Seasonal variability of aerosol optical properties in Darwin,Australia

This paper investigates the annual cycle in aerosol optical thickness (AOT) and Angstrom exponent in Darwin, Australia, a coastal site in the Tropical Warm Pool where the major aerosol sources are biomass burning and sea salt. We have used radiometer measurements from the Tropical Western Pacific Atmospheric Radiation Measurement facility for the period March 2002–June 2003. Strong seasonal cycles in AOT and Angstrom exponent were observed, peaking during the burning season (May–November). Investigation of the spectral dependence of optical thickness showed that the Angstrom formula can be satisfactorily fitted to the AOT data during the burning season but not on summer and autumn afternoons due to the presence of sea salt aerosols.     

The nuclei of atmospheric condensation

Summary Our present knowledge concerning the abundance, size, physical properties, origin and nature of condensation nuclei is reviewed and critically assessed.     

An investigation into the properties of atmospheric freezing nuclei and sea-salt nuclei under maritime conditions at the West Coast of Ireland

Summary Parallel measurements of the concentration of freezing nuclei and sea-salt particles carried out on the west coast of Ireland during summer 1958 to study the activity of sea-salt particles in the process of ice-nucleation of supercooled clouds are described and discussed. The results indicate that the concentration of freezing nuclei, active above –30°C, has no relationship to that of the sea-salt particles. A pronounced increase of the freezing nucleus concentration was observed during advection from the mainland showing that even under maritime conditions the most active freezing nuclei are particles of continental origin.

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Zusammenfassung Es wird über parallele Messungen der Gefrierkerne und der Seesalzkerne berichtet, die im Sommer 1958 an der irischen Westküste durchgeführt wurden, um die Wirkung der Seesalzkerne für den Gefrierprozess in unterkühlten Wolken zu studieren. Die Ergebnisse zeigen, dass zwischen der Konzentration der im Temperaturbereich zwischen 0°C und –30°C aktiven Gefrierkerne und den Seesalzkernen keinerlei Beziehung besteht. Eine starke Erhöhung der Gefrierkernzahl bei Advektion von Aerosolen vom Festland deutet an, dass auch unter maritimen Bedingungen, Teilchen kontinentaler Herkunft besonders aktive Gefrierkerne sind.

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Dedicated to Senior ProfessorL. W. Pollak, Ph. D., M.R.I.A. on his seventieth birthday.

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The research reported in this article has been sponsored and supported in part by the Geophysics Research Directorate of the Air Force Cambridge Research Center. Air Research and Development Command, United States Air Force, under Contracts AF 61(052)-26 and AF 61(514)-927, through the European Office, ARDC in Brussels.     

The oceans as source of cloud-forming nuclei

Summary The number and size of salt particles produced by the bursting of air bubbles in sea water has been measured. Bubbles of diameters varying between 1/2 and 2 mm each produced about 300 nuclei the sizes of which, under the electron microscope, were mainly between 0.1 and 0.5 diameter. They appeared to consist mainly of sodium chloride, the smallest ones containing only 10^–15 g of salt.These results, together with measurements of the size distribution of salt nuclei collected over the oceans in areas of spray formation, indicate that the total concentrations of salt nuclei over the oceans in winds of up to 15 m sec^–1 probably do not exceed 100 cm^–3. The corresponding rate of production of salt nuclei at the sea surface is estimated to be 1000 cm^–2 sec^–1. It is therefore inferred that sea spray contributes perhaps only one-fifth of the nuclei involved in cloud formation, the majority being the products of combustion, either natural or man-made.     

The heterogeneous oxidation of SO2 on aerosol surface

A heterogeneous chemical model is developed by coupling aerosol, gas-phase and liquid-phase chemical model. SO₂ oxidation rates on the aerosol surface are calculated and the influence of some factors is discussed. Model simulations indicate that SO₂ heterogeneous oxidation rates are sensitive to the mass concentration and chemical composition of aerosols, relative humidity, initial values of SO₂ and H₂O₂. The heterogeneous chemical model is coupled with a Eulerian deposition model. Model results show that oxidation of SO₂ on the aerosol surface is found to reduce SO₂ levels by 5%–33%, to increase SO ₄ ^2- - concentrations by 8%–50% in the surface layer. Project supported by the National “85-912” Key Science and Technology Project.     

