AEROSOL CHEMICAL PROCESSES IN THE ENVIRONMENT - CHAPTER 5 pot

5 On the Role of Aerosol Particles in the Phase Transition in the Atmosphere Jan Rosinski CONTENTS Introduction ...81 Modes of Ice Nucleation...83 Liquid → Solid Phase Transition: Freezi

5 On the Role of Aerosol Particles

in the Phase Transition in the Atmosphere

Jan Rosinski

CONTENTS

Introduction 81

Modes of Ice Nucleation 83

Liquid → Solid Phase Transition: Freezing Nuclei 84

Nucleation of Ice During Collision of an Aerosol Particle with Supercooled Water Drop: Contact Nuclei 95

Ice Nucleation from the Vapor Phase: Sorption Nuclei 104

Temperature of Ice Nucleation as a Function of the Size of Aerosol Particles 110

Nature of Ice-Forming Nuclei Present in the Atmosphere 113

Radionuclides as Ice-Forming Nuclei 120

Ice-Forming Nuclei and Climate 121

Formation of Ice in Clouds 121

Freezing of Water Drops 123

Extraterrestrial Particles and Precipitation 125

Acknowledgments 130

Dedication 130

References 130

INTRODUCTION

The dry atmosphere of the earth consists mostly of nitrogen and oxygen In addition to the two permanent gases, there is one variable one: water vapor Water vapor concentration varies from close to 0 to nearly 3% Water is the only constituent of air that, in the range of temperatures present on Earth, can exist as vapor, liquid, or solid The lowest concentration can be found over polar regions where temperatures are mostly far below 0°C The highest concentrations exist in the equatorial region The heat required to vaporize or condense water or water vapor, respectively,

is equal to 595.9 gram-calories (15°C) per gram at T = 0°C The heat of fusion at T = 0°C is 79.7 cal g–1 and, consequently, the heat of sublimation of ice is 675.6 cal g–1 During the vapor → liquid phase transition, the latent heat is released; it is also released during the liquid → solid phase transition The latter constitutes 13.4% of the former.1 Most of the water vapor enters the lower atmosphere through evaporation of liquid water from the surface of the Earth and, to a very small extent, through sublimation of ice At 0°C, water vapor pressure over a flat surface of water is equal that over ice; that is, 4.579 mmHg At a temperature of –15°C, the saturated water vapor L829/frame/ch05 Page 81 Monday, January 31, 2000 2:10 PM

82 Aerosol Chemical Processes in the Environment

pressure over supercooled liquid water is 1.436 mmHg and 1.241 mmHg over ice The vaporpressure over water is always larger than over ice for all temperatures below 0°C Because of thisdifference, liquid water evaporates in the presence of ice, resulting in the ice growing Under suchconditions, the surface temperature of the evaporating water drop decreases and the temperature

of the growing ice surface increases due to condensation The largest difference in vapor pressures,

Pwater – Pice = 0.20 mmHg is for the temperature range between –11°C and –12°C (–11.8°C) This

is the basis of the Wegener-Bergeron-Findeisen mechanism of formation of precipitation in theTemperate Zone

If one starts cooling an air parcel (e.g., in an updraft), eventually at some altitude or somelower temperature, one will discover the presence of first cloud droplets The first cloud dropletsform just below water vapor saturation on aerosol particles that are hygroscopic With subsequentlowering of the temperature, the water vapor will become supersaturated and more cloud dropletswill form They will form on cloud condensation nuclei (CCN) that constitute a fraction of thepopulation of aerosol particles.2,3 When the temperature of a rising and cooling parcel of air reachestemperatures below 0°C, ice can form within a cloud Ice is formed on aerosol (or hydrosol) particlesthat can act as ice-forming nuclei (IFN).4

The CCN initiate a phase transition of water vapor to liquid water; this is called the V → Lphase transition This process has been thoroughly treated in many textbooks, and will be discussed

in this chapter only when it constitutes an integral part of the formation of ice Ice can be formedduring the vapor–solid (ice) transition (V → S phase transition) or during the liquid–solid (L → S)phase transition In all cases, aerosol (or hydrosol) particles are necessary to make the phasetransition possible in the atmosphere at temperatures above ~ –40°C; below this temperature,homogeneous nucleation of ice may take place

It should be pointed out that the L → S phase transition taking place at temperatures below

~–40°C in the atmosphere consists of freezing liquid water suspensions (cloud droplets) of philic hydrosol particles in a water solution of different chemical compounds present in the CCN

hydro-In view of this, the L → S phase transition occurring at very low temperatures should be calledspontaneous freezing of droplets; the term “freezing by homogeneous nucleation” should bereserved for freezing of pure water in laboratory experiments Pure water droplets do not exist inthe atmosphere

There are two major sources of aerosol particles The first one is the Earth’s surface and thesecond one is oceans Particles differ in chemical composition, in solubility in water, and in thestructure of their surfaces and their density Aerosol particles are lifted from the surfaces of theEarth and the oceans by turbulence associated with winds An example of the mass lifted from thetwo surfaces is given in Figure 5.1.5–7 Large water drops settle rapidly to the ocean surface,controlling the mass concentration of aerosol produced by oceans The size distribution of marineaerosol particles is governed by two mechanisms The larger, 0.5- to 10-µm diameter dry sea saltparticles are produced by bursting bubbles, and the very small ones (d < 0.01 µm) through gas-to-particle conversion Over land, soil particles up to 250 µm in diameter are suspended in theatmosphere by strong updrafts associated with storms, and their lifetime is also controlled bygravitational forces Aerosol particles from, for example, Mainland China (115°E) have beenobserved to travel eastward over the Pacific Ocean as far as 170°E longitude (Figure 5.2) Duringtheir residence time over the Pacific, they coagulate with aerosol particles generated by the oceanand produce terrestrial/marine mixed aerosol particles.8 Aerosol particles formed by the PacificOcean travel eastward in the Northern Hemisphere and produce marine/terrestrial mixed aerosolparticles while they cross the North American continent As a result, practically all aerosol particlesare mixed aerosol particles.9–11 Contribution from the oceans consists mostly of sulfates; theseparticles are soluble in water and can act as CCN.12,13 Aerosol particles of soil origin consist mostly

of water-insoluble clay minerals, and of water-soluble sodium chloride and sulfates; the latter twocompounds act as CCN

