JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 98, NO. E6, PAGES 10,933-10,940, JUNE 25, 1993
Adsorptionof HOxon AerosolSurfaces:Implicationsfor the Atmosphereof Mars
A.D.
ANBAR
DivisionoœGeological
andPlanetary Sciences,CaliforniaInstituteoœTechnology,
Pasadena
M-T.
LEU
Earth andSpaceSciences
Division,JetPropulsionLaboratory,CaliforniaInstituteof Technology,
Pasadena
H. A. NAIR AND Y. L. YUNG
DivisionoœGeological
andPlanetary Sciences,
CaliforniaInstituteoœTechnology,
Pasadena
The potentialimpactof heterogeneous
chemistryon the abundanceand distributionof HOx in the atmosphere
of Mars hasbeenassessed
by combiningobservational
dataof dustandice aerosoldistributions
with an updated
photochemical
model. Critical parametersincludethe altitudedistributions
of aerosols,and the surfaceloss
coefficients
(T)of riO2ondustandicein theloweratmosphere,
andH oniceabove40 km. We findthatadsorption
ofriO2ondust(TH02
> 0.01),oricenear30km(?HO•
> 0.1),candeplete
OHabundances
intheloweratmosphere
by 10%or more.Suchdepletions
approach
thoseobtainedby loweringthe watervaporabundance
by an orderof
magnitudebelowthe globalaverageobservedby Viking (• 25%). Sincethe oxidationof CO is catalyzedby HOx
in the lower atmosphere
via the reactionCO + OH -} CO2 + H, lossof OH due to adsorptionof HO2 on dustor
ice at low altitudescouldhavea significanteffectonthe ratioCO' CO2. The adsorptionof H on ice at 50 km (¾H
> 0.0I) canresultin evenlargerOH depletions.However,thiseffectis localizedto altitudes> 40 km, whereCO
oxidation
isrelatively
unimportant.
Laboratory
data
suggest
that?HO•
• 0.01isareasonable
estimate
foradsorption
on dust. Largervaluesare plausible,but are not stonglysupported
by experimentalevidence.The reactivityof
HO2 on iceis unknown,while¾Hon iceappearsto be < 0.001.Thereis a needfor measurements
of HOxadsorption
on surfacesrepresentative
of Martian aerosolsat temperatures
< 220 K.
INTRODUCTION
In a recentstudywhich accountedfor thesefactors,Shirnazaki
Carbondioxidecomprises
over95% of theatmosphere
of Mars,
despitecontinuous
photolysis
of CO2by solarUV radiation.Since
the directrecombinationof CO andO is slow, the balancebetween
CO2production
andlossin theMartianatmosphere
(the "stability
of CO2") is thoughtto be maintainedby a HOx-catalyzedCO
oxidationscheme[McElroyand Donahue,1972;Parkinsonand
Hunten,1972].Thus,the rate of CO oxidationis sensitiveto the
abundance
andaltitude-distribution
of theHOx species
(OH, H and
HO2).
Recentmodelsof gas-phase
chemistry
in theMartianatmosphere
predictratiosof CO to CO2lowerthanthoseobserved,
presumably
dueto an overabtmdance
of HOx [Shimazaki,1989;Krasnopolsky,
1991; Nair et al., 1991,1992].This is largely the resultof two
importantfactorswhichwerenotconsidered
in theclassicexplanationsof CO2stability[McElroyandDonahue,1972;Parkinsonand
Hunten,1972].First,the temperature
dependence
of the CO2 absorption
crosssection[e.g.,DeMoreandPatapoff,1972;Lewisand
Carver,1983]wasnotaccounted
for in earlierstudies.Thiseffect
shoulddecrease
thecalculated
CO2 photolysis
rate,whileincreasing therateof HOxproduction
via photolysis
ofH20 [Parisotand
Zucconi,1984;Anbar et al., this issue].Second,the watervapor
profilesusedin mostof theearlierMarsmodelsassumed
thatwater
was presentprimarilynear the surface(the lower 5-10 kin). In
currentmodels,theloweratmosphere
isconsidered
well-mixedwith
respectto waterto altitudesashigh as2040 kin, consistent
with
observational
data[ffakoskyandFarmer,
1982;Clancyetal., 1992].
This increases
the amountof H20 exposedto photolysis
at higher
altitudes,therebyraisingtherateof HOxproduction.
Copyright1993by the AmericanGeophysical
Union.
Papernumber93JE00132.
0148-0227/93/93
JE-00132505.00
[ 1989]wasabletobalanceCO2production
andlossonlyby imposinganupperlimitonthewatervaporabundance
of 1-2 precipitable
microns(prgrn).Thisconstraint
limitstheabundance
of HOxspecies,whicharelargelyderivedfromphotolysis
of H20. However,
thiswatervaporabundance
isanorderof magnitude
lowerthanthe
globallyaveragedabundance
measured
by Viking [ffakosky
and
Farmer, 1982], and is also substantiallylower than the recent
measurements
of Clancyet al. [ 1992].
Theadsorption
ofHOxonaerosolsurfaces
isanalternative
means
of reducingHOxabundances
JAnbaret al., 1991;Krasnopolsky
et
al., 1991]. Most Martianatmospheric
modelshaveassumed
that
HOxabundances
aregoverned
entirelybygas-phase
chemistry
[e.g.,
Yungetal., 1988;Shimazaki,1989].However,it is well established
thatreactiveHOxradicalsareadsorbed
bya widevarietyof surfaces
(Table1). Adsorption
of thesespecies
on thesurfaces
of Martian
aerosolscould decreaseHOx abundances
significantly,thereby
retardingCO oxidation.
