JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. D16, PAGES 19,575-19,584, AUGUST 27, 1997
The susceptibility of ice formation in upper tropospheric clouds
to insoluble aerosol components
PaulJ. DeMott, David C. Rogers,andSoniaM. Kreidenweis
Departmentof Atmospheric
Science,ColoradoStateUniversity,FortCollins
Abstract. Ice may form by bothhomogeneous
andheterogeneous
freezingnucleationprocessesin cloudsat temperatures
below-35øC.Most investigations
havefocusedon the former
process.This paperpresentsresultsfrom adiabaticparcelmodelcalculations
that includethe
effectsof bothfreezingprocesses
in unactivatedsolutiondroplets.Uncertainties
in predicting
thehomogeneous
freezingratesare discussed
andusedto selectsolutiondropcomposition
and
freezingcharacteristics
thatbracketthoseexpectedin theuppertroposphere.
The heterogeneousfreezingratesof insolubleatmospheric
aerosolsareparameterized
basedonpublished
freezingratesof carbonaceous
particles.Processmodelsimulations
showthatthepotential
variabilityin ice formationin cirruscloudsis muchgreaterif heterogeneous
freezingnucleationis considered
in additionto homogeneous
freezing.The impactof insolubleaerosolson ice
formationis inferredto increasewith insolubleparticlesizeandwith thefractionof soluble
aerosolscomabringinsolublecomponents.
The maximram
impactof heterogeneous
nucleation
isindicated
forvertical
motions
lessthan0.2ms-•andforinsoluble
components
beingassociatedwith at least 10% of all solubleaerosols.The wide rangeof ice crystalconcentrations
observedin cirrusis mostconsistent
with the occurrenceof bothheterogeneous
andhomogeneous
ice formationprocesses.
Theseconclusions
are partiallysupported
by existingobservations
of
aerosolsandcloudmicrophysical
characteristics
in uppertropospheric
cloudsbut requirenew
measurements for confirmation.
1. Introduction
2. Ice Formation Processesin Upper
Tropospheric Clouds
Cirruscloudsgenerallyform at temperatures
below -30øC
andat relativehumidity(RH) fromwell belowsaturationwith 2.1 HomogeneousFreezing Nucleation
respectto water (at coldertemperatures)
to water saturation
Homogeneous
freezingnucleationrefersto the spontaneous
[e.g., Heymsfieldand Miloserich, 1995]. Widespreadcirrus
formationof ice within the metastableliquid phase.The form midlatitudesare producedby gentlelifting processes
with
mationof ice in pure or highly dilute cloud dropletsis exlarge-scale
updraft
velocities
of 1to20cms•. Morelocalized pectedto occur over a narrow range of temperaturesfrom
cirrus-generating
regions
maycontain
updrafts
upto 1.5m s-• about -35ø to -38øC [see, e.g., Sassenand Dodd, 1988;
[Gulteppeet al., 1995]. At the warm end of the cirrustemHeymsfietdand $abin, 1989].Thispredictionandthe validity
peraturespectrum,any existingliquid clouddropletswhich
of a classicalapproachare supportedby a large numberof
areloftedand cooledareexpected
to freezehomogeneously
at
laboratorystudiesof the freezingof purewaterandverydilute
around-36øC. At lower temperatures,unactivatedsolution
solutiondroplets[see, e.g., œruppacher,1995]. At temperadroplets(hazeparticles)areexpected
to freezehomogeneously tures below the fleezing transitionzone for activatedcloud
at a RI-I below water saturation.In contrast,particlesthat act
droplets,unactivatedsolutiondropletscan freezein a manner
as heterogeneous
ice nuclei(IN) are thoughtto be presentin
akin to the pure homogeneous
process.However, elevated
concentrations much lower than cloud condensation nuclei
solute concentrations
lower the freezing rates of solution
(CCN). Consequently,heterogeneous
processeshave been
droplets,suchthat the transitiontemperaturesat which nuthoughtto be of lessimportance
in cirrus.Thispapercritically
cleation rapidly ensues are depressed in proportion
evaluatesthispremisebasedon availableknowledge.
(nonlinearly)with increasedsolutionmolality.TheoreticalinThis paper examinesthe potentialfor heterogeneous
ice
vestigationshave elucidatedthe dynamicsof homogeneous
nucleationby immersionfreezingof solutiondropletsat trofreezingof hazeparticlesandthe microphysical
consequences
pospherictemperatures
below -38øCto impactice formation
within upper tropospheric(UT) clouds [e.g., $assen and
in cirrusclouds.This problemis analyzedusing theoreti- Dodd, 1988, 1989;HeymsfieMand Sabin, 1989;HeymsfieM
cal/empiricalcalculations
of homogeneous
andheterogeneous and Mitosevich, 1993, 1995; DeMott et al., 1994; densen et
freezingnucleation.The expectedsensitivityof cloudmicro- at., 1994a,b].
physicsto naturalaerosolpropertiesis considered.
Data on
For thispaperthe adiabaticparcelmodelwithin whichcalaerosolandcloudmicrophysical
characteristics
arethenexam- culationsof homogeneous
freezingwere madeis the one de-
inedconsidering
thesecalculations
to determine
whichproc- scribedby DeMott et at. [1994]. Solutiondropletsof a given
essor combination
of processes
bestfit the observations.
size and molalityare assumedto have effectivefreezingtemperatures
[SassenandDodd, 1988]givenby
Copyright1997 by the AmericanGeophysical
Union.
Papernumber97JD01138
0148-0227/97/97JD-01138509.00
Teff= T + )• ZSTm
19,575
(1)
19,576
DEMOTT ET AL.' ICE FORMATION IN UPPERTROPOSPHERICCLOUDS
where T is haze droplettemperatureand ATtois the equilibrimn meltingpointdepression,
bothin degreesCelsius.Effective freezingtemperature
is thenusedas solutiondroplettemperaturefor substituting
into the expression
for the classical
nucleationrate for pure water [cf. DeMott et al., 1994, Eq.
(3)]. Anotherapproachwould be to explicitlycalculatethe
effect of soluteon the classicalfreezingnucleationrate by
considering
the theoreticaltemperaturedependence
of water
activity[e.g.,Jensenet al., 1991]. However,equation(1) reflectsthat solutiondropletstypicallysupercool
an amountin
additionto that predictedby their equilibriumfreezingpoint
depression[e.g., Hoffer, 1961' Pruppacherand Neiberger,
1963,Rasmussen,1982]. For ammoniumsulfate,Sassenand
Dodd [1988] suggested
that 3, = 1.7. A polynomialexpressionfor ATtoof (NH4)2SO4
as a functionof solutionmolality
(34) was determinedas
AT,
n= •f'•i
cij•i
(2)
The coefficients
c,are givenin Table 1. Thesecoefficients
differ fromthoseusedby DeMott et al. [1994] because
theirsecond-orderpolynomialwas onlyvalid to aboutM = 2. Higher
solutionconcentrations
occurat the lower temperatures
and
humiditiesconsideredin this paper.The new expression
for
ATto was derived based on a combination of tabulated data
[Weast,1981] andequilibriumfreezingpointscalculatedas in
Robinsonand Stokes[1965], usingrecentwateractivitydata
for (NH4)2SO4from Tang and Munkelwitz[1994]. Calculations otherwisefollow Deg/Iottet al. [1994], includingexpressionsfor othercritical quantities.Solutiondropletswere
assumedto be in equilibrium.Droplet sizesand molalities
were inferred from the well-known Kohler equation.This
givesthe maximumgrowthand dilutionof solutiondroplets
expectedat anyRH, a reasonable
assumption
for slowvertical
is extremelyrapid.The particlesfreezeas solutiondropletsat
a relativehumidityof 94.9%. Depletionof watervaporby ice
crystalgrowththenexceedsthe production
rateby expansion
cooling
so
that
nucleation
ceases
to
occur.