The nuclei of natural cloud formation part I: The chemical diffusion method and its application to atmospheric nuclei

Summary Condensation nuclei which grow into droplets in natural clouds constitute only a small fraction of the total population of nuclei, since they must be activated at low supersaturations (<1%). Such nuclei cannot readily be measured by the usual expansion methods. A chemical diffusion method is described in which water vapour and HCl vapour diffuse between a water surface and an aqueous HCl solution; this produces the continuous small supersaturations required to study these «cloud nuclei». Nucleus numbers are obtained photographically. Results of observations made by this method during the period April–August 1958 are presented and discussed; they suggest that the main source of cloud nuclei is the dry land surface.     

Uniwavelength lidar sensitivity to spherical aerosol microphysical properties for the interpretation of Lagrangian stratospheric observations

The determination of stratospheric particle microphysical properties from multiwavelength lidar, including Rayleigh and/or Raman detection, has been widely investigated. However, most lidar systems are uniwavelength operating at 532 nm. Although the information content of such lidar data is too limited to allow the retrieval of the full size distribution, the coupling of two or more uniwavelength lidar measurements probing the same moving air parcel may provide some meaningful size information. Within the ORACLE-O3 IPY project, the coordination of several ground-based lidars and the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) space-borne lidar is planned during measurement campaigns called MATCH-PSC (Polar Stratospheric Clouds). While probing the same moving air masses, the evolution of the measured backscatter coefficient (BC) should reflect the variation of particles microphysical properties. A sensitivity study of 532 nm lidar particle backscatter to variations of particles size distribution parameters is carried out. For simplicity, the particles are assumed to be spherical (liquid) particles and the size distribution is represented with a unimodal log-normal distribution. Each of the four microphysical parameters (i.e. log-normal size distribution parameters, refractive index) are analysed separately, while the three others are remained set to constant reference values. Overall, the BC behaviour is not affected by the initial values taken as references. The total concentration (N₀) is the parameter to which BC is least sensitive, whereas it is most sensitive to the refractive index (m). A 2% variation of m induces a 15% variation of the lidar BC, while the uncertainty on the BC retrieval can also reach 15%. This result underlines the importance of having both an accurate lidar inversion method and a good knowledge of the temperature for size distribution retrieval techniques. The standard deviation (σ) is the second parameter to which BC is most sensitive to. Yet, the impact of m and σ on BC variations is limited by the realistic range of their variations. The mean radius (r_m) of the size distribution is thus the key parameter for BC, as it can vary several-fold. BC is most sensitive to the presence of large particles. The sensitivity of BC to r_m and σ variations increases when the initial size distributions are characterized by low r_m and large σ. This makes lidar more suitable to detect particles growing on background aerosols than on volcanic aerosols.     

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.     

The measurement of size distributions in aitken nuclei

Summary A method of examining the size distribution of stored Aitken nuclei by obtaining the mobility spectrum of the charged nuclei is described. The results agree-well with those from diffusion coefficient measurements if one assumes that the mobility of an ion is independent of its charge. It was found that stored nuclei tended to form into two size groups. The effect of the heteorogeneity of the nuclei on the methods. of determining their size is briefly discussed.

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Zusammenfassung Eine Methode zur Prüfung der Größenverteilung von Aitkenkernen aufgrund des Beweglichkeitsspektrums der geladenen Teilchen wird beschrieben. Die Ergebnisse stimmen gut mit Messungen der Diffusionskoeffizienten überein, ween man annimmt, daß die Beweglichkeit eines Ions unabhängig von seiner Ladungist. Es wurde gefunden, daß eingeschlossene Kerne zwei Größengruppen bilden. Der Effekt der heterogenen Größenverteilung der Kerne auf die Methoden zu ihrer Größen-bestimmung werden kurz diskutiert.

