozurkewich
[1995]mechanism
andthefactthat lifetime
ofBrpbecomes
toosmall
tosustain
highBrplevels.
thistrajectory
originis alsoassociated
withthehighest
levelsof
Since our method does not discriminatebetween Br2 and
SO2[cf. Bartieet al., 1989]. Unlikebromine,
thehighchlorine HOBr(or C12andHOC1),we examined
theimportance
of the
concentrations
were observedmainly in the dark periodduring different
photochemically
activesources,
usinga simplebox
the earlierpart of the studyand werefrom southerly
air mass
trajectories
(sector1) through
theBaffinInlet(seeFigure6b).
Giventhe factthatBr2hasa photolyticlifetime(•- 1-2 min) that
is much shorterthan that for C12(the sameis true for HOBr
relativetoHOC1),it isinteresting
thatBrpisprese•-•t
in significant
concentrations
aftersunrise,
whileClpisnot. Theseobservations
imply that there may be distinctlydifferentproduction
mechanisms
for photolytically
activechlorineandbromine.This
conclusion
supports
thehypothesis
of Shepson
et al. [1996]that
the levelsof aldehydes
in the Arcticoceanregionareconsistent
with C1 atom chemistryoccurringin the absenceof Br atom
chemistry.
To sustainthe observedlevels of "Br2" after sunrisewould
requirea very largesourceor a continuous
recycling
from
reservoir
species
likeHBr,HOBrandBrONO2
asin theFanand
Jacob [1992] mechanism. As discussed
above,for the
mechanism
of Fan andJacob[ 1992],theproduction
of Br atoms
will ceasein the absenceof 03, sincethere is no meansof
producing
BrO. Thisis in factconsistent
withourobservations
forthemajorozonedepletion
eventof days88-89. Asshown
in
Figure5, whileBrpwasclose
to thedetection
limitwhenozone
wascompletely
removed
(day88),it wasobserved
to increase
at
the end of this event,when ozonewas increasing
to typical
unperturbed
levels(notethatwe werenotsampling
duringthe
start of this 03 depletionevent).This observation
is also Figure 6b. Averagechlorineconcen•ation(0-25 pptv)by sector
consistent
with preliminary
resultsfrom the ARCTOCfield region.
IMPEY ET AL.: PHOTOLYZABLEHALOGENSIN THE ARCTIC
model. The model contains78 reactionswith 49 species,
includingthe standardgas phaseCIOx, BrOx, NOx, and HOx
chemistry.Constant
Br2andCl2fluxesof 22 pptv/hand6 pptv/h,
16,009
[HOC1]
[Cl]:
{kc,
[O3
]+kc,
[HCHd•]O+C2cl
[CH
3CHO]
(2)
4-kC,
IsM4]4-• kcl(i)
[gH]}
respectively, are used to maintain the Br and C1 atom
concentrations
at 107 and104 molecules/cm
3, respectively,
in
Here k, correspondsto the rate constantfor Br or CI atom
accord with the estimatesof dobson et al. [1994].
reaction
with thatparticularreactingspecies.The lasttermin the
Photodissociation
rate coefficients
are as usedin the modeling
of equation
(2) includes
all alkanes
fromC2to C8,as
studyof Tangand McConnell[1996] (with an updatefor HOBr denominator
photolysis from Lock et al. [1996]). Concentrationsof
formaldehyde
(HCHO) and acetaldehyde
(CH3CHO), which are
major sinksfor Br atoms,are maintainedat 200 pptv and 100
pptv, respectively[Shepsonet al., 1996]. Also, all relevant
hydrocarbon,NOs-and 0 3 initial concentrations
are consistent
with measurements
from PSE 1992 [seeBartie et al., 1994] and
well as C2H4, C3H6, and C2H2. Br atoms are assumedto be
removedonly throughreactionwith 03, HCHO, CH3CHO,and
C2H2. Correspondingcalculatedvalues for Br and CI atom
concentrations
can be obtainedassumingthat the measured
photolyrically
activespecies
are Br2 and C12,by appropriate
substitution,
e.g.,ofdBr2O[Br2]
intothenumerator
of equation
(1).
