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Aerosol particles in the atmosphere interact with solar radiation through scattering and absorption. Accurate aerosol optical properties are needed to reduce the uncertainties of climate predictions. The aerosol optical properties can be obtained via optical modeling based on the measured particle size distribution. This approach requires knowledge or assumptions on the particle refractive index and shape. Meanwhile, integrating nephelometry provides information on the aerosol scattering properties directly. However, their measurements are affected by angular non-idealities, and their data need to be corrected for angular truncation and illumination to provide the particle scattering coefficient. We performed an extensive closure study, including a laboratory and a simulated experiment, aiming to compare different nephelometer angular truncation and illumination corrections (further referred to as "angular corrections"). We focused on coarse mode irregularly shaped aerosols, such as mineral dust, a worldwide abundant aerosol component. The angular correction of irregular particles is found to be only ~2 % higher than the angular correction of volume equivalent spheres. If the angular correction is calculated with Mie theory, the particle size distribution is needed. Our calculations show that if the particle size distribution is retrieved from optical particle spectrometer measurements and the irregular shape effect is not considered, the angular correction can be overestimated by about 5 % and up to 22 %. For mineral dust, the traditional angular correction based on the wavelength dependency of the scattering coefficient seems more accurate. We propose a guideline to establish the most appropriate angular correction depending on the aerosol type and the investigated size range.

A numerical procedure is developed, which uses the information contained in the slope of forward and back scattering to estimate the scattering function in the far forward and far back scattering using stepwise extrapolation. This procedure permits the determination of scattering coefficient, backscattered fraction and asymmetry parameter with an accuracy of better than 1%.

The radiance of the night sky is determined by the available light and the scattering properties of the atmosphere (particles and gases). The scattering phase function of the aerosol has a strong dependence on the scattering angle, and depending on the viewing direction different parts of the atmosphere and the ground reflectivity give the most important contribution. The atmospheric radiance cannot be altered by optical instruments. On the other hand the light flux of a distant star increases with the size of the telescope, thus fainter stars become visible. Light extinction, scattering function, atmospheric radiance, ground reflectivity, color effects and others are discussed in detail and a simple theoretical treatment is given.

Mass measurement of ambient PM10 and PM2.5 size fractions is a rather well established technique. However there is a broad consensus in the scientific community about the importance of smaller particle size fractions regarding epidemiological and environmental aspects. We present aerosol mass measurement performed with two different aerosol impactors operated simultaneously – a commercial real cascade impactor and a custom made cascade virtual impaction plate impactor. Both instruments have size segregation for 10, 2.5 and 1 µm in terms of an aerodynamic diameter, hence corresponding to PM10, PM2.5 and PM1. Furthermore the Vienna Telephotometer delivers information about the extinction coefficient in the vicinity of local particle sampling. The time-resolved size distributions of the PM1 fraction are measured by means of the scanning differential mobility analyzer and are merged with the time-resolved aerosol extinction coefficient data in the area of interest, along with the particle surface and mass concentration measurement. Results show the importance of time resolved measurement of the PM1 size fraction and indicate a good correlation between the ratio of PM1 to PM2.5 and PM10 with the extinction coefficient.

Gustav Mie was a professor of physics with a strong background in mathematics. After moving to the University of Greifswald in North-Eastern Germany he became acquainted with colloids, and one of his PhD students investigated the scattering and attenuation of light by gold colloids experimentally. Mie used his previously acquired knowledge of the Maxwell equations and solutions of very similar problems in the literature to concisely treat the theoretical problem of scattering and absorption of light by a small absorbing sphere. He also presented many numerical examples which completely explained all the effects that had been observed until then. Since all calculations were done by hand, Mie had to limit his theoretical results to three terms in infinite expansions, thus he only could treat particles smaller than 200 nm at visible wavelengths. Mie's paper had remained hardly noticed for the next 50 years, most likely because of the lack of computers. It experienced a revival later and up to now it has been referenced more than 4000 times, owing to the widespread use of Mie's approach in sciences such as astronomy, meteorology, fluid dynamics and many others. Gustav Mie did not consider his work on scattering of light by small particles as very important, since he just tried to explain the effects which his students had observed. He concentrated on hot topics in theoretic physics, e.g., the theory of matter. He wrote several textbooks, e.g., on relativity, gravitation theory, and electromagnetism, and all of them had run into several editions.

This paper provides a survey of recent studies on the optical properties of aerosol and cloud particles that have been conducted at the AIDA facility of Forschungszentrum Karlsruhe (Aerosol Interactions and Dynamics in the Atmosphere). Reflecting the broad accessible temperature range of the AIDA chamber which extends from ambient temperature down to 183 K, the investigations feature a broad diversity of research topics, such as the wavelength-dependence of the specific absorption cross sections of soot and mineral dust aerosols at room temperature, depolarization and infrared extinction measurements of ice crystal clouds generated at temperatures below 235 K, and the optical properties of polar stratospheric cloud constituents whose formation was studied in chamber experiments at temperatures well below 200 K. After reviewing the AIDA research activity of the past decade and introducing the optical instrumentation of the AIDA facility, this paper presents illustrative examples of ongoing and already published work on optical measurements of soot aerosols, mineral dust particles, and ice crystal clouds.