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The research reported in this document has been made possible through the support and sponsorship of U.S. Department of Army, through its European Research Office.     

The effects of condensation nuclei in atmospheric electricity

Summary The effects of condensation nuclei in the atmosphere are discussed, particularly in relation to the «sunrise effect» and it is shown that the introduction of positively charged nuclei is the only process yet suggested that can explain the observed results.     

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.     

The condensation nuclei produced by point discharge

Summary The properties of condensation nuclei generated at a metal point corona discharge have been examined. The threshold voltages for nucleus production and for small ion production are the same. The fraction of the nuclei which is charged is initially small and becomes virtually zero after a few seconds. It is demonstrated that in the non-uniform field near the point there is a force on the nuclei driving them to the point. From determinations of the diffusion coefficient it is tentatively concluded that a portion at least of the nuclei are originally of molecular size. No satisfactory explanation has been found for certain anomalous features of the variations in size and in concentration of the nuclei with time after production.

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Zusammenfassung Es wurden die Eigenschaften von Kondensationskernen untersucht, die durch Koronaentladung von einer Metallspitze erzeugt wurden. Die Spannungsschwelle für die Erzeugung von Kondensationskernen und kleinen Ionen ist die gleiche. Der Bruchteil der Kerne, welche geladen sind, ist anfangs klein und wird nach einigen Sekunden praktisch null. Es wird gezeigt, dass in dem inhomogenen Felde nahe der Spitze eine Kraft auf die Kerne wirkt, welche sie zur Spitze treibt. Aus Bestimmungen des Diffusionskoeffizienten wird versuchsweise geschlossen, dass wenigstens ein Teil der Kerne ursprünglich molekulare Grösse hat. Keine befriedigende Erklärung für gewisse anomale Eigenschaften der zeitlichen Änderungen der Grösse und Konzentration der Kerne nach ihrer Erzeugung wurde gefunden.

     

The vertical distribution of condensation nuclei near the ground

Summary Measurements have been made of the concentration of condensation nuclei, Z, at four heights between the ground and 300 cms in country air close to Cambridge. When the results are averaged over sufficiently long periods a systematic variation of Z with height is found and this variation itself has a diurnal variation. The results imply the production of nuclei in the atmosphere in this region. With the aid of simplifying assumptions, estimates of the rate of production are made for various periods of the day and night.     

ENSO对平流层气溶胶分布的影响

本文采用ONI(Oceanic Nino Index)和HALOE(Halogen Occultation Experiment)气溶胶面积密度资料,从其滞后相关性入手分析了ENSO循环对平流层气溶胶的影响,通过对滞后于El Nino和La Nina时气溶胶含量的比较探讨了ENSO强迫的影响程度,并用剩余环流及其输送量解释了平流层气溶胶变化的动力机制.结果表明:ENSO对平流层气溶胶的分布有明显影响,在赤道和低纬度上空尤为显著,El Nino发生后半年内热带平流层低层的气溶胶面积密度较平均值偏大,平流层中层的面积密度则偏小,而La Nina反之.El Nino和La Nina影响的差异显著,在分别滞后于El Nino和La Nina事件2~8个月间的60 hPa气溶胶含量差异甚至高达45%,海表温度变化1 K则在滞后半年内气溶胶面积密度的变化可达到16%.ENSO的强烈影响能够维持大约半年,两年后基本消退.热带的变化幅度明显强于中高纬度,南北半球的变化特征也有所不同.ENSO通过影响剩余环流导致气溶胶输送量发生变化,进而引起气溶胶分布出现上述差异.     