Using our estimates for the d values, and the measured
dataat Alert, the calculated
steadystateBr atom
The input conditionsdiscussedabove simulatethe observed hydrocarbon
for the day 83 episodeis between1.0x106
ozonedecay.The modelpredictsa steadystateconcentration
of concentration
3 (assuming
the precursor
is HOBr)and l.Sx107
•0.5 pptvfor Br2.The simulatedHOBr concentration
depends
on atoms/cm
with the measurements conducted at Alert in 1995.
the value for the effective
first-order transfer coefficient
for
HOBr accommodation
into the fine aerosol.Usingthe valueof
atoms/cm
3 (assuming
it is Br2).Thecorresponding
values
for
[CI]are1.0x104
atoms/cm
3(HOC1)
and6.1x104
atoms/cm
3(C12).
1.4x10
-4s'l usedbyFanandJacob[1992]results
in anHOBr Because
of thelargerangeof estimated
values,we canonlysay
concentration,
when03 is present,of only •10 pptv. However,
laboratorymeasurements
by Abbatt [1994] indicatethat the
uptakecoefficientis considerably
largerthanthat(0.01)usedby
Fan andJacob. If we usean upperlimit uptakecoefficientfor
H¸Br of 1, the first-order
transfercoefficientwouldbe 2.2xl0'3
that our estimates for the absolute concentrationsare consistent
•,1 pptv. That [HOBr] > [Br2]resultsfromthe fact that Br atoms
however,
depend
ontheavailability
ofozone
intheairmass
(see
discussion
above)
andcouldbequitevariable.
Forexample,
if
with the estimatesof dobsonet al. [1994]. However,the
calculated
ratio[Br]/[CI]is muchlesssensitive
to theidentityof
theprecursor,
i.e.,we obtain[Br]/[CI]•100 if theprecursors
are
HOBr andHOCI, andz300 if the precursors
are Br2andC12.
s'q,using
themethod
described
bySander
andCrutzen
[1996]. This rangeof [Br]/[CI] atomratiosis slightlylower than the
Thisvaluethenresults
in a simulated
HOBrconcentration
of only dobson
et al. [1994]estimate
of about800. Thisresultmay,
are relativelyefficientlyconvenedto HOBr via (RS) and(R6)
and becauseBr2 photolysisis 25-30 times fasterthan that for
HOBr.
we assume
the measured
photolyzable
chlorineconcentration
on
day89(a maximum
HOBrof 129ppt)tobe~9pptvasC12(i.e.,
To examine the relative effects of photolyricallyactive at the detection
limit, corresponding
to 45 ppt HOCI), the
bromine in the form of Br2 or HOBr, we can convert our
measurements
of total photolyzablebrominefrom "as Br2"to "as
HOBr". Takinginto accountthe differentphotolysisratesof Br2
andHOBr andthe residence
timewithinthe photoactive
halogen
detector(PHD), as discussed
by lmpeyet al. [thisissue],we find
that ["as HOBr"] - 3.4o["asBr2"],so that the 38 pptv "as Br2"
observedon day 89 is the equivalentof 129 pptv "as H¸Br".
Similarly, ["as HOCI"] - 5.0o["as C12"]. Although our
calculated
(steadystate)[Br]/[CI]ratiohasan upperlimit of
approximately
2000. Thuswe canonly saythatthe Br/C1atom
ratio appearsto be consistent
with the estimates
of dobsonet al.
[1994].
Ourmodelpredicts
a maximum
HOBrconcentration
ranging
between1 and10pptv(when[Br] is fixednearthedobson
et al.
[1994]estimate
of 107),butourmeasurements
indicate
thatas
muchas 129 pptvof HOBr may be presentduringan ozone
measurementsduring PSE 95 did not allow us to discriminate depletionepisodewhenwe are samplingair from the Arctic
betweenBr2 and HOBr, it is importantto do so, as ambientBr2 oceanregion. Increasingthe Br2flux in the modelto asmuchas
photolysisproducesBr atoms •50 times faster than does an 170pptv/h(yieldinga simulated
[Br] neartheupperlimit of the
identicalconcentration
of HOBr. Thus for day 89 we calculate literature
estimates
of 6x107atoms/cm
3) still produces
only
that 38 ppt of Br2 will resultin a photolyticBr atom production 40pptv of HOBr. The Fan and dacob [1992] mechanism
rateof 2.7x107
molecules/cm3os,
while129pptof H¸Br would involvesuptakeof HOBr onto acidicaerosolthat containsBr-,
produce
only1.7x106molecules/cm3os.