The optical properties of strongly absorbing soot particles coated by transparent material are investigated experimentally and described by several modeling approaches. Soot is produced by spark discharge and passed through a Sinclair–La Mer generator where non-absorbing carnauba wax is condensed onto it to obtain internal soot–wax mixtures in a controlled way. Measurements of the extinction and volume scattering coefficient show an amplification of absorption by a factor of approximately 1.8. This behavior was described by different approaches of internally mixed materials for the modal diameters of the measured size distributions: concentric-sphere model, effective medium approximations and heterogeneous ellipsoids. The concentric-sphere model describes the absorption increase quantitatively; and hence, it is chosen to be applied to the entire particle population in the size distribution. The growth of the soot particles by condensing wax is described by a simplified growth model to estimate the different contributions of several soot particle diameters to the overall absorption cross-section.

During the 2002 summer, the VELETA 2002 field campaign has been carried out at the Sierra Nevada Massif, close to Granada in South-Eastern Spain. During the campaign, CIMEL CE-318 robotic radiometer has been one of the key instruments in the characterization of the atmospheric aerosol columnar properties. This kind of radiometers have been operated at Motril, a coastal location at sea level, Pitres (1200 m a.s.l.), located in the South slope of Sierra Nevada Massif, Las Sabinas (2200m a.s.l.), located on the north slope of the mountain range, and Armilla (680m a.s.l.), located in the valley. The principal feature of the locations is that they provide a strong altitudinal gradient. This work presents the evolution of changes followed by the atmospheric aerosol in the vertical column. Columnar aerosol properties have been derived from the CIMEL CE-318 measurements obtained at the four stations. In this sense, the combination of solar direct measurements and sky radiances were used to retrieve the aerosol columnar size distributions by means of Nakajima's code. Marked changes in the atmospheric aerosol along the campaign have been explained in terms of the synoptic conditions. A detailed study of the columnar properties at the different places has been done. Changes in the spectral dependence of the single scattering albedo under the influence of different air masses have been discussed. The size distributions have been fitted to a bimodal lognormal distribution function model, and the associated parameters have been correlated with aerosol optical depth or Angstrom exponent. In some cases, it has been necessary to develop different correlations for cases affected or non-affected by the arrival of Saharan dust. In the case of Armilla (680 m a.s.l.) and Las Sabinas (2200 m a.s.l.), the small horizontal distance allow us to consider that both stations are representative of the same atmospheric column, a fact that has been used to analyse the daily evolution of the mixing layer in the north slope. (C) 2007 Elsevier Ltd. All rights reserved.

The atmosphere consists mainly of nitrogen, oxygen, as well as argon, water vapor, carbon dioxide, and other trace gases and particles. Whereas the fractions of nitrogen, oxygen, and the noble gases are practically constant, the others undergo temporal and spatial variations.

The oak processionary moth (Thaumetopoea processionea L.) is found in oak forests in most European countries. The caterpillars bear urticating hairs (setae) as a chemical defence. These hairs break off and are small enough to become airborne and be transported by the wind. Upon contact with humans the toxin can cause an allergic reaction that ranges from a skin rash to respiratory distress. In order to measure the terminal settling velocity of this bioaerosol, we used a small elutriator and tested its functionality with particles of known aerodynamic diameter. We determined that the mean settling velocity of the setae is about 1 cm/s, corresponding to an aerodynamic diameter of 19 μm for setae with a diameter of 6 μm and a length of 190 μm. The dispersion of the hairs in the atmosphere for a typical summer day was calculated by means of an Eulerian model. The results of this calculation revealed that the maximum concentrations in the atmosphere on a typical summer day reach 20–30% of the concentration found directly at the source. Those maximum concentrations are reached at a distance from the source that varies between 174 and 562 m, depending on the atmospheric stability and the settling velocity.

The optical properties of composite particles are calculated by means of the discrete dipole approximation. In particular, efficiency factors for extinction’ scattering and absorption, asymmetry parameter, linear polarization, and phase function S 11 are obtained for particles in which (a) the individual components of different materials are distributed randomly within the particle body, and (b) the individual constituents are not distributed randomly over the particle volume, but are clumped together into larger compact blocks. Particles are assumed to have non-spherical forms (ellipsoids, cuboids, and an irregular shape U2015B10 are discussed in detail). Computational results are compared to those obtained for homogeneous particles of the same morphology using an effective medium theory (EMT). The greatest discrepancies between the use of the homogeneous and the inhomogeneous particles are found for polarization when particle size is larger than the wavelength of incident radiation. An important finding for S 11 is that the particles built from compact homogeneous blocks contribute more efficiently to backscattering than other analyzed particles, leading to a consistently lower asymmetry parameter. The homogeneous particles scatter least in the backscatter region, resulting in the highest asymmetry parameters of the three classes of particles. We find that the internal heterogeneities and surface structure have a comparable effect on the asymmetry parameter. It is shown that the efficiency factor for scattering is reduced when the real material configuration is ignored and the optical properties are modeled using the EMT; however, the use of the EMT and homogeneity assumption leads to significantly increased absorption.