Thus
onlyabout0.5
-3
cm of theCCN solutiondropletsfreeze.
Theoretical
studieshavealsodemonstrated
thathomogeneous freezingof solutiondropletsis characteristically
quite
sensitive
to temperature
andverticalvelocity.Coldertemperaturescausefasternucleationrates.Increasedverticalvelocity
increasesthe maximumparcelhumidityleadingto greater
droplet dilution and increasedfreezing rates. The dashed
"fence"in Figure2 outlinesmaximumrelativehumidityand
maximumice crystalconcentration
observedin 25 numerical
simulationsfor (NH4)2SO4aerosols(3, = 1.7). Simulations
were initializedat temperatures
rangingfrom -38ø to -54øC
andconstant
vertical
velocities
ranging
from1 to50cms4. It
is seenthat solutiondropletsare predictedto freezehomogeneouslyat progressively
lowerrelativehumidities(belowwater saturation)as temperaturedecreases.Sassenand Dodd
[ 1989]usedthisapparentrelationshipto proposea simplelinearregression(for climatemodeluse)for the thermodynamic
conditions
requiredfor formingcirrusclouds.
Uncertainties
in the amountsof ice formedby homogeneous freezingand the relativehumidityrangeat which the
processensuesmay be gleanedfrom previousliterature.It is
importantto note theseuncertainties
becausethey affectthe
frequency,persistenceand radiative properties of cirrus
clouds.For givenverticalmotionsand thermodynamic
conditions, at least three factorslead to uncertaintiesin the amounts
of ice formedby homogeneous
fleezing and the humidity
rangeat which this occursin the atmosphere.
Theseare the
nonequilibriumeffects of different solutes on freezing
[Pruppacherand Neiberger, 1963], the actual solutecommotions and RH less than 95%.
positionand the aerosolsizedistribution.The valueof 3,used
To compareand contrastto casesincludingheterogeneous for ammoniumsulfate(3, = 1.7) is basedon the observednunucleation,we review resultsthat characterizethe sensitivities cleationtemperatures,
or nonequilibriumfreezingpoint deand uncertainties
of the homogeneous
freezingprocess.The pressions,of a numberof both ionic and nonionicsolution
curvesin Figure1 demonstrate
theevolutionof thermodynam- emulsions(Rasmussen,
1982). Nevertheless,
rigorousexperiics andice formationthat typifyrisingair parcelscontaining mentalinferencesto 3, havenot beenmadefor (NH4)2SO4nor
solubleaerosolsat low temperatures.
The three simulations for othersubstances
of relevenceto the atmosphere.
To demshownwereinitializedat -46øC.The initial relativehumidity onstratethe impactof this uncertainty,
a simulationassuming
was 81% (assumeddeliquescence
point). Vertical velocity ideal ammoniumsulfatesolutions(3, = 1) is also shown in
was constantat 10 cm s'. Dry aerosolsizewas basedon the Figure 1 (long-dashed
curves).This assumption
lowersthe
CCN watersupersaturation
spectrum
([CCN]= 200 S1'5) peak relativehumidityin the air parcelsbut only lowersby
given by Heymsfield and Sabin [1989] for UT conditions. 10% themaximumice crystalconcentrations
nucleated.
Simulationresultsfor the caseof homogeneous
freezingonly
The potentialuncertaintyintroducedinto calculationsdue
by (NH4)2SO4aerosols(3, = 1.7), shownby the thick solid to uncertaintyin solutecompositionis investigatedby also
curvesin Figure 1, demonstrate
thatthe onsetof ice formation performing calculations of homogeneousfreezing for
Table1. Polynomial
Coefficients
forSome
Critical
Parametem
Solute
Parameter
(NH4)2SO4 ATm
H2SO4
ATm
1-12SO4
aw
co
+0.02199
.
H2SO4
H2SO4
I•
+0.99901
* Coeflidents
fromChen[1994]
c•
C2
C3
C4
+4.12
-1.33055
+0.66822
-0.12445
+3.513627
+0.471638
+0.033208
+0.02505
+2.561806x10'2
+1.076046
x 10'2
+6.184438
x 104
+4233388x 10'5
+0.18993
+0.31201
-0.07029
+0.004896
+0.06212
-0.00268
C5
+0.00832
DEMOTT ET AL.' ICE FORMATION
IN LIPPER TROPOSPHERIC CLOUDS
-46
lOO
95
-47 ?
=
85
-48 E
..,.
*•
80
75
-49
7O
o
1000
500
1500
2000
2500
3000
Time (s)
500
a
•
450
•
400
.o 350
•
300
u 250
o
o
200
•
150
o
lOO
_.u 50
500
1000
1500
2000
2500
3000
19,577
pressionin degreesCelsius was determinedfrom Weast
[1981] and a fit to (2) was determinedwith the coefficients
listed in Table 1. The expressions
for p, and AT,, accurately
representtabulateddatafor H2SO4weightpercentsfrom 0 to
about35%. This coversmostof the rangerelevantto the upper
troposphere.
Much more guidanceis availableon the choiceof L for
sulfuricacid solutions.Ohtake [1993] mademeasurements
of
actualfreezingpoint depression
of bulk volumesof sulfuric
acid solutions.He found that the freezingpoint depression
was almostindistinguishable
from ATto(k.: 1) for solution
weightpercentsbetweenabout30 and37% but exceeded
AT,,
by up to 1.5timesat lowerweightpercents
(L: 1.5).Freezing
temperaturesof 5 gL H2SO4 solution drops measuredby
Beyeret al. [1994] from about25 to 35 wt. % H2SO4are also
in excellentagreementwith the sum of the freezingtemperature of pure dropsof this size plus publishedequilibrium
fleezingpointdepression
(e.g.,L: 1). Their freezingtemperaturesat lower solutionweightpercents,suggesting
L < 1, may
indicateheterogeneous
effectsunder those conditions.More
recently,Bertram et al. [1996] measuredthe freezingratesof
solutiondropletsizesof H2SO4.A. Tabazadeh
et al. (A model
descriptionfor cirrus cloud nucleationfrom homogneous
freezingof sulfateaerosols,
submitted
to Joumalof Oeophysical Research,1997) inferthat the Bertramet al. measurements
areconsistent
with a loweringof thefreezingpointdepression
belowthe equilibriumvalueof only3øC,constant
acrossthe
rangeof solutionconcentrations
measured.