followedby (R3). Onetestof thismechanism
is to compare
the
As discussedby dobsonet al. [1994] and Shepsonet al. estimatedBr2 productionrate with the amount of available
[1996], the relative rates of decay of various reactive aerosolbromine.If the Br2is generated
from(R3), the lifetime
hydrocarbons(e.g., alkanes and acetylene; aldehydes and of brominein the aerosolwould then be given by q;aeroso
I Br=
ketones)are highly dependenton the [Br]/[C1] ratio. For days [aerosolBr]/2o[Br2flux]. Using the Liet al. [1994] measured
when there is radiation and when the photolyticallyactive aerosol
Brconcentration
of about1.25xl
0-9molBr/m
3during
an
halogensare well above the detectionlimit, it is possibleto ozone depletionevent when air was comingfrom the Arctic
estimatethe resultingsteadystateBr andCI atomconcentrations. oceanboundarylayerand assuming
a Br2production
rate from
A good casefor this is day 83, when the ozone concentration theaerosol
of 170pptv/h(2.82x10
© molBr/m
3min)leads
to an
ranged between •5-20 ppb. Using the measuredmaximum effectivelifetime for the aerosolBr of 2.2 minutes. However,
concentrations
on day 83 of 43 and97 pptv of HOBr and HOCI, following the method of Sander and Crutzen [1996], our
respectively
(i.e.,assuming
theserepresent
BrpandClp),a steady estimatedcharacteristictime for gas-phasediffusion of the
state concentration
of Br and C1 atoms can be estimated from
equations(1) and(2):
limitingreactant
HOBr to theaerosolsurfaceis 8 min (usingan
averageparticleradiusof 1.Sx10
-5 cm, an accommodation
coefficient
ctof 1,anddimensionless
LWC= 4xl0'12[Staebler
et
1994]). Using this value for ct yieldsthe lower limit to the
[Br]=
kBr
[O3]+kBr[HCH•)I•O_m_B;Br[CH3CHO]+kBr[C2H2
] (1)al.,
HOBr characteristic
transporttime to the aerosol. However,
16,010
IMPEYETAL.:PHOTOLYZABLE
HALOGENS
IN THEARCTIC
sinceour lower limit estimateis greaterthanthe effectiveaerosol Keene, W.C., J.R. Maben, A.A.P. Pszenny,and J.N. Galloway,
Measurementtechniquefor inorganicchlorinegasesin the marine
Br lifetime, it appearsthat the availableaerosolbrominewould
boundarylayer, Environ.Sci. Technol.27, 866-874, 1993.
be rapidly depleted if the only mechanismfor generating
Li, S.M., Y. Yokouchi,L.A. Barrie,K. Muthuramu,P.B. Shepson,
J.W.
photolyticallyactive bromine was (R3) occurringon aerosol
Bottenheim,
W.T. Sturges,
andS. Landsberger,
Organicandinorganic
particles. Thereforeit appearsthat othermechanisms,
suchas
brominecompounds
andtheircomposition
in the Arctictroposphere
duringpolarsunrise,J. Geophys.Res.,99, 25,415-25,428,1994.
oxidationof Br- in the surfaceice [Tang and McConnell, 1996],
Thereactionof gaseous
N205
may play a more importantrole in generationof photolytically Livingston,F. E., andB. J. Finlayson-Pitts,
active bromine in the Arctic.
