Charged meteoric smoke as ice nuclei in the mesosphere: Part 1—A review of basic concepts

The role of meteoric smoke as condensation nuclei for mesospheric ice has recently been challenged by model simulations on the global transport of meteoric material. At the same time a considerable fraction of smoke particles is charged in the mesosphere. This has significant effects on nucleation processes as it can remove the Kelvin barrier. We suggest that in particular nucleation on negatively charged smoke is likely to be a dominant mechanism for mesospheric ice formation. This is in contrast to nucleation on positive ion clusters as the latter is largely hampered by efficient ion/electron recombination. Surprisingly, the large potential of nucleation on charged smoke has so far not been considered in mesospheric ice models. A challenging question concerns the fraction of mesospheric smoke that is actually charged. An improved understanding of mesospheric charging and nucleation will require laboratory experiments on nuclei in the transition regime between molecular and particulate sizes.     

News from the Lower Ionosphere: A Review of Recent Developments

Current knowledge concerning the lower ionosphere (D- and E-region) is reviewed with an emphasis on new aspects of empirical results. Starting with an overview of experimental techniques and corresponding data bases, both regarding charged as well as the most relevant neutral constituents of this altitude range, the ionospheric variability is discussed both concerning regular (e.g. diurnal and seasonal) as well as irregular variations (e.g. driven by the variability of nitric oxide). We then turn to ‘new players’ in the lower ionosphere, i.e. charged aerosol particles such as mesospheric ice particles in noctilucent clouds or polar mesospheric summer echoes and meteor smoke particles originating from ablated meteoric matter. These species have received considerable attention in recent years, in part because it is speculated that observations of their properties might be useful for the detection of climate change signals. The available experimental data base regarding these species is reviewed and we show that there is now compelling evidence for the ubiquitous presence of these very heavy charge carriers throughout the lower ionosphere. While many fundamental details regarding these charged species are not yet completely understood, this emphasizes that charged aerosol particles may not be neglected in a comprehensive treatment of the lower ionospheric charge balance and related phenomena. Finally, we close with suggestions for future research.     

The role of sodium bicarbonate in the nucleation of noctilucent clouds

It is proposed that a component of meteoric smoke, sodium bicarbonate (NaHCO₃), provides particularly effective condensation nuclei for noctilucent clouds. This assertion is based on three conditions being met. The first is that NaHCO₃ is present at sufficient concentration (10⁴ cm^–3) in the upper mesosphere between 80 and 90 km. It is demonstrated that there is strong evidence for this based on recent laboratory measurements coupled with atmospheric modelling. The second condition is that the thermodynamics of NaHCO₃(H₂O)_n cluster formation allow spontaneous nucleation to occur under mesospheric conditions at temperatures below 140 K. The Gibbs free energy changes for forming clusters with n = 1 and 2 were computed from quantum calculations using hybrid density functional/Hartree-Fock (B3LYP) theory and a large basis set with added polarization and diffuse functions. The results were then extrapolated to higher n using an established dependence of the free energy on cluster size and the free energy for the sublimation of H₂O to bulk ice. A 1-dimensional model of sodium chemistry was then employed to show that spontaneous nucleation to form ice particles (n > 100) should occur between 84 and 89 km in the high-latitude summer mesosphere. The third condition is that other metallic components of meteoric smoke are less effective condensation nuclei, so that the total number of potential nuclei is small relative to the amount of available H₂O. Quantum calculations indicate that this is probably the case for major constituents such as Fe(OH)₂, FeO₃ and MgCO₃.     

Seasonal and diurnal variability of the meteor flux at high latitudes observed using PFISR