Therefore
we will
assumeL: 1 for sulfuricacidthroughout
the restof this pahumidity
andicecrystal per.Thisshouldgivetheminimumimpactof solutioneffects
Time (s)
Figure1. Temporal
evolution
of•ture,
concentration
in threenumerical
simulations
of adiabatic
air FarceIs on freezingof solubleatmospheric
particles.
rising
at10cms4.Onlyhomogeneous
freezing
nucleation
isallowed Resultsof a simulationassumingsulfuricacid (L: 1) as
in simulations
of (a) (NH4)2SO4
CCN fieezir•attheirestimated
nonequilibrium
fi'eeh• points(Z = ].7 in (1)), (b) (NH4hSO4CCN
at theirequmbrium
freeingpoints½ =
and(c) H=SO
CCNfiew_Jng
attheirequilibrium
freehng
points.
T•mres
varied
slightly
in thedifferent
simulations,
butonlyoneprofileis shownfor
thesakeofclarity.
theCCN composition
arepresented
in Figure1 (short-dashed
curves).It is seenby comparingthesecurvesto the 3,: 1
curvesfor ammoniumsulfate,that the degreeof sulfateneutralizationis inherentlya minor factorin homogeneous
freezlOO
H2804/H20 solutiondroplets.The sameinitial dry size distribution is assumed as for ammonium
-381
sulfate. For calculations
of dropletsize, interfacialsurfacetensionat the solution/air
interfacewas determinedas a function of temperatureand
molalityfor pureH2SOa/H20as
(3)
CoefficientsCi are given in Table 1. This parameterization
followsPruppacherand Klett [1978] and usesdata givenby
Weast[1981]. It is valid for data compiledat T > -40øC and
M < 4.2 but is usedoutsidetheseboundsin this study.Water
activity(a,•)versusmolalitywasbasedon a formsuggested
by
Homogeneous-4:2•_.
-- --
95
-r
ot/a= 76.10 - 0.155T+ Y',,c,M'
5 10
,..21L•:50
90
n•
E
•0
E
ß•
85
-5(
80
Chen [1994]. This is
a,•= l/(l + •-]iciJ•')
(4)
This semiempirical
formulationof a,• was foundto agreequite
well with the data of Rard et al. [1976]. The temperaturedependenceof water activitywas not considered.
Molality effectson solutiondensity(p,) weredeterminedbasedon Weast
[1981] as
P,= •']iCiM i
(5)
with the c, coefficients
givenin Table 1. For computation
of
effectivefreezingtemperature,
equilibriummeltingpoint de-
0.1
I
10
100
1000
10000
MaximumIce CrystalConc(L'•)
Figure2. Maximtan
icecrystal
concentration
versus
relative
humidity
in 50 simulations
ofairparcels
initiated
atthe
deliquescence
humidity
1.5
ofammonium
sulfate
CCN ([CCN]= 200& ) fork = 1.7in Eq. (1).
Initialtemperatures
(degrees
Celsius)
areindicated
bythickcmwes
and
vertical
motions
(centimeters
persecond)
aregivenbythethincurves.
Dashedcurvesassume
ice formation
by homogeneous
freezhag
of
CCNsolution
&opsonly,whilesolidcurves
assrune
alsothatallsolutiondrops
contain
heterogeneous
fieez• nuclei
whichrepresent
10ø,4
of CCN mass.
19,578
DEMOTT ET AL.' ICE FORMATION IN UPPERTROPOSPHERIC
CLOUDS
ing nucleation.For the equivalentassumption
on 3, sulfuric
aciddropletsare shownto freezeat 91.2% RH anda temperature of-47.5øC, while ammoniumsulfatedropletsfreezeat
91.3% RH at -47.6øC. Jensenet al. [1994a] statedthat they
observeda similarresultusingtheirmodel.
The solutecompositionmay be an issueas concernsthe
deliquescence
humidity.In particular,the deliquescence
humidityof ammoniumsulfateis not knownfor cirrustemperatures. Chen [1994] has theoreticallyprojectedthat smaller
(Nm4)2SO
4 aerosolsmay not even deliquescebelow water
saturationat -40øC. Nevertheless,Chen did note that if these
of maximumice crystalconcentrations
by factorsof up to 3.5
at higher
updrafts
(100cms'l).Thiscouldonlybe accomplishedby varyingCCN spectraoverextremerangesthathave
neverbeenobserved.A somewhatlargerrangeof maximum
concentrations could be achieved if it was assumed that CCN
concentrations
wereon occasiondepletedto valueslessthan 1
cm'3 [Heymsfield
andMilosevich,
1995].Jensen
andToon
[1992] calculatedthat the 2 ordersof magnitudeincreasein
aerosolslargerthan 0.5 [tm that can occurin the uppertropospherefollowingmajor volcaniceruptionsmight enhance
ice concentrations
by a factorof 5. Theypredictedthisto occur
aerosolshavebeenthrougha humidificationcyclepreviously, onlyfortemperatures
below-55øCandupdrafts
near5 cms4.
theywill existas supersaturated
solutiondropletsat RH val- We noteherethat conditionspresentin the casestudyof Sasues below ice saturation. Sulfuric acid aerosols have no such
sen et al. [1995], in which the authorsspeculated
that vollimitation,sotheyshouldbe morediluteat RH below90%.
canicCCN increasedice concentrations
in a cirruscloudby a
Since the exact compositionof UT sulfateaerosolsand factor of 3 or more, did not meet thesecriteria.
To summarize,we notefrom previousliteratureand calcutheir variabilityin spaceand time are still uncertain,we consider computationsmade for ammonium sulfate solution lationspresented
that 1) uncertainties
in the composition
and
dropletswith 3, = 1.7 and sulfuricacid solutiondropletswith nonequilibrium
freezingbehaviorof solubleatmospheric
par• = 1 to bracketthe potentialhomogeneous
freezingbehavior ticlesaffectmainlythe onsethumidityat whichcirrusformby
freezingprocess,and 2) the expecteduncerof atmosphericaerosols.The dashed"fence" in Figure 3 a homogeneous
showsthe result of homogeneous
freezingcalculationsfor tainty in the concentrationof ice crystalsformed due to
sulfuricacid solutionsin the sameparameterspaceas shown changesin aerosolsizedistributionis < 50% exceptin very
for ammoniumsulfatehazedropletsin Figure2. It is seenthat specialsituations.Theseresultswill be comparedand conice formation is extended to lower RH values for the same
trastedwith the impactof considering
the addedprocessof
simulation conditions.
heterogeneous
freezingnucleation.
The sensitivityof ice formationby homogeneous
freezingto
solubleaerosolsize distributionhas beenextensivelystudied, 2.2 HeterogeneousIce Nucleation
sincethis is an importantaerosol-climate
interactionissue.
While thepotentialroleof heterogeneous
icenucleiON) in
Previousstudiesare consistentin predictingthat veryparticuice formationin UT cloudshasbeenstated[Sassenand Dodd,
lar changesin the sizedistributionof the solubleaerosolspe1988;Detwiler, 1989;HeymsfieMand Sabin, 1989;Heymsciesdeliquescing
arenecessary
to haveany significantimpact
fieM andMilosevich,1995;Jensenet al., 1994a,b], derailed
on the maximum ice crystalconcentrations
formed.For exanalysesof IN effectson cirruscloudformationhavenot been
ample,Jensenand Toon[1994] haveshownthat increases
of
presented.