4. Conclusions
This work providesconclusiveevidencefor the presenceof
substantialconcentrations
of photolyticallyactive chlorine and
brominein the Arctic near the time of polar sunrise. For both
photolyticallyactive chlorine and bromine it is apparentthat
there exist dark productionmechanisms,as large concentrations
were observedbeforesunrise. Furthermore,largeconcentrations
of photolyticallyactive brominewere observedafter sunrise,
whenthe lifetimeof thesespecies(presumably
Br2or HOBr) are
short, implying a large source,and indicatinga large Br atom
source. For example,for day 89, the ~38 pptv of bromine("as
Br2") would be equivalentto 3.8 ppbv of CHBr3 &3 ordersof
magnitudegreaterthanobserved)in termsof the relativeratesof
photolyticgenerationof Br atoms. It is thusclear from our data
that CHBr3 (or other organo-brominecompounds)is an
unimportant source of bromine atoms in the Arctic at polar
sunrise.
with solid NaC1 at 298 K: Estimated lower limit to the reaction
probabilityand its potentialrole in tropospheric
and stratospheric
chemistry,Geophys.Res.Lett., 18, 17-20, 1991.
Lock, M., R.J. Barnes,and A. Sinha,Near-threshold
photodissociation
dynamicsof HOBr: determination
of productstatedistribution,
vector
correlation,and heat of formation,J. Phys.Chem.,100, 7972-7980,
1996.
Maben,J.R.,W.C. Keene,A.A.P. Pszenny,
andJ.N. Galloway,Volatile
inorganic
C1in surfaceair overeastern
NorthAmerica,Geophys.
Res.
Lett., 22, 3513-3516, 1995.
McConnell,J.C., and G.S. Henderson,Ozone depletionduringpolar
sunrise,in tropospheric
chemistryof ozonein polarregions,Global
EnvironmentalChange,edited by H. Niki and K.H. Becker,NATO
ASI Set. I, 7, 89-103, 1993.
McConnell,J.C., G.S. Henderson,L.A. Barrie, J. Bottenheim,H. Niki,
C.H. Langford, and E.M. Templeton, Photochemicalbromine
productionimplicatedin Arctic boundary-layer
ozone depletion,
Nature, 355, 150-152, 1992.
Mozurkewich,M., Mechanismsfor thereleaseof halogensfrom sea-salt
particlesby free radical reactions,J. Geophys.Res.. 100, 14,19914,207, 1995.
Muthuramu, K., P.B. Shepson,J.W. Bottenheim,B.T. Jobson,H. Niki,
and K.G. Anlauf, Relationshipsbetweenorganicnitratesand surface
ozonedestructionduringPolar SunriseExperiment1992,J. Geophys.
Res., 99, 25,369-25,378, 1994.
The observationof high bromineon the "wings"of an ozone Pszenny, A.A.P., W.C. Keene, D.J. Jacob, S. Fan, J.R. Maben, M.P.
Zetwo,M. Springer-Young,
andJ.N. Galloway,Evidenceof inorganic
depletionepisodeseemsto indicateeitherthat we are measuring
chlorine gasesother than hydrogenchloride in marine surfaceair,
HOBr or that ozoneis an importantoxidant(or directlyrelatedto
Geophys.Res.Lett., 20, 699-702, 1993.
the oxidant)involvedin productionof Br2. It is importantthat
Sander,R., and P. J. Crutzen,Model studyindicatinghalogenactivation
the exact natureof this Br atom sourcebe determined(i.e., Br2
and ozone destructionin pollutedair massestransportedto the sea,
versusHOBr).
d. Geophys.Res., 101, 9121-9138, 1996.
Shepson,P.B., A.-P. Sirju, F.J. Hopper,L.A. Barrie,V. Young,H. Niki,
and H. Dryfhout, Sourcesand sinks of carbonylcompoundsin the
Acknowledgments. We thank Natural Sciencesand Engineering
Arctic oceanboundarylayer: A polar icefloeexperiment,J. Geophys.
ResearchCouncil of Canadaand the AtmosphericEnvironmentService
Res., 101, 21,081-21,089, 1996.
for their financial support and D. Worthy for back trajectoriesand
meterologicaldata. We also thank Jack McConnelland Apollo Tang for Solberg,S., N. Schmidbauer,A. Semb, and F. Stordal,Boundary-layer
ozonedepletionas seenin the Norwegian Arctic in spring,d. Atmos.
helpful discussions.
Chem., 23, 301-332, 1996.