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 83

Air parcel trajectories must be known in studies of aerosol particles participating in phasetransitions in the atmosphere; they will establish the origin and contribute to the knowledge of thelife history of aerosol particles under investigation.14

MODES OF ICE NUCLEATION

There are three basic modes of ice nucleation: freezing, contact, and sorption The same aerosolparticle present in a cloud may nucleate ice by any of the three mechanisms Usually, the differencewill be in the temperature at which phase transition into solid (ice) takes place

FIGURE 5.1 Concentration of soil particles ( × – Chepid, 1957;  – Rosinski et al., 1973) and sea salt particles (  , Reference 115;  , Reference 9) as a function of wind speed.

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84 Aerosol Chemical Processes in the Environment

L IQUID →→→ S OLID (I CE ) P HASE T RANSITION : F REEZING N UCLEI

Different-sized aerosol particles will act as cloud condensation nuclei (CCN) at different watervapor supersaturations (Sw) A dry particle, for example, ammonium sulfate of 6 × 10–2µm diameterwill act as CCN at critical supersaturation of 0.2%; a supersaturation of 1% is required to activate

a particle of 1.6 × 10–2µm diameter.15 Liquid water droplets formed in the atmosphere are thereforewater solutions of portions of an aerosol particle that acted as CCN The water-insoluble part ofthat particle may be wetted, transferred into the interior of a droplet, and later act as an IFN.Transfer of aerosol particles into the liquid phase (aerosol particles become hydrosol particles)takes place during the following processes active in the atmosphere:

1 Transfer of aerosol particles through condensation of water vapor on aerosol particlesactive as CCN This process can be subdivided into three separate groups:

a Condensation of water vapor at subsaturations with respect to saturation over liquidwater, Sw, (hygroscopic particles); S ≤ Sw

b Condensation of water vapor at conditions of slight supersaturation, S > Sw; this takesplace at and just above cloud bases

c Condensation of water vapor at high supersaturations, S >> Sw, that are present in thevicinity of freezing drops or wet hailstones (freezing water)

In the above three cases of V → L phase transition, the liquid phase consists of a solution

of the water-soluble part of the CCN and of the water-insoluble particles present ashydrosol (hydrophilic) particles; if particles are not wetted, hydrophobic particles willremain floating on the surface of a drop A water solution droplet can freeze at highertemperatures than the freezing temperature of pure water, or can be frozen at differenttemperatures with the help of a hydrosol (hydrophilic) or a hydrophobic particle

2 Transfer of aerosol particles into cloud droplets and raindrops This transfer takes place

in the atmosphere by means of several different mechanisms, including:

a Brownian diffusion of submicron aerosol particles

b Phoretic forces associated with condensation and evaporation of droplets: and micron-sized aerosol particles are affected by this scavenging mechanism

submicron-FIGURE 5.2 Transport of non-sea-salt sulfate particles over the Pacific Ocean.

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 85

c Aerodynamic capture: larger particles will be captured by this process

d Turbulent diffusion: this mechanism is responsible for bringing different-sized cles together

parti-e Electrostatic forces: these forces act on particles of all sizes

The above-listed scavenging mechanisms will act in the atmosphere simultaneously.Not all of the mechanisms act at the same time, but in different combinations mostprobably with electrostatic forces always present

3 Formation of hydrosol particles in the liquid phase of clouds In addition to the aboveprocesses that transfer existing aerosol particles into the liquid phase, there are someadditional mechanisms taking place in a cloud that introduce newly formed hydrosolparticles directly into the condensed water They can be grouped into two major catego-ries The first category consists of:

a Formation of solid hydrosol particles during cooling of a water solution of dissolvedsalts (CCN); this takes place in an updraft

b Formation of solid hydrosol particles in evaporating droplets

c Formation of solid particles through chemical reactions between different chemicalcompounds supplied by the CCN and other scavenged water-soluble salts

The second category consists of submicron- and micron-sized hydrosol particles that areshed from the surfaces of larger particles when they are transferred into the condensedwater drop These particles are shed upon contact with water droplets larger than 40 µm

in diameter Concentration and size of the shed particles vary with the size of the parentparticle and type of soil (Figure 5.3) This process is not the breaking of aggregates; it

is a separate process.5-7,16

4 Hydrosol particles as IFN Most aerosol particles consist of aggregates of water-solubleand water-insoluble particles Aerosol particles that can act as CCN are generally watersoluble; they consist of some water-insoluble particles found together in the water-solublematrix A droplet formed on a CCN particle consists of a water solution of water-solublesalts and a suspension of water-insoluble particles Particles that will not be wetted(hydrophobic particles) will float on the surface of a droplet; they may nucleate ice bydelayed-on-surface nucleation Concentrations of salts and suspended (hydrophilic) par-ticles will decrease during the growth of a droplet growing by condensation in an updraft

