GEOPHYSICAL RESEARCH LETTERS, VOL. 16, NO. 8, PAGES 941-944, AUGUST 1989

STRATOSPHERIC AEROSOLS FROM CI-I4 PHOTOCHEMISTRY ON NEPTUNE

Paul. N. Romani

ScienceSystems and Applications,Inc. SushilK. Atreya Departmentof Atmospheric,Oceanic and Space Sciences, The Universityof Michigan

Abstract. We have used a combined photochemical- them to surviveto much lower presstires.PH3 and/orNH3 condensationmodel to studyhydrocarbon ices produced from photochemistrywhich can also produce aerosols [West et CH4 photolysisin the stratosphereof Neptune. We predicta 1986], will not occur on Neptune becauseof the removal of total stratospherichaze production rate of 4.2 X 10-15grams theparent species in the lower troposphereby condensation. cm-2 s-1 (75%ethane, 25% acetylene, trace diacetylene). The Previously in our modelling we had assumedan abrupt total productionrate is insensitiveto within a factorof two to conversionof the photochemicalprofile to a saturationlimited orderof magnitudechanges in the eddy diffusioncoefficient one. We now include a loss processdue to condensationin and methane mixing ratio, which is within our estimate of the model. This improvementallows for a bettercalculation of uncertaintyfor this number. The condensationtemperatures the total haze productionrate, the partitioning of the haze are 97 K for C4H2, 71 K for C2H2, and 64 K for C2H6. productionrate amongthe condensingspecies, and the effects Voyager 2 images of Neptune will be able to confirm the condensationhas upon the mixing ratio profiles of the presenceof stratosphericaerosols and provide constraintson condensingspecies. theirproduction rate andlocation. Numerical Model Introduction Phot0chemical Analysisof highphase angle Voyager 2 imagesof Uranus The photochemicalpart of this modelused in this studyhas done in Pollack et al. [1987] showedfirst, the presenceof beendescribed previously [Romani and Atreya, 1988]. The stratospherichazes and second,combined with a cloud major changesince then is the inclusionof C4H2 chemistry microphysicsmodel placed constraints on theproduction rate into the model;no longeris it solvedfor separately.Acetylene and location of the aerosols. On the eve of Voyager 2's is the parent molecule for diacetylene, but diacetylene encounterwith Neptune we presentour latest modelling undergoesrecycling to acetylene. Including both speciesin predictions about the production rates, location, and the photochemicalmodel allows for feedbackto occurbetween compositionof hazesfrom methanephotochemistry in the the two. The modelis onedimensional and solves the coupled stratosphereof Neptune. continuityequations for methaneand its photolysisproducts There is evidence for stratosphericaerosols on Neptune by using an iterative Newton-Raphsontechnique. Eddy from groundbased observations in the near IR. The 0.9gm mixing is parameterizedby the use of an eddy diffusion methaneband of Neptunehas a residualintensity of 1%. In coefficient, K. The molecular diffusion coefficients for the the absenceof any stratosphericaerosols it would have a chemicalspecies are the sameas beforewith the exceptionof residual intensity of only 0.1% [Bergstralhet al., 1987]. C4H2 whichis estimatedusing techniques given in Reid et al. Bergstralhet al. deducean opticaldepth of 0.1 to 0.25 at [1977]. Methane (CH4), methyl radical (CH3), ethylene 0.9gm dependingon the pressurelevel of the hazes(0.001- (C2H4), acetylene(C2H2), ethane(C2H6), diacetylene(C4H2) 0.02 bar). RecentlyHammel [1988] from an analysisof the and atomic hydrogen (H), undergo eddy and molecular center-to-limbprofile of this methaneband deducedthe diffusion and photochemicalreactions. The radicals; CH, presenceof somescattering material above the 0.005 bar level. 3CH2, 1CH2, C2H, C2H3, C2H5, C4H, and C4H3 are The likely source of these aerosolsare hydrocarbons assumed to be in photochemical equilibrium. From producedby the photolysisof methane. We have shown comparison of model results to observations of the previouslythat photochemical production of ethane,acetylene, hydrocarbonsin the stratospheresof Jupiter, Saturn, and and diacetylenefrom methanephotolysis will lead to Uranus, we estimate an uncertaintyof a factor of two in the productionof theirrespective hydrocarbon ice hazesin the predictedmixing ratios. 0.002-0.01bar region [Romani and Atreya, 1988]. The other Fixed-pointboundary conditions and a convergencecriteria possiblesource of stratosphericaerosols is methaneice of 1% was used for all studies. Changes in the lower crystalsbeing transportedfrom the methaneice cloud boundaryvalues of C4H2, C2H2, and C2H6 by an order of formationregion in thetroposphere (= 1.5 barlevel) into the magnitudedid not propagatemore than 2 levels above the stratosphere.Presently it is believedthat sublimating CH4 ice lower boundary. The upper boundary values for the crystalsare the sourceof the observedsupersaturation of hydrocarbonswere adjustedso therewas zero net flux at the methaneabove the cold trap on Neptune,2% vaporphase upperboundary (placed within an atmosphericscale height mixingratio [Orton et al., 1987]. However,the sublimating above the homopause),while the upper boundaryvalue for methaneice crystalswill be in equilibriumwith thismixing atomic hydrogenwas adjustedso there was a net downward ratio at =64K, (about0.02 bar level), and we do not expect flux of 4 X 107molecules cm -2 s-1. The H productionby solar EUV (photons and photoelectrons) at Jupiter correspondingto the time of the Voyager encountersin 1979 was 1.3 X 109cm -2 s-1. Scalingthis to Neptunewould Copyright 1989 by the American Geophysical Union. givea productionrate of 3.5X 107.The EUV fluxat the time of the Neptuneencounter is expectedto be greaterthan that in Paper number 89GL00936. 1979, as it is alreadyat that level now. So we adjustedthis 0094-8276 / 89 / 89GL-00936503.00 numberslightly upward.