Aerosol optical data obtained by means of ground-based methods are applied to determine microphysical properties of aerosols in the atmosphere of Vienna-city. The measured aerosol extinction coefficient σ A serves as a source of information on the ambient aerosols. A large database of extinction efficiency factors for a set of irregularly shaped as well as the spherical particles of various sizes is pre-calculated and employed in the inversion procedure. The assumed particle models differ in chemical composition and are representative for most typical aerosol systems in the urban atmospheres. All database records are taken into a regularization scheme to solve the inverse problem for aerosol size distribution using measured extinction data. In addition, subsidiary data on spectral sky radiance are successfully incorporated into the mathematical model to retrieve the information on aerosol effective refractive index in the visible. As for Vienna, the aerosol extinction is a decreasing function of wavelength in visible spectrum—it indicates the predominance of sub-micrometer-sized particles in the atmosphere. The surface distribution function s(r)=dS/dr of aerosol particles customarily peaks at radii r≈0.2–0.3 μm, while the volume distribution function v(r)=dV/dr∼rs(r) has a mode at radii about 0.3–0.4 μm. Analysing size distributions dV/d log(r) for irregularly shaped particles it is shown that the daily profile of this function is smoothly evolving and almost typically accounts for a first mode at radii between 0.8 and 0.9 μm.

The optical and microphysical properties of aerosols in highly urbanized region Bratislava–Vienna were determined by means of ground-based optical methods during campaign in August and September 2004. Although both cities are close to each other forming a common metropolitan region, the features of their aerosol systems are distinct. While urban and suburban zones around Vienna have mostly a clean air without major influences of emissions from industry, Bratislava itself need to be classified as polluted area—the optical data collected in the measuring site are influenced mainly by Technicke Sklo factory (NW positioned), Matador (SSE), Istrochem (ENE) and Slovnaft (ESE). In contrary to an observed smooth evolution of the aerosol system in Vienna, the aerosol environment is quite unstable in Bratislava and usually follows the day changes of the wind directions (as they correspond to the position of individual sources of pollution). The particle sizes in Bratislava are predominately larger compared to Vienna. A subsidiary mode within surface size distribution frequently occurs at radius about 0.7 μm in Bratislava but not in Vienna. The size distribution of airborne particles in Vienna is more dependent on relative humidity than in Bratislava. It suggests the particles in Bratislava are larger whenever, or non-deliquescent to a great extent. The spectral attenuation of solar radiation by aerosol particles shows a typical mode at λ ≈ 0.4 μ m in Bratislava, which is not observed in the spectral aerosol extinction coefficient in Vienna. In Bratislava, the average aerosol optical thickness grows from morning hours to the evening, while an opposite effect can be observed in Vienna in the same time.

The formation of aerosol layers in the earth's stratosphere and mesosphere under the influence of gravito-photophoretic forces is considered. The minimum set of effective particle parameters for describing gravito-photophoretic forces, and their critical values for vertical transport and levitation of particles are discussed. The behaviour of two types of particles are investigated specifically: (1) soot-like particles strongly absorbing both solar (visible) and IR radiation, and (2) sulfate-like particles which are transparent in the visible, but strongly absorb the terrestrial IR radiation. Only particles larger than one micrometer can levitate under the influence of gravito-photophoresis. Particles larger than 1μm can be suspended in the atmosphere. They need to have a variation in accommodation coefficient over the surface. As a rule transparent particles should have a density smaller than the one of water. Both black particles and white particles can be levitated. Gravito-photophoresis explains the existence of narrow aerosol layers at altitudes of approximately 20 and 50km, and also at approx. 70 and 80–83km. The thickness of the layers can be less than 1km if the accomodation coefficient exhibits only small differences over the surface. Transparent particles may form thick layers above 50km rather easily.

We are dealing with retrieval of aerosol size distribution using multispectral extinction data collected in highly industrialized urban region. Especially, a role of the particle morphology is in the focus of this work. As well known, at present, still many retrieval algorithms are based on simple Lorenz-Mie's theory applicable for perfectly spherical and homogeneous particles, because that approach is fast and can handle the whole size distribution of particles. However, the solid-phase aerosols never render simple geometries, and rather than being spherical or spheroidal they are quite irregular. It is shown, that identification of the modal radius aM of both, the size distribution f(a) and the distribution of geometrical cross section s(a) of aerosol particles is not significantly influenced by the particle's morphology in case the aspect ratio is smaller than 2 and the particles are randomly oriented in the atmospheric environment. On the other hand, the amount of medium-sized particles (radius of which is larger than the modal radius) can be underestimated if distribution of non-spherical grains is substituted by system of volume equivalent spheres. Retrieved volume content of fine aerosols (as characterized by PM2.5 and PM1.0) can be potentially affected by inappropriate assumption on the particle shape.

