GEOPHYSICAL RESEARCH LETTERS,
VOL.
16,
NO.
8,
PAGES 941-944,
STRATOSPHERIC AEROSOLS FROM CI-I4 PHOTOCHEMISTRY
AUGUST 1989
ON NEPTUNE
Paul. N. Romani
ScienceSystemsandApplications,Inc.
SushilK. Atreya
Departmentof Atmospheric,OceanicandSpaceSciences,
The Universityof Michigan
them to surviveto muchlower presstires.
PH3 and/orNH3
photochemistry
whichcanalsoproduceaerosols[Westet
Abstract. We have used a combined photochemicalcondensation
model to studyhydrocarbonicesproducedfrom
CH4 photolysisin the stratosphere
of Neptune. We predicta
totalstratospheric
hazeproductionrateof 4.2 X 10-15grams
cm-2 s-1 (75%ethane,
25%acetylene,
tracediacetylene).
The
total productionrate is insensitiveto within a factorof two to
orderof magnitudechangesin the eddy diffusioncoefficient
and methanemixing ratio, which is within our estimate of
uncertaintyfor this number. The condensationtemperatures
are 97 K for C4H2, 71 K for C2H2, and 64 K for C2H6.
Voyager 2 images of Neptune will be able to confirm the
presenceof stratosphericaerosolsand provide constraintson
theirproductionrate andlocation.
1986], will not occur on Neptunebecauseof the removal of
theparentspeciesin the lower troposphere
by condensation.
Previouslyin our modelling we had assumedan abrupt
conversionof the photochemicalprofile to a saturationlimited
one. We now include a loss processdue to condensationin
the model. This improvementallowsfor a bettercalculationof
the total haze productionrate, the partitioning of the haze
productionrate amongthe condensingspecies,and the effects
condensationhas upon the mixing ratio profiles of the
condensingspecies.
Numerical
Model
Introduction
Phot0chemical
The photochemical
part of thismodelusedin this studyhas
Analysisof highphaseangleVoyager2 imagesof Uranus
done in Pollack et al. [1987] showedfirst, the presenceof
stratospherichazes and second,combined with a cloud
microphysics
modelplacedconstraints
on theproduction
rate
beendescribed
previously
[Romani
andAtreya,1988]. The
major changesincethen is the inclusionof C4H2 chemistry
intothemodel;no longeris it solvedfor separately.Acetylene
is the parent molecule for diacetylene, but diacetylene
undergoesrecycling to acetylene. Including both speciesin
thephotochemical
modelallowsfor feedbackto occurbetween
thetwo. The modelis onedimensionalandsolvesthecoupled
continuityequationsfor methaneand its photolysisproducts
by using an iterative Newton-Raphsontechnique. Eddy
mixing is parameterizedby the use of an eddy diffusion
and location of the aerosols. On the eve of Voyager 2's
encounterwith Neptune we presentour latest modelling
predictions about the production rates, location, and
composition
of hazesfrom methanephotochemistry
in the
stratosphere
of Neptune.
There is evidencefor stratosphericaerosolson Neptune
from groundbasedobservations
in the nearIR. The 0.9gm
methanebandof Neptunehasa residualintensityof 1%. In
the absenceof any stratospheric
aerosolsit would have a
residualintensity of only 0.1% [Bergstralhet al., 1987].
Bergstralhet al. deducean opticaldepthof 0.1 to 0.25 at
0.9gm dependingon the pressurelevel of the hazes(0.0010.02 bar). RecentlyHammel [1988] from an analysisof the
center-to-limbprofile of this methaneband deducedthe
coefficient, K. The molecular diffusion coefficients for the
chemicalspeciesarethe sameasbeforewith theexceptionof
C4H2whichis estimatedusingtechniques
givenin Reid et al.
[1977]. Methane (CH4), methyl radical (CH3), ethylene
(C2H4),acetylene(C2H2),ethane(C2H6),diacetylene(C4H2)
and atomic hydrogen (H), undergo eddy and molecular
diffusion and photochemicalreactions. The radicals; CH,
presence
of somescattering
materialabovethe0.005barlevel.
The likely sourceof these aerosolsare hydrocarbons
producedby the photolysisof methane. We have shown
previously
thatphotochemical
production
of ethane,acetylene,
and diacetylenefrom methanephotolysiswill lead to
3CH2, 1CH2, C2H, C2H3, C2H5, C4H, and C4H3 are
assumed to be in photochemical equilibrium. From
comparison of model results to observations of the
hydrocarbonsin the stratospheresof Jupiter, Saturn, and
Uranus, we estimatean uncertaintyof a factor of two in the
production
of theirrespective
hydrocarbon
ice hazesin the predictedmixing ratios.