Previous
discussions
of theinfluence
of heterogeneous
chemistry
ontheMartianatmosphere
havelargelyfocussed
onheterogeneous
catalysis
of CO oxidation[e.g.,Clark, 1971;Hugueninetal., 1977;
Atreya and Blamont,1990;Leu et al., 1992], althoughsurface
catalyzedCO oxidationhasnot yet beenobservedunderMartian
atmospheric
conditions.HOx adsorption
hastypicallybeenoverlooked,althoughthisphenomenon
hasbeenstudiedin the laboratory at temperatures
andpressures
approaching
thoseof the lower
Martian atmosphere
(Table 1). Hunten [1974] was the first to
suggest
that "coldtrapping"of waterandHOx species
on surfaces
couldstronglyperturbgas-phase
HOx catalyticchemistry.However,thiswasdiscussed
asa low-temperature
equilibriumcondensationprocess,
ratherthanasanadsorption
reaction
governed
bythe
kineticsof gas-surface
reactions,
capableof occurringat temperaturestypicalof the Martiansurfaceandloweratmosphere.
Kong
andMcElroy [1977] madethe only attemptto quantitativelyconsiderthe impactof heterogeneous
destruction
of HOx, but confid-
10,933
10,934
ANBAR•r nL.: ADSORPTION
ov HOx oN AEROSOL
SURFACES
TABLE 1. SurfaceLossCoefficientson InorganicOxides
Surface
¾
T, K
Reference
H Atom
H20(s:amorphous)
> 0.1
< 150
1'
Caltech/JetPropulsionLaboratorygeneralizedplanetaryatmospherephotochemistry
codeis described
elsewhere[Allenet al.,
1981]. The modelusedhereis a modifiedversionof themodelof
Yungetal. [ 1988].Detailsof thisMarsmodelaredescribed
below.
Heterogeneous
Chemistry
K2CO3
NaNO3
0.039
0.013
298
298
8
8
Pyrex
Quartz
6.0x10
'3
2.8x10'3
298
298
3
3
ZnO
1.2x10'3
298
2
Cr203
lx10'3
298
2
H20(s)
MgO
l.tx10'4
lx10'4
253
298
4
2
298
4
298
4
> 0.4
0.4
0.24
0.10
250
298
298
250
4
8
8
4
0.12
298
8
[Michelangeliet al., 1991]. The temperatures
usedto calculatev
> 0.08
0.045
0.04
249
298
250
5
8
4
are described below. The dust and ice surfaces available for reaction
0.036
298
8
250
275
4
5
PbO
H2SO4
1.9x10'3
_<5x10'5
298
2
OH Radical
H2SO4
H20 (s)
Fe(NO3)3- FeOs
Pb(NO3)2
A1203(on AI metal)
FeSO4 ß nH20
H2SO4(28 wt.%)
Zn(NO3)3
FeOs(on steel)
NaNO3
Quartz
H20(/)
I
6.3xl0'3
> 3.5x10
'3
HO2 Radical
H20 (/)
H2SO4(28 wt.%)
H20 (/)
StainlessSteel
Glass
0.2
> 0.05
>0.01
1.7x10'2
8x10'3
298
249
275
320
298
7
5
5
6
6
FeOx(onsteel)
Quartz
3x10'3
4xl0'3
298
298
9
6
The rate of eachHOx adsorption
reactionwas setequalto the
collisionfrequency
of thereactingspecies
witha surface,
multiplied
by a surfacelosscoefficient,¾, whichrepresents
the fractionof
collisionswhichresultin lossof the speciesfrom the gasphase
(0 < ¾< 1). Publishedmeasurements
of ¾ for HOx speciesare
summarizedin Table 1, anddiscussed
furtherbelow. The collision
frequency
of a gas-phase
species
withan aerosolsurface
at a given
altitudeis a functionof the meanthermalmolecularspeed(v; cm
s'l),andthesurface
area(S;cm2/cm
3)available
atthataltitude,
followingthe equation:
Rate
(cm
'3s']) = (1/4)¾SvNi,
where
Ni(cm'3)isthenmnber
density
ofthespecies
being
adsorbed
in themodelareplottedin Figure1 as extinctioncoefficients
(x).
For spherical
particles,surfaceareaandextinctioncoefficientare
approximately
relatedby S = 2x, assuming
thattheopticalcross
sectionis equalto twicethegeometrical
crosssection.
For dust,the availablesurfacearea was calculatedfrom dust
distributions
andparticledimensions
supplied
by D. Michelangeli,
basedon the work of Michelangeliet al. [in press].An overall
opticaldepthof approximately
0.2 wasassumed;
thisrepresents
a
relativelyclearMartianatmosphere,
anddoesnotsimulate
themuch
higheropacities
observed
duringglobalduststorms.Thesedataare
similarto thoseobtainedby Phobos2 [Blamontet al., 1991]. We
note that the total surface area available for reaction on these
particlesis comparable
to theareaof theplanet'ssurface.