We report in this and a companion paper [Fentzke, J.T., Janches, D., Sparks, J.J., 2008. Latitudinal and seasonal variability of the micrometeor input function: A study using model predictions and observations from Arecibo and PFISR. Journal of Atmospheric and Solar-Terrestrial Physics, this issue, doi:10.1016/j.jastp.2008.07.015] a complete seasonal study of the micrometeor input function (MIF) at high latitudes using meteor head-echo radar observations performed with the Poker Flat Incoherent Scatter Radar (PFISR). This flux is responsible for a number of atmospheric phenomena; for example, it could be the source of meteoric smoke that is thought to act as condensation nuclei in the formation of ice particles in the polar mesosphere. The observations presented here were performed for full 24-h periods near the summer and winter solstices and spring and autumn equinoxes, times at which the seasonal variability of the MIF is predicted to be large at high latitudes [Janches, D., Heinselman, C.J., Chau, J.L., Chandran, A., Woodman, R., 2006. Modeling of the micrometeor input function in the upper atmosphere observed by High Power and Large Aperture Radars, JGR, 11, A07317, doi:10.1029/2006JA011628]. Precise altitude and radar instantaneous line-of-sight (radial) Doppler velocity information are obtained for each of the hundreds of events detected every day. We show that meteor rates, altitude, and radial velocity distributions have a large seasonal dependence. This seasonal variability can be explained by a change in the relative location of the meteoroid sources with respect to the observer. Our results show that the meteor flux into the upper atmosphere is strongly anisotropic and its characteristics must be accounted for when including this flux into models attempting to explain related aeronomical phenomena. In addition, the measured acceleration and received signal strength distribution do not seem to depend on season; which may suggest that these observed quantities do not have a strong dependence on entry angle.     

Homogeneous nucleation of amorphous solid water particles in the upper mesosphere

Condensed water particles are known to exist in the high latitude upper mesosphere during the summer months. However, the mechanism or mechanisms through which they nucleate remains uncertain. It is postulated here that particles of amorphous solid water (ASW, condensed water with a non-crystalline structure) may nucleate homogeneously in the summer mesosphere. Using classical nucleation theory and a one-dimensional model, it is shown that more than 10⁵ cm^?3 amorphous solid water particles can nucleate homogeneously under mesopause conditions. Furthermore, it is shown that homogeneous nucleation competes with heterogeneous nucleation on meteoric smoke particles when the cooling rate is >0.5 K/h. The homogeneous nucleation of amorphous solid water could provide an explanation for the high density of ice particles (many thousands per cm³) thought to be required for electron depletions in the upper mesosphere. A parameterisation for homogeneous nucleation is presented which can be used in other mesospheric cloud models.     

SOFIE PMC observations during the northern summer of 2007

This work examines the first season of polar mesospheric cloud (PMC) observations from the Solar Occultation for Ice Experiment (SOFIE). SOFIE observations of temperature, water vapor, and PMC frequency, mass density, particle shape, and size distribution are used to characterize the seasonal evolution and altitude dependence of mesospheric ice and the surrounding environment. SOFIE indicates that ice is nearly always present during summer, and that the ice layer is continuous from about 81 km altitude to the mesopause and above. Ice particles are observed to be more aspherical above and below the extinction peak altitude, suggesting a relationship between particle shape and mass density. The smallest particles are observed near the top of the ice layer while the largest particles exist at low concentrations near cloud base. A strong correlation was found between water vapor and particle size with small particles existing when H₂O is low. This relationship holds when examining variability in altitude, and variability over time at one altitude.     

积云动力和电过程二维模式研究 Ⅰ．理论和模式

提出了一个模拟积云动力和电力发展的二维时变轴对称模式。考虑了１０种主要微物理过程，它们包括凝结（凝华）、蒸发、自动转换、粒子间的碰撞以及冰晶核化，次生冰晶等。在起电过程中除了考虑常规的扩散和电导起电外，重点引入了感应和非感应起电，以及次生冰晶起电的作用。作者认为后三个过程是形成积云电结构的主要物理因子。     

Measurements of meteor smoke particles during the ECOMA-2006 campaign: 2. Results

The first sounding rocket of the European ECOMA-project (ECOMA, Existence and Charge state Of Meteoric smoke particles in the middle Atmosphere) was launched on 8 September 2006. Measurements with a new particle detector described in the companion paper by Rapp and Strelnikova [2008. Measurements of meteor smoke particles during the ECOMA-2006 campaign: 1. Particle detection by active photoionization. Journal of Atmospheric and Solar-Terrestrial Physics, this issue, doi:10.1016/j.jastp.2008.06.002] clearly showed meteor smoke particle (MSP) signatures in both data channels. The data channels measure particles directly impacting on the detector electrode and photoelectrons from the particles actively created using ionization by the UV-photons of a xenon-flashlamp. Measured photoelectron currents resemble model expectations of the shape of the MSP layer almost perfectly, whereas derived number densities in the altitude range 60–90 km are larger than model results by about a factor of 5. Given the large uncertainties inherent to both model and the analysis of our measurements (e.g., the composition of the particles is not known and must be assumed) we consider this a satisfactory agreement and proof that MSPs do extend throughout the entire mesosphere as predicted by models. The measurements of direct particle impacts revealed a confined layer of negative charge between 80 and 90 km. This limited altitude range, however, is quantitatively shown to be the consequence of the aerodynamics of the rocket flight and does not have any geophysical origin. Measured charge signatures are consistent with expectations of particle charging given our own measurements of the background ionization. Unfortunately, however, a contamination of these measurements from triboelectric charging cannot be excluded at this stage.     

