The nucleation and growth of ice particles in the upper mesosphere

Adv. Space I&s. Vol. 20, No. 6, pp. 128%1291,1997

Publishedby BlsevierScience Ltd on behalf of COSPAR Printed in Great Britain 0273-l 177/97 $17.00 + 0.00 PII: 50273-l 177(97)00788-6

THE NUCLEATION AND GROWTH OF ICE PARTICLES IN THE UPPER MESOSPHERE George C. Reid Aero~m~ ~ora~o~, NUAA and ~~~er~‘ve ~nstit~te~or Research ik E~viro~~en~a~Sciences, University ofColorado, Boulder, CO 80303, U.S.A.

ABSTRACT The highest clouds and the coldest temperatures in the Earth’s atmosphere exist in the region of the summer mesopause at high latitudes. The presence of ice particles in the region leads to several unique phenomena, including sharply bounded layers in which electrons, and occasionally positive ions, are severely depleted ~biteou~“), and intense radar echoes. This paper reviews some of the recent advances in our ~de~t~~g of the ways in which these ice particles form and grow. Nucleation can occur on heavy positive ions, but the smoke and dust particles resulting from meteor ablation are more likely condensation nuclei. Ice particles probably form mainly near the mesopause and grow as they sediment downward, but the presence of strong horizontal and vertical winds in the region complicate this simple picture. While biteouts are now generally recognized as being due to scavenging of electrons by particles, the reasons for their existence in narrow sharply bounded layers remain unclear. While the subvisible ice particles are likely to be negatively charged under normal conditions, it is pointed out that under conditions of low ionization in the region, negatively and positively charged particles probably exist in roughly equal numbers, leading to the possibility of enhanced growth by coagulation of oppositely charged particles. Published by Elsevier Science Ltd on behalf of COSPAR INTRODUCTION The coldest tem~ratures that exist anywhere in the Earth’s environment are found in the region of the summer polar mesopause at heights of 85-90 km, where strong upwelling and adiabatic cooling are driven by the interaction of large-amplitude gravity waves with the mean zonal flow. Associated with these low temperatures are a variety of phenomena that are unique to the region, including noctilucent clouds (NLC) and polar mesospheric clouds (PMC), localized layers with strong depletions of electron density, known as electron “biteouts”, and the radio-wave scatterers that give rise to the intense radar echoes known as Polar Mesosphe~~ Summer Echoes (PMSE). It is now generally recognized that all of these phenomena are the result of the existence of supersa~ation conditions and the growth of small ice particles, producing sheets of visible or subvisible cirrus that make up the highest ice clouds of the Earth’s atmosphere, This paper will review some of the recent advances in our understanding of the nucleation and growth of these ice particles. The large-scale properties and climatology of the clouds themselves have been reviewed by Gadsden and Schrijder (1989) and Thomas (1991), and the observational and theoretical aspects of PMSE by Cho and Kelley (1993). While significant progress has been made since these reviews, these topics will not be discussed here except in passing. THE ENVIRONMENT OF THE SUMmR

POLAR MESOPAUSE REGION

Many direct m-situ measurements of temperatures in the region have been carried out by rocket experiments, using the falling-sphere technique, and a number of individual examples have been published by von Zahn and Meyer (1989), Inhester et al, (1994), and Liibken et al. (1996). More recently ground-based lidars have also begun to provide temperature profiles (e.g., Hansen and von Zahn,1994), and are ultimately likely to supplant the rocket measurements as a source of climatological information, The temperature minima that define the mesopause generally range between 110 and 140 K and are usually found between 85 and 90 km altitude at the latitudes at which most of the measurements have been made 1285

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70” N in northern Scandinavia). Model calculations of the thermal and dynamical structure in the region are in general agreement with the observations (e.g., Garcia, 1989), and predict temperatures that decrease with increasing latitude toward the pole, caused by vertical upwelling with velocities of a few centimeters per second. ( -

Less is known about the water-vapor concentration, which is the other key ingredient in ice-cloud formation. The same upwelling motion that causes the low temperatures brings up water vapor from below, and this advective supply of water is balanced by photoche~c~ destruction, mainly by solar Lyman-a radiation. No in-situ me~u~ments of water-vapor co~cen~ations have been made, but models predict volume mixing ratios of 1 or 2 ppmv above 80 km. Even in this extremely dry environment, however, frost points are often above the local temperature, and supersaturation conditions are common. For 1 ppmv at 85 km, the frost point is about 140 K, and the extreme sensitivity to temperature under these cold conditions leads to a supersaturation ratio of about 20,000 at 120 K. There is thus no mystery about the existence of ice particles in the region, but the process of nucleation of the particles remains uncertain and to some extent controversial.