A spark of short duration between two carbon electrodes produces carbon vapor which immediately condenses and forms carbon particles of about 30 nm size. The spark is obtained by discharging a capacitor triggered by a small high-frequency spark by means of a Tesla transformer. Thus the initiation of the spark and the discharge are decoupled. Both the distance of the electrodes, the energy of the discharge and the frequency of the sparks can be varied independently. Furthermore it is possible to charge the capacitor to voltages of only ≈100 V for the discharge. A very stable and continuous operation of the generator has been achieved for tens of hours, particles sizes between 10 and 100 nm can be produced.

The UV-VIS-NIR spectral optical properties of soot and soot containing aerosols were investigated in detail during the AIDA Soot Aerosol Campaign 1999. One aim of the campaign was a comprehensive comparison of the microphysical properties of Diesel and spark generator soot. The mass specific extinction cross section at λ=450 nm of Diesel soot is 10.6±0.5 m 2 g −1 which is almost a factor of two larger than the corresponding value of 5.7±0.3 m 2 g −1 measured for spark generator soot. Coagulation-induced particle growth does not affect the soot extinction cross section and has a weak influence on the scattering properties of the soot aggregates. Atmospheric processing of freshly emitted soot was simulated in mixing experiments. The formation of mixed Diesel soot and dry ammonium sulfate particles by coagulation has only a minor effect on the soot absorption cross section. The coating of spark generated soot with organic material results in a strong increase of the single scattering albedo. A significant increase of the absorption coefficient at λ=473 nm during the coating process can be attributed to an enhancement of the specific soot absorption cross section by more than 30%.

In a campaign in Vienna, the horizontal extinction over the central part of Vienna, the size dependent light absorption, and the mass size distribution have been measured and a size resolved PIXE analysis has been performed. PIXE did not give a complete chemical analysis; therefore the chemical compounds and the amount of nitrogen and organic carbon had to be guessed. The extinction coefficient has been determined by Mie calculation using the measured size distribution, density and refractive index from the chemical compounds. The closure was successful and a linear relationship between measured and calculated extinction coefficient was found, with the measured extinction coefficient about 20% higher than the calculated one. The Angstrom exponent was also compared, the averages agreed but the measured values were more variable than the calculated ones. A sensitivity analysis shows that the accuracy of the determination of the optical properties of the aerosol is governed by the accuracies of all input parameters and the generally unknown state of mixing. This leads to uncertainties of the optical parameters of at least ±20%. The accumulation mode makes a considerable contribution to the extinction coefficient; therefore knowledge of the size distribution, mass density and refractive index is critical. Lack of information in the coarse mode or the nucleation mode is less critical. Usually extinction coefficients are not measured on a routine basis, instead they are calculated using available aerosol data such as size distributions obtained from models, and densities and refractive indices obtained from chemical analyses or models. In that case larger uncertainties are to be expected.

A total of 56 weekly samples from Oporto and 40 from Vienna were collected and analysed, for 23 n-alkanes and 16 polycyclic aromatic hydrocarbons (PAH), by GC-MS after extraction with a toluene/methanol mixture. Total and elemental carbon were in the same range of values for both sampling sites. Although parts of the spectrum of species in both sampling sites were constant over the sampling period there is no evidence for suggesting a universal tracer for alkanes. For both Oporto and Vienna, Principal Component Analysis (PCA) identified two PCs and in both cases the first PC contained only PAHs while the second contained only alkanes. This separation between alkanes and PAHs and the observation of an alternating pattern with higher concentrations of odd carbon numbered from C27 to C30 (natural emissions from biological origin) is believed to result from a separation between anthropogenic and biological contributions associated with the first and second PC, respectively.

A separated turbulent flow around a cube with slightly rough surfaces of 3×3 cm area was produced to simulate particle deposition on edges or other small scale structures of building walls. Polydisperse soot particles with a mass mean diameter of 0.8 μm produced by atomising a suspension of India ink and monodisperse fluorescent 0.6 μm latex spheres were used. The deposition velocity of the soot particles was determined by measuring the change in reflectance of the surrogate surfaces and the light absorption coefficient of the aerosol. The deposited fluorescent spheres were counted by means of a fluorescence microscope. Deposition velocities of the fluorescent spheres used in outdoor experiments were equal to deposition velocities of the soot particles used in a small wind tunnel. The density of the deposit was different on each side of the cube and also inhomogeneously distributed on each individual surface. The highest deposition was always found on the edges of the cube. This peculiar deposition pattern could be explained by the characteristics of the turbulent flow field around the surfaces which was measured by laser Doppler anemometry. Modelled soiling constants calculated with the help of the measured deposition velocities were up to a factor of 10 higher than values obtained for a flat plate in a simple boundary layer flow.