One reasonfor this is that heterogeneous
ice fortypicalCCN concentrations
at all sizesby a factorof 20 inmationprocesses
havenot shownthemselves
to be readily
creasedmaximumice crystalconcentrations
by abouta factor
tractable through the application of classical theory
1.5 in the extremecase.DeMott et al. [ 1994] notedsimilarre[Pruppacherand Klett, 1978]. The surfacephysicaland
sultswhenaerosolconcentrations
largerthan0.5 lamwereinchemicalrequirements
for IN are complexand poorlyundercreasedby an orderof magnitude.In both instances,the exstood.It hasbeenrecognizedthatIN maybe typedaccording
treme caseswere for updraftvelocitiescharacteristic
of the
lowervalues
expected
in cirrusclouds
(<10cms'l).Heyms- to a numberof hypotheticalseriesof processesor mechanisms:deposition,
condensation
freezing,immersionfreezing
fieM and Sabin [ 1989] simulatedCCN-influencedincreases
andcontactfreezing[see,e.g., Vali, 1985].In considering
ice
formation
for
conditions
existing
in
most
UT
clouds,
the
lOO
deposition
andimmersionfreezingprocesses
appearto be the
5 10,•
20,,•5•.
onesof mostimportance.
Deposition,the formationof ice di-38 •'
95
rectlyfromthe vaporontoa particlesurface,is onlyrelevant
for "dry"andinsolubleparticles.Immersionfreezing,thenu-42 "-- "-- •'
'
cleationof a liquidparticleby an insolubleparticlewithin it,
Homogeneous
'
•
•"
-r
9O
-46 •
is relevantfor liquid solutiondroplets.With deepconvective
E
-50 '.
transportof cloud water into the uppertroposphere,
other
E
processes
might becomeimportant,althoughbelow -38øC
ß85
homogeneous
freezingwill efficientlyconvertany remaining
-3
0
waterto ice in strongupdrafts.
10
80
DeMott et al. [1994] treatedthe caseof depositionIN in
-4(•
wlHeterogeneous
the uppertroposphere.
If presentin concentrations
that have
-5
been
measured
in
boundary
layer
air,
then
ice
formation
could
75
be dominated
by deposition
nucleiin cloudswith verticalmo0.1
I
10
100
1000
10000
,
ß•
.
,
ß
.
........
i
........
i
,
,
,
.....
!
,
.......
i
MaximumIceCrystalConc.(L4)
,
.....
,,
tionslessthanabout
0.5m s4. If deposition
IN wereassumed
to be depletedat higheraltitudes,thena complexcompetition
processand homogeneous
freezFigure3. AsinFigure2,butformlfia/caeidAvater
aerosols
asstaned betweenthis heterogeneous
tofieezeassolution
droplets
attheirequilibrium
fieefi• point&pres- ing was inferred.The effectof the presenceof depositionIN
sions(L = 1 in (1))
wasto decrease
thenumbersof nucleated
icecrystals
by up to
DEMOTT ET AL.' ICE FORMATION IN LIPPERTROPOSPHERIC CLOUDS
an order of magnitude.densenand Toon [1994] found the
same result when nonspecificheterogeneous
ice formation
was startedat 5% ice supersaturation
in numericalmodelcalculations.The decreases
in ice crystalconcentrations
are due
to the vapordepletioneffectsof ice crystalgrowthat low ice
supersaturations
in coolingand humidifyingair parcels.The
existence
of evenlow numbersof deposition
nucleiin the upper tropospherehas not been validated. Also, observations
that particlesin many regionsof the uppertroposphere
and
lower stratosphere
are primarilyliquid in regionsof higher
humidity[Pueschelet al., 1994] suggesta weakpotentialrole
of depositionIN. This studywill assumethe lack of deposition IN in the uppertroposphere
and focuson the potential
roleof immersionfreezingnucleationin cirrusclouds.
Some proportion,perhapsa large proportion,of atmosphericaerosolparticlesconsistof a mixture of water soluble
and insolublecomponents.
Thesemixed aerosolswill become
solutiondropletsandmay becomethe nucleifor naturalcloud
formation.Laboratorystudieshavebeenperformedon freez-
19,579
In (6),a = 1.04x 10'4(notethatthere
wasa typographical
error in the exponentof a givenby DeMott [1990]), b = 7.767,
rpis equivalent
radius,andT, is supercooling
below0øC.This
expression
quantifiedresultsto -34øC.Homogeneous
freezing
of thefreelysuspended
5 •tm clouddropletsoccurred
at lower
temperatures.
The resultsimply that the populationof active
sitesincreaseswith increasingsootsurfaceareaand decreasing temperature.The freezingfractionwas not found to be
time dependent
in the experimental
temperature
range.These
resultsare typicalof mostobservations
of freezingnuclei.In
contrast,classicaltheorypredictsthat all particlesof a given
size and chemistrywill nucleateat the sametemperature
if
given enough time. Thereforean empirical approachto
parameterizing
freezingby sootaerosolsis preferredfor this
studybecausea classicalcalculationwould overestimatethe
heterogeneous
freezingcontribution.
The heterogeneous
freezingefficiencies
givenby (6) were
extrapolated
for unactivatedsolutiondropletsat coldertem-
peratures
by settingT, = -Teff.An exampleof resultsfor an
ing initiatedby insolubleparticlesin larger(10 to 2000 •tm) equivalent
simulation
to theonegivenin Figure1 is shownin
solutiondroplets[Hoffer, 1961;Pruppacherand Neiberger, Figure4. The CCN composition
and sizedistributionin these
1963; Reische!and Vali, 1975]. These studiesdemonstrated numerical simulations were the same as for the simulations
that dropswould freezeat higher temperaturesthan without for homogeneous
freezingnucleationalone. It was also asinsolublecomponents.
Thishasbeena majorcomplication
in sumedthat every CCN containedan insolubleparticlethat
studyingthe freezingof larger volumesof "pure" water represented10% of its mass. It is seen that this liberal as[Langhamand Mason, 1958; Pruppacherand Neiberger, sumptionon the availabilityof heterogeneous
freezingnuclei
1963]. Certainclaymineralshavebeenidentifiedas particu- leadsto a dominance
of ice formationby the heterogeneous
larly efficientfleezingnuclei [Hoffer, 1961]. Althoughthe process,independentof the assumptions
made on solution
concentrations
of highlyefficientfreezingnucleimaybe small composition
andZ. A factorof 10 fewerice crystals
arepro-
in theuppertroposphere
dueto precipitation
scavenging,
any ducedin the simulations
includingheterogeneous
freezing.
insolubleparticlemayactas a freezingnucleusat highlysu1 O0
-46
percooled
temperatures.
For example,DeMott [1990] showed
95
that sootparticlesbeginto act as heterogeneous
freezingnuclei in cloud dropletsat temperatures
below about -25øC.
-47 o
O
Thusit is reasonable
to assumethat someinsolubleparticles '• 90
will also cause unactivated(mostly submicron)solution
s5
dropletsto freezeat a highersolutionconcentration
thanthey
.>_
wouldfreezehomogeneously.
• 80
The possibleinterplaybetweenhomogeneous
and heterogeneousfreezingprocesses
in cirrusconditionswas examined
75
by incorporating
a parameterization
of heterogeneous
freezing
nucleationinto numericalcalculations.