Persistent,
geographically
extensive
detached
hazesof waterice
are commonin the Martianatmosphere
[e.g.,Kahn, 1990].Two
differentice distribution
profileswereincludedin thisstudy,becausethesehazeshavebeenobserved
overa rangeof altitudes.The
firstprof'de,
basedondatasupplied
by D. Michelangeli,
reaches
a
maximin iceabundance
at 30 kin.Thesecond
prof'deassumes
the
eredonlyadsorption
ontheregolithat theMartiansurface;reac- samedistribution,
but slfifted20 km higherto matchobservations
tionson aerosolsurfaces
(dustor ice) werenotincluded,although by Phobos2 [Blamontet al., 1991].
the surfaceareaavailablefor reactionon aerosolparticlesin the
Onceadsorbed,
HOxspecies
areassumed
toreactwithgasphase
Marti.anatmosphere
is oftencomparable
to theareaof theplanet's OH or H, resultingin conversion
to H20 orH2, whicharerapidly
surface.Their studyalsodid not accountfor the temperature-de- rettm•edto thegasphase.Tlfismechanism
isnecessary
topreserve
pendence
of the CO2 absorption
crosssection,andassumed
that mass balance m•d maintain the overall oxidation state of the atmoswaterin theatmosphere
waspresentonlyverycloseto thesurface.
In thisstudy,we explorethesuggestion
thattheadsorption
and
reactionof HOxon aerosolsurfaces
is analternativemechanism
for
suppressh•g
HOxabundances
andretarding
therateofCOoxidation,
6O
withoutviolatingobservational
constraints
on the abundance
of
H20. Our intexat
is not to rigorouslyquantifythe effectson CO
chemistry,
butto determine
thefeasibilityof heterogeneous
chem4O
istryas a significant
HOx-suppression
mechanism.
We assess
the
potentialimpactof heterogeneous
processes
involvingtheMartian
References:1, Buch and Zhang [1991]; 2, Thrush[1965]; 3, Woodand
Wise[1962]; 4, Gershenzon
et al. [1986]; 5, Hansonet al. [1992];6, Gershenzonand Purmal [1990]; 7, Mozurkewichet al. [1987]; 8, dechet al.
[ 1982]; 9, Rozhenshtein
et al. [ 1985].
*Theoreticalvalue.
surface,as well as dust and ice aerosols,on the abundanceand
altitudedistributions
of HOxspecies.
Theeffectsof theseprocesses
are comparedto thoseof loweringthe H20 abundance,
and the
implications
for CO2chemistry
arediscussed.
MODEL
DESCRIPTION
2O
o
lO
Dust
........
7
i
........
i
........
10-s
i
........
i
........
10-3
i
.......
-1
Extinction
Coefficient
(km'4)
The potentialimpactof HOx adsorption
wasassessed
by calcu- Fig. 1. Extinctioncoefficients
of icehazeat 30 km (solidline),icehazeat
latingHOxaltitudeprofilesin thepresence
of iceand/ordustusing 50 km (dottedline), anddust(dashedline). A totalopticaldepthof • 0.2
a onedimensional
photochemical
model,including
transport.The due to dust is assumed.
ANBA•ETAL.' ADSORPHON
OVHOx ONAErOSOl.SU•,FACœS
10,93$
phere. For example,in the caseof HO2 adsorption
(H andOH solved
forthespecies
CO2,CO,02,O,O(•D),03,H20,H202,OH,
adsorption
canbetreatedsimilarly),
thesurface
chemist•ispara- HO2,H2,andH. Thegas-phase
reactions
incorporated
inthemodel
meterized as
and their rate constantsare listed in Table 2. Rate constantsfor
HO2 + Surface---} (HO2•
(1)
three-body
reactions
wereincreased
by a factorof two fromthe
tabulated
values
toaccount
fortheefficiency
ofCO2asa thirdbody
(HO2)s+ OH •
H20 + 0 2
Net: HO2 + OH •
H20 + 0 2
(2)
[YungandDeMore, 1982].
BoundaryConditions
In ourmodelcalculations,
the numberdensitiesof 02, CO and
values
(2.34x 1014
cm'3,1.26
where(HO2)sdenotes
theadsorbed
species.
At steadystate,therate CO2at0 kmwerefixedtoobserved
'3,and2.05x 10•7cm'3,
respectively).
Theabtmdance
of
of thenetreaction
mustequaltherateofreaction
(1). Thus,surface x 10•4cm
H20 wasalsofixedat theground.Two H20 casesweremodeled;
chemistry
canbethought
of ascatalyzing
thedestruction
of HOx.