International audience; Larvae of the processionary moths of the Palaearctic region bear urticating setae that are released against vertebrate predators, especially insectivorous birds. A few species are pests of forest and urban trees and, consequently, may threaten human and animal health during outbreaks, causing dermatitis, conjunctivitis and respiratory distress. Although some studies provide detailed information about the setae, particularly those of the pine processionary moth Thaumetopoea pityocampa, there is little knowledge on the morphological traits of the setae and their release by the larvae. In the present study we identify major traits of the setae of 3 species of processionary moth, T. pityocampa, T. pinivora and T. processionea, which are potentially helpful in the understanding of setae dynamics in the environment: (i) diameter and length of setae and (ii) analysis of dynamical properties of the setae in the airborne state. Setae are highly variable in size, with bimodal distribution in T. pityocampa and T. pinivora; in these 2 species, short and long setae are interspersed within the integument fields where they occur. The difference in the seta size has important consequences in dispersion, as smaller setae can spread 5 times further than their bigger counterparts. This information is relevant for a full understanding of the defensive importance of larval setae against natural enemies of the processionary moths, as well for elucidating the importance of the processionary setae as air pollutants, both close to the infested trees and at longer distances.

The light absorption coefficient of the aerosol of Santiago de Chile has been measured by means of the integrating plate method. Values ranging between 0.01 and 0.2 km−1 have been found. Variations of the absorption coefficient by a factor of 8 within a few hours have frequently been found and have their causes in the specific meteorology of the Santiago air basin. In the summer a minimum occurs between 1 and 4 a.m. and a maximum between 8 and 10 a.m., whereas in the winter a maximum is found during the night and minimum values are in the afternoon with a smaller amplitude. Similar variations of the light absorption coefficient have been found also inside uninhabited rooms, since the aerosol was transported there almost without any losses. The daily pattern is similar but delayed and has a lower mean value and amplitude. When converting the light absorption to mass of black carbon, concentrations of elemental carbon of 1.3–25 μg m−3 were obtained. This is similar to other large cities, where a considerable part of the mobile sources are equipped with diesel motors. The light absorption contributes to about 15% of the light extinction of the aerosol, and considering an equal scattering of the soot particles, the contribution of soot to light attenuation in the atmosphere is about 30%. The single scattering albedo of the aerosol is 0.85, thus the heating effect of the absorption dominates. The light absorption coefficient is wavelength dependent with an average relationship of σa = k(λλ0)−α, the exponent being α = 0.92. The exponent of the wavelength dependence of the extinction coefficient of the aerosol was 1.22.

The deposition velocity of particles to a model surface has been determined experimentally in a wind tunnel by producing an aerosol consisting of monodisperse spheres containing a fluorescent substance. Simultaneously to the deposition a diluted sample of the aerosol is produced on a Nuclepore filter. By counting the particles on the deposition surface and the filter the deposition velocity can easily be determined.

The vertical extinction of the aerosol has been measured with a multiwavelength sunphotometer at a rural location in southern Italy. All measurements were performed during a period with relatively stable weather condition, thus it could be expected to have measured always the same air mass at least within each day. The direct solar radiation was recorded at wavelengths between 400 and 1100 nm with a resolution of 0.5 nm. Calibration of the instrument was performed by means of a Langley plot at an elevation of approximately 3000 m. From these measurements, vertical optical depths have been derived for a series of days in June 1993. The spectral extinction coefficient of the atmospheric aerosol at the same location has been measured by means of a telephotometer. The radiance of a target at a distance of 12 km was measured at nine wavelengths in the visible. Thus the extinction coefficient of the aerosol contained within a conical volume of about 450,000 m 3 and a length of 12 km has been measured. The daily variation usually followed the same pattern : a decrease in extinction coefficient in the morning and in the early afternoon and an increase toward the evening. This variation correlated well with the change in humidity, so that it can be assumed that the aerosol particles have decreased their size due to evaporation of water in the morning and uptake of water in the evening. The regular daily pattern of the horizontal aerosol extinction coefficient normally was not followed by the vertical aerosol optical density, but on the other hand, changes in vertical extinction have occurred on some days, which had no effect on the horizontal extinction. Vertical extinction measurements were inverted to obtain aerosol size distributions. Inversions were obtained by the Twomey-Phillips method and they confirm the presence of a hygroscopic aerosol as suggested directly by the variation of the extinction coefficients against the relative humidity. The aerosol undergoes very clear processes of growth which are well correlated with the behavior of the humidity. Overall, the type of aerosol we have measured could be classified as rural aerosol. This kind of aerosol is typical of the boundary layer of clean continental area and is composed prevalently by water-soluble substances and in minor abundance by dustlike aerosol.