941 942 Romaniand Atreya: Neptune--Stratospheric Aerosols

Condensation Loss and Uranus [West et al., 1986, Tomasko et al., 1984, Pollack et al. 1987]. The lossprocess of the condensinghydrocarbons; C4H2, First the model was run without condensation and the C2H2, and C2H6 from is modelledby the diffusive growth condensation levels were determined to be where the rate for ice crystals- vapor phasemolecules strike an ice equilibriumsaturation abundances fell below the respective crystaland stick to it. Thediffusive mass growth rate (gm s-1) photochemicalones. The resultantcolumn densities were then per crystalis takenfrom Pruppacherand Klett [1980]; usedas input into the model, but now with the condensation loss turned on. The new condensation levels and photochemicalproduction rates were usedto calculatenew dm 4•CS VpD'M (1) column densities which were then fed back into the model. dt RT Thiswas repeated until convergenceoccurred. For simplicity we assumedthat eachspecies condenses only on its own ice Where C is a proportionalto the crystalsize and a functionof particle. We also assumedthen ice crystalswere distributed the crystal geometry,S the supersaturation,T the absolute uniformlyin heightfrom the respectivecondensation level to temperature,R the universal gas constant,Vp and M are the tropopause. respectivelythe vapor pressure and the molecularweight of the condensing species, and D' is the molecular diffusion Discussion coefficientof the condensingspecies corrected for gaskinetic effectsfor smallcrystals; Mixing Ratio Profilesof C4H9.,C•.H•, and C•H6