The equation of motion for a small aerosol in a continuum

Summary The motion of an aerosol can be described by a general force balance equation, independent of the detailed structure of the flow, provided that the interaction between the external flow field and the local flow induced by the aerosol is weak. A necessary and sufficient condition for the interaction to be weak is that the length scale of the aerosol is much less than that of the external flow. High and low Reynolds number regimes can be distinguished for the motion of an aerosol relative to the external flow. In some extreme conditions the equation of motion reduces to an algebraic equation for the aerosol velocity.After submission of this article the author was made aware of a similar treatment published inTopics of Appl. Phys., 12 (Turbulence, Chapter 7), Springer Verlag, Berlin, which at the time of the article's submission had not yet appeared in Australia. (Ed.)     

Some results of aerosol measurements

Summary In brief a newly developed freezing nuclei meter is described. Results of measurements made with this instrument in New Jersey, California and in the Thule area, Greenland, are shown. These data are compared with measurements of condensation nuclei, simultaneously made with the General Electric and the Aitkennuclei counters. Also the results obtained from some measurements of the lightscattering function of the aerosol in industrial areas near New York City are presented.     

An investigation of the size frequency resolution of an aerosol by the static method of diffusion

Summary Theoretical considerations are put forward for identifying the Dynamic and Static Methods of Diffusion measurements of aerosol particles. On the basic of this equality the Exhaustion Method of Diffusion developed byPollak andMetnieks for the size-frequency resolution of a heterogeneous aerosol by Dynamic Diffusion Methods should be applicable to Static Methods. This hypothesis is tested using a Pollak-Nolan nucleus counter as decay vessel. It is shown that turbulence in the counter is the most serious obstacle against this type of approach to finding the size components of an aerosol. The time involved, the high humidity in the counter and a hypothetical nucleus-free zone are other major difficulties for relatively large Aitken nuclei. It is established, however, that for particles whose diffusion coefficient is of the order of 10^–3 cm² sec^–1, the proposed method should be quite workable.     

Study of vertical structure of aerosol optical properties with sun photometers and ceilometer during the MACRON campaign in 2007

This paper presents the measurements of a vertical structure of aerosol optical properties performed during the MACRON (Maritime Aerosol, Clouds and Radiation Observation in Norway) campaign, which took place in July and August 2007 at ALOMAR observatory on Andøya island (69.279°N, 16.009°E, elevation 380 m a.s.l.). The mean value of the aerosol optical thickness (AOT) at 500 nm during campaign was 0.12. Significant increase of the AOT above longtime mean value was observed on 7 and 8 August 2007 when the AOT exceeded 0.4 at 500 nm. Analyses of back trajectories show the aerosol transported from over Africa and Central Europe. The aerosol extinction coefficient obtained from the synergy of ceilometer and sun photometer observations reached 0.05–0.08 km^?1 (at 1064 nm) in the dust layer. The single scattering albedo at the ALOMAR observatory decreased during the dust episode to 0.93–0.94, which indicates some absorptive aerosols in the lower PBL.     

The approach to charge equilibrium in a stored aerosol during aging

Summary The approach during natural aging to charge equilibrium of an aerosol produced and stored in a polyester «Mylar» balloon gasometer with metallized internal surface and 4200 litres content is studied. The number of charged condensation nuclei at any instant is related to the number of charged nuclei present at the same moment in electrical equilibrium produced by alpha-irradiation of the aerosol.It is shown that a stored aerosol consisting of small nuclei of moderate concentration is not later than 15 minutes after their birth in electrical equilibrium. With increasing size and concentration of the nuclei it takes longer and longer until the stored aerosol attains charge equilibrium. Stored large nuclei appear not to reach exact charge equilibrium even after several days.The reasons for replacing the rubber balloon gasometer, used previously, by the Mylar balloon for this investigation are given.In connection with the ionizing radiation required for producing charge equilibrium, the generation of condensation nuclei by alpha-radiation in stagnant and moving nucleus-free air has been investigated. It has been found that no condensation nuclei are being produced in filtered (nucleus-free) air for air-flows of between 0.1 and 20 litres/min in a tube of 4.2 cm diameter by the alpha-rays of Polonium 210 of about 125 c.

