EC Advanced Study Course ����

Environment and Climate Programme

Atmospheric E�ects of Aircraft Emissions in the UpperTroposphere and Lower Stratosphere �ASTAIRE�

Bergen� Norway� August ��� ����

Particle Formation on Global Scales

by

Priv��Doz� Dr� Bernd K archerDeutsches Zentrum f ur Luft� und Raumfahrt e�V� �DLR�

Institut f ur Physik der Atmosph are �PA�

D���� We�ling� Germany

PARTICLE FORMATION ON GLOBAL SCALES

Bernd K�archerInstitut f�ur Physik der Atmosph�areDeutsches Zentrum f�ur Luft� und Raumfahrt �DLR�D����� WesslingGermany

Abstract� A brief survey of types sources and classi�cations of atmospheric aerosolsis given� New particle formation in the troposphere and stratosphere is discussed withthe help of observed case studies and model simulations� Some problems that arise frommeasuring and modeling particle nucleation and growth are illustrated in the discussion�

�� INTRODUCTION

The objective of the lecture held at the Summer School was �rstly to give an introduc�tion to the fundamentals of aerosol physics in order to enable the students to understandand follow subsequent lessons where atmospheric aerosols and aerosol�related processesplayed a role� Secondly the lecture presented an overview on the formation of particlesin the global troposphere and stratosphere�

While preparing this contribution I decided to skip the former topic since excellentintroductions to the physics and chemistry of atmospheric aerosols are available Prup�pacher and Klett ����� Seinfeld and Pandis ����� Jacobson ����� among other text�books�� Hence after some general remarks concerning atmospheric aerosols I present acondensed overview on major new particle formation pathways in the Earth�s atmospherefrom the planetary boundary layer up to the upper stratosphere�

This contribution is far from being comprehensive� A complete coverage of the liter�ature was not intended� the citations are to document statements made in the text andto provide sources for further in�depth studies� I do not consider aerosol chemistry andtransport� radiative properties of aerosols and interaction with clouds� aviation�producedaerosols and contrails� The latter issue is subject of a separate contribution to this volume�

�� GENERAL REMARKS

The atmospheric aerosol is the combination of condensed�phase �liquid or solid� ma�terial and the air within which the suspended material resides� Aerosol particles areubiquitous components of the atmosphere�

Particles are important to atmospheric photochemistry through processes occuring attheir surfaces or in their interior� they exert direct and indirect e�ects on climate by scat�tering and absorbing solar and terrestrial radiation and by serving as cloud condensationnuclei �CCN�� together with hazardous gases they determine air quality and visibility��nally particulate matter may a�ect human health by penetrating into the lungs�

Many problems of chemistry and climate of the Earth cannot adequately be addressedwithout considering the e�ects of atmospheric aerosols on the processes of interest�

���� SOURCES AND TYPES OF ATMOSPHERIC AEROSOLS

Aerosol particles can be classi�ed as being of primary or secondary origin accordingto di�erent sources and chemical composition� Among primary particles we list crustal

�

Ice L and

Clouds

VegetationT rafficIndustryVolcanoes

Radiation

Absorption

S cattering

Diffus ion

Advection Convection

S edimentation

E vaporation ofPrecipitation

ChemicalT ransformation

Nucleation

Condensation

Coagulation

E miss ionE ros ionDeposition

R ainoutWashout

Ocean

S cavenging

Fig��� Schematic of processes a�ecting the atmospheric aerosol in the climate system� Besides show�

ing radiative and transport processes� the �gure illustrates sources �land� ice� water surfaces� vegetation�

industrial combustion sources incl� �res� volcanoes� trac incl� motorcars and airplanes� in situ nucle�

ation and transformation processes �condensation� coagulation� evaporation of cloud particles� chemical

transformations� many of which are discussed in this work�

material and minerals� sea salt� biogenic particles �e�g� pollen or bacteria�� soot �ele�mental carbon including organic material�� Examples for secondary particles are sulfatesoriginating from DMS �dimethyl sul�de� or SO�� nitrates from NOx� organic particlesfrom biogenic or anthropogenic hydrocarbons�

There are three principal types of aerosol sources� ��� Dispersion of material at theEarth�s surface� ��� high�temperature combustion sources� ��� chemical reactions andgas�to�particle conversion� Examples for the mechanical generation of particles ��� are�dust particles created by erosion of soils� wind�driven release of biogenic particles fromplants� droplet formation over the oceans from bursting water bubbles� Combustionsources and biomass burning ��� produce �y ash and soot particles and smoke particlesin smoldering and open �res� Examples for source ��� are the formation of new particlesdue to photochemical activity from volcanic SO� or in water clouds� Figure � sketchesprincipal sources and transformations pathways of aerosols in our atmosphere�

Global source strengths are ���� � ���Tg yr�� for surface�derived aerosols �� ����Tg yr�� for particles formed by gas�to�particle conversion and ��� ���Tg yr�� foraerosols from anthropogenic origin Georgii and Warneck in� Zellner ������ Aerosolmasses are often of the order ����gm�� at the Earth�s surface �rarely lower than ��gm���and may exceed several ��� �gm�� in desert dust storms Graedel and Crutzen ������Masses of aerosols and reactive gases are comparable in background �unpolluted� air�

�

���� CLASSIFYING ATMOSPHERIC AEROSOLS

Aerosols in the lower atmosphere can also be classi�ed speci�c to the regions in whichthey are observed� Jaenicke suggests polar� background� maritime� remote continental�

desert dust storm� rural� and urban aerosols in� Hobbs ������ This classi�cation basedon earlier suggestions made by Christian Junge turned out to be very helpful for thediscussion of general particle properties such as abundances and radiative parameters�

Aerosol particles pertinent to a speci�c class as noted above are further subdividedinto di�erent populations according to their size� Microphysical particle properties suchas total mass or surface area are derived from discrete or continuous size distributions�It turns out that atmospheric particles often occur in distinct size regions called modes�Junge ����� suggested dividing the particles into three size categories� A more recentclassi�cation into size modes goes back to Whitby ����� who described atmosphericaerosol size distributions by the sum of three lognormal functions� Each of these functions�Gaussian curves on a logarithmic size scale� are fully determined by a mean diameter Da total number concentration n and a width parameter�

Aerosols in the nucleation mode indicate freshly generated particles from gas�to�particleconversion processes or emitted small particles� Their mean diameters range from thesize of molecular clusters �� nm� up to D � ����m� Aged nuclei which have already grownby condensation from the vapor phase may appear as a secondary maximum referred toas Aitken mode actually leading to a bimodal structure of the nucleation mode� Particlesgrown and aged in the atmosphere belong to the accumulation mode ���� � D �m� � ���Also this mode is sometimes observed to be bimodal for example due to selective growthprocesses occuring in clouds� Emission from the Earth�s surface is the primary origin ofthe coarse�particle mode consisting of particles generated mechanically �dust sea sprayplant fragments� with mean diameters exceeding ��m� Usually these modes do not oc�cur simultaneously and sometimes two or more modes overlap so that they cannot bedistinguished by experimental means�

Several transformation and removal mechanisms �deposition scavenging rainout andwashout� see Figure �� determine the residence time � of particles in the atmosphere whichis mainly a function of D e�g� Warneck ������ Nucleation mode particles are depletedby colliding with pre�existing larger particles �scavenging� in which case � is only of theorder � hr� Accumulation mode particles may serve as cloud condensation nuclei �CCN�and can be fully incorporated into cloud drops �wet deposition�� Their lifetime in thefree �cloudy� troposphere does rarely exceed a few days� Particles larger than about���m undergo gravitational settling �sedimentation� and eventually dry deposition atthe surface on time scales of a few hours or so�

���� SOME FEATURES OF ATMOSPHERIC AEROSOLS

Tropospheric aerosols are extremely variable in terms of number or mass concentra�tions both in time and space� In the boundary layer at altitudes below about � kmcontinental aerosol particles may reach concentrations of several ��� cm��� Maritimeaerosol concentrations may range between �� and several ��� particles per cm�� In thefree troposphere a typical background number density is ��� cm��� Polar aerosols do notexceed ��� cm�� on average above � km altitude strongly decreasing to values of several�� cm�� near the surface e�g� Jaenicke in� Hobbs ������

Much of this variability is caused by the large range of residence times� Small � valuestogether with non�uniform sources lead to signi�cant spatial and temporal variability� In

�

contrast long � values in non�cloudy regions lead to the comparatively constant back�ground of free tropospheric particles�

Stratospheric aerosols have a much longer lifetime of the order of � yr before theypenetrate the tropopause and undergo tropospheric loss processes �mainly scavengingby clouds and coagulation with larger aerosol particles�� The background stratosphericaerosol layer with a typical number density and mean diameter of accumulation modeparticles of �� cm�� and ����m respectively has been �rst investigated in detail by Jungeand co�workers Junge ������ Large volcanic eruptions are known to strongly modifystratospheric aerosols along with related dynamical chemical and radiative processes WMO ���� and references therein��

In terms of chemical composition optical properties and abundance aerosols inthe tropopause region may be of stratospheric or tropospheric character K�archer andSolomon ����� as demonstrated by in situ observations that became available only re�cently e�g� Sheridan et al� ���� Talbot et al� ����� Murphy et al� ������ Their life�time and hence potential impact on cirrus cloud formation and chemistry depends onstratospheric�tropospheric exchange processes among other details�

�� NEW PARTICLE FORMATION IN THE TROPOSPHERE

The production of new particles especially in the troposphere has received growinginterest during the past decade� Experimental case studies exist considering nucleation inrural and urban environments in the marine boundary layer in the free troposphere inthe vicinity of clouds and in the tropopause region� I selected a few of them on a purelysubjective basis which I discuss subsequently� In the discussion I highlight some of theprincipal problems that arise from measuring and modeling new particle formation�

���� NUCLEATION IN THE MARINE BOUNDARY LAYER

Marine aerosols were among the �rst which received attention in both measurementsand modeling� Partly because of the spatial extent of oceans they are believed to exert asigni�cant in�uence on global climate Charlson et al� ������ Model calculations by Raesand van Dingenen ����� suggest that the formation of new particles by homogeneousnucleation can occur in the remote marine boundary layer�

Aerosol measurements were made from a ship in the marine boundary layer during thePaci�c Stratus Sulfur Investigation Covert et al� ����� showing that nucleation indeedoccurs there� New particle formation took place in the form of bursts in situations withlow surface area of already existing particles re�ected by distinct spikes in the time seriesof the total number concentration of particles larger than � nm in diameter� The smallerthe surface area the longer the freshly�formed particles can grow by condensation from thevapor phase to sensible sizes� In contrast at high available surface areas condensationof molecules on pre�existing particles is favored over new particle production and thisquenching of the nucleation is proportional to the aerosol surface area in most cases�

Because homogeneous nucleation produces very small particles �nm�sized or ultra�ne�the concentration ratio n�D � � nm� � n�D � �� nm���n�D � �� nm� turned out to bea good indicator of new particle production and growth in the measurements� �Heren�D � Dmin� denotes the cumulative number density of particles with diameters exceedingDmin� Such quantities are measured with condensation particle counters �CPCs��� Duringthe observed intense bursts total particle numbers increased by more than a factor of

�� while the above ratio increased from ��� to � over a period of only one hour� At thesame time the background aerosol surface area declined by more than a factor of �� Thetotal particle concentration remained high for over � hours and declined to backgroundvalues within � hours�

However only those aerosols can be detected which have grown beyond a minimumdetectable size �� � nm� at the time of observation� This implies that nucleation eventsin which particles remain smaller than � nm due for example to the lack of condensablematerial cannot be observed with ultra�ne CPCs� Further this implies that the measure�ments of Covert et al� ����� likely underestimate the number of actual particle formationevents�

Particles can also be formed above and then be mixed downward into the marineboundary layer Raes ������ The higher the altitude where the new particles are producedthe longer their transport time the bigger the particles can grow and the less probableit is that they are observed as a new particle formation event�

���� NUCLEATION AT A CONTINENTAL SITE

New particle formation from the gas phase is also observed at the Meteorological Ob�servatory Hohenpeissenberg operated by the German Weather Service in collaborationwith the Institute of Tropospheric Physics in Leipzig Berresheim et al� ������ Hohen�peissenberg is a rural continental site between Munich and the Alps� the observatory islocated � km above sea level�

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

Tageszeit, MEZ

0

1

2

3

4

5

6

N(ges)

H2SO4

NO

Fig��� Diurnal variations of number density of ultra�ne particles

n�D � � nm� and H�SO� and NO concentrations� representing

a measurement on April ��� ���� at Hohenpeissenberg� While

the �rst maximum at � am is due to vertical mixing of polluted

air from the valley� the second maximum at noon illustrates a

nucleation burst� Figure courtesy of Harald Berresheim�

One goal of the ongoingstudies there is to investigateparticle formation at di�er�ent meteorological conditionsand to clarify the role ofH�SO� volatile organic com�pounds and meteorology in nu�cleation� For example OHand H�SO� are measured us�ing chemical ionization massspectrometry and particles aremeasured using a twin di�eren�tial mobility particle sizer for��� ��min scans of the aerosolsize distribution between � ���� nm diameter and high timeresolved measurements of parti�cles � � nm and � �� nm withtwo CPCs�

A case study from April ������ is depicted in Figure �where the total number of par�ticles n�D � � nm� and the concentrations of H�SO� and NO are shown versus the timeof day� At night all species show low concentrations� A night�time inversion breaks upin the morning hours transporting moderately polluted air from the valley to altitudesabove the boundary layer� At the observatory this transient pollution event results in

�

enhanced NO H�SO� and particles with D � �� nm �as indicated by the CPCs�� WithNO levels dropping and indicating again relatively clean air conditions �wind changing towesterly directions� a strong increase in H�SO� levels was observed preceding a similarstrong increase in total �mainly ultra�ne� particle concentrations indicative of a nucle�ation burst� The CPC data �not shown� reveal the growth of ultra�ne particles from �nmup to �� nm from �� am until � pm with peak concentrations of several ��� cm�� aroundnoon�

Note the shift of � � � hours between the maxima in H�SO� and particle number inFigure � indicating that H�SO� may have been the precursor for the observed new particleformation and the following particle growth� All observed particle formation events atHohenpeissenberg were accompanied by relatively high H�SO� levels �� ��� cm��� andoccurred exclusively around noon time under intense solar illumination conditions�

However nucleation did not always take place when H�SO� and OH were high in�dicating that other parameters also play a role� No signi�cant bursts were observed inthe summer of ���� which may be linked to enhanced background area surface areasobserved in this season� Another interesting feature that is not yet fully understood is therelatively constant level of ultra�ne particles �several ��� cm��� that is usually recordedduring daytime in addition to the distinct burst events�

Observed cases exist which point to a greater in�uence of transport processes� Themeasurements suggest that new particle formation may occur on regional scales notonly locally� In general care has to be taken in order to distinguish between in situformation of new particles and transport �including large�scale horizontal advection� offreshly nucleated particles from a location near the measurement site� Nucleation theremay take place under quite di�erent chemical and meteorological conditions�

���� NUCLEATION AT A FOREST SITE

The main source of biogenic hydrocarbons is besides oceans the terrestrial vegetation�Among the most abundant organic trace gases especially from the foliage of forestsare isoprenes �C�H�� and monoterpenes �C��H��� Main source regions are the tropicalrain forests� in summer months emissions of biogenic hydrocarbons also play a role atmidlatitudes cf� Ehhalt in� Zellner ����� Simpson et al� ������

Once emitted biogenic compounds become oxidized mainly by reactions with OHradicals and ozone in the gas phase leading to carbonic acids and other oxidation productswith extremely low vapor pressures ������ torr� su�ciently low to explain nucleationunder typical tropospheric conditions in the presence of H�O Seinfeld and Pandis ������The small freshly nucleated particles may attach to existing aerosol particles causing theformation of �blue haze� due to enhanced scattering of light from these grown aerosols Went ������

Measurements of new particle formation in a boreal forest in Southern Finland havebeen reported by M�akel�a et al� ������ Aerosol formation events occurred on sunny dayswith high global radiation large temperature di�erences between early morning and after�noon and relative humidity decreasing over the day� Nucleation was usually accompaniedby vertical mixing causing the disappearence of nocturnal inversion layers� Nucleationpeaks producing ultra�ne particles with D � � nm were observed during the morninghours� With growth rates of several nm per hour the nucleation modes increased in sizeand decreased in concentration eventually merging with an existing mode centered atabout � nm in the late afternoon�

�

While the actual chemical composition of particles in this size range is very di�cult todetermine in general and has not been measured in this study two arguments underscorethe potential role of biogenic sources �from plant and�or soil microbial processes�� Inthe sunny morning hours biological activity and emission rates of biogenic hydrocarbonsdriven by solar radiation and elevated temperatures were high� the absence of local sourcesof pollution rendered the participation of other gaseous aerosol precursors �e�g� H�SO�

from oxidation of emitted SO�� inside the forest unlikely�Laboratory studies allowing the preparation of controlled conditions in reaction cham�

bers are needed to complement �eld studies as described above e�g� Ho�mann et al������� Recently Yu et al� ����� have suggested that atmospheric oxidation of biogenichydrocarbons could be a signi�cant source of the global aerosol� Whether or not parti�cles from biogenic origin serve as CCN is not yet fully understood� In any case organicaerosols may provide an interesting link between the biosphere and the atmosphere Elli�son et al� ������ Another such link one between oceanic phytoplankton and climate hasbeen suggested earlier by Charlson et al� ������

���� NUCLEATION IN THE FREE TROPOSPHERE

Progress in studying new aerosol formation has been made by measuring simultane�ously ultra�ne �D � � � �� nm� particles and one or more of their presumed gaseousprecursors� Weber et al� ����� presented such observed particle size distributions deter�mined from CPC data along with H�SO� measurements using an ion chemical ionizationmass spectrometric technique similar to the Hohenpeissenberg measurements� �Theseauthors also give a summary of other �eld measurements of new particle formation from�����������

Weber et al� ����� report measurements at a remote marine site �Mauna Loa Hawaii�and at a remote continental site �Idaho Hill Colorado� both in the lower free tropospherein the vicinity of the boundary layer� The data con�rm previous �ndings that new aerosolformation is anticorrelated with pre�existing surface area� Further the data show that al�though H�SO� is involved in the nucleation and growth processes the observed nucleationrates are much faster than predicted by the classical binary �H�SO��H�O� homogeneousnucleation models and particle formation takes place at much lower H�SO� levels thanpredicted by this theory�

The detailed analysis of the CPC data indicates that the nucleation rate varies in pro�portion to H�SO��

� suggesting a collision�controlled rather than an evaporation�conden�sation�controlled particle formation process� The latter forms the basis of classical nucle�ation theory cf� e�g� McDonald ����� ������ Further Weber et al� present argumentsthat the discrepancies between observations and theory can be attributed to the presenceof a species such as ammonia �NH��� Ammonia has not been measured during these �eldexperiments but it reacts at a near collision�controlled rate with liquid sulfuric acid andmarkedly reduces the vapor pressure of H�SO��H�O droplets� NH� could stabilize a frac�tion of the near�critical clusters �������� ������ as inferred from the data� and therebyfacilitate nucleation and subsequent growth by signi�cantly reducing the evaporation rateof H�SO� from the ternary clusters�

On a climatological basis CCN and optically active particles �D � ����m� exhibitmarked seasonal variations throughout the troposphere with maxima in the summer sea�son Hofmann ����� �balloon�borne CPC measurements over Wyoming LA a continentalsite at midlatitudes�� The region of the upper troposphere near the tropopause appears

�

to be an e�ective source of new particles presumably mainly driven by photochemistrywhereby the observed high mixing ratios of new particles are regulated by the relativelylow levels of larger �optically active� particles as compared to the lower troposphere�

���� NUCLEATION IN THE TROPOPAUSE REGION

Fig��� Airborne measurement of a tropospheric nucleation burst

event� located between ��� � � km altitude� Shown are time se�

ries of cumulative particle number densities n�D � � nm� and

n�D � �� nm�� atmospheric stability d��dz� and horizontal wind

speed WS� The relatively neutral atmosphere and the uniform wind

pro�le make vertical mixing as a cause for the burst likely� which

occurred inside a frontal system over western Europe� The hor�

izontal scale over which the burst was observed is �� km� Figure

taken from Schr�oder and Str�om � �����

A comparison of observedparticle formation events inthe troposphere at various sitessuggests that in remote �ma�rine� regions above about kmbinary nucleation of H�SO� andH�O may be mainly responsi�ble for the production of newparticles Weber et al� �����although uncertainties in thenucleation theory limit decisiveconclusions� This is in contrastto observations in regions closerto the Earth�s surface whereother �perhaps ternary� nucle�ation mechanisms may be ofgreater relevance� see above�

Together with indicationsfrom other measurements e�g�Clarke ����� such observationsserve as a motivation to esti�mate the potential of new parti�cle formation in the upper tro�posphere and tropopause regionwith the help of the nucleationof aqueous H�SO� particles�

Observations during the air�borne �eld campaign STREAM�Stratosphere�Troposphere Ex�periment by Aircraft Measure�ments� revealed an intriguingcorrelation between the num�ber density of particles withD � � nm and the H�O mix�ing ratio DeReus et al� ������For a given value of H�O� noparticles are observed below athreshold value n

�of the aerosol

concentration� The value of n�

shows almost no variation inthe troposphere above � km ����� cm��� but varies between about ��� ��� cm�� in the lowermost stratosphere�

�

These observations suggest that points with n � n�at a given H�O� would lie in a

region of the atmosphere where new particle formation is likely to occur� If some processwould bring the particle number density �and hence surface area� into that region newparticles would form quickly increasing n above n

�� Indeed with the help of binary

H�SO��H�O nucleation calculations the authors were able to identify the backgroundaerosol surface area as an important factor for aerosol nucleation in this region� Thesimulations indicate a strongly nonlinear relationship between the increase of nucleationmode particles and the decrease of the pre�existing particle concentrations�

In situ measurements of sub��m particle size distributions and other aerosol param�eters by Schr�oder and Str�om ����� suggest that new particle formation seems to be afrequently ongoing process in the troposphere above km altitude� They report severalnucleation bursts �one example is presented in Figure �� whereby the available surfacearea of background aerosol did not always suppress nucleation� Bursts of new particleformation most frequently occurred above the local tropopause�

In many cases nucleation is preceded by the photochemical production of aerosol�forming molecules� see above� As additional factors triggering nucleation events in theupper troposphere and tropopause region Schr�oder and Str�om ����� discuss dynamically�induced mixing of air masses with di�erent temperatures and relative humidities cf� Bigg����� and turbulent �uctuations of temperature and concentrations of water vapor cf�Easter and Peters ���� and other gaseous aerosol precursors� On the basis of their dataSchr�oder and Str�om argue that such dynamical processes could at times serve as triggerevents for new particle formation but further experimental e�ort is needed to strengthenthis hypothesis�

���� SOURCES OF TROPOSPHERIC BACKGROUND AEROSOL

Nucleation by IonIon Recombination� The recombination of molecular cluster ionswhich are known to exist both in the troposphere and stratosphere associated with galacticcosmic rays may in principle lead to the formation of stable neutral clusters Arnold����� which subsequently grow by vapor condensation� Turco et al� ����� have proposedthat large ion clusters could provide a stable source of tropospheric aerosols through ion�ion recombination� In the lower atmosphere positive core ions consist of H�O

NH�

and organic ions� Preferred negative core ions are HSO�� CH�SO�

� and NO��� The

fraction of ion�ion recombination events producing stable nuclei is a strong function ofthe precursors that make up these core ions� Typically the number of ion clusters largeenough to create stable nuclei is only a small fraction of the total ion population�

Turco et al� ����� have used this proposed particle source in combination with asimulation model including the e�ects of ambient ionization on particle microphysics toassess the behavior of the resulting aerosol in relation to observations� The authorsfound their results to be consistent with a variety of observations of �ne aerosols andconcluded that nucleation by ion�ion recombination o�ers a possible explanation for themaintenance of the tropospheric background aerosol� In the case of very large precursorsupersaturations that could for example precede burst events transient homogeneousbinary nucleation is not precluded by the model�

However this mechanism for aerosol formation is not yet well de�ned at the molecularlevel �as are the nucleation processes discussed above�� Especially measurements of thecomposition and the stability of large atmospheric ions and their neutral recombinationproducts are required to ascertain the model predictions�

�

Evaporating Tropospheric Clouds� Water clouds form on aerosols� Aerosol particlescan become incorporated into cloud drops by various collision mechanisms �see Figure ���Many gases are soluble in water and will enter cloud and raindrops by di�usion wherethey will dissociate into ions� They may become chemically oxidized in the liquid phaseand turn into essentially nonvolatile components of the drop� When the cloud particleevaporates some or all dissolved material will crystallize in which manner an aerosolparticle results with physico�chemical characteristics di�erent from those possessed beforeentering the drop� This transformation of aerosols is called cloud processing� The e�ectsof cloud processing � increases of dry aerosol mass and associated shifts in the numberspectra toward larger sizes � have been observed in several �eld studies Pruppacher andKlett ���� and references therein�� In a similar fashion aerosols may also be processedby evaporating ice clouds whereby chemical transformations take place at the surfaceof the cloud ice crystals �di�usion of molecules into the bulk of the crystal is very slowcompared to di�usion within liquid particles��

An example illustrating tropospheric cloud processing is as follows� growth of sulfateaerosols in a humid environment with associated production of CCNs� rapid water con�densation and drop formation by surpassing a critical supersaturation determined by theK�ohler curve of the sulfate aerosol� di�usion and uptake of gaseous SO�� oxidation of SO�

in the water droplet by H�O� and�or ozone with associated formation of sulfate� evapo�ration of the cloud drops upon warming and release of a processed aerosol with enhancedsulfate mass� The oxidation of SO� in clouds is much more important than the gas phaseoxidation of SO� by OH which is insu�cient to explain observed levels of conversionof SO� toward sulfate in the global troposphere Warneck in� Zellner ����� Liang andJacobson ������ Thus clouds act both as a source and a sink for atmospheric aerosolparticles� New particles may also form photochemically in the vicinity of evaporatingclouds Clarke et al� ������

�� NEW PARTICLE FORMATION IN THE STRATOSPHERE

Our knowledge of the characteristics of the stratospheric aerosol appears to be muchmore complete than our knowledge of tropospheric particles� Stratospheric particles havebeen observed by balloons since the ����s Junge et al� ����� Rosen ���� by lidar Fiocco and Grams ���� and since the late ����s by satellites McCormick et al������� Stratospheric aerosols are strongly supercooled liquid solution droplets mainlycomposed of H�SO� and H�O Junge et al� ����� Rosen ������ The observed depletion ofstratospheric ozone is strongly linked to heterogeneous chemical processes involving liquidstratospheric aerosol particles Solomon ������

The major source of particles in the stratospheric aerosol �or Junge� layer is the up�per tropical troposphere� To a minor extent new stratospheric particles are also pro�duced in the polar vortices� There is also an extraterrestrial source of particulates forthe stratosphere and upper troposphere Hunten et al� ����� Turco et al� ������ It iswell�established that large volcanic eruptions signi�cantly enhance the mass of the layerfollowed by gradual decreases in mass� We arrived at a relatively robust picture of thelife cycle of stratospheric aerosol particles Hamill et al� ����� which is illustrated inFigure � In what follows I focus on some of these aspects in more detail� I do notdiscuss the formation of polar stratospheric or noctilucent clouds although the formerare important in the context of polar ozone depletion�

��

Nucleation

Coagulation

TropicalStratosphericReservoir

TropicalTransportBarrier Polar

Vortex

Evaporation

Removal

TropopauseFold

Nucleation

8 km

17 km

CloudScavengingandSedimentation

Transport

Condensation

Precursors

VolcanicEruption

Fig��� Schematic of the life cycle of stratospheric aerosol particles� Three main processes a�ect the life

cycle� formation in the tropics� transport to the midlatidutes and growth by condensation of oxidized

sulfur gases� removal from the stratosphere by transport through the tropopause� isentropic� and diabatic

exchange processes� Nucleation processes are discussed in the text� Modi�ed after Hamill et al� � �����

���� NUCLEATION IN THE UPPER TROPICAL TROPOSPHERE

Atmospheric measurements obtained in the midlatitude polar and tropical regions ofboth hemispheres were analyzed by Brock et al� ����� in order to investigate the role ofthe upper tropical troposphere as a source of new particles for the stratospheric aerosolduring times which are not strongly in�uenced by volcanic eruptions �see below�� Themeasurements and numerical calculations indicate that binary homogeneous nucleationof H�SO� and H�O occurs in this region� The nuclei coagulate and are transported up�wards in the rising tropical air masses� Oxidized sulfur gases condense onto the resultingH�SO��H�O droplets�

The observed vertical pro�les of total particle mixing ratios exhibit a maximum inboth the tropical and extratropical regions� Particle abundances decrease smoothly withincreasing altitude above the tropopause due to coagulation� The maximum in particlenumber in the tropics is about � times larger than in the extratropics suggesting that newparticles are preferably formed near the tropical tropopause� It is worth noting that highnumbers of new particles are formed in the tropical tropopause region despite the potentialof high clouds to scavenge the small H�SO��H�O droplets at the time of nucleation� Thenet upwelling in the moist cold upper tropical troposphere causes frequent formation ofconvective and �mostly subvisible� cirrus clouds e�g� Crutzen and Ramanathan ������

The weaker extratropical maximum of the particle mixing ratios may be due to insitu nucleation and�or transport of particles and aerosol precursors by convective pump�ing� Quasi�horizontal transport from the tropics is likely not a signi�cant source for theparticles at and below the extratropical tropopause due to ine�cient mixing between thetropics and the extratropics in contrast to regions in the lowermost stratosphere�

��

The measurements show a greater proportion of nonvolatile particles at extratropicallocations as compared to the mostly volatile tropical particles� As Brock et al� �����conjecture a possible cause for the reduced volatility is the incorporation of ammonia overthe time of quasi�horizontal transport from the tropics toward midlatitudes� Ammoniaacts to partially neutralize sulfuric acid solutions thereby increasing the boiling point ofthe mixed particle�

���� NUCLEATION IN THE POLAR STRATOSPHERE

An anomalous enhancement of springtime condensation nuclei �CN� near �� km al�titude in Arctic air has been �rst noted by Rosen and Hofmann ������ Measurementsover Antarctica have likewise revealed an enhanced particle layer between �� and �� kmaltitude with peak concentrations exceeding ��� cm�� e�g� Hofmann and Rosen ������More recent measurements con�rm the repeated occurrence of this particle layer in thespringtime polar vortices� In the absence of nucleation particle concentrations shoulddecrease with air density as observed during summer� Thus these observations suggestthat new particle formation takes place during polar night� It is conceivable that the layerevents in both hemispheres have similar formation mechanisms�

The CN layer is con�ned to altitudes between about �� and �� km� Below the increas�ing surface area suppresses new particle formation� Above temperatures are high enoughto prevent H�SO� and H�O from nucleating� Model calculations have been performed tosimulate and understand the formation of the particle layer� Zhao et al� ����a� inves�tigated the layer with the help of a one�dimensional dynamical�chemical�microphysicalmodel allowing vertical subsidence of vortex air and with �xed SO� and H�SO� concen�trations at �� km altitude� Mills et al� ����� used a two�dimensional version of a similarmodel and additionally included new photolysis rates of fully oxidized sulfur gases andseasonal and vertical variations of the partitioning between SO� and H�SO��

The models show that accumulation of sulfur gases in air descending from the meso�sphere in the vortex during the polar night followed by rapid radiative cooling and thereturn of sunlight leading to high supersaturations in regions lacking existing particlescan explain the formation of the CN layer� However Zhao et al� ����a� found that theconversion of carbonyl sul�de �OCS� into H�SO� is insu�cient to trigger new particleformation and growth to observable sizes� The simulations of Mills et al� ����� show thataccumulation of SO� in the upper stratosphere and mesosphere generated by photolysisof SO� and H�SO� bring enough SO� into the lower stratosphere to produce the spring�time layers� However comparison with observations indicate another source of SO� in theupper stratosphere not yet considered�

In contrast to the particles produced in the tropics and injected into the stratospherethe residence time of the particles generated in the descending air in the polar vortices ismuch shorter as a result of their transport� The same holds for new particles occasionallygenerated in the midlatitude stratosphere near �� km Rosen and Hofmann ������ Neitherof these particle sources have signi�cant impact on the abundance of aerosols in the lowerstratosphere� As noted above a stratospheric source of new particles is meteoritic debrisbut again the net contribution of meteorites to particles in the stratosphere is small�

���� NUCLEATION IN VOLCANIC ERUPTION PLUMES

During the past two decades lidar and satellite observations have shown that thestratospheric aerosol is at times in�uenced by volcanic eruptions for example in terms

��

76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 9910

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YEAR

TROPOPAUSE + 1 km TO LAYER TOP 694.3 nm694.3 nm

1979

Fig��� Time series of the aerosol backscatter measured by lidar at Garmisch�Partenkirchen from ����

��� �last data point from August ���� The signal can be converted to optical depth or column mass

of the stratospheric aerosol applying appropriate conversion functions �J�ager et al�� ����� Arrows and

corresponding legends mark volcanic eruptions in�uencing the aerosol layer� The horizontal dashed lines

suggests a baseline value of the column particle backscatter signal attributed to the year ���� which also

seems to be representative for the current situation� The two largest eruptions occurred in ��� �El

Chichon and �� �Mt� Pinatubo� Figure courtesy of Horst J�ager�

of mass and optical depth e�g� WMO ������ Besides major explosions with volcanicplumes reaching into the lower stratosphere emissions from small volcanic eruptions mayalso contribute to the aerosol� A review of the observed global radiative and chemicale�ects of a major volcanic eruption is given by McCormick et al� ������

Due to the relatively high frequency of major eruptions in the recent past �El Chichonin ���� Mt� Pinatubo in ����� and the relatively slow decay of the associated pertur�bations �several years� it turns out to be di�cult to ascertain exactly when the aerosolin the stratosphere reaches a steady�state condition and what the exact background levelof aerosol mass and other parameters is� This situation is shown in Figure � where thecolumn�integrated backscatter from stratospheric aerosol particles is plotted from ����until early �����

Several model studies investigated new particle formation e�g� Zhao et al� ����b��For example the Pinatubo eruption injected about �� Mt of SO� into the stratospheretogether with volcanic ash particles and other substances� After chemical conversionto sulfuric acid this generated high numbers of small H�SO��H�O particles by binaryhomogeneous nucleation in the �rst few weeks after the eruption� This is consistent withobservations indicating that almost all of the volcanic aerosols were volatile�

The new particles grew rapidly �rst by condensation and subsequently by coagula�tion in the following months� A stabilized volcanic aerosol mode with a mean diameternear ����m developed after one�half year� Heterogeneous nucleation on ash grains andpre�existing particles did also take place but this mechanism alone cannot explain theobserved timing of the aerosol loading peak and the magnitude of the surface area per�turbation among other details� Large ash and dust particles in the size range ���� ��msedimented quickly and disappeared from the stratosphere after a couple of months afterhaving been injected�

��

�� CONCLUSION

After some introductory remarks about types sources and classi�cations of atmo�spheric aerosols I have discussed case studies of new particle formation in the troposphereand stratosphere� Among important problems of current research are� ��� The lack ofsystematic data on the formation and evolution of ultra�ne particles� More continuousmonitoring periods are needed to explain the variety of nucleation processes and to bet�ter assess the impact of the new particles upon the atmosphere� ��� Particle formationis still not well de�ned on a molecular level� Re�ned theoretical methods to investigatenew particle formation �e�g� molecular dynamics� have to be considered and applied toatmospheric conditions� ��� Interpretation of ultra�ne particle data is often problematicbecause only apparent nucleation rates � equal to the rates of appearance of particleslarger than the threshold sizes of the CPCs � are observed� Local particle formationmay be masked by transport processes and may be in�uenced by di�erences in the con�centrations of chemical precursors often not measured simultaneously with the particles�Comparisons of apparently similar case studies then can easily cause confusion�

Further laboratory studies improved long�term satellite measurements and plannedobservations of aerosol properties with lidars from space along with enhanced modelinge�orts are required to arrive at a more consistent picture of tropospheric aerosol formationand to better represent atmospheric aerosols in climate models leading to an improvedcapability to simulate the future climate� In comparison the life cycle of stratosphericaerosols is relatively well de�ned� Uncertainties remain for example concerning aerosoltransport through tropopause and deep convection of SO�� Only few studies have so farconcentrated on nucleation mode particles in the stratosphere�

Acknowledgments� I thank Chuck Brock and Rich Turco for interesting discussionsand Harald Berresheim and Horst J�ager for providing material used in this work�

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