As the parcel rises, it will eventually pass through the 0°C temperature level Above thisaltitude, cloud droplets become supercooled suspensions of hydrosol particles in solutions

of CCN in water The liquid → solid (ice) phase transition can now take place It wasfound from experiments performed over the years that for all modes of ice nucleation,each particle size — even if monodispersed and chemically and physically homogeneous

— is always associated with a freezing temperature spectrum.17 Hydrosol particles anddissolved chemical compounds participate in the initiation of the L → S (ice) phasetransition To see if there is any relation between aerosol particles (aerosol particlestransferred into liquid water), Rosinski introduced the concept of a water-affected fraction

of aerosol particles.14 The water-affected fraction (by number) in a given size range i is

(5.1)

where L is the concentration of aerosol particles and N is the concentration of water-insolublehydrosol particles

Transfer of aerosol particles from and into the i size range when they become hydrosol particles

is shown in Figure 5.4 Group A consists of aerosol particles that are insoluble in water The affected fraction for aerosol particles in Group A is equal to zero; they are transferred into waterwithout changing size Another extreme is when the aerosol population consists of water-soluble

water-f i= −1 N L i i

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86 Aerosol Chemical Processes in the Environment

FIGURE 5.3 Shedding of micron-size particles from a surface of a 180- µ m diameter particle immersed in water at 15, 30, 60, and 90 seconds and lost from a dry surface (A) on impact (B).

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 87

particles only The water-affected fraction is equal to 100%, indicating the complete absence ofhydrosol particles for Group B of the aerosol particles Group C consists of mixed aerosol particles,that is, of particles that are aggregates of water-soluble and water-insoluble particles When thesoluble part dissolves in water, the insoluble particle becomes a hydrosol particle It can remain inthe i size range, it can be transferred into the (i – 1), or even into the (i – n) size range and becompletely lost if that lower size range is outside the size range under investigation Category Dconsists of aggregates of smaller particles that may produce even larger numbers of hydrosolparticles For large concentrations of aerosol particles in the D category, the water-affected fractionbecomes a negative number Some of the results from experiments performed during 1969 and

1970 are presented in Figure 5.5 The negative values of f i were found in experiments in whichliquid impinger was used; they were for the lower size ranges of i equal to 1.5–3 and 3–5 µmdiameter size ranges (experiments I, 0–0) However, there were aerosols that did not producenegative values of f i (I, x–x), indicating the presence of aerosol particles that did not consist ofaggregates that could be broken either during the contact of particles with water or the mechanicalforce present in the impinger that is exerted on particles That force does not exist in nature whenaerosol particles are transferred into the liquid phase of a cloud The f i values determined on filtersclearly show the presence of two different classes of aerosol particles The f i values for aerosolparticles of marine origin were found to be around 99% (II, ) For pure continental air masses,the f i values were around 1 to 5% (II, –) Aerosol particles present in mixed air masses have f i

values between the extreme values For continental–marine air (II, ) f i values were about 30%and for marine–continental air (II, –x) they were 72 to 90% Generally, f i values were higher (55

to 90%) in the presence of southerly winds; for westerly winds, they were from 1 to 83% inColorado

Part of the aerosol population acts as CCN; if the water-insoluble parts of mixed particles canact as IFN, then there should exist a direct relation between IFN and CCN The ratio of CCN toIFN concentrations is about 106 in an unpolluted atmosphere An example of that relation is shown

FIGURE 5.4 Transfer of aerosol particles into water.

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88 Aerosol Chemical Processes in the Environment

in Figure 5.6.18 Khorguani, et al.19 found a correlation between concentrations of CCN and IFN in

40% of measurements made over the North Caucasus Mountains Results of these measurements

strongly suggest a relation between CCN and IFN; they also suggest that, at first, condensation

takes place on aerosol particles active as CCN and, after cloud droplets have formed, ice particles

(frozen droplets) are produced through ice nucleation The liquid phase is the solution phase, which

is generally more difficult to nucleate than pure supercooled liquid water The molal depression of

the freezing point was found to be proportioned to the molality of a solution; this is known as

Blagden’s law It was published in 1778, but R Watson discovered the depression of the freezing

point in 1771; his findings somehow went unnoticed Junge9 pointed out that the salt concentrations

in cloud droplets just formed on CCN are too high for the L → S phase transition to take place at

cloud temperatures Experiments by Sano et al.20 completely changed the understanding of freezing

of droplets formed on CCN They showed, in experiments using 8 µm average diameter water

solution droplets, the existence of temperature maxima at which L → S transitions take place; this

temperature was a function of the concentration of dissolved chemical compounds in water In

nature, the temperature at which droplets freeze will depend not only on the concentration of

dissolved CCN, but also on the type of insoluble particle or particles that were part of the aerosol

particle acting as CCN and remained within a droplet as hydrosol particles The consequence of

this finding is that not only can a droplet growing by condensation reach critical dilution and freeze,

but also an evaporating droplet can come to the same critical concentration and also freeze This

FIGURE 5.5 Water affected fraction for aerosol particles present in different air masses (I, f i from liquid

impinger; II, f i from filters).

