81 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 © 2000 by CRC Press LLC 82 Aerosol Chemical Processes in the Environment pressure over supercooled liquid water is 1.436 mmHg and 1.241 mmHg over ice. The vapor pressure over water is always larger than over ice for all temperatures below 0°C. Because of this difference, liquid water evaporates in the presence of ice, resulting in the ice growing. Under such conditions, 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, P water – P ice = 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 the Temperate Zone. If one starts cooling an air parcel (e.g., in an updraft), eventually at some altitude or some lower temperature, one will discover the presence of first cloud droplets. The first cloud droplets form just below water vapor saturation on aerosol particles that are hygroscopic. With subsequent lowering of the temperature, the water vapor will become supersaturated and more cloud droplets will form. They will form on cloud condensation nuclei (CCN) that constitute a fraction of the population of aerosol particles. 2,3 When the temperature of a rising and cooling parcel of air reaches temperatures below 0°C, ice can form within a cloud. Ice is formed on aerosol (or hydrosol) particles that 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 → L phase 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 formed during 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 phase transition 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 hydro- philic hydrosol particles in a water solution of different chemical compounds present in the CCN. In view of this, the L → S phase transition occurring at very low temperatures should be called spontaneous freezing of droplets; the term “freezing by homogeneous nucleation” should be reserved for freezing of pure water in laboratory experiments. Pure water droplets do not exist in the atmosphere. There are two major sources of aerosol particles. The first one is the Earth’s surface and the second one is oceans. Particles differ in chemical composition, in solubility in water, and in the structure of their surfaces and their density. Aerosol particles are lifted from the surfaces of the Earth and the oceans by turbulence associated with winds. An example of the mass lifted from the two 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 marine aerosol particles is governed by two mechanisms. The larger, 0.5- to 10- µ m diameter dry sea salt particles are produced by bursting bubbles, and the very small ones (d < 0.01 µ m) through gas-to- particle conversion. Over land, soil particles up to 250 µ m in diameter are suspended in the atmosphere by strong updrafts associated with storms, and their lifetime is also controlled by gravitational forces. Aerosol particles from, for example, Mainland China (115°E) have been observed to travel eastward over the Pacific Ocean as far as 170°E longitude (Figure 5.2). During their residence time over the Pacific, they coagulate with aerosol particles generated by the ocean and produce terrestrial/marine mixed aerosol particles. 8 Aerosol particles formed by the Pacific Ocean travel eastward in the Northern Hemisphere and produce marine/terrestrial mixed aerosol particles while they cross the North American continent. As a result, practically all aerosol particles are mixed aerosol particles. 9–11 Contribution from the oceans consists mostly of sulfates; these particles 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 two compounds act as CCN. L829/frame/ch05 Page 82 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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 phase transitions in the atmosphere; they will establish the origin and contribute to the knowledge of the life 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 aerosol particle present in a cloud may nucleate ice by any of the three mechanisms. Usually, the difference will 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. L829/frame/ch05 Page 83 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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 water vapor supersaturations (S w ). A dry particle, for example, ammonium sulfate of 6 × 10 –2 µ m diameter will act as CCN at critical supersaturation of 0.2%; a supersaturation of 1% is required to activate a particle of 1.6 × 10 –2 µ m diameter. 