Fixed-pointboundaryconditionsanda convergence
criteria
0.002-0.01barregion[RomaniandAtreya,1988]. Theother
possiblesourceof stratospheric
aerosols
is methaneice of 1% was used for all studies. Changes in the lower
crystalsbeing transported
from the methaneice cloud boundaryvalues of C4H2, C2H2, and C2H6 by an order of
formation
regionin thetroposphere
(= 1.5barlevel)intothe magnitudedid not propagatemore than 2 levels above the
stratosphere.
Presently
it is believed
thatsublimating
CH4ice lower boundary. The upper boundary values for the
hydrocarbons
were adjustedso therewas zero net flux at the
crystalsare the sourceof the observed
supersaturation
of
methaneabovethe cold trap on Neptune,2% vaporphase upperboundary(placedwithin an atmosphericscaleheight
mixingratio[Ortonet al., 1987]. However,thesublimating abovethe homopause),while the upper boundaryvalue for
methaneice crystalswill be in equilibriumwith thismixing atomic hydrogenwas adjustedso there was a net downward
cm-2 s-1. TheH production
by
ratio at =64K, (about0.02 bar level), and we do not expect flux of 4 X 107molecules
solar EUV (photons and photoelectrons) at Jupiter
corresponding
to the time of the Voyager encounters
in 1979
Copyright
1989 by the American Geophysical
was 1.3 X 109cm-2 s-1. Scalingthisto Neptunewould
giveaproduction
rateof 3.5X 107.TheEUV fluxatthetime
Union.
of the Neptuneencounteris expectedto be greaterthanthatin
1979, as it is alreadyat that level now. So we adjustedthis
numberslightlyupward.
Paper number 89GL00936.
0094-8276 / 89 / 89GL-00936503.00
941
942
RomaniandAtreya: Neptune--Stratospheric
Aerosols
Condensation
Loss
and Uranus [West et al., 1986, Tomasko et al., 1984, Pollack
et al. 1987].
The lossprocessof the condensinghydrocarbons;
C4H2,
C2H2, and C2H6 from is modelledby the diffusive growth
rate for ice crystals- vapor phasemoleculesstrike an ice
crystal
andsticktoit. Thediffusive
massgrowth
rate(gms-1)
per crystalis takenfrom Pruppacher
andKlett [1980];
First the model was run without condensation and the
condensation levels were determined to be where the
equilibriumsaturationabundances
fell below the respective
photochemical
ones. The resultantcolumndensitieswere then
usedas input into the model,but now with the condensation
loss
dm
4•CS
dt
VpD'M
(1)
RT
Where C is a proportionalto the crystalsizeanda functionof
the crystal geometry,S the supersaturation,
T the absolute
temperature,R the universal gas constant,Vp and M are
respectively
thevaporpressure
andthemolecularweightof the
condensing species, and D' is the molecular diffusion
coefficientof the condensing
speciescorrectedfor gaskinetic
effectsfor smallcrystals;
D' -
D
c+-••
turned
on.
The
new
condensation
levels
and
photochemicalproductionrates were usedto calculatenew
f2
•M•1/2
RT j
(2)
column densities which were then fed back into the model.
Thiswasrepeateduntilconvergence
occurred.For simplicity
we assumedthat eachspeciescondenses
only on its own ice
particle. We alsoassumedthen ice crystalswere distributed
uniformlyin heightfrom the respectivecondensation
level to
the tropopause.
Discussion
Mixing Ratio Profilesof C4H9.,C•.H•, and C•H6
In Figures1 - 3 we comparethe mixing ratio profilesof
C4H2, C2H2, andC2H6 from the combinedphotochemicalcondensationmodel to their respectivesaturationlimited
profiles. The relevant model parametersfor this standard
model (for the purposeof this discussion)are as follows;an
eddydiffusioncoefficientof 106cm-2 s-1 at the methane
õ isthe"thermal
jumpdistance"
whichistakentobethemean
free path of the atmospheric
molecules,a the sticking
efficiency,andD the standardmoleculardiffusioncoefficient.
In our modelswe assumeda stickingefficiencyof unity. For
small crystals the second term in the denominatorof (2)
dominates
andthecrystalgrows
proportional
toC2;forlarge
crystalsthefirst termdominatesandD' approaches
thelimit of
D and the crystalgrowsas only C. For consistency
with our
othercalculationswe assumedthe crystalsto be spherical,in
whichcaseC equalstheradius.For crystalsof approximately
equalsizethe variabilityof C is within a factorof 3. The total
ice haze productionrate is controlledby the photochemical
destruction
of methaneandthe condensation
lossrateadjusts
the supersaturation
to match this. So this variability of C
introduces
a variabilityin thesupersaturations.