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 89

is shown in Figure 5.7; the hypothetical curve represents actual data but cannot be used to determine

the temperature of the L → S phase transition taking place in different clouds.21,22 The role of

hydrosol particles of different origin on this freezing phenomenon is shown in Figure 5.8.8 The

maximum freezing temperatures at a given concentration of ammonium sulfate in water solution

were –4°, –9°, and –12°C for marine and continental aerosol particles acting as IFN in a pure salt

solution They were all recorded at an ammonium sulfate water solution of 10–4 M At that salt

concentration, the difference between the highest temperature of drop freezing of a water solution

of pure ammonium sulfate and a solution of IFN present (aerosol particles) of marine and continental

origins is 8° and 3°C, respectively For 10–1 M solutions, the difference was 10° and 6°C; these

differences were the largest observed It is clear that there is a difference between aerosol particles

of marine and continental origin active as IFN through freezing

Cloud condensation nuclei consist mostly of sulfates and chlorides.10,11,23–25 Sulfate-bearing

aerosol particles are predominant in the marine atmosphere The ratio of sulfate-bearing aerosol

particles to the number concentration of aerosol particles in the 0.1 to 0.3 µm diameter size range

was found to be between 0.99 and 1.0 Sulfates, most probably ammonium sulfate, are therefore

present in practically all cloud droplets in the marine atmosphere The sulfate ion constitutes an

integral part of IFN of marine origin Results of experiments performed with aerosol particles of

continental and marine origins are shown in Figures 5.9 and 5.10.26,27 It was found that the

concentration of IFN present in marine air masses increases with increasing Sw at constant

tem-perature On the other hand, the concentration of IFN of continental origin remained constant over

a wide range of Sw at constant temperature This suggests that the marine atmosphere contains

aerosol particles with a wide size range Larger aerosol particles will act as CCN at lower water

vapor supersaturation; smaller particles will nucleate liquid water (vapor → liquid phase transition)

at higher Sw An aerosol particle of 0.1 µm diameter (9.26 × 10–16 g) acting as CCN will initiate a

water droplet that will grow in an updraft The concentration of ammonium sulfate in water solution

will reach the critical concentration of 10–4 M when the growing droplet reaches ~4 µm in diameter

The critical concentration is the concentration of the solute at which the L → S phase transition

FIGURE 5.6 Concentration of IFN (–20°C) and of CCN (Sw = 1.5%) at an altitude of 3000 m in Colorado.

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FIGURE 5.7 Hypothetical curve based on experimental data showing temperature of freezing of droplets.

Critical concentration curve, C cr: , evaporating, and , growing droplets, , at Sw = 10%, , at Sw = 0.3%.

FIGURE 5.8 Maximum freezing temperatures are a function of ammonium sulfate concentration (, marine aerosol particles; , continental aerosol particles; , no particles).

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 91

will take place at a critical temperature The critical temperature of ice nucleation is the maximumtemperature at which the L → S phase transition takes place For drop freezing, there are threetemperature maxima: one is for a pure solution of ammonium sulfate in water, and the second andthird are for the solution in contact with aerosol particles of continental and marine origins The

~4 µm diameter droplet must therefore cross the altitude corresponding to one of the criticaltemperatures to freeze If the temperature is higher than the critical temperature when the growingdroplet reaches ~4 µm in diameter, then it will continue to grow by condensation in an updraft andfreeze later at some lower temperature corresponding to a freezing temperature of a more dilutesolution The diameter of a droplet formed on a 0.3 µm diameter ammonium sulfate particle (2.5

× 10–4 g) is 17 µm for 10–4 M solute concentration This cloud droplet size is a better candidate

for freezing than the 4 µm diameter droplet because it will reach this diameter at a lower temperaturewithin a cloud

All three concentrations of solute vs temperature of ice nucleation curves (see Figure 5.8) aremore or less parallel with the maximum for the L → S phase transition occurring at a concentration

FIGURE 5.9 Cumulative concentrations of IFN of

marine and continental origin as a function of water

vapor supersaturation, Sw%.

FIGURE 5.10 Differential concentrations of IFN of

marine and continental origin as a function of water vapor supersaturation, Sw%.

of 10–4 M If particles of marine or continental origin would be solely responsible for the nucleation

of ice, then the temperatures of drop freezing should be the same as the temperature of ice nucleation

of the individual particles This is not the case, however, and the temperature of ice nucleation isgoverned by the concentration of the solute to a large degree Particles seem to change temperature

in an orderly manner The internal structure of liquid water is not uniform Water has local regions

in its interior of hydrogen-bound clusters In the presence of particles, ions present in the solutionmust somehow either increase the number or size of ice-like clusters and move the ice nucleationtemperature upward Ammonium and sulfate ions are larger than H+ and OH– ions, and they probablyform their own network of ions, thus “squeezing” or “caging” water molecules and maybe stabilizinghydrogen-bound clusters All this must take place on the surface of hydrosol particles Particles ofmarine origin always contain organic matter Particles of continental origin, on the other hand,contain everything that is picked up by wind from the surface of the ground The ice nucleatingability of particles of continental origin is associated with the presence of clays and other minerals.Adsorption of hydrogen and hydroxyl ions — and maybe hydrogen-bounded clusters — and ofammonium and sulfate ions is different for organic and inorganic surfaces, and this may accountfor the difference in temperatures of ice nucleation for the two different classes of IFN surfaces.The population of IFN generally increases with decreasing temperature This was observed allover the world The variations were up to a factor of ten at any given temperature Using data ofBigg and Stevenson,28 it is possible to derive an expression for the IFN concentration (C IFN , m–3,

the median number concentration) as a function of temperature (T°C):

(5.2)

The C IFN is equal to 10 m–3 and 1000 m–3 for the temperatures of –11°C and –20°C, respectively.Equation 5.2 suggests that the IFN concentration increases tenfold for every 4.5°C temperaturedecrease It should be emphasized that this relation exists for the identical method of detection ofIFN under investigation Different techniques will yield different concentrations of IFN because ofdifferent modes of activation of aerosol particles in different chambers

There are exceptions It was found, for example, that the aerosol particles of marine originexisting in the equatorial region of the Pacific Ocean act as IFN that are independent of Sw andtemperature (see Figure 5.11) The mode of ice nucleation was condensation-followed-by-freezing