onewithanintegrated
coltann
abtmdance
of 16prgm,andtheother
Homogeneous
Chemistry
with1.6prgm.Following
Shimazala'
[1989],H20 wasmodeled
as
We haveconsidered
the gasphasechemistry
of a CO2-H20 well-mixedupto a "criticalaltitude,"abovewhichsaturation
was
atmosphere.The continuityequation,includingtransport,
was assumed.Theheightof thecriticalaltitudeis determined
by the
TABLE 2. Gas-Phase
Reactions
in theMartianAtmosphere
Reaction
Rla
02+hv
Rib
R2a
R2b
R3a
03 + hv
H20+hv
R3b
R3c
R4
R5a
H202+ hv
CO2+ hv
R5b
R6
R7
R8
R9
RI0
Rll
R12a
R12b
R13
R14
RI5
R16
R17
R18
R19a
R19b
R19c
R20
R21
R22
R23
R24
R25
R26
R27
R28
R29
R30
R31
R32
R33
20 + M
20 + 02
O + 02 + CO2
O + 03
O + CO + M
RateCoefficient*
.-•
20
1.1x10'7
-•
O + O(ID)
6.8x 10'7
.-•
02 + O
3.3 x 10'4
-•
02 + O(ID)
2.1x 10'3
-•
H2+ O(ID)
-}
H+OH
2.7x10 '6
1.7x 10'7
-}
-.•
-.•
2H+O
2OH
CO + O
-•
CO+ O(ID)
1.4x 10'7
-}
--}
-•
--}
-}
02 + M
03 + O
03 + CO2
202
CO2+ M
4.3 x 10'28T-2.00
6.4 x 10'35½663//'
5.0x 10'35e724/1'
8.0 x 10'12e
'2ø•ø/r
6.5 x 10'33e'2184/r
2.0 x 10'7
2.9 x 10'5
3.7 x 10'7
O(ID)+O2 -• O+O2
O('D) +O3 -}
--}
3.2X10'11e7ø/r
202
1.2X 10'10
02 +20
-•
-•
H + 03
H +HO2
--}
--}
-•
--}
O+H2
--}
O+OH
--}
O + HO2 -•
O + H202 -•
2OH --}
2OH+ M -}
OH + 03
OH + H2
OH + HO2
OH + H202
-•
--}
--}
-}
H2+ M
HO2+ M
OH + 02
2OH
H2+ 02
H20 + O
OH+H
O2+H
OH + 02
OH + HO2
H20 + O
H202+ M
HO2+ 02
H20 + H
H20 + 02
H20 + HO2,
3
3
1.2x 10'1ø
3
3
3
3
1.5x 10'29T -1.30
ko= 5.2x 10'28T-l.60
5
3
k•o= 7.5 x 10'11
1.4x 10'10e'470//'
6.8x 10'11
2.9 x 10'12
1.4x 10'12
1.6x 10'lIe '457ø/r
2.2x10 '11e12ø/r
3.0 x 10'11e2øøIt
1.4x 10'12e'20ø0//'
4.2 x 10'12 e'240//'
/Co
= 6.6x 10'29T'0'80
• = 1.5x 10'11
1.6x 10'12•-940/T
5.5x 10'12e-2ooo/:r
4.8 x 10'11e23ø/r
3.3x 10'12e '200//'
OH+CO -• CO2
OH ++"
202
HO2+ 03 --}
2HO2 -•
2HO2+ M -}
1
2
2
3
4
1.0x 10'lø
7.4x 10'11e120//'
2.2x 10'10
O(ID)+ H2 --} H + OH
O(!D)+ CO2 --} O+CO2
O(•D)+ H20 -• 2OH
2H + M
H + 02 + M
Reference
H202+ 02
H202+ 02 + M
3
6
6
6
1
3
3
3
3
3
3
3
3
3
3
1.5
l0'13
3
1.1x
x 10'
14(1
½-•-000/•.6Patm)
3
2.3x 10'13e6ø0/T
3
1.7x 10'33e1000/I'
3
References:
1,Hampson
[1980]; 2,LinandLeu[1982];3,Demore
etal. [1990]; 4,Baulchetal. [1976];5,Tsang
andHampson[1986];6, Yunget al. [1988].
õCross
sections
used
tocalculate
photodissociation
rateconstants
forCO2,02andH20aredescribed
inAnbar
etal. [thisissue].
Cross
sections
for0 3andH202photodissociation
calculations
aresimilartothoseusedin Yung
et al. [ 1988].
'Units
ares'1forphotolysis
reactions,
cm3s'l fortwo-body
reactions,
andcm
6s'1forthree-body
reactions.
Photolysis
rateconstants
refertotheoptically
thinregion.Reactions
withCO2asa thirdbodyhavehadtheirrate
constants
increased
by a factorof twofromthevaluesgivenhere.
10,936
ANBA•ETAL.:ADSOP,•ION
O•'HOx ONAœROSOL
SU•ACœS
H20 columnabundance.In our 16 p•m and 1.6prgrnmodels,the
criticalheights
were• 20 km and3$ km,respectively.
ThetwoH20
profiles,shownin Figure2, are in agreement
with thoseof Shimazaki [1989] below 50 lan, but are somewhatlargerat higher
altitudes.
FollowingYunget al. [ 1988],the escapevelocitiesof H andH2
cales,aerosoleffects on the radiation field shouldnot have much
impactonourresults.
All computations
employed
a diurnallyaveragedradiationfield at equinoxfor • 30ølatitude.
MODEL RESULTS
HOx profileswerecalculated
for a numberof differentmodels
in Figures3-6.ModelsA
werefixedto6.76x l03cms'] and1.70x 102cms']. TheOescape (Table3), andtheresultsaresummarized
chemist•, but assanne
fluxwassetto6 x 107½m
'2s'1, inagreement
withMcElroy
I1972]. andB (Figure3) invokeno heterogeneous
water vaporabundances
of 16 and 1.6 pr[tm,respectively.The
resultsof modelB arecloseto thoseofShimazala'
[ 1989],whoalso
The eddydiffusivityprofilein theMartianatmosphere
is uncer- assumed
• 1pr[tmH20. Thedepletions
of riO2,H andOH inmodel
tian, and has been the subjectof somedebate[e.g., Kong and B relativeto modelA arelargestin theloweratmosphere
(below50
McElroy, 1977; Kahn, 1990; Atreya and Blamont,1990]. Our km), sincethelargestdifferences
in watervaporabundances
occur
resultsshouldbe insensitive
to thisprofile,overtherangeof values nearthe surface(Figure2), andthe chemicallosstimescales
for
that has been suggested.