The generation of condensation nuclei by ionising radiation

Summary The production of condensation nuclei by the irradiation of filtered nuclei-free air by X-rays, alpha particles and beta particles is described. It is found that nuclei are created in large numbers by such irradiation and that there appears to be a threshold radiation dose at which the nucleus concentration is increased by a factor of more than a hundred.The effect of varying sulphur dioxide content of the irradiated air has been investigated and for concentrations of between one and ten milligrams per cubic metre of air there is some proportionality between the sulphur dioxide content of the air and the nucleus concentration produced by a given radiation dose.It is shown, however, that sulphur dioxide cannot be exclusively responsible for the generation of condensation nuclei by irradiation of filtered air and other possible agents and mechanisms are suggested.     

Possible impact of interplanetary and interstellar dust fluxes on the Earth’s climate

This article considers the process of entry of cosmic substance into the Earth’s atmosphere and the further evolution of the formed extraterrestrial aerosol. It is shown that meteorite-derived aerosol generated in the atmosphere may affect the Earth’s climate in two ways: (a) particles of meteoric haze may serve as condensation nuclei in the troposphere and stratosphere; (b) charged meteor particles residing in the mesosphere may markedly change (by a few percent) the total atmospheric resistance and, thereby, affect the global current circuit. Changes in the global electric circuit, in turn, may influence cloud formation processes. The obtained results argue for the fact that the meteoric dust in the Earth’s atmosphere is potentially one of the important climate-forming agents. It is shown that the amount of interstellar dust in the Earth’s atmosphere is too small to have a considerable affect on atmospheric processes.     

Noctilucent clouds seen at 60°N: origin and development

A noctilucent cloud is seen at a particular time from a specified place. The journey of the cloud particles from nucleation to observation can be calculated by using a simple model of growth and taking account of the fall speed of the cloud particles. Cloud particles can be backtracked by bringing together growth and fall speed equations and a model of mesospheric winds to find where the particles of a cloud seen at a particular time and place have originated. The wind model that is used here suggests that there is a distinct outer edge to the summertime polar circulation pattern in which water vapour is being carried up from the lower mesosphere to the mesopause. The change in latitude of this outer edge during the summer season may well account for the observed seasonal change in occurrence of mesospheric clouds. Polar mesospheric clouds cause a drying of the upper mesosphere. It is suggested here that diffusion of water vapour dumped at the level of polar mesospheric clouds will take an appreciable time to carry water vapour back up to the mesopause. In consequence, there will be a significant separation between the observed location of a noctilucent cloud and its precursor polar mesospheric cloud.     

Mesospheric cloud observations at unusually low latitudes

Noctilucent clouds (NLC) are a beautiful, high-latitude, summertime phenomenon that was first reported over 100 years ago. They are seen during the hours of twilight by the scattering of sunlight from sub-micron-sized ice particles that form in the vicinity of the cold mesopause region. NLC are quite distinctive, often appearing silvery-blue in color. In recent years there has been a marked increase in their frequency of occurrence, possibly due to an increase in mesospheric water vapor and/or to a cooling of the mesopause region, prompting speculation that they are “harbingers” of potentially serious changes in the mesospheric climate. In concert with this trend there are also a growing number of ground-based NLC sightings at significantly lower latitudes than expected. Here we report two unusual NLC displays photographed from Logan, UT, USA (∼42°N) during June 1999, well over 10° lower in latitude than expected and implying a major, yet temporary, departure from normal mid-latitude summertime conditions. These data provide new evidence for the occasional expansion of NLC to unusually low latitudes possibly due to exceptional dynamical forcing. Alternatively, they may be an early indicator of significant long-term changes taking place in the upper mesospheric summertime environment.     

Variations in atmospheric transparency under the action of galactic cosmic rays as a possible cause of their effect on the formation of cloudiness

The possible mechanism by which cosmic rays affect the formation of neutral water droplets and ice crystals in the Earth’s atmosphere has been considered. This mechanism is based on changes in atmospheric transparency and vertical temperature distribution. It has been indicated that a change in the optical thickness for visible and IR radiation by several percents, which can take place when cosmic-ray particles penetrate into the atmosphere, results in a change in the temperature vertical distribution, affecting the growth of water droplets, concentration of active condensation nuclei, and the formation of ice particles. This mechanism makes it possible to explain the correlation between the intensity of galactic cosmic rays at low altitudes and the absence of this correlation at middle altitudes.     