15 Liquid water droplets formed in the atmosphere are therefore water solutions of portions of an aerosol particle that acted as CCN. The water-insoluble part of that 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 particles active as CCN. This process can be subdivided into three separate groups: a. Condensation of water vapor at subsaturations with respect to saturation over liquid water, S w , (hygroscopic particles); S ≤ S w . b. Condensation of water vapor at conditions of slight supersaturation, S > S w ; this takes place at and just above cloud bases. c. Condensation of water vapor at high supersaturations, S >> S w , that are present in the vicinity 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 as hydrosol (hydrophilic) particles; if particles are not wetted, hydrophobic particles will remain floating on the surface of a drop. A water solution droplet can freeze at higher temperatures than the freezing temperature of pure water, or can be frozen at different temperatures 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: submicron- and micron-sized aerosol particles are affected by this scavenging mechanism. FIGURE 5.2 Transport of non-sea-salt sulfate particles over the Pacific Ocean. L829/frame/ch05 Page 84 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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 parti- cles together. 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 most probably with electrostatic forces always present. 3. Formation of hydrosol particles in the liquid phase of clouds. In addition to the above processes that transfer existing aerosol particles into the liquid phase, there are some additional mechanisms taking place in a cloud that introduce newly formed hydrosol particles 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 dissolved salts (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 chemical compounds supplied by the CCN and other scavenged water-soluble salts The second category consists of submicron- and micron-sized hydrosol particles that are shed from the surfaces of larger particles when they are transferred into the condensed water drop. These particles are shed upon contact with water droplets larger than 40 µ m in diameter. Concentration and size of the shed particles vary with the size of the parent particle 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-soluble and water-insoluble particles. Aerosol particles that can act as CCN are generally water soluble; they consist of some water-insoluble particles found together in the water-soluble matrix. A droplet formed on a CCN particle consists of a water solution of water-soluble salts 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 by delayed-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 this altitude, 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 was found 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 and dissolved chemical compounds participate in the initiation of the L → S (ice) phase transition. To see if there is any relation between aerosol particles (aerosol particles transferred 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-insoluble hydrosol 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 water- affected fraction for aerosol particles in Group A is equal to zero; they are transferred into water without changing size. Another extreme is when the aerosol population consists of water-soluble fNL iii =−1 L829/frame/ch05 Page 85 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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). L829/frame/ch05 Page 86 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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 of hydrosol 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 the soluble part dissolves in water, the insoluble particle becomes a hydrosol particle. It can remain in the i size range, it can be transferred into the ( i – 1), or even into the ( i – n ) size range and be completely lost if that lower size range is outside the size range under investigation. Category D consists of aggregates of smaller particles that may produce even larger numbers of hydrosol particles. For large concentrations of aerosol particles in the D category, the water-affected fraction becomes 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 which liquid impinger was used; they were for the lower size ranges of i equal to 1.5–3 and 3–5 µ m diameter size ranges (experiments I, 0–0). However, there were aerosols that did not produce negative values of f i (I, x–x), indicating the presence of aerosol particles that did not consist of aggregates that could be broken either during the contact of particles with water or the mechanical force present in the impinger that is exerted on particles. That force does not exist in nature when aerosol particles are transferred into the liquid phase of a cloud. The f i values determined on filters clearly show the presence of two different classes of aerosol particles. The f i values for aerosol particles of marine origin were found to be around 99% (II, ⅷ ). 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% in Colorado. Part of the aerosol population acts as CCN; if the water-insoluble parts of mixed particles can act as IFN, then there should exist a direct relation between IFN and CCN. The ratio of CCN to IFN concentrations is about 10 6 in an unpolluted atmosphere. An example of that relation is shown FIGURE 5.4 Transfer of aerosol particles into water. L829/frame/ch05 Page 87 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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. Junge 9 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 µ m 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). L829/frame/ch05 Page 88 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 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 µ m 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 S w at constant tem- perature. On the other hand, the concentration of IFN of continental origin remained constant over a wide range of S w 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 S w . 