This is due to the exceedinglyshort thesespecies
dominateovertransport
timescales.The abundances
lifethnesof theHOx speciesrelativeto thetransport
timescale.At of OH andHO2 aremoststronglyaffected;theircoluntodensities
the ground,the lifetime of the HOx family is on the orderof aredepletedby • 25% and• 65%, respectively.
Significantly,
in
[HOx]/k2810H][H02]
• 104s,whilethetransport
thnescale
canbe bothmodels,HO2isthedominantHOx species
betweentheground
OtherInputParameters
approximated
byH2/Kz
(H isatmospheric
scale
height;
Kziseddy
diffusivity)
•, 1012/105
= 107s. Therefore,
theHOxfamilyisina
and • 35 km, which is where most of the dust surfacearea is
avaflable. This is also where most of the ice surface is available h•
the low-altitudeice hazecasewe haveconsidered
(Figure1). At
higheraltitudes,wherethe ice abundance
may exceedthatof dust
(Figure1),H istheprimaryHOxspecies.
Thus,theinfluenceof HOx
adsorption
is determined
by the rateof adsorption
of HO2 on dust
and regolith,and on low-altitudeice, and of H on ice at higher
altitudes;
therateof adsorption
of OH oneithersurfaceis insignifiDue to the reactionswhichrelatethe HOx
tlfismodel
grade
exponentially
from105cm2 s'] atthesurface
to cantby comparison.
lossof oneof theseradicals
5 x 107cm2s'] at 100km,andremain
constant
athigher
altitudes.radicalsto oneanother,heterogeneous
propagates
throughtheentireHOxfamily.
Thisis similarto Shimazala'
's [ 1989]profileA.
Heterogeneous
chemistry
wasincorporated
inmodelsC1,C2,C3,
The COSPARtemperature
profilehasbeenadoptedbelow 100
16 prgtmof water
km [Seiff,1982],with an exospheric
temperature
of 365 K [Kong D 1, D2, E 1, E2 andE3, all of whichassumed
ondustandonthesurface(C1, C2 andC3)was
andMcElroy, 1977]. The temperature
profileof theupperatmos- vapor.Adsorption
considered
separately
fromadsorption
on ice(D 1,D2, El, E2 and
pherewasthm•foundby fittinga splinecurveto thesedata.
Solar flux values at 1 AU were obtained fromMount andRottman
E3) to allow the effectsof reactionson differentsurfacesto be
h• modelsC1, C2, C3, D1 andD2, onlyHO2adsorp[ 1983]andTorrandTorr [ 1985],andscaledto 1.52AU. Thecross distinguished.
ModelsE 1,E2 andE3 includeonlyH adsorpsectionsusedto calculatephotodissociation
rateconstants
are de- tionwasconsidered.
HOx-catalyzed
CO oxidationis mostrapid
scribed
inAnbaretal. [thisissue]andYungetal. [ 1988].CO2cross tion. Sincegas-phase
1989],the
sectionswere adoptedfrom Lewisand Carver [1983], and the below40 km [e.g.,KongandMcElroy,1977;Shimazala',
HOx depletionon CO chemistrycanbe
telnperature
dependence
of CO2absorption
wasmodeled
followi•g impactof heterogeneous
theprocedureofAnbar et al. [thisissue].No adjustment
wasmade assessed
to firstorderby comparing
theHOxaltitudedistributions
for aerosolscattering
or absorption
in the resultspresented
below. to this40 km "threshhold."
The abundance
of OH is particularly
We find thatincorporation
of theseeffectsintothemodelchanges critical,sincetherateof CO oxidationis limitedby therateof R30
photolyricrateconstants
by a maximumamountof 10-20%at the (CO + OH --} CO2 + H20).
ground.The magnitudeof this effect decreases
with increasing
Examination
of theC models(Figure4) revealsthatadsorption
altitude.Sincethe bulk of HOx formationoccursnear20 kin, and of HO2 on dustcansignificantly
depletesomeHOx species
in the
sinceHOx lifetimesarevery shortwith respectto transport
times- loweratmosphere,
evenfor?•-io:
--0.001(model
121).
If¾ao:= 0.01,
thetotalHOxbudgetin modelC2 approaches
thatof the 1.6prgm
lOO
watervaporcase(modelB). Althoughtheabundance
of OH isonly
reduced
by g 10%ona column-integrated
basis(Table3), thebulk
80
of thisreductionoccursbelow20 kin, wherethe rateof HOx-catalyzedCO recombination
is highestwhenheterogeneous
chemistry
isnotincluded.Thus,therateof CO recombination
mayberetarded
•
60
stateof photochemical
equilibrium. The abundances
of longerlivedspecies
(e.g.,CO and02) whichcouldbeimpacted
by differera
eddydiffusivityprofries,andhenceaffectthe partitioningof the
HOx species,havebeenfreedto observedvalues.Thus,thedistributionof the HOx speciesshouldbe independent
of the choiceof
theeddydiffusionprofile. The eddydiffusioncoefficients
usedin
ß
,
40
substantially
byadsorption
ofriO2ondust,
with?•_io:
= 0.01.If an
evenlargersurface
losscoefficient
isconsidered
(¾HO•
= 0.1;model
20
model B, and the OH abundancecomeswithin 25% of model B.
C3),thenthetotalHOxabundance
isactuallyloweredbelowthatof
o
1 oe
1 08
1 010
1 012
1 014
The plausibilityof thesevaluesof ¾is assessed
below.