The parameterization
7O
o
is basedonDeMott's[1990]dataon thefreezingefficiencyof
-48 E
-49
500
lOOO
sootparticles
immersed
in clouddroplets.
Sincesootis a very
inefficientfreezingnucleus,this parameterization
is a conservativeonefor representing
theactionof atmospheric
insoluble
species
by heterogeneous
freezing.Nevertheless,
it is a potentially relevantonesincesootis a knowninsolublespecies
in
the uppertroposphere
and hasan anthropogenic
sourcethere
in theformofjet aircraftemissions.
Theactualcomposition
of
fleezingnucleiin the uppertroposphere
is unknown.We describetheparameterization
for heterogeneous
freezingin this
sectionand characterize
its impacton ice formationin the
adiabaticparcelmodel. The followingsectionthen investigatesthe impacton ice formationof associating
different
amountsand sizesof insolubleaerosolcomponents
with the
CCN
distribution.
2000
2500
3000
2000
2500
3000
Time (s)
5OO
•
450
--. 400
._o350
ß• 300
• 250
o
(.) 200
'•
150
r,.) lOO
b
_.u 50
o
,
o
In theexperiments
onheterogeneous
freezingnucleation
by
15oo
500
1000
1500
Time (s)
sootparticles,DeMott [1990] fit the fractionsF of immersed
Figure4. As in Figure1, butheterogeneous
freefir•nucleation
of
sootparticlesfrozento therelationship,
solutionsis includedin simulationsas •bed
F= a 4Irrp
2(Ts)
b
(6)
in section2.2. The in-
soluble
heterogeneous
fieehr•nuclei
areassaaned
torepresent
10ø,4
of
themassofeveryCCNparticle.
19,580
DEMOTT ET AL.' ICE FORMATION 1NUPPERTROPOSPHERIC
CLOUDS
sults from 45 selectedsimulationsfor (NH4)2SO4 (•, '- 1.7)
hazeparticlesthat includebothheterogeneous
andhomogeneousfreezingprocesses.
The fractionsof CCN of all sizescontainingan insolublecomponent(representing
10% of particle
mass)were varied at decadalvaluesfrom 0.0001 to 1. Maximum ice crystalconcentrations
formedfor initial parceltemlowerRH at whichheterogeneous
freezingensues.
Furtherresultsfor the maximum influenceof the heteroge- peratureof-38 ø, -46ø and -54øC and verticalvelocitiesof 1,
in Figure5. Simulations
were
neousfreezingprocesson ice formationas a functionof com- 10 and50 cms'• areshown
conditions
but arenot shown.The right
position,temperature,
and verticalvelocityare indicatedby donefor intermediate
resultsdominatedby heterogethe thick solidline "fences"in Figures2 and 3. Theseresults borderof the figurerepresents
whereasthe left sidedepictsresults
demonstrate
that if all aerosolsin the uppertroposphere
were neousfreezingnucleation,
freezing.It is seenthat the presmixed aerosols,then the actionof a heterogeneous
freezing dominatedby homogeneous
nucleationprocessin additionto homogeneous
freezinglow- ence of 1 insolubleparticlein every 100 CCN can have a
nucleated,but
erstherangeof maximumicecrystalconcentrations
in all cir- markedinfluenceon ice crystalconcentrations
per secruswithvertical
motions
lessthan50 cms-]. Forupdraftsonlyfor cirrusascentratesbelowa few centimeters
lessthan20cms-] therangeofmaximum
icecrystal
concen- ond.Fractions_>0.1 of CCN containinginsolubleparticlesare
trations
is about10to 2000L-• fora homogeneous
freezing necessaryto affectice formationat all ascentratesbelow 50
These form soonerduring parcel ascent,suppressing
the
maximumhumidityachieved.The onsetof ice formationis
earliestand at the lowesthumidityfor the assumption
of sulfuric acid CCN composition.
This occursbecauseH2SO4solution dropletsgrow more quicklythan (NH4)2SO4at the
that,foranyascent
rate,thelowering
of
processactingalone.In contrast,ice crystalconcentrationscms-].It isalsoseen
by heterogeneous
nucleation
is greatrange
fromabout1to500L-•whena heterogeneous
process
is ice crystalconcentration
Homogeneous
freezingnucleation
included.Theprimaryimpactof uncertainties
in thechemical est at highertemperatures.
composition
(solublecomponent
of aerosols)
andfreezingbe- becomesthe more dominantprocesswhen ascentratesexceed
arebelow-50øC.
hahor (•,) is on the humidityrequiredfor the onsetof ice for- 50cms-•andtemperatures
An
equivalent
series
of
simulationsto thosein Figure 5,
mation. The differentpositions,breadth,and slopesof the
of sulfuric
solidfencesin Figures2 and 3 demonstrate
that ice formsat but assumingthat the hazeparticlesare composed
the lowestRH, and over the widesthumidityrangethe more acid (•, = 1), are shownin Figure6. Resultsare quitesimilar
hygroscopic
is the soluteand the closeris its freezingpoint to thosefor (NH4)2SO4aerosols,althoughthe impactof heterogeneous
freezingnucleationis strongerfor H2SO4.If 10%
depression
to theequilibriumvalue.
of theparticlescontainan insolublecomponent
thatrepresents
just 10% of theirmass,thenthe heterogeneous
processlowers
3. Susceptibilityof Ice Formation to Mixed
the maximumice crystalconcentrations
nucleatedby up to an
Particle Composition
To address the uncertainties
orderof magnitude.This occursfor verticalmotionslessthan
in the abundance and frac-
about
20cms-•andtemperatures
higher
than-50øC.
Sensitivity
to thesizesof insoluble
aerosolcomponents
was
tionationof mixedparticlesin the atmosphere,
the fractionof
CCN containinginsolubleparticlesand the sizesof insoluble examined in two other series of numerical simulations for
components
were variedin numericalsimulations
at several sulfuricacid solutiondroplets.Figure7 showsmodelresults
updraftsand severalinitial temperatures.
Figure5 showsre- for the assumptionthat eachparticlecontains50% insoluble
mass.This assumption
impliesmuchlargerinsolubleparticle
sizeacrossthe CCN sizespectrum.
The fractionof all aerosols
10000
containingan insolublecomponentneededto causethe transitionfrom a homogeneous
to a heterogeneously
dominated
ice
formation
process
was
just
a
few
hundredths
for
thiscase.
1000
In simulationsresultsnot shown here, a soot diameter of 0.08
pm was assumedfor all aerosolsizesfor which sucha size
.10cms-1.........
100
10000
G
o,
o
n
10
1000
•
I
'- -..,
100
0.0001
0.001
0.01
0.1
l'O'cm,s
ß
-- ..:.:2222::...
1
Fraction with insoluble component
Figure5. Maximumicecrystal
concentration
inparcel
modelsimulationsversus
thenumberfraction
of ammonium
sulfate
CCN (L = 1.7)
containing
aninsoluble
particle,
asa function
ofvertical
velocity
and
initial
parcel
•ture.
Vertical
velocities
of1,10and50cms-]are
0.1
, , ill
0.001
i
I
I
I
lllll
I
0.01
I
I
I .I I II
i
0.1
I
I
I
!