Concentrations of 100 m–3 active at a temperature of –3.3°C and of 3 × 104 m-3 active at and below–4°C were located over the South Equatorial Current These concentrations were patchy and by

no means represent the IFN concentration over the Pacific Ocean Concentrations of IFN collected

in the coastal region of the Pacific Ocean increased with increasing Sw; but for each water vaporsupersaturation, they were independent of temperature (Figure 5.12) The data are scattered due tothe different times of day of sampling the aerosol particles from the same air mass for each of the

temperature curves There are C IFN vs T curves that exhibit a concentration plateau for some

temperature ranges.29–31 It was assumed that the part of the curve showing C IFN independent of T

was due to the presence of aerosol particles of marine origin

It has been shown up until recently that CCN are the source of IFN active by freezing; but this

is not the general case In storms, it was found that the changes in concentrations of IFN do notfollow the curves showing the concentrations of CCN vs time; it looks, as a matter of fact, likethese two curves are completely independent of each other The IFN concentration vs time curvesare parallel to the wind speed curves, indicating that the source of IFN is aerosol particles liftedfrom the ground surface by winds It was also found that IFN storms exist; the concentration ofIFN rises, quite often two orders of magnitude and lasts sometimes for a few hours (see Figures5.13 and 5.14)

The size distribution of aerosol particles lifted from the surface of land depends on the condition

of the land surface (either wet or dry, covered with grass or open soil) and wind speed Particles

=0 036 10 × − 0 222

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 93

found in updrafts were generally up to 250 µm in diameter (Table 5.1); particles larger than thatsize were found in severe storms Aerosol particles therefore extend size distribution of clouddroplets before their full size distribution can be developed inside a cloud (given in Table 5.1) Thelarge aerosol particles accrete cloud droplets; they become wet practically from the time they enterthe cloud, and continue to grow and form large droplets or raindrops The liquid phase of suchdroplets consists of one large hydrosol particle, and sometimes of large (several hundred) numbers

of submicrometer-sized and a few micrometer-sized particles The smaller hydrosol particles areshed from the surfaces of larger particles (see Figure 5.3)

Results from laboratory experiments have shown that the parent large particles nucleate ice attemperatures higher than the temperatures of ice nucleation of the shed particles (see Figure 5.15).The large hydrosol particles freeze large drops within clouds, and the frozen drops serve astransparent hailstone embryos.32–38 The dimensions of the largest soil particle located centrally in

a transparent embryo was 265 × 750 µm

FIGURE 5.11 Concentrations of IFN active by condensation-followed-by-freezing as a function of

temper-ature for different ranges of Sw% (continental aerosol particles: , South Africa; , United States marine aerosol particles; , Pacific Ocean equatorial region).

There are many hydrosol particles in a droplet or drop One of the particles will start the L → Sphase transition This particle will nucleate ice at the highest temperature; the rest of the hydrosolparticles may be capable of nucleating ice at temperatures infinitesimally lower than the one thatstarted the transition Formation of ice nucleating sites on the surfaces of hydrosol particles is timedependent.39,40 At the time of contact of the surface of a particle with liquid water, the surface isexposed to a highly turbulent water layer The dissolution of water-soluble chemical compoundsand the shedding of submicrometer and micrometer hydrosol particles starts from the moment aparticle is wetted Heat of immersion (i.e., the sum of different heats associated with immersion,whether positive or negative) is released during the time of wetting of aerosol particles Microscaleturbulent diffusion moves the dissolved solids and the shed hydrosol particles away from the surface

of the parent particle In a few minutes, the system (hydrosol particles and solution) returns tothermal equilibrium and the process of shedding small particles ceases During the period of particleshedding and dissolution of water-soluble chemical compounds, high rates of ice nucleation areobserved The shed particles, and the parent particle when resuspended again in supercooled water,did not exhibit enhanced ice nucleation rates and nucleated ice at lower temperatures It is clear,therefore, that refreezing of collected precipitation cannot produce ice nucleation temperaturespectra of aerosol particles ingested by a storm Aerosol particles undergo chemical and physicalchanges when they become hydrosol particles

FIGURE 5.12 Concentrations of IFN from a Pacific Coastal region as a function of Sw% for different temperatures.

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 95

N UCLEATION OF I CE D URING C OLLISION OF AN A EROSOL P ARTICLE WITH

S UPERCOOLED W ATER D ROP : C ONTACT N UCLEI

A cloud consists of cloud droplets and “dry” aerosol particles Cloud droplets grow rapidly fromthe time of the V → L phase transition taking place at a cloud base Aerosol particles that acted

as CCN are removed from the aerosol population Aerosol particles larger than about 40 µm indiameter act as an accretion center for cloud droplets from the time they enter a cloud and areremoved from the aerosol population The temperature at the cloud base is often not low enoughfor these particles to act as IFN at the time of first contact with liquid droplets They act as IFNthrough freezing after they form larger droplets that are lifted by an updraft into lower temperatures

at higher altitudes The remaining aerosol particles may collide with supercooled drops They maybounce off the surface of a supercooled drop (water non-wettable particles), be captured by asupercooled drop on contact (wettable particles), or penetrate through the surface of a supercooleddrop and be captured Particles that are transferred into the interior of drops become hydrosolparticles and will nucleate ice only through freezing The relationship41 between the impact velocityv(cm–1), its component normal to the surface of a drop required for penetration of an impacting

non-wettable aerosol particle (V PN cm sec–1), water surface tension, T W (dynes cm–1), and waternon-wettable aerosol particle diameter (d, µm), and its density (ρ, g cm –3), is:

FIGURE 5.13 An example of changes in CN and IFN (–21°C) concentrations and of wind speed during a

24-hour period.