NUCLEATION PROCESSES At the extreme supersaturation conditions that exist occasionally, homogeneous nucleation could occur, but under more normal conditions nucleation must require the existence of some kind of condensation nuclei. The presence of ionization in the region has led to ion-induced nucleation being a strong candidate (e.g., Witt, 1969), and Sugiyama (1994) has recently investigated the physical chemistry of proton hydrates in the mesosphere, and the formation of neutral condensation embryos from large proton hydrates, following the mechanism originally proposed by Arnold (1980). His results show that ion nucleation is a possibility, but its occurrence appears to be highly sensitive to temperature and to water-vapor concentrations. A more IikeIy possibility is that the condensation nuclei are the smoke and dust particles that are probably produced in large numbers at mesopause altitudes as a result of meteor ablation (Hunten et aZ., 1980, Turco et al., 1982). Hunten et al. (1980) have modeled the size and height distribution of these particles, assuming a fixed total mass input to the atmosphere and various initial particle radii. Figure 1, taken from data in their paper, shows the calculated particle concentration at 90 km as a function of the assumed initial radius. For the smaller radii, concentrations of several thousand per cm3, comparable to or greater than electron concentrations, may exist. Even if they were electrically neutral, these tiny particles would act as efficient condensation embryos, but

lo4 103 lo2 10’ loo 10-r 1 Radius (nm) Fig. 1. Concentration of meteoric smoke and dust particles at 90 km for the assumed initial particle radii indicated on the bottom axis. From data in Hunten et al, (1980).

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they are likely to be electrically charged, making their role as nucleation centers for polar water molecules even more effective. Figure 2 shows the result of a calculation of charge balance conditions based on the analysis of .Reid (1990) at a height of 85 km, for a temperature of 130 K, and particle radius and concen~~on of 1 nm and 3000 cm-3 respectively. Conc~~~tions are normalized to the concen~tion in the

0.01

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0.1

1

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Ionization Rate (cm-3S-‘) Fig. 2. Charge distribution for 3000 particles cmm3at 85 km. Particle radius = 1 nm. absence of charging, and are shown as functions of the ionization rate. For a typical high-latitude daytime upper mesosphe~c io~ation rate of 10 cm-s s-1, over 90% of the particles carry a single negative charge, resulting in an electron depletion of about 40%. At the very low ionization rates of 0.1 cm-3 s-1, however, such as might exist during the night in the absence of energetic-particle precipitation, most of the particles remain electrically neutral, and nucleation could become less efficient. As will be mentioned later, the existence of electron biteouts and the corresponding absence of ion biteouts (except under special conditions, also to be discussed later) implies that the positive ions and the ice particles are more or less independent of each other, arguing against ions as nucleation centers for the particles. PARTICLE GROWTH Once the initial ice nucleation has taken place, the subsequent growth phase of the particles involves a number of factors that are poorly understood. Under the kinetic regime of the mesopause region the rate of increase in radius of a spherical particle by vapor deposition is independent of the radius, and is given by

where cx is the sticking efficiency, p is the density of the ice, p and pi are the water-vapor pressure and the saturation vapor pressure respectively, and m, is the mass of a water molecule (Hesstvedt, 1961; Reid, 1975). Assuming a = 1, p = 1000 kg m-3 (the density of normal water ice), and high supersaturation conditions (i.e., p >> pi) for 1 ppmv at 85 km, this growth rate becomes about 2.3 nm hr-1. Growth to the lo-nm radius thought to be mainly responsible for electron biteouts thus takes about 4 hours, while growth to the larger sizes comprising visible NLC takes correspondingly longer times. Liibken et al. (I 996) have shown the results of s~ult~eous measu~ments of temperate by rockets and NILC particles by ground-based lidar, and have shown that the particles almost always seem to be located several kilometers below the temperature minimum, consistent with a scenario in which nucleation occurs near the temperature minimum, and the particles then grow as they sink downward under gravity. The visible