Similarly,if we
hadreducedthe stickingefficiencytherewouldhavebeena
corresponding
increasein the supersaturation.
To determine
the supersaturations
we used the samevapor pressuresas
before[RomaniandAtreya, 1988]with theexceptionof C4H2
wherewe haveusedthe new laboratorydataof Hudsonet al.
homopauseand inverselyproportionalto the atmospheric
numberdensity, same model atmosphereas Romani and
Atreya [ 1988], solarmaximumfluxes(presentat the time of
theVoyager2 encounter),
diurnallyaveragedsolarfluxesand
a solarzenithangleof 50ø (globalaverageconditions)anda
methanemixing ratio of 2% at the lower boundary. The
condensationlevels are as follows; C4H2, 97K and 0.005 bar,
C2H2, 71K and 0.017 bar, and C2H6, 64K and 0.027 bar.
Since C2H2 and C2H6 are controlledby eddy diffusionthe
effectsof thecondensation
sinkpropagate
severalscaleheights
above the condensationlevel. Diacetylene is close to
photochemical
equilibriumso its profile showsthe sharpest
turnoverfrom a photochemical
profile to a saturationlimited
one.
Diacetylene,unlikeacetylene
or ethane,is supersaturated
in
its condensation
region. C2H2andC2H6areproduced
above
their condensationregion and are transportedby eddy
diffusioninto it. While someof this occursfor C4H2, mostof
its haze productionis in situ productionwith chemical
[1988];
10-7
log10
(Vp)
=9.5822023.8
T
(3)
whereVp is the vaporpressureof C4H2 in mm Hg, andT is
the temperaturein degreesKelvin. We are still forced to
extrapolate
thevaporpressure
for C4H2over30øK. Thelikely
bias in doing so is to predictvapor pressurestoo large and
thus a condensationlevel that is higherin temperaturethan
........ I ........ I ........ I ........ I ........ I ........ [ ........ I I I IllIll] ........
-
_
10-6
_
m 10-$
_
•
10-4
actual.
_
Sincethe abovevaporphaselossprocessis per crystal,a
number density proffie of the hydrocarbonice crystals is
required. We assumedthat all downwardtransportof the
hydrocarbons through the cold trap is by ice crystal
sedimentation.At equilibrium,the photochemicalcolumn
productionrate of each speciesis balancedby the column
densityof aerosolstimesan inverse-lifetime.For lifetimeswe
used the time
it takes for the aerosols
to fall
from
the
condensation
levelsto the tropopause
(0.1 barlevel). For the
sizesof particlesin our analysis(0.1, 0.2, 0.5, and 1.0 gm
radii) the fall velocitieswere calculatedby multiplyingtheir
respectiveStokesvelocitiesby the Cunninghamcorrection
factor. This sizerangeof particlescorresponds
to therangeof
particlesobservedfor stratospheric
hazeson Jupiter,Saturn,
• 10-a
__
_
10-z
.........................
•
-
_
-
_
1 -• ----,,..,,..I. ,.,.,,.I,,,.,.,,I , ,..,,,,I,,,, ....
I .,,,.,,,I , ,,.,.,l , ,,,i,,,I,
010_i
8 10_i
? 10_i
o 10_i
$ 10_i
4 10_i
a 10-•z 10
-ll 10
-•ø 10
-ø
C4Hz MIXING RATIO
Fig. 1.Diacetylene
mixingratiovs.pressure
onNeptune.The
dashedlineis themixingratiofromsaturation
equilibrium
over
theice andthesolidline is from thecombined
photochemicalcondensationmodel. See text for model atmosphere
assumptions.
RomaniandAtreya: Neptune-- Stratospheric
Aerosols
10-?
943
UV, haze productionis primarily a photonlimited process.
However, the location of the t = 1 level does affect the
chemistrythat occursafter methanephotolysis.If this moves
to higher numberdensities(decreasingthe eddy diffusionor
the methane mixing ratio at the lower boundary), the
conversionof CH4 to higher order hydrocarbonsdecreases
due to changes in which reaction pathways dominate.
Nevertheless,
the totalhazeproductionratevariedby lessthan
a factorof two to the orderof magnitudechangesin the eddy
diffusion coefficient and methanemixing ratio described
10-•
_
_
_
_
_
above.
_
10-z
......
__
_
_
_
_
_
10-14 10-la 10-•'
10-•
10-•0 10-9
10-8
10-? 10-6
C•.H•. MIXING RATIO
Fig. 2. Sameasfigure 1 exceptfor Acetylene.