Adsorptionof HO2 on ice particlescenteredat 30 lcm was
considered
in modelsD1 andD2 (Figure5), whileadsorption
of H
onahazeat 50lcmwasconsidered
in models
El,E2 andE3 (Figure
6). Adsorbed
H wasassumed
to reactwithgas-phase
H to produce
Fig. 2. H20 profilescorresponding
to globallyaveraged
watervaporabun- H2 gas,by analogywith the HO2 surfacechemistrydescribed
Number
Density
(cm'3)
dancesof 16 priam(solidline) and 1.6 prlum(dottedline).
earlier. The influenceof ice surfaceson HOx abundances,
as
ANBARETAL.:ADSORPTION
OFHOx ONAEROSOL
SURFACES
10,937
TABLE 3. Summaryof ModelR•ults: HOs ColumnAbundances
H20
Model prpm
H
Surfaces
¾
OH
x 1014
x 1012
HO2
x 1014
Z;HOx
x 1014
A
16
--
--
2.5g
1.20
5.05
7.64
B
1.6
•
•
2.51
0. gg
1.74
4.25
Cl
C2
C3
DI
D2
16
16
16
16
16
7Hoa
= 0.001
•HO2= 0.01
7HOa
= 0.1
7HOa
= 0.01
•HO2= 0.1
2.58
2.52
2.36
2.58
2.48
1.17
1.10
1.00
1.19
1.0g
4.56
7.15
E1
E2
E3
16
16
16
7H= 0.001
• -- 0.01
7H= 0.1
2.30
1.21
0.71
1.11
0.86
0.74
DustandRegolith
DustandRegolith
DustandRegolith
Iceat 30 km
Iceat 30 km
Ice at 50 km
Ice at 50 km
Ice at 50 km
3.20
5.75
1.76
4.13
4.98
7.57
4.45
6.94
5.06
7.37
5.03
6.25
5.01
5.72
Abundances
ofH, OH,HO2and]•HOsrepofi•d
as½m
'2
:
50
smaller
titaninthe30Ioncases
utilizingcomparable
values
of7(e.g.
compare
modelsD1 andE2, or D2 andE3). Evenif 7H= 0.001
(modelEl), OH isdepleted
bynearly10%relative
tomodelA. The
sensitivity
to the altitudedistribution
of the ice particlesis due
largelyto thefactthat,in thepurelygas-phase
case,thechemical
losstimescale
of OH (and,hence,itsabundance)
hasa maximum
near50lcm.Thus,theintroduction
of a heterogeneous
lossprocess
hasitsgreatesteffecton OH at thisaltitude.
Of all theheterogeneous
casesstudies,
modelsE2 andE3 have
thelargestimpacton theOH abundance,
resulting
in evenlower
H
40
30
•
20
lO
103
105
107
6o
109
Number
Density
(cm'3)
Fig. 3. Distribution
of HOxspecies
calculated
for modelA (solidline)and
5o
E
40
ß
30
modelB (dottedline).
OH //
/#
6O
<
20
ii
5O
II
10
E
40
o
1 03
•:
02
//
.............
10•
10?
10•
Number
Density
(½m'
•)
20
Fig.5. Distribution
of riO2 (dottedline),H (solidline)andOH (dashed
line)
in modelsA, D 1, andD2. Thehighestabundances
for all threespecies
are
thoseof modelA. Abundances
decrease
as¾HO
from0' 01 (model
2 increases
DI) to 0.1 (modelD2). A watervaporabundance
of 16 prpmis assumed.
10
0
103
Number
Density
(cm'3)
Fig.4. Distribution
of riO2(dottedline),H (solidline),andOH (dashed
line)
in modelsA, C1, C2, andC3. Thehighestabundances
for all threespecies
arethoseof modelA. Abundances
decrease
as¾HOfrom0.001
2 increases
(modelCI) to 0.1 (modelC3). A watervaporabundance
of 16 prpm is
8O
60
assumed.
40
expected,
isverysensitive
tothealtitudeof theicecloud.Whenthe
20
-!,
"'.,,HO2
icedistribution
iscentered
at30Ionand7HO2
= 0.01(model
D1),
onlya verysmallimpactonHOxis seenin theloweratmosphere.
Valuesof 7HO2
ontheorderof 0.1 (modelD2) arenecessary
to
0
103
105
107
10ø
achievean OH abundance
comparable
to thatof modelC2. Even
highervalueswouldbenecessary
to approach
thedepletions
found
Number
Density
(cm'3)
in thecaseof low waterabundance
(modelB). The inclusionof H
adsorption
on ice in theloweratmosphere
wasfoundto havelittle Fig. 6. Distributionof HO2 (dottedline), H (solidline),andOH (dashed
effecton HOx abundances.
If theicedistribution
maximumisshiftedto • 50 km (modelsE 1,
line) in modelsA, El, E2, andE2. The highestabundances
for all three
speciesarethoseof modelA. Abundances
decrease
as¾Hincreases
from
0.001(modelEl) to 0.1 (modelE3). A watervaporabundance
of 16priam
E2andE3),thepredicted
abundances
of H andOH aresubstantially is assumed.