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particles consistently have their peak concentration where the temperature is close to 150 K, and disappear rapidly below that level. The saturation mixing ratio at T = 150 K and a height of 82 krn is about 10 ppmv, which is much larger than ambient mixing ratios, so the particles must already be sublimating, as the authors point out. The simple one-dimensional picture of particles forming near the mesopause and slowly growing as they sediment down, eventually sublimating in warmer temperatures is likely to be misleading, however. Zonal and meridional winds are strong at these levels, as has been shown by many radar measurements. Hansen and von Zahn (1994) have shown that there is generally little correspondence between temperature conditions at the mesopause and the occurrence of NLCs at lower levels over the same location, and they have pointed out that the strong and variable horizontal winds must carry the particles long distances as they grow. EISCAT measurements in August 1989 quoted by Hansen and Hoppe (1996) show predominantly westward zonal winds with speeds as high as 60 m s-1 and predominantly southward meridional winds with speeds as high as 35 m s-1. In a growth time of 8 hours, these winds could carry particles as far as 1700 km westward and as far as 1000 km southward from their point of origin. The southward transport suggests that the particles seen in visible NLCs at lower latitudes where twilight conditions are suitable may have originated in the higher and colder environment of the polar region and been carried equatorward as they grew. Vertical winds are also strong and variable in time and space. Hansen and von Zahn (1994) and Hansen and Hoppe (1996) have shown vertical profiles of the vertical winds measured by EISCAT with speeds as high as 1 m s-l, and with only a slight indication of the wave-like structure expected from a monochromatic gravity wave. One can speculate that this semi-chaotic vertical structure may arise from nonlinear interactions between large-amplitude gravity waves at these altitudes, in which case it must give rise to a corresponding pronounced small-scale structure in the temperature. Once again the simple picture of a gentle upward motion of a few cm s-1 cooling the region and opposing the downward sedimentation of ice particles is oversimplified. While this is probably the average situation, the particles must in reality undergo a fairly random succession of up and down motions as they travel horizontally from their point of origin, and their net growth may well represent the sum total of short periods of growth and decay as they encounter colder and warmer regions. If this is the real situation, it suggests that efforts to predict the particle behavior on the basis of one-dimensional arguments may be doomed to failure. A further uncertainty lies in the common assumption that the particles are spherical in shape. The spherical shape is certainly the easiest to deal with mathematically, but the physical arguments in favor of it are not particularly compelling. Since spherical particles have the maximum mass for a given linear dimension, growth rates for non-spherical particles are likely to be greater and sedimentation velocities smaller than for spherical particles, relaxing some of the conditions on such factors as the supply of water vapor needed to produce a given particle concentration. ELECTRON

BITEOUTS

Electron biteouts are narrow (- 1 km) regions in which the electron density is severely depleted relative to regions above and below. They were first observed by Pedersen et al, (1970), and subsequent rocket measurements (e.g., Ulwick et aZ., 1988) have shown that they are a common feature of the polar upper mesosphere in summer. Theoretical models of charge balance in the presence of small particles (Reid, 1990; Klostermeyer, 1994) have shown that the depletions can be explained by scavenging of electrons by ice particles with spherical radii of the order of 10 nm, i.e., by subvisible particles not yet grown to the size of visible NLC particles. Since the electrons have much higher thermal velocities than the positive ions, and their impact rate with ice particles is much larger, the particles are predominantly negatively charged. Since these small particles can only possess a single charge, their concentration must be at least comparable with that of the electrons in the absence of a biteout, i.e., a few thousand cm-3 under typical conditions. Figure 3 shows the results of a charge balance calculation similar to that shown in Figure 2, but for particles with a radius of 10 nm instead of 1 nm. For an ionization rate of 10 cm-3 s-1, there is now a large electron depletion, with about 90% of the electrons removed, while over 90% of the particles carry a negative charge. Positively charged particles are almost non-existent at this ionization rate, since there are enough electrons to neutralize them rapidly. This is not the case at very low ionization rates, however. For rates less than about 0.1 cm-3 s-1, which might apply at night with only weak energetic-particle precipitation, concentrations of positively and negatively charged particles become almost equal, since there are too few electrons to neutral-