FIGURE 5.14 An example of an IFN storm (dashed line: data from NCAR ice nucleus counter, and solid

line: data from the filter technique).

FIGURE 5.15 The ratios of ice nucleation temperatures of parent particles (T) to the shed particles (Tsp).

Average Concentrations of Different-Sized Cloud Droplets and Aerosol Particles in a Severe Storm (Number of Particles per Cubic Meter

per Given Size Interval)

Figure 5.16) Particle trajectories deviate from the streamlines of airflow Particles followingtrajectories 1 and 2 collide with the surface of the spherical sector confined between angles α2 and

α1 A tangent particle trajectory defines the zone of the sphere downstream from the circle ofintersection AA′ (corresponding to angle γ) This zone is free of particles because particles deposited

on the circle AA′ block deposition downstream The number of particles collected at time t on the

surface of a spherical sector positioned at α is:

(5.5)

and at saturation:

(5.6)

where α is the mean of α2 and α1, and where (α2 – α1) is very small The equations permit calculation

of an angle of approach of an aerosol particle during aerodynamic capture N is the number of

particles per square centimeter

An example of the angle at which aerosol particles are captured by a sphere is given in Figure5.17 Aerosol particles are also collected on the lee side of a sphere; results of experiments with2.8 µm diameter particles have shown that higher number of particles were collected at lower airvelocities Every collision following deflection of a particle or its capture can start the L → S phasetransition A question remains, however: Is the angle at which an aerosol particle colliding with aliquid surface of a drop or droplet important?

Some of the aerosol particles may start the L → S phase transition during penetration throughthe skin of a drop In this case, they can nucleate ice through mechanical disturbance of the surface

of a supercooled drop; to do this, they may or may not possess ice nucleating sites active at thetemperature of a supercooled drop Smaller aerosol particles than the minimum size will collidewith drops, but they will not penetrate through the surface of the drops Particles may be capturedthrough aerodynamic capture or, if aerosol particles are in the submicron diameter size range,

DC m

PN PN

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 99

through Brownian motion Pure Brownian motion can be modified by thermophoresis Electrostaticforces can also play an important role in transferring aerosol particles toward the surface of adroplet Diffusiophoresis must be taken into account Nucleation of ice by contact can be subdividedinto three subgroups using time as a variable The first one might be nucleation on contact during

a collision and subsequent bouncing-off of an aerosol particle This will be most pronounced whendealing with particles that are hydrophobic The time of nucleation is a fraction of a second Thesecond one is during collision and capture of an aerosol particle; in this case, nucleation takes place

at the time of collision or between the collision and extremely short residence time of an aerosolparticle on the surface of a drop The third category is when an aerosol particle is captured on or

in the surface of a drop and spends some time floating before nucleating ice; this is called thedelayed on-surface ice nucleation Pitter and Pruppacher,47 in an elegant experiment using homo-geneous sources of aerosol particles, have shown that aerosol particles present on the surface ofsupercooled drops nucleated at temperatures higher than when they were present inside drops ashydrosol particles Particles of montmorilonite and koalinite nucleated ice at –3°C and –5°C when

FIGURE 5.16 Schematic drawing showing aerodynamic capture of an aerosol particle.

present on the surface of supercooled drops (nucleation by contact), but the temperatures of icenucleation were –12°C and –14°C for particles suspended in water and acting as hydrosol particles(nucleation by freezing) The above temperatures are the highest temperatures and there are tem-perature distributions connected with each experiment.

Ice nucleation by contact starts on or in the surface of a drop It has not been possible, however,

to identify a particle that nucleated ice because it could act alone or maybe in a cluster of particlesfloating on the surface of a drop Experimental evidence indicates that the fraction of aerosolparticles nucleating ice by contact at the time of collision increases with increasing size of collidingparticles and with decreasing temperature.39,40,48 The results of these experiments are given in Figure5.18 for different natural aerosol particles derived from two different soils The diameter range ofthe aerosol particles was 5 to 40 µm If these “dust” particles (soil-derived aerosol particles) arelifted by an updraft and subject to scavenging by cloud droplets or drops, they will produce iceparticles through delayed on-surface ice nucleation as a function of temperature, aerosol particlesize, and time of residence of particles on the surface of droplets

The number of ice crystals formed in a cloud parcel at time t by delayed on-surface ice

nucleation is given by:

FIGURE 5.17 Angle (β) at which aerosol particles were captured by a sphere at position defined by angle

α for different sized particles (d, µm) and two different air flow velocities U, (m s–1 ).

t

d s

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 101

Delayed on-surface ice nucleation should therefore be considered as an important process forprimary ice particle formation in clouds Older clouds should have larger concentrations of iceparticles because of the longer residence time available for aerosol particles captured by droplets

to nucleate ice At temperatures of ~ –10°C, 10 to 20 µm diameter aerosol particles or larger must

be present to have nucleation of ice by contact Captured aerosol particles start to nucleate ice withtime; smaller particles will require longer residence times and lower temperatures, and largerparticles require shorter times and higher temperatures At temperatures of –15°C, the ice particleconcentration during the lifetime of a cumulus cloud should reach ~104 m–3 (it is clear that thisconcentration will depend on the ice nucleating temperature spectrum of aerosol particles ingested

by a cloud, the size distribution of aerosol particles, temperature, size distribution of cloud droplets,and the updraft velocity) However, estimated ice concentrations are far below the observed ones

in cumulus clouds Blyth and Latham,49 studying development of ice in New Mexican summertimecumulus clouds, found ice particle concentrations of up to 1.3 × 106 m–3 These large concentrationsare far greater than concentrations of IFN active at –15°C present in that part of the country IFNconcentrations of up to 104 m–3 were observed at a temperature of –16°C on some occasions.10,11