Electron-microscope study of snow crystal nuclei: II

Summary Experimental work was conducted in an igloo, half way up Mt. Taisetsu (1050 m), Mt. Tokachi (1030 m) and Mt. Okuteine (820 m) in Hokkaido. One solid nucleus (center nucleus) was observed almost always in the central portion of a snow crystal. From their electron micro-diffraction patterns, the materials of the nuclei were classified into three kinds; a single crystal, polycrystalline or amorphous substances. The patterns of the single crystal gave a hexagonal cross grating, the atomic arrangement in their basal plane had some similarity to that of an ice crystal. Some polycrystalline center nuclei were small sea salt, particles. Their shape was similar to that of the nuclei of sea sprays, and their patterns coincide with that of the sea salt. Some amorphous center nuclei were observed which looked like a liquid droplet under the operating condition of the electron-microscope. It seemed that they are chemical components in the sea salt such as KCl or MgCl particles.In the other parts of snow crystal, many smaller nuclei (condensation nuclei) were observed. These nuclei are also aerosols in the atmosphere; their size was 0.10.01 in diameter. They gave the Debye-Scherrer ring; their patterns were different in each specimen. Their materials consist of various substances. From these data, the origin of nuclei and the mode of action of center nucleus were discussed.     

The cloud imaging and particle size experiment on the aeronomy of ice in the mesosphere mission: Cloud morphology for the northern 2007 season

The Aeronomy of Ice in the Mesosphere (AIM) mission was launched from Vandenberg Air Force Base in California at 4:26:03 EDT on April 25, 2007, becoming the first satellite mission dedicated to the study of noctilucent clouds (NLCs), also known as polar mesospheric clouds (PMC) when viewed from space. We present the first results from one of the three instruments on board the satellite, the Cloud Imaging and Particle Size (CIPS) instrument. CIPS has produced detailed morphology of the Northern 2007 PMC and Southern 2007/2008 seasons with 5 km horizontal spatial resolution. CIPS, with its very large angular field of view, images cloud structures at multiple scattering angles within a narrow spectral bandpass centered at 265 nm. Spatial coverage is 100% above about 70° latitude, where camera views overlap from orbit to orbit, and terminates at about 82°. Spatial coverage decreases to about 50% at the lowest latitudes where data are collected (35°). Cloud structures have for the first time been mapped out over nearly the entire summertime polar region. These structures include ‘ice rings’, spatially small but bright clouds, and large regions (‘ice-free regions’) in the heart of the cloud season essentially devoid of ice particles. The ice rings bear a close resemblance to tropospheric convective outflow events, suggesting a point source of mesospheric convection. These rings (often circular arcs) are most likely Type IV NLC (‘whirls’ in the standard World Meteorological Organization (WMO) nomenclature).     

Role of condensation nuclei as carriers of radioactive particles

Summary The effect on the deposition of thoron and radon decay products of attachment to condensation nuclei is described. Methods are suggested of using these decay products as markers for condensation nuclei and comparison is made between radioactive and non radioactive determinations of the diffusion coefficients of marked nuclei. The spontaneous production of condensation nuclei by irradiation of clean air by alpha and beta particles and by X-rays is described.     

Some recent investigations on the nature and properties of natural and artificial freezing nuclei