The spectral extinction coefficient of the atomospheric aerosol at a rural location in Southern Italy was determined by means of a telephotometer: the radiance of a target at a horizontal distance of 12 km was measured at nine different wavelengths in the visible. Therefore the extinction coefficient of the aerosol contained within a conical volume of about 450,000 m3 and a length of 12 km was measured. All measurements were performed in summer 1993 during a period of stagnant air, presumably always with the same air mass and thus a similar type of aerosol. The daily variation usually followed a similar pattern: decrease in extinction coefficients in the morning and early afternoon and an increase towards the evening. This variation correlated well with the change in humidity. The Angstrom exponent of the spectral extinction coefficient, was lowest at high humidity and highest at low humidity. Comparing the horizontal attenuation measurements and vertical transmission measurements done with a solar photometer, a considerable vertical extent of the aerosol was found. This is important for climate considerations, since additional light absorption and scattering by the aerosol can lead to an increase or a decrease of the temperature. Inversion of the spectral extinction coefficient data to obtain particle size distributions shows that the dry ( Light absorption by the aerosol varied little on a day to day basis, amounting to 20 to 40% of the light extiniction coefficient. Values this high are common in Europe, also outside densely populated areas. The average single scattering albedo of the dry aerosol was 0.76, thus the aerosol will have a heating effect due to its light absorption.

At a suburban location near Vienna and in the center of Vienna the aerosol was sampled with a 10 stage rotating cascade impactor permitting the classification of the aerosol particle sizes between aerodynamic diameters of 15 nm and 16 um. Both sampling sites were at an elevated location. Gravimetric, light absorption and PIXE analysis have been performed for all samples. The light absorption analysis also gives the concentration of black carbon, the PIXE analysis yielded concentrations of Si, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, and Pb. The mass size distribution of the aerosol for most of the elements had two modes. One peak occurred between 0.2 and 0.8 um, the coarse particles had a slight peak usually above 3 μm. On occasions an indication of a third peak below 50 nm could be observed. The total mass of the submicrometer particles was more than half of the total mass, except for calcium and iron. Little difference between the mass of the submicrometer particles at the two locations was found, except for Ca, Pb, and black carbon. The particles were generally larger at the suburban location. The light absorbing particles mostly had only one peak at a diameter of 0.2 um, thus being smaller than the other particles. Soot at the suburban location was a factor of 2 less and slightly larger than at the urban site. The similarity of the size distribution for almost all elements between the urban and the suburban location, as well as similarities to the arctic haze and the aerosol in Northern Europe suggests that the aerosol predominately originates from non-local sources and might be considered the central and north European average aerosol. Differences to aerosol characteristics at other locations exist. On a day-to-day basis the aerosol shows a very high variability.

The light absorption coefficient of atmospheric aerosols in the visible can be determined by depositing the particles on a filter and measuring its “transmission” in a special optical arrangement. With an impactor with rotating impaction plates producing a homogeneous deposit, it is possible to extend this technique to size segregated aerosol samples. A simultaneous determination of the mass size distribution is possible. Test measurements with black carbon aerosol have shown the feasibility of this method. Samples of the atmospheric aerosol have been taken in and near Vienna, in Naples and near Bologna. The light absorption of the aerosol is always highest for particle diameters between 0.1 and 0.2 μm. Only in the humid environment of the Po valley it had a slightly larger peak size, whereas the size of the nonabsorbing particles increased considerably. The light absorption of the atmospheric aerosol is always higher in an urban environment. 'The mass absorption coefficient of the aerosol at all four locations was very similar, and completely different from values which could be. expected using effective refractive indices which are frequently used in models. Using the data measured in this work two alternate models for the effective refractive index and black carbon content of the aerosol are suggested: (a) a size-dependent refractive index, where the imaginary part varies from −0.25 for particles smaller than 30 nm to − 0.003 for particles larger than 2 μm; this could especially be applied if an internal mixing of the aerosol is to be expected, or (2) a size-dependent fraction of elemental carbon in the case of external mixing with 43% of carbon particles for sizes below 30 nm decreasing to 10% for sizes up to 0.4 μm.

The Integrating Nephelometer is a widely used instrument which permits the determination of the scattering coefficient of aerosols, having concentrations from background levels to high pollution. The traditional calibration is done by filling the instrument with a gas having a high refractive index and using the Rayleigh scattering of the gas molecules as a reference. Mostly halocarbon gases are used. Since one calibration of the instrument has the potential of destroying 40–80 tons of ozone, alternate methods for calibration have been investigated. Most gases with a high refractive index are difficult to handle or are toxic; also they have a completely different wavelength dependence of the scattering coefficient as the aerosol, thus gases appear unsuitable for calibration. As an alternative, a calibration with an aerosol containing particles which have no light absorption has been developed. The light extinction coefficient of the aerosol can be measured easily in a 1 m transmission cell, and with a subsequent dilution an aerosol having a light scattering coefficient in the measuring range of the Nephelometer is obtained. By varying the dilution many calibration points can be obtained. Since the calibration aerosol can be adjusted to have similar optical properties to the aerosol to be measured, the truncation errors and deviations due to different wavelength dependence of the calibration medium and the aerosol can be avoided. As a secondary standard an improved version of the original magnesium oxide disk calibration is suggested.