I Ill
1
dmoted
byalternate
solidordashed
linegroupir• goingfrombottom
Fraction
with
insoluble
component
totopinthefigure.
Linesbetween
datapoints
arelinearextrapolations.
The'initial
temtxommres
were-38(squares),
-46(triangles),
and-54øC
acidwatersolution
droplets
(circles).
Thesizesof thesoluble
andinsoluble
components
werethe Figure6. As in Figure5, butfor sulfufic
•=1).
sameasforFigure4.
0.0001
DEMOTT ET AL.' ICE FORMATION
IN UPPER TROPOSPHERIC CLOUDS
nation for observations
10000 •
of increased
ice concentrations
19,581
in the
volcanicallyaffectedcirrusis a switchto a morepredominant
homogeneous
freezingprocesscomparedto surroundingUT
cirrus.That is, the lack of insolubleparticlesin the exchanged
stratospheric
air resultedin a hypothetical
shiftfrom the right
to theleft sideof Figure7.
n
1000
•...................
.• 50
cm
s-1
10
4. Comparison to Atmospheric Observations
of Aerosols
1
and Ice Formation
Observations
of the microphysics
and thermodynamics
of
UT cloudssupportthat bothhomogeneous
andheterogeneous
nucleationprocessesplay roles in differentsituations.The
0.0001
0.001
0.01
0.1
1
generalimportanceof the homogeneous
freezingprocessis
Fraction with insoluble component
clearlysupported
by therareobservation
of liquidwatercolder
Figure7. As inFigure6, butasstnm•thattheinsoluble
coinFonentthan about -38øC. However,it is possiblethat the predomiof mlfi,dcacidCCN represent
50ø/6of aerosol
massacross
thesize nance of this mechanism for ice formation is overstated due to
thenatureof the cloudssampledby variousauthors.Most observationswhich clearlydocumentice initiationat temperarepresented
lessthan 10% of the total mass.In this case,a tures below -35øC are from orographicwave clouds. These
heterogeneous
freezingimpact on cirrus ice formationwas cloudsprovideexcellentoutdoorlaboratories
for studyingthe
only predictedif the fractionof all aerosolscontainingin- formationof ice due to their relativelylaminar flow fields.
solublematterapproached
a valueof 1. This is dueto the low- Nevertheless,
the typicalupdraftsin thesecloudsrangefrom
eredfreezingefficiencyof thesesmallerinsolubleparticulates. about
0.5m s-•to 10m s'•.Asshown
inFigure
2, anyheteroThis sizeis characteristic
of the largerparticleswithin the ex- geneousfreezingprocessis predictedto be maskedby a
haustfromjet aircraft.Therefore,a minimumpotentialimpact dominanthomogeneousprocessfor vertical motionsexceedof widelydispersed
aircraftexhaustaerosolson heterogeneous ingabout
0.5m s4. Thisisconsistent
withtheobservations
of
ice formationprocesses
in cirrusmightbe inferred.However, HeymsfieMandMilosevich[ 1993, 1995] in orographicwave
this resultcoulddependon chemicalprocessing
and surface cloudsand $assenand Dodd [1988] in an altocumuluscloud.
activationof aerosols
at latertimesaftergeneration.
Withinjet Vertical cloudmotionsexceeded1 m s-• in both studies.Both
contrails,it is quitepossiblethat evensmallersootparticles studiesalsonotedice formationin agreement
with the expeccouldact as heterogeneous
freezingnucleidue to the higher tationsof a homogeneous
freezingnucleationprocess.Howhumiditiesgenerated.
Karcher et al. [1996] provideevidence ever,SassenandDodd alsonotedthatheterogeneously
formed
that sootaerosolsas smallas 0.02 pm are responsible
for ice ice crystalscouldhavebeenpresentin concentrations
as high
0.1
I
,
llll
,
,
I
I
,
,Ill
,
I
,
,
11111
,
I
I
I
,
Ill
as50 L-• without
beingdetected.
Suchconcentrations
of het-
formation in some contrails.
In additional
simulations
not shown here the numbers of
CCN containinginsolublematter (10% by mass) at sizes
greaterthan0.5 gm were increased
by a factorof 20. This was
doneby alteringthe CCN spectrumto the form [CCN] = 80
erogeneous
ice nucleicouldplay an importantrole in clouds
withupdrafts
below0.5 m s'i, a category
thatprobably
in-
cludesmanywidespread
cirrusclouds.
The mechanisms
of ice formationin cirrusformedby a
5ø'6.Thisbhange
waspreviously
shown
toincrease
icecrystal gentlelifting processshouldin principlebe distinguishable.
concentrations
nucleatedby homogenous
freezingby a maxi- Ice crystalconcentrations
predictedfor a homogeneous
ice
mum of 30% [DeMott et al,. 1994]. The effecton the aerosol formationprocessare in the rangefrom a few hundredto a
fractioncontainingan insolublecomponent
neededto causea fewthousand
perliterforanupdraft
rangeof 10to 20 cms'•
transitionto a heterogeneous
freezingnucleationprocesswas (Figures5 to 7). In contrast,the ice crystalconcentrations
nearlyequivalentto the effectof raisingthe insolublemass predictedwhenincludinga heterogeneous
ice formationprocpercentto 50% (i.e., Figure7). The presenceof an insoluble essare only 1 to a few hundredper liter for theseconditions.
particulatewithin just a few percentof an enhancedpopula- Dowling and Radke [1990] haveestimateda typicalrangeof
tion of larger CCN lowered ice crystal concentrations
by ice crystal concentrations
for cirrus cloudsto be between
nearlyan orderof magnitude.Aerosolsfrom volcanicerup- about
1 and100L'•. These
concentrations
arequitelowcomtionsare known to enhancethe largerpart of the UT aerosol paredto the theoreticalpredictions
for homogeneous
freezing,
size distribution. The influence of volcanic aerosols on cirrus
but agreewell with thoseexpectedwith the inclusionof a hetcloudscouldthereforestronglydependon the freezingnuclea- erogeneousfreezing process.However, we must note that
tion activityof containedinsolublecomponents
and changes measurements
using improvedin situ methodsfor detecting
in composition
of the volcaniccloudovertime. Volcanicaero- concentrations
of small ice crystalsin cold UT clouds[e.g.,
sols in the stratosphere
are known to transformover time, He?msfieldand Milosevich, 1995; Strom and Heintzenberg,
suchthat the larger insolublematteris depositedto the tro- 1994] suggestthat averageice crystal concentrations
have
posphereand the remainingaerosolsare pure sulfuricacid probablybeen underestimated
in the past. Nevertheless,the
[Zhaoet al,. 1995]. Thereforeit is likely that a pure sulfuric estimatedrange of concentrations
presentin cirrus (1 to
acid aerosolcloudexistedin the stratosphere
at the time that 10,000
L4) hasnotchanged.