The number of aerosol particles in the micron and submicron diameter size range collidingwith and captured by a drop due to aerodynamic capture is very low Aerosol particles in thesesize ranges are in constant random motion due to collisions with gas molecules The steady-state

flux of aerosol particles of radius r to a surface of a droplet of radius R is given by:

(5.8)

where N p (r) is the aerosol particle number density at some distance from a drop surface (number

cm–3 cm–1); D p (r) is the diffusion coefficient for aerosol particles in air (cm2 s–1); and Ψ w (r) is the

factor by which the pure Brownian transport is modified in order to include phoretic effects Thisfactor is:

FIGURE 5.18 Delayed on-surface ice nucleation as a function of temperature and size of aerosol particles.

4πD r RN r dr p( ) p( ) Ψw( ),r number cm–2s–1

where

(5.10)

The first term in Equation 5.9 describes the thermophoretic effects, while the second term

describes the diffusiophoretic effects T w and T∞(°C), respectively, are the water droplet temperatureand the mean temperature of the cloud parcel, and ρw and ρ∞ are the water vapor density at a dropletsurface and the mean water vapor density in a cloud parcel (g cm–3), respectively The upper limit

of applicability of this equation is d ≤ 1 µm

An example of the rate of aerosol particle capture (cm–3 s–1) is given in Figure 5.19 The factor

β depends on the thermodynamic state of the surrounding cloud parcel In-cloud conditions used

in this example are T = –15°C, N ice = 0.1N droplets, and cloud water content = 4 gm–3 Three (5,

10, and 15 m s–1) updraft and three (–5, –10, and –15 m s–1) downdraft velocities were used incalculations Capture of aerosol particles by evaporating droplets in the downdraft is larger for alldowndraft velocities than in an updraft; droplets evaporate not only due to the presence of ice, butalso due to the downward transport of an air (cloud) parcel This latter effect is the predominantcause of enhanced particle capture (collision), and the collision rate is proportional to the downdraftvelocity All collision rates in the downdraft regime are higher than the one predicted by pureBrownian theory In the downdraft at 0 m s–1, cloud droplets evaporate in the presence of the icephase only At 10 m s–1 updraft, the rate of evaporation of droplets is compensated by the rate ofcondensation of water vapor on droplets; these two practically cancel one another and the modifi-cation of the pure Brownian theory is eliminated At this and higher updraft velocities, the ther-mophoretic effect is practically eliminated and nucleation of ice by contact ceases

Slinn and Hales50 were the first to point out that ice nucleation in clouds can take place onsubmicron particles by means of thermophoresis The fraction of aerosol particles nucleating ice

is a function of the particle diameter The smaller the size of the particle, the smaller the fraction

of particles starting the phase transition Below 1 µm in diameter, this fraction is reduced by 4 or

5 orders of magnitude Consequently, the large number of collisions is counteracted by smallnumbers of particles capable of nucleating ice However, this last statement might be incorrect.Different size fractions of aerosol particles nucleating ice were determined for different temperaturesusing supercooled drops that were neither condensing nor evaporating For a water drop in equi-librium with temperature, the number of condensing and evaporating water molecules are equal toeach other in any time interval During the thermophoretic collision of a submicron-diameter aerosolparticle with a water droplet, water molecules are leaving the surface; the configuration of watermolecules in the evaporating surface may be different from that in the surface at rest and, conse-quently, the fraction of aerosol particles nucleating ice may be different It should be pointed outthat the captured submicron aerosol particles will not float separately on the surface of a drop, butthey will form clusters Some of these may be capable of nucleating ice

Aerosol particles in the phoretic size range, when moving toward an evaporating drop, mustcross a layer of high water vapor supersaturation adjacent to the surface of an evaporating drop Ifthe particle can nucleate ice through the vapor → ice phase transition, then it will be coated with

at least a molecular layer of ice Nucleation of ice in an evaporating drop will take place, not by

an aerosol particle, but by an ice-coated particle In natural clouds, this process will be restricted

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 103

to lower temperatures (maybe below –20°C) In the presence of silver iodide particles, ice nucleationshould take place at temperatures just below 0°C

There is also an internal temperature gradient within an evaporating droplet Hydrosol particlesmoving in that gradient will collide with the internal surface of an evaporating droplet Again, theconfiguration of water molecules in that surface (the surface facing the interior of a drop) may bedifferent from that on the evaporating surface The possibility of the nucleation of ice by internalcollisions was suggested by Weickmann.51

Experiments have been performed to study capture of submicron- and micron-sized aerosolparticles by evaporating and condensing water drops.17,44,45,52 Hydrophilic and hydrophobic particleswere used to see if there was a difference in capture efficiency of particles with different wettabil-ities The experiment was not designed to duplicate processes taking place in clouds The resultsare given in Figure 5.20 Cooling of a drop was provided by a thermoelectric element of a single-stage bismuth telluride p-n junction Heat of condensation of water vapor was removed by excessivecooling, which was necessary to keep the temperature of a condensing drop below the temperature

of the chamber for the entire duration of the experiment The results shown in the right side ofFigure 5.20 therefore represent the capture of different-sized aerosol particles by three mechanismsacting simultaneously: Brownian diffusion, thermophoresis, and diffusiophoresis For particles of0.05 µm diameter, Brownian diffusion modified by thermophoresis is the most effective collisionand capture mechanism Capture of aerosol particles decreased with increasing size of particles,and capture increased for all aerosol particle sizes with increasing rate of condensation of watervapor (in this experiment, it corresponds to increased cooling) The fastest increase was for 0.05