In Figures1 - 3 we comparethe mixing ratio profilesof D' - D C4H2, C2H2, and C2H6 from the combinedphotochemical- condensationmodel to their respectivesaturation limited f2•M•1/2 (2) profiles. The relevant model parametersfor this standard c+-•• RT j model (for the purposeof this discussion)are as follows;an eddydiffusion coefficient of 106cm -2 s-1 at the methane õ is the"thermal jump distance" which is takento bethe mean homopauseand inverselyproportional to the atmospheric free path of the atmosphericmolecules, a the sticking number density, same model atmosphereas Romani and efficiency,and D the standardmolecular diffusion coefficient. Atreya [ 1988], solarmaximum fluxes (present at the time of In our modelswe assumeda stickingefficiency of unity. For the Voyager2 encounter),diurnally averaged solar fluxes and small crystals the second term in the denominatorof (2) a solarzenith angle of 50ø (globalaverage conditions) and a methanemixing ratio of 2% at the lower boundary. The dominatesand the crystal grows proportional to C2; for large condensationlevels are as follows; C4H2, 97K and 0.005 bar, crystalsthe first termdominates and D' approachesthe limit of C2H2, 71K and 0.017 bar, and C2H6, 64K and 0.027 bar. D and the crystalgrows as only C. For consistencywith our other calculationswe assumedthe crystalsto be spherical,in Since C2H2 and C2H6 are controlledby eddy diffusionthe whichcase C equalsthe radius. For crystalsof approximately effectsof thecondensation sink propagate several scale heights equalsize the variabilityof C is within a factorof 3. The total above the condensationlevel. Diacetylene is close to ice haze productionrate is controlledby the photochemical photochemicalequilibrium so its profile showsthe sharpest destructionof methaneand the condensationloss rate adjusts turn over from a photochemicalprofile to a saturationlimited one. the supersaturationto match this. So this variability of C introducesa variabilityin the supersaturations.Similarly, if we Diacetylene,unlike acetylene or ethane,is supersaturatedin had reducedthe stickingefficiency there would have beena its condensationregion. C2H2and C2H6 are produced above correspondingincrease in the supersaturation.To determine their condensationregion and are transportedby eddy the supersaturationswe used the same vapor pressuresas diffusioninto it. While someof this occursfor C4H2, mostof before[Romani and Atreya, 1988] with the exceptionof C4H2 its haze productionis in situ productionwith chemical where we have usedthe new laboratorydata of Hudsonet al. [1988]; 10-7 ...... I ...... I ...... I ...... I ...... I ...... [ ...... I I I IllIll] ......

- _ log10(Vp) =9.582 2023.8 T (3) 10-6 whereVp is the vaporpressure of C4H2 in mm Hg, andT is _ the temperaturein degreesKelvin. We are still forced to m 10-$ extrapolatethe vapor pressure for C4H2over 30øK. The likely bias in doing so is to predict vapor pressurestoo large and _ thus a condensationlevel that is higher in temperaturethan • 10-4 actual.

_ Sincethe abovevapor phaseloss process is per crystal,a • 10-a number density proffie of the hydrocarbonice crystals is

__ required. We assumedthat all downwardtransport of the _ hydrocarbons through the cold trap is by ice crystal 10-z ...... • -

sedimentation. At equilibrium, the photochemicalcolumn _ productionrate of each speciesis balancedby the column - _ densityof aerosolstimes an inverse-lifetime.For lifetimeswe 1 -• ----,,..,,..I. ,.,.,,.I ,,,.,.,,I , ,..,,,,I,,,, ....I .,,,.,,,I , ,,.,.,l , ,,,i,,,I, used the time it takes for the aerosols to fall from the 010_i8 10_i ? 10_io 10_i$ 10_i4 10_ia 10-•z 10-ll 10-•ø 10-ø C4Hz MIXING RATIO condensationlevels to the tropopause(0.1 bar level). For the sizesof particlesin our analysis(0.1, 0.2, 0.5, and 1.0 gm Fig. 1. Diacetylenemixing ratio vs. pressure on Neptune.The radii) the fall velocitieswere calculatedby multiplyingtheir dashedline is themixing ratio from saturationequilibrium over respectiveStokes velocities by the Cunninghamcorrection theice andthe solidline is from thecombined photochemical- factor. This sizerange of particlescorresponds to therange of condensationmodel. See text for model atmosphere particlesobserved for stratospherichazes on Jupiter,Saturn, assumptions. Romaniand Atreya: Neptune-- StratosphericAerosols 943

10-? UV, haze productionis primarily a photonlimited process. However, the location of the t = 1 level does affect the