Observations
of crystal
concenSassenet al. [1995] inferredexchangeof Pinatuboaerosols trationsbelowa few hundredper liter in recentlyformedcirinto the troposphereand subsequent
modificationof cirrus rus canbe only be accounted
for by a singularhomogeneous
cloudpropertiesby increasedCCN. Thus an alternateexpla- freezinghypothesis
if updraftsaretypicallyjust a few centime-
19,582
DEMOTTET AL.' ICE FORMATIONIN UPPERTROPOSPHE•CCLOUDS
terspersecond
or if wideregionsof depleted
CCN concentra- positionof CCN which couldfreezeat 70% RH at -50øC.A
tionsare presentin the uppertroposphere
[Heymsfield
and possibleexplanationis nitric aciduptakeby solutiondroplets
Milosevich, 1995]. Unfortunately,the measurement
of small
vertical motionsremainshighly uncertainand coincident
measurements
of CCN with in situ microphysicalobservationsin theuppertroposphere
arefew in numberat thetimeof
thiswriting.
The relativehumidityregimeof cirruscloudsshouldalso
[Lamb et al., 1996]. However,the saturationratios of nitric
acidin the uppertroposphere
in midlatitudesappearto be too
low for such an effect to occur.
The potentialimportanceof heterogeneous
freezingnuclei
mightalsobe inferredfrommeasurements
of the development
of ice crystalconcentrations
in cloudswarmerthantheh6moindicate their formative mechanisms. The definition of this
geneousfreezingtemperature
regime;for example,cloudsat
regimeis somewhat
complicated
by theverticalmicrophysical around-30øC.HeymsfieldandMilosevich[ 1993, 1995]noted
structure
of cirrusthatplacesnucleation
at thecloudtopwith a generalabsenceof ice particleswarmerthan -34øC in the
wavecloudstheystudied.Thismightindicatethe
regions of ice crystal growth and evaporationbelow supercooled
in
[HeymsfieM
andMilosevich,
1995].Nevertheless,
Heymsfield absenceof freezingnuclei. However,other observations
and Milosevichwere able to definea minimumRH required wave clouds(W.A. Cooper,Ice formationin wave clouds:
duringevaporation,
preprint,Ameriforcirrusformation
by usingthemaximumrelativehumidities Observedenhancement
on CloudPhysics,15observedin clearair conditionsduringNASA's FIRE II pro- canMeteorologicalSocietyConference
gram[of.Heymsfield
andMilosevich,1995,Eq. (1)]. Their 20 January,Dallas,Texas,pp. 147-152,1995)andorographic
RH limit is displayed
as a functionof temperature
in Figure8. capclouds[Cooperand Vali, 1981]haveshownthe presence
attemperatures
downto-34øC.
Thethreshold
values
(arbitrarily
> 0.001L'l) required
forho- ofupto 10L'1icecrystals
Direct informationon the abundance
of heterogeneous
IN
mogeneous
and heterogeneous
freezingof differentsolution
couldresolvetheissueof theirimpordropletsare also shownin Figure8. For the assumptions in theuppertroposphere
made,it is seenthat the calculations
includingheterogeneous tancethere,but few measurements
of IN existanywhereother
freezingnucleation
for sulfuricacidsolution
droplets
and3,= thanat the Earth'ssurface.Bigg [1967] collectedaerosolson
1 are mostcloselyin accordwith observations.
Heymsfield filtersduringlong latitudinalaircrafttransectsat 4 to 12 km
and Milosevichstatedthat their observations
might be ex- andprocessed
thesein a diffusionchamber.He foundIN con-
centrations
ranging
from0.05to > 1 L4 whenprocessed
at
plainedif a heterogeneous
ice formationprocess
occurred
at
an RH of 10% lowerthanhomogeneous
freezing.This is approximately
thevaluewe predictfor thetemperature
rangeof
their measurements.
The lowestcirrustemperaturesampled
by Heymsfieldand Milosevichwas about-47øC,and the
minimumRH requiredfor cloud formationwas near 80%.
Thereforeit is possiblethat the slopeof their extrapolated
curvemaybe toosteepandmayunderestimate
required
RH at
coldertemperatures.
Thisis supported
by ourmodelingcalcu-
havebeenprocessed
for the lower cloudtemperatures
that existedat the samplingaltitudes.D.C. Rogersand P.J. DeMott
(Measurementsof natural ice nuclei, CCN, and CN in winter
clouds,preprint,AmericanMeteorological
SocietyConference
on CloudPhysics,15-20 January,Dallas,Texas,pp. 139-144,
1995) collectedbulk bag samplesof aerosolsnear wave
lationsand the fact that it is difficult to hypothesize
a tom-
clouds(4 to 9 km altitudes)for processing
to detectIN in the
water saturationat -15øC. It seemspossiblethat higher IN
concentrations would have been detected if the filters could
lOO
homogeneouslimit
•
3:
13::90
I::
E•
i• 85
(NH4)2SO4 ...............
'-•
• ....................
..................
/
-'
homogeneous
limit
.e 80
75
-65
Observed
Min.
RH
vs.Ttoform
cirrus
H2SO4/H20 •
_
/
heterogeneouslimit
./
'
ß•
I:Z::
•
/
heterogeneous
limit
H2SO4/H20 /
-60
-55
,,
-50
.
-45
-40
-35
Temperature of Min. RH (øC)
Figure8. Threshold
humidity
versus
temperature
fortheformation
oficeinnumerical
s'andations
including
onlyhomogeneous
freehng
(homogeneous
limit)andhomogeneous
plusheterogeneous
fieezhzg
nucleation
(heterogeneous
limit)of
(NH4)2SO4
(• = 1.7)and
H2SO4/•-I20
½= 1.0)solution
droplets.
Cloud
model
iceconcentrations
> 0.001
L4arepredicted
above
eachthreshold
curve.
Theheterogeneous
nucleation
thresholds
areforthecase
represented
inFigure
7 (soluble
fraction
= 0.5,with10%
ofaerosols
conta'lni•
aninsoluble
particle).
Thehomogeneous
limiting
curves
areforanutxtmff
of1cms4
andwould
beoffset
toward
higher
humidifies
forhigher
updrafts.
Theheterogeneous
limiting
curieisonlyslightly
sensitive
to
uv•ft since
heterogeneous
fieehng
rates
werepmmnetmz•
asa fimction
oftem•uae alone.
Alsoplotted
areobserved
threshold
humidities
required
forcinuscloud
formation
estimated
byHeymsfield
andMilosevich
[1995].
DEMOTT ET AL.' ICE FORMATION [N UPPER TROPOSPHERIC CLOUDS
19,583
laboratory.Aerosolswere processed
over a rangeof satura- sizes of insoluble matter. It also dependson the relative
freezingefficiencyof differentinsolubleaerosolsin comparisimulations
ofwavecloudascent
(1 to8 m s-1)in a controlledsonto the sootspeciesassumedin this paper.Other,moreefexpansion
cloudchamber.IN concentrations
measuredat or ficient, freezing nuclei may be presentin the upper troabove
watersaturation
at-30øCranged
from1 to 100L'•. posphere.Resultswill remain speculativeuntil appropriate
tion conditions in a continuous flow diffusion chamber and for
These
concentrations
represented
fromabout1 partin 105to
greater
than1 partin 103ofCCNconcentrations
measured
in
new measurements
are obtained.