[1] Photophoretic forces act on a particle which has an inhomogeneous surface, especially with respect to the accommodation coefficient. Gas molecules incident on it leave the surface after more or less accommodation, resulting in a force and torque on the particle. Due to Brownian rotation of the particle the net force is zero. If the particle is also exposed to an orientating torque, the photophoretic force points in a preferred direction resulting in net photoporetic force. The orienting torque can be caused by gravity or by the earth magnetic field. Photophoretic forces are important if the mean free path of the gas molecules is larger than the size of the particles. This is the case for all particles found in the stratosphere and mesosphere. The direction of the photophoretic force can be opposite to gravity and it can exceed the weight of the particle. In that case the particle is lifted upwards, until it reaches conditions where the two forces are equal. The stratification of the stratospheric and mesospheric aerosol is given fact. The temperature profile of the atmosphere, the incoming solar flux density, and the emitted terrestrial infrared radiation, are used to investigate systematically the role of photophoresis in the formation of aerosol layers at specific altitudes for various types of particles around the globe. Photophoresis can be responsible for aerosol layer formation at altitudes of 20, 30, 50, 70 and 85 km. The layers form in specific geographic regions and at certain seasons.

The spectral extinction coefficient and the volume scattering function both of a laboratory generated aerosol and atmospheric aerosol have been measured for a wavelength range between 400 and 700 nm. The mass size distribution (for aerodynamic diameter) of the aerosols was determined simultaneously using a low pressure impactor. It was converted to a volume size distribution (for geometric diameter). The size distribution of the aerosol and its optical properties are related by an integral equation which is ill-posed; thus the inversion of the optical data to size distribution is difficult. Using a set of several million functions which are the sum of two lognormal size distributions, solutions of the integral equation may be found by selecting those functions giving an extinction coefficient which agreed within the experimental error margin with the measurements. By this it was also possible to estimate the error margin of the inverted size distributions. It is concluded that the approximate size and volume of the accumulation mode can be determined. It appears impossible to make more than a guess about the size and volume of particles larger than 1 μm.

Optical techniques permit a rapid and non-invasive technique to measure the properties of aerosols. Since optical properties are influenced by the properties of the particles, a determination of the size distribution of the particles by inversion of the optical data is desirable. Unfortunately, the inversion problem is ill-posed, the solutions are ambiguous and many techniques have been developed to select realistic solution functions. An information parameter κ close to 1 is necessary for a meaningful inversion. For extinction measurements in the visible, this applies to the particle size range between 0.3 and 2 μm in diameter. Inversions of simulated extinction data have shown that a determination of the mean diameter of monomodal size distributions is possible for diameters between 0.35 and 1 μm, a determination of the volume (mass) of the suspended particles is possible within a factor of 1.6, but that the determination of the width of the size distribution is difficult. For bimodal distributions having an accumulation mode and a coarse mode, only the accumulation mode can be retrieved, with a tendency to wider size distributions. Inversions of angular scattering data give similar results. Since the scattering pattern of particles larger than 1 μm is quite distinct, the range of retrievable sizes is extended to larger particles compared to extinction measurements.

Light-absorbing aerosol particles are present in the atmosphere all over the world, and they play an important role in the energy transfer to the atmosphere. The main substance causing light absorption in the atmosphere is graphitic (black) carbon, the major constituent of soot. It is generated at any kind of combustion, and in source regions soot particles are mostly mixed externally with other particles. After some time the soot particles may become attached to other particles, thus becoming mixed internally. Many methods for the determination of the light absorption by particles exists. The integrating plate technique offers a fast and reproducible way to determine light absorption coefficients of aerosol particles deposited on filters. Comparisons of this method with other determinations of light absorption coefficients have given partially agreement and partially disagreement. Therefore a comparison with an absolute method has been performed in order to determine possible inaccuracies. An externally mixed aerosol was made by independently generating a light-absorbing aerosol and a non-light-absorbing aerosol and mixing them such that within the measuring period coagulation was negligible. The light-absorbing aerosol was generated by a spark discharge between carbon electrodes. The sizes of the particles were small, usually more than 90% of the particle mass was below 50 nm. The non-absorbing was generated by a constant output atomizer. For small carbon particles the light extinction is dominated by the light absorption, therefore a measurement of the light extinction coefficient in a long path photometer provides an absolute method to determine the absorption coefficient. A comparison with the absorption coefficient determined simultaneously with the integrating plate method permits the estimation of the error. For pure carbon the light absorption coefficient determined by the integrating plate method is approximately 20% higher than the “true” value. For mixed aerosols the deviation depends on the mixing rate, but the integrating plate method always overestimates the light absorption coefficient.