Summary Working on the hypothesis that atmospheric ice-forming nuclei are largely of terrestrial origin, the nucleating ability of various types of soil particles and mineral dusts has been investigated. Of the thirty substances tested, twenty-one, mainly silicate minerals of the clay and mica groups, were found to produce ice crystals in supercooled clouds and also on supercooled soap films at temperatures of – 18° C, or above, and of these, ten were active above – 12° C. The most abundant of these is kaolinite with a threshold temperature of – 9° C. Ten natural substances, again mainly silicates, were found to become more efficient ice nuclei having once been involved in ice-crystal formation, i.e. they could be pre-activated or «trained». Thus, ice crystals grown on kaolinite nuclei, which are initially active at –9° C, when evaporated and warmed to near 0° C in a dry atmosphere, leave behind nuclei which are thereafter effective at – 4° C. Particles of montmorillonite, another important constituent of some clays, and which are initially inactive even at –25° C, may be pre-activated to serve as ice nuclei at temperatures as high as –10° C. It is suggested that although such particles can initially form ice crystals only at cirrus levels, when the ice crystals evaporate they will leave behind some «trained» nuclei which may later seed lower clouds at temperatures only a few degrees below 0° C. On this hypothesis, the fact that efficient nuclei are occasionally more abundant at higher levels would not necessarily imply that they originate from outer space. Indeed, in view of our tests on products of stony meteorites, produced both by grinding and vaporization, which show them to be ineffective at temperatures above – 17° C, it seems likely that atmospheric ice nuclei are produced mainly at the earth's surface, the clay minerals, particularly kaolinite, being a major source.Although a good deal of work has been carried out in different laboratories on the ice-nucleating ability of a wide variety of inorganic compounds, there has been little agreement in the results. Careful tests carried out in our laboratory have revealed a number of reasons for this. Spurious results may be obtained because of the presence, in the air or the chemicals, of small traces of silver or free iodine, leading to the formation of silver iodide: if all such trace impurities are removed, many of the substances that have been claimed to provide efficient ice nuclei are found to be quite ineffective. It is dangerous to infer that all twinkling particles in a water cloud are ice crystals since particles of some seeding agents glitter even at positive temperatures. The threshold temperature of a nucleant will depend upon the criterion adopted for the onset of nucleation, i.e. upon the fraction of the total number of particles of seeding agent which are activated; this, in turn, will depend upon the fraction of particles which happen to possess suitable crystallographic faces for nucleation. Much may also depend upon the manner in which the test is performed. Since some nucleating materials produce ice crystals only after a delay of 30 seconds or more, they may appear to be ineffective if tested in the transient cloud of an expansion chamber but highly effective if allowed to remain in an ice-supersaturated atmosphere for a minute or more. Again, we have found that the efficiency of some nuclei is governed by the supersaturation as well as the temperature of the environment, and the supersaturation regimes in expansion, diffusion, and mixing-cloud chamber may be widely different. Highly soluble particles, although able to act as «sublimation» nuclei in atmospheres super-saturated relative to ice but sub-saturated relative to water, on entering a water cloud go quickly into solution and lose their nucleating ability.Inorganic substances which definitely nucleate a supercooled water cloud in a mixing-cloud chamber at temperatures of –15° C and above are: AgI (–4° C), PbI₂ (–6° C), CuS (–6° C), Ag₂S (–8° C), Ag₂O (–9° C), HgI₂ (–8° C), V₂O₅ (–14° C), Cu₂I₂ (–15° C), the figures in brackets indicating the threshold temperatures at which about one particle in 10⁴ becomes active as an ice nucleus. Cadmium iodide (–12° C), ammonium fluoride (–9° C) and iodine (–14° C) are examples of salts which will act as sublimation nuclei in an ice-supersaturated atmosphere and will nucleate a supercooled soap film, but which are ineffective in a water cloud because of their solubility.Although the most efficient nucleating agents tend to be hexagonal in structure, there are some striking exceptions e.g. Ag₂S, Ag₂O, HgI₂, but in most cases, we have been able to find a low-index crystal surface on which the ice lattice could grow with a misfit of only a few per cent.In an attempt to investigate the nucleation mechanism in more detail, we have studied the growth of ice on single crystals of various nucleating agents. Perfect orientation of ice crystals has so far been observed on the basal faces of silver iodide, lead iodide, cupric sulphide, cadmium iodide, and freshly-cleaved mica, on the (001) plane of iodine, and on the (010) plane of mercuric iodide.     

An ionometric counter for condensation nuclei

Summary Condensation nuclei detectable in the range of supersaturation used in the expansion method do not play the role expected earlier, for the production of water droplets in natural clouds.However, these nuclei exist in the atmosphere, and they can be of importance in other processes in the air; for example, they can be partners for annihilation of small air ions there. Therefore, a direct investigation of the interaction between the condensation nuclei and the small air ions may be of interest. For this purpose an, ionometric counter for condensation nuclei is designed, in which the air under examination is mixed in a mixing chamber with small air ions produced by a tritium ion-generator, and the presence of condensation nuclei is measured by the decrease of the concentration of air ions recorded with an aspiration ion counter as compared to the concentration recorded when filtered air was originally sucked through the mixing chamber.Comparatively rapid variations of the concentration of nuclei can be continuously recorded. The electrical properties of the aerosol particles destroying air ions can be investigated by inserting an electrical filter. By the ionometric counter condensation nuclei can be investigated without adding water particles to them.