10-• chemistrythat occursafter methanephotolysis. If this moves to higher numberdensities (decreasing the 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 total hazeproduction rate varied by lessthan a factorof two to the orderof magnitudechanges in the eddy diffusion coefficient and methanemixing ratio described _ above. The compositionof this haze also remainedunchanged to within a factor of 2 to these changes;the C2H2/C2H6 ratio 10-z ...... __ stayedthe sameand C2H6 productionwas = 75% of the total. _

_ _ However, the C4H2 productionrate changedby factorsof 3-

_

_ - 15, andin the oppositesense as the totalhaze production rate. •, Lowering the eddy diffusion coefficient in the lower 10-14 10-la 10-•' 10-• 10-•0 10-9 10-8 10-? 10-6 10-s 0-4 stratosphereincreases the vapor phase C2H2 mixing ratio and C•.H•. MIXING RATIO thusincreases C4H2 production;lowering the CH4 mixing Fig. 2. Sameas figure 1 exceptfor Acetylene. ratio loweredthe recyclingrate of C2H2 after it underwent photolysisand thus increased C4H2 production. The ultimate source for these hazes is the solar UV. Thus productionbalanced by condensationloss. Throughout this theproduction rate varies with solaractivity and the seasonal hazeformation region the production rate is relativelyconstant, changein the solarinsolation (the cosineof the solarzenith but the temperatureand, thus, the vaporpressure is dropping angletimes the length of theilluminated day). At thetime of exponentially.As canbe seenin Eqn. (1) for the condensation theVoyager 2 encounterwith Neptune it will benear southern lossto balancea constantphotochemical production requires summer solstice;the productionrate will be zero in the the supersaturationto increaseas the vapor pressuredrops. northernpolar night and a maximumin thesummer pole (twice C4H2 production ceases below the level of C2H 2 the global productionrate given here). The C4H2 haze is condensation,and we do not haveconfidence in the predicted producedin situso it will bedirectly affected by thevariation C4H2mixing ratio there. Supersaturationscould also develop in solar insulation. However the sourcefor C2H2 and C2H6 is for C2H2 and C2H6 if there were insufficient numbersof ice above the condensationlayer and the relevant time is the crystals on which they could condense. For the column transporttime from the sourceregion to the condensation densitieswe calculated,balancing photochemical production region.For our standardcase, the eddy transport time in the by sedimentation,this neveroccurred. lower stratosphereis on the order of 0.4 Neptune years, The magnitudeof the eddydiffusion coefficient in the lower longerthan a seasonand the 11 year solarcycle. Also stratospherecontrols how muchthe condensation of C2H2 and latitudinaltransport of thehaze particles themselves can make C2H6 affects their mixing ratio profiles above their the hazes more uniform. Here the appropriatetime is the condensationlevels. To illustratethis, in Figure 3 we show particlefall outtime. The fall timesrange from 0.2 Neptune the changesin the C2H6 profile causedby loweringthe eddy yearfor 0.1 gm radiusparticles to 0.01 for 1.0gm. diffusion coefficient from 106 to 105 cm-2 s-1 at the methane Thus the hazes will be affected by local dynamics. In homopausebut with the samevariation in numberdensity. particular,the location of thecondensation levels is controlled Lowering the eddy diffusion coefficientlowers the ethane by the temperaturesince the vaporpressures change by an productionby only 7 %. As the eddy diffusioncoefficient is orderof magnitudefor a changein temperatureof 5K. Any loweredthe gradientin mixing ratio mustincrease, since the photochemicalproduction is balancedby eddy transportinto 10-7 the condensationregion. This resultsin a more abruptcross over from the photochemical to saturation profile. We presentlyfavor a K in therange of 105to 106cm -2 s-1based 10-6 2 _ K = 1 ]K = 106 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 eddy diffusionin the stratosphere • 10-4 _ of Neptune. _ _

- _

The C2H6 andC2H2 mixing ratio profilesare not sensitive _ _ to the choice of the methane mixing ratio at the lower • 10-a boundary. Orton et al. [1987] estimate a 2% stratospheric - _ - _ mixing of CH4 which we adoptedhere. But if they usetheir - _ "warm"thermal profile the CH4 mixing ratio would drop to 10-z __a