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 (S w = 1.5%) at an altitude of 3000 m in Colorado. L829/frame/ch05 Page 89 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC 90 Aerosol Chemical Processes in the Environment 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 S w = 10%, ᭺, at S w = 0.3%. FIGURE 5.8 Maximum freezing temperatures are a function of ammonium sulfate concentration. (᭹, marine aerosol particles; ᭺, continental aerosol particles; ᮀ, no particles). L829/frame/ch05 Page 90 Monday, January 31, 2000 2:10 PM © 2000 by CRC Press LLC [...]... parent particles (T) to the shed particles (Tsp) © 2000 by CRC Press LLC Cloud droplets Aerosol 107 0 .5 5 5–10 106 109 0.1–0 .5 1 .5 × 107 106 1 05 10– 15 15 20 20– 25 25 30 30–40 40 50 50 –60 60–70 70–100 3 × 107 2 .5 × 107 1 .5 × 107 5 × 106 5 × 106 3 × 106 2 × 106 102 104 9000 150 –200 200– 250 50 0 50 0 50 10 2 × 104 100– 150 104 104 104 104 104 104 97 © 2000 by CRC Press LLC L829/frame/ch 05 Page 97 Monday, January... 2:10 PM 114 Aerosol Chemical Processes in the Environment TABLE 5. 2 Size of Natural Ice Forming Nuclei Size of IFN (Size Range) 0 .5 8 1–80 0.2–8 0.1–7 0.1–.13 0 .5 7 I* 0 .5 5 O* 0 .5 10 I 1 5 O 1 .5 10 I 1 5 O 0.2–7 I 0 .5 3 O 0 .5 10 I 1 5 O 0.1– 15 I 0.1–13 O 0.017–0.1 O d; µm (Maximum Frequency) 3 5 15 Temperature (°C) > –20°C > –20°C > –21°C Location Observer, Date Kumai, 1961 Souilage, 1 953 Kumai, 1961... Particles in the Phase Transition in the Atmosphere 113 FIGURE 5. 29 Fractions of hydrosol particles nucleating ice by freezing (solid lines) and of aerosol particles nucleating ice by condensation-followed-by-freezing (Sw = 0.6%) (dotted lines) for different temperatures and not on the diameter of the particle The lifetime of these very small aerosol particles in the atmosphere is short They coagulate... and they probably form their own network of ions, thus “squeezing” or “caging” water molecules and maybe stabilizing hydrogen-bound clusters All this must take place on the surface of hydrosol particles Particles of marine 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 nucleating... Hallett ,55 using different minerals, found the threshold temperature 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 a freezing drop or when exposed to a controlled supersaturation in a dynamic chamber depends on the size of particles, the nature of particles, and the. .. the size of aerosol particles and temperature of ice nucleation, it is hardly noticeable The lower the temperature, the smaller the size of aerosol particles acting as IFN; the particle size here is the size present in the maximum frequency in the size distribution Rosinski classified separated IFN into two groups: the first group consisted of inorganic particles and the second of organic particles Inorganic... Press LLC L829/frame/ch 05 Page 92 Monday, January 31, 2000 2:10 PM 92 Aerosol Chemical Processes in the Environment 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...  ρd  1/ 2 (5. 4) where α is the angle of incidence (°), D is the raindrop diameter (mm), m is mass of an aerosol particle (g), and C is equal to 3πµwd In the Stoke’s laws, µw is the viscosity of water (g cm–1 sec-1).41 The angle of incidence was determined experimentally by Rosinski for different geometrical objects.42–46 The minimum diameter of aerosol particles transferred into the interior of a... probably ammonium sulfate, are an integral part of the ice-forming nuclei population in marine atmospheres (see Figure 5. 30); also, aerosol particles of marine origin in the presence of sulfates nucleate ice at higher temperatures than the temperatures of ice nucleation by aerosol particles acting alone Mason and colleagues79,80 examined 35 minerals for their icenucleating ability; ten were active at... constants (Figure 5. 24) The constant α was found to be equal to about 3 for aerosol particles present over Colorado and Wyoming, and about 8 in the vicinity of St Louis, MO The relation is of the same form as to the one expressing concentration of CCN vs water vapor supersaturation The nature of aerosol particles is governed by their origin and their life history in the atmosphere The concentration . Size Interval) Particle diameter (µm) 0.01–0.1 0.1–0 .5 0 .5 5 5–10 10– 15 15 20 20– 25 25 30 30–40 40 50 50 –60 60–70 70–100 100– 150 150 –200 200– 250 Cloud droplets 10 6 1 .5 × 10 7 3 × 10 7 2 .5 ×. of marine 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 nucleating ability. LLC 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