10,938
,•qBARETAL.:ADSOP,•T•ON
OFHOx ONAEROSOL
SURFACES
abundancesthan in model B. However, due to the ice particle
DISCUSSION
AND CONCLUSIONS
distribution,
thisdepletionoccursprimarilyabove40 km (Figure
Our findingsindicatethatheterogeneous
chemistryon aerosols
5), wheretherateof HOx-catalyzed
CO recombination
isquitelow
of HOxspecies
in theMartian
[e.g.,Kong
andMcElroy,1977;Shimazaki,
1989].Thus,heteroge- is capableof loweringtheabundance
Adsorption
of riO2oneitherdustoricesurfaces
below
neousprocesses
onicesurfaces
at thesealtitudes
arelikelyto have atmosphere.
40 km couldhavea non-trivialimpacton the abtmdance
of HOx,
onlya smalleffectonCO2stability.
andhenceontheratioof CO: CO2; thepredicteddepletions
of OH
(• 10% or more)approachthoseachievedby loweringthe H20
COMPARISONS WITH LABORATORY ADSORPTION DATA
abundance
from 16 prgm to 1.6 pr•m (• 25 %), which could
retardtherateof CO oxidation.The)' valuesrequired
Theplausibility
of therangeof), valuesdescribed
abovecanonly significantly
be assessed
by makingcomparisons
to laboratory
data. Unfortu- forsuchaneffect(),HO2
> 0.01ondust;),HO2
> 0.1 onice)arenot
whencompared
to the few laboratorydataavailable.
nately,thenmnberof studiesof HOx adsorption
is small,andmost unreasonable
on sursurfacesthat have been studiedare not directlyrelevantto the However,thereis a clearneedfor laboratoryexperiments
of Martianaerosols,and at temperatures
apMartiansurfaceor aerosols.Theyare,at best,crudeanalogsof the facesrepresentative
reactivesurfaces
availableonMars,whicharelikelytobecomposed proachingthoseof thelowerMartianatmosphere.
Adsorptionof H on ice near 50 km altitudecan alsoresultin
of complex,iron-richoxide and silicateweatheringproductsof
HOx depletions,
if),H > 0.01.However,thesedepletions
basalts[e.g.,Bell et al., 1990;Morris et al., 1990;Pinet and substantial
Chevrel,1990].Nonetheless,
we believethe publisheddataare are confinedto altitudesat which the rate of HOx-catalyzedCO
oxidationisalreadylow. Additionally,thesevaluesof•-i arehigher
usefulto provideorder-of-magnitude
estimates
of),.
Table 1 summarizes
the publishedvaluesof HOx surfaceloss than observedin the only relevantlaboratorystudy. Thus,little
on
coefficientson inorganicoxides. HOx adsorption
dataon metal impacton the chemistryof CO is expectedfrom H adsorption
surfaces
havenotbeenincluded,althoughthesetendtobe substan- ice.
While this studyhas demonstrated
the potentialinfluenceof
tiallylargerthanthevaluesonoxides[e.g.,Thrush,
1965].We have
che•nistry,
a criticalre-examination
of Martianatalsoomitteddatafromearlystudies
whichindicaterelativelylarge heterogeneous
valuesof •-Io, on oxidesurfaces
[e.g.,Smith,1943];we arenot toosphericchemistryis requiredto rigorouslyquantify'theimportanceof suchreactions.For example,theuncertainties
associated
confidentof thereliabilityof theseearlymeasurements.
usedin thismodelareas largeas
Most datawerecollectedat or nearroomtemperature.At lower with someof the rate constants
with somewhathigheruncertainties
at
temperatures,
it is expectedthat ¾shouldincrease.This canbe +30% at roomtemperature,
understood
if thedatarepresent
anequilibriumbetweenanenergeti- Martian temperatures.Thus, it may be possibleto reducethe
of HOxradicalsin themodeledatmosphere
by manipucallyfavorableadsorption
reaction,andanendothermic
desorption abundance
process.Such behaviorhas been observedfor OH and HO2 on
H3PO4,wherelosscoefficientsincreaseby an orderof magnitude
as temperature
is loweredfrom298 K to 220 K [Margitan,1976;
Howard, 1979]. This observation
is of specialimportance
to the
Martianatmosphere,
wheretypicaltemperatures
are< 220 K.
lating key rate constantswithin the reporteduncertainties.The
abundance
of HOx is controlled
by therateof production,
via H20
photolysis
withacontribution
fromO(•D)+ H20--}2OH(R15)at
the surface,andtherateof loss,dominated
by thereactionOH +
HO2 --} H20 + 02 (R28). Thus, the abundanceof HOx can be
bydecreasing
theproduction
rate(R3a+ K15[O(lD)])
TheHO2datademonstrate
thata ¾HO2
value> 0.001is entirely decreased
[H20], increasing
therateconstant
K28, or a combination
of these
changes.The impactof reasonable
"re-evaluations"
of thesegasphasedatamustbe quantified.
Additionally,reactions
involvingHOxareconsidered
criticalto
manyprocesses
in theMartianatmosphere
(e.g.,theabundance
of
03, theescaperateof H, andthecouplingof H escapeto O escape).
mustbe suppressed,
we mayneedto reviseour
[Jechetal., 1982];inTable1,¾oH
and¾Ho2
aretypically
withinan IfHOx abundances
of thechemistry
governing
suchprocesses.
Forexamorderof magnitudeof eachotherfor comparable
surfaces.
More- explanations
that03 lossis dominated
by
over,the OH dataarethemostcomprehensive,
sincetheycovera ple, althoughit is generallyassumed
varietyof complexoxides,includingsomecontaining
iron.If the reactionwith HOx [e.g.,Lindner,1988],catalyticNOx chemistry
OH dataare usedto supplement
the HO2 databy inference,then may becomeimportantif HOx levelsare depressed.Hence,the
of nitrogen[Yunget al., 1977]shouldbe re-exam¾Ho•
• 0.01seems
a reasonable
estimate
foradsorption
onMartian photochemistry
dustandregolith,particularly
at lowtemperatures.