References
Staebler,R.M., G. den Hartog,B. Georgi,and T. D•lsterdiek,Aerosolsize
distributionsin Arctic hazeduringthe PolarSunriseExperiment1992,
J. Geophys.Res.,99, 25,429-25,437, 1994.
Tang, T., and J.C. McConnell, Autocatalyticreleaseof bromine from
Arctic snowpack duringpolar sunrise,Geophys.Res.Lett., 23, 2633-
Abbatt, J.P.D., Heterogeneousreactionof HOBr with HBr and HC1 on ice
surfacesat 228K, Geophys.Res.Lett., 21, 665-668, 1994.
2636, 1996.
Barrie, L.A., J.W. Bottenheim, R.C. Schnell, P.J. Crutzen, and R.A.
Taube,H., Reactions
in solutions
containing
03, H202,H* andBr-.The
Rasmussen,Ozone destructionand photochemicalreactionsat polar
specific rate of the reaction 03 + Br-, d. Am. Chem. Sot.. 64,
sunrisein the lower Arctic atmosphere,
Nature, 334, 138-141, 1988.
2468-2474, 1942.
Barrie, L. A., G. Den Hartog, J. W. Bottenheim,and S. Landsberger, Yeatts, R.B., and H. Taube, The kinetics of the reaction of ozone and
Anthropogenicaerosolsand gasesin the lower troposphereat Alert,
chlorideion in acidic aqueoussolution,d. Am. Chem.Soc., 71, 400,
Canadain April 1986,d. Atmos.Chem.,9, 101-127,1989.
1949.
Barrie, L.A., J.W. Bottenheim,and W.R. Hart, PolarSunriseExperiment
Zetzsch, C., and W. Behnke, Heterogeneousreactions of chlorine
1992 (PSE 1992): Preface,d. Geophys.Res.,99, 25,313-25,314, 1994.
compounds,in troposphericchemistryof ozone in polar regions,
Eigen, M., and K. Kustin, The kinetics of halogenhydrolysis,d. Am.
Global EnvironmentalChange,editedby H. Niki and K.H. Becker,
Chem. Soc., 84, 1355-1361, 1962.
Fan, S.-M. and D.J. Jacob, Surface ozone depletion in Arctic spring
sustainedby bromine reactionson aerosols,Nature, 359, 522-524,
1992.
Hausmann, M., and U. Platt, Spectroscopicmeasurementof bromine
oxide in the high Arctic during Polar SunriseExperiment 1992, d.
Geophys.Res.,99, 25,399-25,413, 1994.
Honrath,R. E., and D. A. Jaffe, The seasonalcycle of nitrogenoxidesin
the Arctic troposphereat Barrow, Alaska, d. Geophys. Res., 97,
20,615-20,630, 1992.
Impey, G.A., P.B. Shepson,D.R. Hastie, L.A. Barrie, Measurement
techniquefor the determinationof photolyzablechlorineand bromine
in the atmosphere,
J. Geophys.Res.,this issue.
Jobson,B.T., H. Niki, Y. Yokouchi, J. Bottenheim,F. Hopper, and R.
Leaitch,Measurements
of C2-C6 hydrocarbons
duringthe 1992 Polar
SunriseExperiment:Evidenceof Cl-atomand Br-atomchemistry,d.
Geophys.Res.,99, 25,355-25,368,1994.
NATO ASI Ser. I, 7, 291-306, 1993.
Zetzsch,
C., G. Pfahler
andW. Behnke,
Heterogeneous
formation
of
chlorineatomsfrom NaC1in a photosmogsystem,d. AerosolSci., 19,
1203-1206, 1988.
K. Anlauf and L. A. Barrie, AtmosphericEnvironmentService,4905
Dufferin St., Downsview, Ontario, Canada M3H 5T4. (e-mail:
[email protected])
D. R. Hastie and G. A. Impey, Departmentof Chemistry,York
University, Room 124 CCB, 4700 Keele St., North York, Ontario,
CanadaM3J 1P3. (e-mail: [email protected])
P. B. Shepson,Departmentof Chemistry, Purdue University, 1393
Brown Bldg., West Lafayette, IN
47907-1393. (e-mail:
pshepson•chem.purdue.edu)
(ReceivedSeptember
27, 1996;revisedFebruary19, 1997;
acceptedMarch 7, 1997.)