3. The rangeof concentrations
of ice crystalsnucleatedand
the samebag samples.Since,basedon DeMott [ 1990], only the humidityregimefor the onsetof ice formationin cirrus
about 1% of available freezingnuclei might be active at a predictedwhen includinga heterogeneous
nucleationprocess
temperatureof-30øC, the observedIN concentrations
could agree with existing atmosphericobservations.In contrast,
indicatefrom 0.01 to 0.1 fractionsof CCN containinginsolu- predictionsthat consideronly a homogeneous
freezingprocess
ble freezingnuclei.This is within the rangeof predictedhet- actingin cirrusdo not agreewith most existingobservations
erogeneous
effectsbasedonFigures5 to 7.
of ice crystalconcentrations
and the humidityrequiredfor the
Existingmeasurements
of UT aerosolproperties
lend some onset of ice formation.
supportto the potentialfor a heterogeneous
ice formation
4. Existingmeasurements
of aerosolsand ice nucleiin the
processin UT clouds.Hagen et al. [1994] foundthat UT par- upper tropospheresuggestthat the numbers and sizes of
ticlessmallerthan 0.1 gm were usuallymixed particleswith mixed aerosolscapableof acting as heterogeneous
freezing
solublefractionsof from 0.3 to 0.5. Particleslarger than 0.1 nucleiare sufficientto reducecirruscrystalconcentrations
and
grnwerealsopredominately
mixedparticleswith 5 to 20% of lowerthe RH requiredfor cloudformation.
the particlescontaininginsolublespeciessuch as carbon,
The resultspresentedin this paperunderscore
the needfor
aluminum,and titanium. ?ueschelet al. [1994] found Si and in situ measurementsof ice nucleatingaerosolsin the upper
includingidentifyingnaturaland anthropogenic
Mg in 10 to 20% of "liquid" particlesin backgroundUT air. troposphere,
Consideringour numericalcalculations,suchhigh percent- sourcesof theseaerosols.Suchan effortwas initiatedduring
ages and large sizes of insolublespeciesassociatedwith the springof 1996 [Thompsonet al., 1996, (pp.175-199)]. In
solubleaerosolssuggesta strongpotentialfor the actionof addition,laboratorystudiesare neededto deœme
solutionefheterogeneous
ice nucleationprocesses.However, we must fectson the freezingratesof solubleand insolublespeciesat
alsonotetwo casespresentedby Sheridanet al. [ 1994] indi- or neartheir actualdeliquesced
sizesat coldtemperatures.
Theresultspresented
alsohavesignificance
to theradiative
catingthatUT aerosols
werepredominantly
composed
of sulfates and did not contain detectable amounts of insoluble
propertiesof cirrus.Sincelargenumbersof smallice crystals
matter. Sheridanet al. also notedthat the severalpercentof absorband scatterradiationmore effectivelythan a smaller
aerosolsthey sampledcontaininginsolublecrustalmaterials numberof largeice crystals[e.g.,Kinne and Liou, 1989], the
and metals were not coated with sulfates at a detectable level.
sensitivitiesof ice formationto temperatureand verticalvelocityareveryimportant.It was shownthat the functionalde5. Summary
pendenciesof ice crystal concentrations
versustemperature
and verticalvelocityare quite differentdependingon the naA numericalanalysisof the impactof homogeneous
and
ture of the ice fonuationprocess(homogeneous
versusheteroheterogeneous
ice formationprocesses
on initial cirruscloud
geneous)in the uppertroposphere.
A variablepopulationof
composition
has beenpresented.
The salientconclusions
are
freezingnucleusaerosolscouldlead to a switchin the prethe following.
dominanceof heterogeneousversus homogeneousfreezing
1. A numberof uncertainties
existwhichimpactthe pre- and a large difference(orderof magnitude)in maximumice
dictedhomogeneous
freezingof unactivated
solutiondroplets crystal concentrationsnucleated.This sensitivityto aerosol
in theuppertroposphere.
Solutecomposition
doesnot appear number concentrations
doesnot theoreticallyexist, exceptin
to be a criticalparameter,with two notableexceptions:
the
veryspecialcircumstances,
for cirrusformedonlyby homogehigh deliquescence
humidityof ammoniumsulfateand the
neous freezing of CCN. Ice crystal concentrations
are ineffectof nitricaciduptakeby solution
droplets.
Uncertainty
in verselyrelatedto effectiveice crystalradius[e.g.,Jensenet
the freezingconditions(nonequilibrium
freezingpoint deal., 1994b].Thereforelower maximumice crystalconcentrapression)of relevantsolutesis a moreimportantfactor.The
tion for any set of conditionsimpliesa smalleropticaldepth
impactof this uncertainty
is primarilyan indirectoneon the
andlowernet radiativeforcingin cloudsinfluencedby heteroapparentrelativehumidityneededfor ice formationto occur.
geneousnucleation.At the sametime a heterogeneous
freezConsidering
the expected
rangeof atmospheric
solutionefing processfavorsthe morefrequent(at lowerRH values)and
fectsonfreezingpointdepression,
it was shownthatan uncerwidespreadformationof ice clouds.
taintyof severalpercentRH existsfor predicting
cirruscloud
In conclusion,heterogeneous
ice nucleicouldexerta much
formation
by homogeneous
freezing.Variations
in icecrystal
strongerandmorevariableclimate-forcing
influencein cirrus
concentrations
are only about30% due to this factor.Varicloudsthanthat which is exertedby solubleaerosolsacting
abilitiesin solubleaerosolsizedistribution
mayleadto simisolelyby a homogeneous
freezingprocess.This studyprolar uncertainties
in icecrystalconcentrations
formed.
videsa quantitative
exampleof theimportance
of considering
2. Insolublematerialactingas heterogeneous
freezingnu- ice nuclei in the overall aerosol-cloud-climatescenario,as
cleiwithinsolution
droplets
in UT air maydecrease
icecrystal statedby Rogers[ 1994].
concentrations
in someckruscloudsby up to a factorof 10.
Theapparent
humidityrequiredfor ice formation
mayalsobe
decreased
by upto 10%.Theimportance
of thisheterogeneous Acknowledgments.This researchwas primarilysupportedby the
freezingprocesscompared
to the competing
homogeneousNationalAeronauticsand SpaceAdministrationundergrantNAG-2freezingnucleation
process
depends
ontheconcentrations
and 924. Partialfundingwas providedby the U.S. Departmentof Energy
19,584
DEMOTTETAL.' ICEFORMATION
1NUPPERTROPOSPHERIC
CLOUDS
undergrantDE-FG03-95ER61958andby theNationalScience
Foun- Lamb, D., A.M. Moyle, and W.H. Brune,The environmentalcontrol
of individualaqueousparticlesin a cubicelectrodynamic
levitation
dationundergrantATM-9311606.The contents
of thispaperevolved
system,AerosolSci. Technol.,24, 263-278, 1996.
from materialpresentedat the 1995 NASA Atmospheric
Effectsof
and homogeneous
AviationProgramAnnualMeeting,April 23-28, VirginiaBeach,Vir- Langham,E.J.,and B.J.Mason,The heterogeneous
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ginia.We thankthereviewersfor theirusefulcomments.
1958.
Ohtake,T., Freezingpointsof H2SO4aqueoussolutionsand formation
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P.J. DeMott. S.M. Kreidenweis,and D.C. Rogers,Departmentof At-
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80523.(email:[email protected])
(ReceivedDecember18, 1995;revisedJanuary17, 1997;
acceptedApril 10, 1997.)