10-s
The compositionof this haze alsoremainedunchangedto
within a factor of 2 to thesechanges;the C2H2/C2H6 ratio
stayedthe sameandC2H6 productionwas = 75% of the total.
However, the C4H2 productionrate changedby factorsof 315, andin the oppositesenseasthe totalhazeproduction
rate.
•, Lowering the eddy diffusion coefficient in the lower
0-4 stratosphere
increases
thevaporphaseC2H2mixingratioand
thusincreasesC4H2 production;loweringthe CH4 mixing
ratio loweredthe recyclingrate of C2H2 after it underwent
photolysis
andthusincreased
C4H2production.
The ultimate source for these hazes is the solar UV.
Thus
theproduction
ratevarieswith solaractivityandtheseasonal
productionbalancedby condensation
loss.Throughoutthis
hazeformationregiontheproduction
rateis relativelyconstant, changein the solarinsolation(thecosineof the solarzenith
day). At thetimeof
but the temperature
and,thus,thevaporpressure
is dropping angletimesthelengthof theilluminated
withNeptuneit will benearsouthern
exponentially.As canbe seenin Eqn. (1) for thecondensation theVoyager2 encounter
lossto balancea constantphotochemical
productionrequires summer solstice;the productionrate will be zero in the
northern
polarnightanda maximumin thesummer
pole(twice
the supersaturation
to increaseas the vapor pressuredrops.
the globalproductionrate given here). The C4H2 hazeis
C4H2 production ceases below the level of C2H 2
in situsoit will bedirectlyaffectedby thevariation
condensation,
andwe do not haveconfidencein the predicted produced
C4H2mixingratiothere. Supersaturations
couldalsodevelop in solar insulation. However the sourcefor C2H2 and C2H6 is
for C2H2 and C2H6 if there were insufficient numbersof ice
crystals on which they could condense. For the column
densitieswe calculated,balancingphotochemical
production
by sedimentation,
this neveroccurred.
The magnitudeof the eddydiffusioncoefficientin the lower
stratosphere
controlshow muchthecondensation
of C2H2and
C2H6 affects their mixing ratio profiles above their
condensation
levels. To illustratethis, in Figure 3 we show
the changesin the C2H6 profile causedby loweringthe eddy
diffusion coefficient from 106 to 105 cm-2 s-1 at the methane
homopausebut with the samevariationin numberdensity.
Lowering the eddy diffusion coefficientlowers the ethane
productionby only 7 %. As the eddydiffusioncoefficientis
loweredthe gradientin mixing ratio mustincrease,sincethe
photochemical
productionis balancedby eddy transportinto
the condensationregion. This resultsin a more abruptcross
over from the photochemicalto saturation profile. We
presently
favora K in therangeof 105to 106cm-2 s-1based
above the condensationlayer and the relevant time is the
transporttime from the sourceregionto the condensation
region.For ourstandard
case,theeddytransport
timein the
lower stratosphereis on the order of 0.4 Neptuneyears,
longerthan a seasonand the 11 year solarcycle. Also
latitudinaltransport
of thehazeparticles
themselves
canmake
the hazesmore uniform. Here the appropriatetime is the
particlefall outtime. The fall timesrangefrom0.2 Neptune
yearfor 0.1 gm radiusparticlesto 0.01 for 1.0gm.
Thus the hazeswill be affectedby local dynamics. In
particular,
thelocationof thecondensation
levelsis controlled
by the temperature
sincethe vaporpressures
changeby an
orderof magnitudefor a changein temperature
of 5K. Any
10-7
10-6
on comparison of model output to IR heteroydyne
observationsof C2H6 on Neptune (Kostiuk, pets. comm.).
m 10-s
The Voyager2 UltravioletSpectrometer
will be ableto detect
the level to which the hydrocarbonsare mixed and directly
constrainthe magnitudeof eddydiffusionin the stratosphere • 10-4
of Neptune.
The C2H6 andC2H2mixingratioprofilesare not sensitive
to the choice of the methane mixing ratio at the lower
• 10-a
boundary. Orton et al. [1987] estimatea 2% stratospheric
mixing of CH4 which we adoptedhere. But if they usetheir
10-z
"warm"thermalprofile the CH4 mixing ratio would drop to
0.2%. Usingthe lower amountof methanereducesthemixing
ratiosby a factor of 1.5.