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Zusammenfassung Es wurde die während natürlicher Alterung stattfindende Annäherung an das elektrische Gleichgewicht eines Aerosols, welches in einem «Mylar» Ballon-Gasometer aus Polyester mit metallisierter Innenfläche und von 4200 Litern Inhalt erzeugt und gespeichert wurde, studiert. Die in jedem Augenblick vorhandene Anzahl der geladenen Kondensationskerne wurde zu der Zahl der geladenen Kerne im elektrischen Gleichgewicht für den gleichen Moment in Beziehung gesetzt, welches durch Bestrahlung des Aerosols mit Alpha-Teilchen erzeugt wurde.Es wird gezeigt, dass ein gespeichertes Aerosol, welches aus kleinen Kernen von mässiger Konzentration besteht, nicht später als 15 Minuten nach der Geburt der Teilchen im elektrischen Gleichgewicht ist. Mit zunehmender Grösse und Konzentration der Kerne dauert es länger und länger bis das gespeicherte Aerosol sein Gleichgewicht erreicht. Gespeicherte grosse Kerne scheinen selbst nach mehreren Tagen nicht das volle Gleichgewicht zu erreichen.Die Gründe für die Einführung des Mylar Ballons statt des früher benützten Gummiballons werden gegeben.In Zusammenhang mit der radioaktiven Bestrahlung, welche für die Erzeugung des Ladungsgleichgewichtes gebraucht wird, wurde die Produktion von Kondensationskernen durch Alpha-Teilchen in stagnierender und bewegter kernfreier Luft untersucht. Es wurde festgestellt, dass keine Kondensationskerne erzeugt werden, wenn kernfreie Luft, welche sich mit 0.1 bis 20 Litern per Minute durch ein Rohrsystem mit 4.2 cm Durchmesser bewegt, von Polonium 210 mit 125 c bestrahlt wird.

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The research reported in this article has been supported in part by the Geophysics Research Directorate of the Air Force Cambridge Research Laboratories, through the European Office of the Air Research Division, United States Air Force under Contract AF 61 (052)-26 and by the Instrumentation Engineering Physics & Analysis Laboratory of the General Electric Co., Schenectady, New York under Retainer Agreement.     

Source apportionment and secondary organic aerosol estimation of PM2.5 in an urban atmosphere in China

PM2.5 is the key pollutant in atmospheric pollution in China.With new national air quality standards taking effect,PM2.5 has become a major issue for future pollution control.To effectively prevent and control PM2.5,its emission sources must be precisely and thoroughly understood.However,there are few publications reporting comprehensive and systematic results of PM2.5 source apportionment in the country.Based on PM2.5 sampling during 2009 in Shenzhen and follow-up investigation,positive matrix factorization(PMF)analysis has been carried out to understand the major sources and their temporal and spatial variations.The results show that in urban Shenzhen(University Town site),annual mean PM2.5 concentration was 42.2μg m?3,with secondary sulfate,vehicular emission,biomass burning and secondary nitrate as major sources;these contributed30.0%,26.9%,9.8%and 9.3%to total PM2.5,respectively.Other sources included high chloride,heavy oil combustion,sea salt,dust and the metallurgical industry,with contributions between 2%–4%.Spatiotemporal variations of various sources show that vehicular emission was mainly a local source,whereas secondary sulfate and biomass burning were mostly regional.Secondary nitrate had both local and regional sources.Identification of secondary organic aerosol(SOA)has always been difficult in aerosol source apportionment.In this study,the PMF model and organic carbon/elemental carbon(OC/EC)ratio method were combined to estimate SOA in PM2.5.The results show that in urban Shenzhen,annual SOA mass concentration was 7.5μg m?3,accounting for 57%of total organic matter,with precursors emitted from vehicles as the major source.This work can serve as a case study for further in-depth research on PM2.5 pollution and source apportionment in China.