µm, followed by 0.37 µm and 1.9 µm diameter particles Capture of hydrophilic aerosol particles

FIGURE 5.19 The rate of capture of aerosol particles as a function of their size (in-cloud conditions are

given in text).

was found to be up to four times larger than that of hydrophobic particles The number of collisionswas the same for both types of particles (number cm–2 s–1) Hydrophobic particles are thereforebetter candidates for the delayed-on-surface ice nucleation because of their indefinite residencetime on the surface of a drop In a real cloud, evaporation of droplets takes place in a downdraft,

in the presence of ice particles or during dilution with outside drier air; condensation of watervapor takes place in an updraft Phoretic forces are an important mechanism in capturing aerosolparticles by cloud particles and, consequently, in contact nucleation of ice.48

I CE N UCLEATION FROM THE V APOR P HASE : S ORPTION N UCLEI

Aerosol particles present in the rising parcel of a cloud are exposed to water vapor present atdifferent supersaturations with respect to ice The degree of supersaturation in a cloud depends onthe updraft velocity and the concentration of cloud condensation nuclei.53 The most active CCNwill produce the first droplets, which will continue to grow by further condensation and coagulationwith adjacent droplets A droplet or a drop may freeze through freezing or contact nucleation Atthe time of initiation of the liquid → solid (ice) phase transition, the temperature of a drop is thesame as the temperature of the surroundings From time zero, the heat of the phase transition offreezing is released and the temperature of a freezing drop is higher than that of the surroundingcloud parcel; the temperature of a freezing drop is 0°C during the entire process of freezing Heatand water vapor are released into the surrounding air in proportion to the size (diameter) of thefreezing drop Due to nonequilibrium thermal conditions, a region of supersaturated water vaporsurrounds a freezing drop The released water vapor condenses on water droplets, ice crystals, andaerosol particles present in the vicinity of a freezing drop Water vapor supersaturation in naturalclouds reaches about 3 and 1% in marine and continental atmospheres, respectively, for a 10 m s–1

updraft velocity; at 1 m s–1 updraft, the Sw is 0.8% and 0.3%, respectively For these updraft

FIGURE 5.20 Aerosol particle capture by an evaporating and condensing water drop.

On the Role of Aerosol Particles in the Phase Transition in the Atmosphere 105

velocities, the concentration of cloud droplets is about 95 and 60 cm–3 for marine, and 925 and

515 cm-3 for continental clouds, respectively Supersaturation of water vapor around a freezingdrop, on the other hand, reaches values in excess of 20% Supersaturation lasts for a fraction of asecond around freezing cloud droplets, but it persists for the entire time of freezing of large waterdrops, which may take over one minute (Figure 5.21) Water vapor supersaturation in an ascendingcloud is distributed more or less uniformly above the cloud base, but the supersaturation presentaround the freezing drops is spread non-uniformly in time and space within a storm Weickmanngave the name “Rosinski-Nix effect” to the formation of ice crystals on aerosol particles at watervapor supersaturations present around a freezing drop (see “Acknowledgments”)

Wegener4 suggested the existence of natural aerosol particles that absorb water vapor on theirsurfaces and grow ice crystals directly from vapor The existence of such particles was shown byFindeisen54 (in 1938), who grew ice crystals at water vapor supersaturation with respect to ice butbelow liquid water saturation Roberts and Hallett,55 using different minerals, found the thresholdtemperature to be –19°C and the minimum supersaturation with respect to ice about 20% Rosinski

et al.,56,57 using different-sized soil particles, have shown that ice nucleation in the vicinity of afreezing drop or when exposed to a controlled supersaturation in a dynamic chamber depends onthe size of particles, the nature of particles, and the temperature (Figure 5.22) For particles largerthan 40 µm in diameter, the temperature of ice nucleation was –16.8°C and independent of size.The temperature of ice nucleation on particles below 40 µm diameter increased with increasingsize; at 15 µm diameter, it was below –20°C On the left side of the demarcation line in Figure

FIGURE 5.21 Water vapor supersaturation around a freezing droplet (d = 40 µm).

5.22, soil particles acted as condensation nuclei; only water droplets were observed down to thetemperature of –21°C In the water vapor → solid (ice) phase transition zone around a freezingdrop, formation of liquid droplets was observed on some occasions Consequently, one obtains icenucleation through two mechanisms: condensation-followed-by-freezing, and sorption In storms,particles larger than 40 µm in diameter will always be “wet” through collisions with cloud dropletsand, therefore, they will not nucleate ice through sorption Small aerosol particles will require lowtemperatures to form ice directly from water vapor, even in the vicinity of a freezing drop Thesoil particles used do not represent the entire aerosol population, but it is probably safe to statethat the Rosinski-Nix effect produces insignificant numbers of ice crystals in a natural cloud.58

The distribution of water vapor is not uniform around a freezing drop This is shown in Figure5.23 for the phase transition on silver iodide particles It should also be noted that on some occasions,there was a delay of ice particle formation — with a maximum of a few seconds Water vapor →solid phase transition took place through ice nucleation by sorption only; liquid droplets andsubsequently freezing droplets were not observed The threshold temperature was –9.8°C ± 0.1°C.Dessens59 and Gagin60 have shown that the IFN concentration increased with relative humidity at

a constant temperature Huffman61 plotted normalized data of IFN concentrations vs water vaporsupersaturation over ice for natural aerosol particles and for laboratory-prepared particles of silver iodide

The IFN concentrations, N IFN, were found to be independent of temperature and obey a power law:

FIGURE 5.22 Formation of liquid and solid (ice) phases on different-sized aerosol particles.