0.2%. Using the lower amountof methanereduces the mixing - _ ratiosby a factor of 1.5. - _ xO-•o_..•, .•.-•..,.,•,,.,,,I10-'a 10-1•- 10-'•,. i,..,,[10-'0 ,,,,,,,,I 10 -9 ,, ,,,,,,I10 -o .. ,,,,,,I10 -? ,,,,,,.I 10-o ,, ,,,.,1 10 -s ,, ,,,• 10-4 Haze Productionand Variability C•.Ho MIXING RATIO

The total haze productionrate in the standardmodel is 4.2 Fig. 3. Ethane mixing ratio vs. pressureon Neptune. The X 10-15 gramscm -2 s-1, of which75% is ethane,25% dashedline is the mixingratio from saturationequilibrium over acetylene,and only a tracediacetylene. This is controlledby the ice, the solid curves are from the model with two different the photochemistry,i.e., the conversionrate of CH4 to C4H2, valuesof K at theCH4 homopause, 105 cm 2 s-1and 106 cm 2 C2H2 andC2H6. Sincefor evena saturationlimited abundance s-1 (K proportionalto the inversesquare root of the of CH4 there is enough methaneto producea t of 1 in the atmosphericnumber density). 944 Romaniand Atreya: Neptune-- StratosphericAerosols processthat causeda localized lowering of the temperature Pruppacher,H. R., and J. D. Klett, Microphysicsof Clouds would cause haze formation at lower pressures. For and Precipitation,714 pp., D. Reidel PublishingCompany, observationsin methane bands (e.g. Hammel, 1988), this Dordrecht, Holland, 1980. would causean apparentincrease in the opticaldepth as the Reid, R. C., J. M. Prausnitz, and T. K. Sherwood, The scatteringnow takesplace below a lower columnabundance of Properties of Gases and Liquids, 3rd edition, 688 pp., methane. McGraw-Hill Book Co., New York, 1977. Romani, P. N. and S. K. Atreya, Methane photochemistry Acknowledgments.We thank J. Allen, C. Hudson,and R. and haze production on Neptune, Icarus, 74, 424-445, Khanafor assistancein interpretingtheir C4H2 vaporpressure 1988. data. P. N. Romaniacknowledges support at NASA Goddard Pollack, J. B., K. Rages, S. Pope, M. Tomasko, P. N. Space Flight Center, Greenbelt, Maryland, from NASA Romani, and S. K. Atreya, Nature of the stratospherichaze contract NAS5-3014, S. K. Atreya from NSG-7404, and on Uranus: evidence for condensedhydrocarbons, J. Universityof ArizonaVoyager Project Number 070485. Geophys.Res., 92, 15037-15065, 1987. Tomasko, M. G., R. A. West, G. S. Orton, and V. G. References Teiffel, Clouds and aerosolsin Saturn's atmosphere,in Saturn,edited by T. Gehrelsand M. S. Matthews,pp. 150- 194, The Univ. of Arizona Press, Tucson, Ariz., 1984. Bergstralh,J. T., K. H. Baines, R. J. Terrile, D. Wenkert, J. West, R. A., D. F. Strobel, and M. G. Tomasko, Clouds, Neff, and B. A. Smith, Aerosols in the stratosphereof aerosols,and photochemistryin the jovian atmosphere, Neptune:constraints from near-IRbroadband imagery and Icarus, 65, 161-218, 1986. UV, blue, and near-IR spectrophotometry(abstract), Bull. 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-bandand continuum wavelengths, Sushil K. Atreya., Departmentof Atmospheric,Oceanic Ph D. thesis,128 pp. The Univ. of Hawaii, May 1988. andSpace Sciences, The Universityof Michigan,Ann Arbor, Hudson, C. M., J. E. Allen Jr., R. K. Khanna, and G. M148109-2143. Kraus, Vapor pressuremeasurements of hydrocarbonsand 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 revised:April 17, 1989; Neptuneat 8-14 and 17-23 gm, Icarus,70, 1-12, 1987. accepted:April 27, 1989.)