Haze ProductionandVariability
The total haze productionrate in the standardmodelis 4.2
X 10-15 gramscm-2 s-1, of which75% is ethane,25%
acetylene,and only a tracediacetylene.This is controlledby
the photochemistry,
i.e., the conversionrate of CH4 to C4H2,
C2H2andC2H6. Sincefor evena saturation
limitedabundance
of CH4 there is enough methaneto producea t of 1 in the
_
K
=
1
K = 106
2
]
_
_
_
_
_
_
_
_
_
_
_
-
_
_
_
-
_
-
_
-
_
__a
-
_
-
_
xO-•o_..
•,.•.-•..,.,•,,.,,,I
,,,,,,,,I
10-'a 10-1•- 10-'•,.i,..,,[
10-'0
10-9,,,,,,,,I
10-o..,,,,,,I
10-?,,,,,,.I
10-o,,,,,.,1
10-s,,,,,•
10-4
C•.Ho MIXING RATIO
Fig. 3. Ethane mixing ratio vs. pressureon Neptune. The
dashedline is themixingratiofrom saturation
equilibriumover
the ice, the solid curves are from the model with two different
valuesof K attheCH4homopause,
105cm2 s-1and106cm2
s-1 (K proportional
to the inversesquareroot of the
atmospheric
numberdensity).
944
RomaniandAtreya: Neptune-- Stratospheric
Aerosols
processthat causeda localized lowering of the temperature
would cause haze formation at lower pressures. For
observationsin methane bands (e.g. Hammel, 1988), this
would causean apparentincreasein the opticaldepthas the
scatteringnow takesplacebelow a lower columnabundanceof
methane.
Pruppacher,H. R., and J. D. Klett, Microphysicsof Clouds
andPrecipitation,714 pp., D. Reidel PublishingCompany,
Dordrecht, Holland, 1980.
Reid, R. C., J. M. Prausnitz, and T. K. Sherwood, The
Propertiesof Gases and Liquids, 3rd edition, 688 pp.,
McGraw-Hill Book Co., New York, 1977.
Acknowledgments.We thankJ. Allen, C. Hudson,and R.
Khanafor assistance
in interpretingtheirC4H2vaporpressure
data. P. N. Romaniacknowledges
supportat NASA Goddard
Space Flight Center, Greenbelt, Maryland, from NASA
contract NAS5-3014, S. K. Atreya from NSG-7404, and
Universityof ArizonaVoyagerProjectNumber070485.
Romani, P. N. and S. K. Atreya, Methane photochemistry
and haze production on Neptune, Icarus, 74, 424-445,
1988.
Pollack, J. B., K. Rages, S. Pope, M. Tomasko, P. N.
Romani,and S. K. Atreya, Nature of the stratospheric
haze
on Uranus: evidence for condensedhydrocarbons,J.
Geophys.Res.,92, 15037-15065, 1987.
Tomasko, M. G., R. A. West, G. S. Orton, and V. G.
References
Bergstralh,J. T., K. H. Baines, R. J. Terrile, D. Wenkert, J.
Neff, and B. A. Smith, Aerosolsin the stratosphereof
Neptune:constraints
from near-IRbroadband
imageryand
UV, blue, andnear-IR spectrophotometry
(abstract),Bull.
Teiffel, Clouds and aerosolsin Saturn'satmosphere,in
Saturn,editedby T. GehrelsandM. S. Matthews,pp. 150194, The Univ. of Arizona Press, Tucson, Ariz., 1984.
West, R. A., D. F. Strobel, and M. G. Tomasko, Clouds,
aerosols,and photochemistryin the jovian atmosphere,
Icarus, 65, 161-218, 1986.
Am. Astron. Soc., 19, 639, 1987.
Paul N. Romani, Code 693.2, NASA/Goddard Space
Hammel, H. B., The atmosphereof Neptune studiedwith
Flight Center,Greenbelt,MD 20771.
CCD imagingat methane-band
andcontinuum
wavelengths,
SushilK. Atreya., Departmentof Atmospheric,Oceanic
Ph D. thesis,128 pp. The Univ. of Hawaii, May 1988.
andSpaceSciences,The Universityof Michigan,Ann Arbor,
Hudson, C. M., J. E. Allen Jr., R. K. Khanna, and G.
Kraus, Vapor pressuremeasurements
of hydrocarbons
and
M148109-2143.
nitfiles (abstract), Bull. Am. Astron. Soc., 20, 857, 1988.
Orton, G. S., D. K. Aitken, C. Smith, P. F. Rouche, J.
(Received: March 9, 1989;
Caldwell, and R. Snyder, The spectra of Uranus and
Neptuneat 8-14 and 17-23 gm, Icarus,70, 1-12, 1987.
revised:April 17, 1989;
accepted:April 27, 1989.)