ice particles in the Upper Atmosphere

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Ionization Rate (cmm3S-‘) Fig. 3. Charge distribution for 3000 particles cme3at 85 km. Particle radius = 10 nm. ize the positively charged particles. This could have an ~terest~g consequence for the growth of particles, since the inhibition of particle coagulation that would apply if all the particles were charged negatively (e.g., Jensen and Thomas, 1991) would no longer be active. The opposite charges on the particles would in fact encourage coagulation, allowing the particles to grow much more rapidly than they would if vapor deposition were the only mecha~sm. It is thus possible that particle growth takes place preferentially under low ionization-rate conditions, i.e., at night and in the absence of energetic-particle precipitation. Low ioniza~o~ con~tions also favor the existence of subst~ti~ ion bi~outs as a result of ion capture by the particles, and this situation has been seen by Balsiger et al. (1996) during a rocket flight of a mass spectrometer and a particle impact detector over Kiruna in the summer of 1993. The positive ions were found to be depleted by an order of magnitude in a narrow layer centered at about 83 km, and roughly coincident with an NLC layer. Ionization rates at the time were estimated at 0.01 to 0.1 cm3 s-l, and the authors point out that the existence of an ion biteout is consistent with these low ionization conditions and with the existence of scavenging particles with dimensions of order 10 nm. The charged aerosol particles responsible for biteouts now appear to have been detected directly by dust probes carried on rockets launched from Andoya (Havnes et al, 1996). In one flight, launched in the presence of strong PMSE, but no NLC, sharply bounded layers were found in which negative charge densities reached several thousand cm-3, with particle dimensions less than 100 nm, consistent with the theoretical models of charge balance in biteout regions (Reid, 1990; Klostermeyer, 1994). The association with PMSE suggests that the steep gradients in particle and electron concen~ation may be responsible for at Ieast some of the strong radar echoes (Rottger and La Hoz, 1990). The reason for the existence of biteouts in narrow sharply bounded layers remains uncertain. Klostermeyer (1996) has suggested that gravity waves are responsibIe, since their associated temperature structure effectively determines the minimum particle size that can exist through the variation in vapor pressure with radius over a spherical particle (the Kelvin effect). There will thus be a variation in particle size with height, smaller particles being concentrated around the temperature minima. An alternative possibility is that the layers form as a result of the ~~~haotic vertical motions resulting from nonlinear gravity-wave interactions, described above. These motions are likely to lead to transient regions of vertical convergence and horizontal divergence, leading to layer-like spreading of particles, much as cloud layers form in the lower atmosphere when upward-moving air parcels encounter a temperature inversion, or as deep cumulonimbus clouds form outward-spreading anvils as their updrafts meet the tropopause. The mechanisms of layer formation are important for our unders~ding of several aspects of the behavior of the a~osphere, and there is a need for

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experiments designed to determine the relative importance of the various possible mechanisms. SUM~RY

AND CONCLUSIONS

In summa~, our underst~ding of the unique properties of the summer polar mesopause region has advanced in major ways in recent years, largely as a result of several rocket campaigns and the advent of gro~d-bred lidar and radar facilities. In particular, an overall underdog of the processes by which ice particles nucleate and grow in this region seems to have been reached, but many impo~~t problems remain. The long”stan~ng question of ions versus meteoric dust as nucleation centers has not been completely resolved, but most of the evidence points toward dust nucleation. The simple picture in which the initial nucleation occurs near the mesopause, and the ice particles grow as they settle down under the influence of gravity is probably valid in general terms, but the wind meas~men~ have shown that the particles are likely to have a rough and bumpy ride on their way down and to travel large distances ho~ont~y from their point of origin. These considerations cast doubt on the usefulness of one-~me~sional modeling of the growth of clouds, since the ideal conditions such models have to assume may not have much bearing on the real situation. The continuing uncertainty about the shape of ice particles formed under the extremely dry and cold conditions of the region raises another ~fflc~~ that has not been seriously addressed. On the topic of biteouts, a general ~nde~~~g seems to have been reached. Scavenging of electrons by ice particles with ~me~ions of order 10 nm, and of positive ions under the special con~tio~ of low ionization rates, is probably the basic mech~ism. The positive-ion biteouts predicted from ch~g~b~~~e theory when ionization is weak have now been seen, as have layers of negatively charged particles with condensations comparable with those of electrons in the ~~s~r~d regions. An ad~~on~ prediction of roughly equal concentra~ons of oppositely charged particles under low io~zation conditions suggests that growth of particles by coa~lation could be an important factor at times. Under more normal conditions, electrostatic repulsion is likely to inhibit coagulation since most of the particles will carry a negative charge. The formation of narrow sharply bounded layers of particles remains an interesting and important topic, with potential applications in a wider range of atmospheric behavior. Future rocket experiments could provide important info~ation by addressing some of the questions raised by the existence of these layers.

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