Highervaluesof inedin lightof thiswork.
It must be realizedthat the inclusionof heterogeneous
HOx
¾arepossible,
butarenotwell supported
bythelaboratory
data.The
adsorption
affinity of HO2 for ice is unknown,but is likely to be chemistryintroducesa complicatedvariableinto Martian atmosof H, OH andHO2
> 0.01onthebasisof comparison
with dataforadsorption
onliquid phericmodeling,sincethealtitudedistributions
consistent with the few studies that have been done. A value of 0.01
onpotentialMartiansurfaces
isplausibleif temperature
effectsare
considered
(seeabove). T'nisconclusion
is strengthened
if theOH
dataareconsidered.
Althoughadsorption
of OH is probablynotan
importantHOx sinkin the Martian atmosphere,
we includethese
databecauseOH andHO2 havesimilaraffinitiesfor manysurfaces
water,aswell astheOH data.A valueashighas•-Io• • 0.1 is
are sensitive to the altitude distribution of the aerosol surfaces
plausible.
The adsorptionof H on ice is more problematic. The only
laboratorystudyindicates¾H< 0.001 [Gershenzon
et al., 1986],
which is too small for an appreciableeffect in the atmosphere.
However,a theoreticalstudyof adsorption
onamorphous
ice suggestSyH>0.1 at < 150K[BuchandZhang,1991]. Theapplicability
of this resultto Martian ice surTaces
is doubtful,sincetypical
Martianatmospheric
temperatures
are higherthanthoseat which
amorphous
iceis stable.Thus,theadsorption
of H is unlikelyto be
an importantprocess.
(comparemodelsC2,D 1,andE2). Like thewatervaporabundance,
thealtitudedistribution
of aerosolsurTaces
(especiallyicesurfaces)
varieslatitudinally,
seasonally
anddiurnally.Thus,heterogeneous
HOxchemistry
mustbeintroduced
withcare,andshouldnotbeused
to "solve"modelingdifficultiesuntil all thereasonable
permutationsof gas-phase
chemistryhave been explored. Despitethis
caveat,the importanceof quantifyingthe role of heterogeneous
chemistryin theatmosphere
of Mars cannotbe overlooked.In the
comparative
planetology
of atmospheres,
Marsissaidtopossess
the
"dustiest"atmosphere.
The impactof aerosols
onMartianatmos-
ANBAR
ETAL.:ADSORPTION
OFHOxONAEROSOL
SURFACES
10,939
12, 529-535, 1974.
phericchemistry
is largelyunknown,
despite
someimaginative
but
Jakosky,
B. M., andC. B. Farmer,Theseasonal
andglobalbehavior
ofwater
unproven
speculations
[Huguenin,
etal. 1977;AtreyaandBlamont,
vaporin the Marsatmosphere:
Completeglobalresultsof the Viking
1990].Thisstudyhascombined
laboratory
dataandphotochemical atmospheric
waterdetectorexperiment,
J. Geophys.
Res.,87, 2999-3019,
modelingto demonstrate
thatheterogeneous
chemistrycouldbe
1982.
between
importantin regulatingthe abundance
of HOx. This opensthe Jech,D. D., P. G. Easley,andB. B. Krieger,Kineticsof reactions
freeradicalsandsurfaces
(aerosols)
applicable
to atmospheric
chemistry,
excitingpossibility
thattheMartianatmosphere
maybe a natural
Atmospheric
Chemistry,
Geophys.
Monogr.26, edited
laboratory
for the quantitative
studyof heterogeneous
reactions, in Heterogeneous
by D. R. Sehryer,10%121,AGU, Washington,
D.C., 1982.
whichare now considered
importantin the terrestrialatmosphere Kahn, R., Ice haze,snow,andthe Mars water cycle,J. Geophys.Res., 95,
[Molinaetal., 1987;Tolbertet al., 1988;Leu, 1988;Michelangeli 14,677-14,693, 1990.
etal., 1991].
Kong,T. Y., andM. B.MeElroy,Photochemistry
oftheMartianatmosphere,
Icarus, 32, 168-189, 1977.
Krasnopolsky,
V. A., Photochemistry
of the Martianatmosphere
(mean
Acknowledgments.
Theauthors
thankD. Michelangelifor providingaeroconditions),
Bull.Am.Astron. Soc.,23, 1212,1991.
sol data in electronicformat. The assistance
of M. Allen was greatly
Leu, M-T., Laboratorystudiesof stickingcoefficients
andheterogeneous
appreciated.The commentsof two anonymous
reviewerswereextremely
reactions
important
in theAntarcticstratosphere,
Geophys.
Res.Lett., 15,
helpful.Thisresearch
wassupported
by NASA grantNAGW-2204. Divi17-20, 1988.
sionof GeologicalSciences,
CaliforniaInstitute
of Technology,
contribution
Leu, M-T., J. E. Blamont, A.D. Anbar, L. F. Keyser,and S. P. Sander,
5114.
Adsorptionof CO on water-iceand oxide surfaces:Implicationsfor the
Martianatmosphere,
J. Geophys.Res.,97, 2621-2627, 1992.
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(ReceivedFebruary3, 1992;
revisedNovember 13, 1992;
acceptedJanuary14, 1993.)