The atmosphere interacts both with incoming as well as outgoing light. Two main processes take place: light scattering and light absorption. Whereas light scattering redistributes any ligh energy in the atmosphere, light absorption converts the light energy to internal energy of the absorbing molecules and eventually transfers it to the surrounding gas as heat. Atmospheric gases absorb light in distinct spectral regions usually at more or less broad bands. Best known is the broad absorption of ozone in the far u.v., being essential for the existence of the biological macromolecules on Earth. Gases known as greenhouse gases, e.g. CO2, CH4, N2O and water vapor absorb a wide range of infrared radiation, and thus are responsible for the greenhouse effects. Since the lifetime of these gases (except water vapor) is considerable, their distribution around the globe is fairly homogeneous. The atmospheric aerosol gives the major contribution to the atmospheric light absorption in the visible and near u.v. and near i.r. Graphitic (black) carbon, the main constituent of soot, is almost exclusively responsible for the light absorption of the particles. The light absorption by aerosols is continuous and covers the whole visible spectral range. It only depends slightly on wavelength. The optical properties of elemental carbon are determined by the size of the particles and their complex refractive index. A variety of refractive indices can be found in the literature for elemental carbon, most likely caused by different production and thus composition of the particles. Soot particles are very efficient in attenuating light; for the average size the particles have more than twice the mass extinction coefficient compared to transparent particles such as ammonium sulfate. The light absorption coefficient of a mixture of elemental carbon and transparent materials is higher for internal than for external mixtures. When incorporated into transparent particles, the absorption properties of elemental carbon can be multiplied and the single scattering albedo will decrease in comparison to an external mixture of the same materials. There are different methods to measure the light absorption coefficient of suspended particles. They can be separated in three groups, depending on the effect or methodology they use, but no standard procedure has been adopted so far. Soot is produced by all combusttion processes. Since most fires on Earth are due to humans, then indirectly humans are the major source of light-absorbing aerosol particles. On a global scale black carbon amounts to 1.1–2.5% of the anthropogenic particles and to 0.2–1% of the total emitted particles. The emission factors for elemental carbon are highest for small sources such as diesel motors or fireplaces. The light-absorbing aerosol consists of fine particles, with most particles having diameters less than a few tenths of a micrometer. Particles in the size range of soot particles have an average lifetime of 7 days in the atmosphere, therefore they can be transported over large distances and are also found in remote regions. Since light-absorbing particles have a variety of sources and sinks and they are involved in precipitation cycles, their distribution on the globe is inhomogeneous. Light-absorption coefficients of the atmospheric aerosol reported in the literature differ by more than four orders of magnitudes at different locations, but nevertheless black carbon particles have been found even at very remote areas, such as the South Pole. Although light-absorbing particles are a minority component in the atmospheric aerosol, their effects cannot be neglected: since the mass extinction coefficient of soot is higher by a factor of two to three compared to transparent particles, light-absorbing substances in the atmosphere can cause at some locations up to half of the visibility reduction; light-absorbing substances in the atmosphere can be responsible for the brown appearance of urban hazes and the discoloration of the sky. The light absorption of the atmosphere in the visible (which mainly is due to particulate matter) has to be taken into account when considering radiative properties and climatic consequences. A small temperature increase due to absorption in the visible is to be expected. The value is around a few tenths of a Kelvin, but no general statement on its magnitude is possible, since a large spatial and temporal variation exists and other factors like surface albedo, the optical depth of the aerosol, its incorporation in clouds and humidity growth of the aerosol have to be considered.

The atmosphere of Santiago de Chile has been investigated by telephotometry in several parts of the city. Spectral extinction coefficients were measured during the time period between September 1988 and January 1989 (mid-spring to mid-summer) and again between February 1990 and June 1990 (autumn to mid-winter). The values measured show a distinct daily pattern with high values in the morning and a steady decrease in the afternoon. This pattern is attributed to a steady increase in the height of the mixing layer during daytime which allows an increasing dilution of the aerosol; unfortunately systematic measurements of vertical temperature profiles are not available for Santiago. In contrast to the urban area, measurements at an elevated site in the Andes Mountains showed a constant increase in light extinction during daytime. In the morning the extinction coefficients were similar to the background values off the coast on the Pacific. In the afternoon after expansion of the mixing layer extinction coefficients in the mountains were comparable to those in the urban area. Some sight paths were reached at later hours by the mixing layer and had a maximum of light extinction coefficient later in the morning hours. The transport of aerosols can cause dramatic changes in extinction coefficients within a short time; changes of up to a factor of five have been observed within an hour. In such cases the wavelength dependence of the aerosol underwent changes with the advection or production of a different aerosol. A comparison with the pollution of other large cities shows that the daily pattern of variation of the extinction coefficient in Santiago is different; the values of the extinction coefficients observed in Santiago de Chile are compared to those in European and Asian cities. With this study part of the complex behaviour of the Santiago atmosphere has been understood but much further work is needed. The optical technique used here has proven useful in investigating the atmosphere in real time with adequate time resolution and the possibility to make space-resolved measurements also.

[1] So far, levitation of solid particles in the atmosphere by photophoretic forces has been attributed exclusively to gravito-photophoresis. In this paper attention is drawn to magneto-photophoresis, a motion of ferromagnetic particles related to a magnetic field during their irradiation by light. In regions showing an approximately vertically directed magnetic field, magneto-photophoretic levitation is likewise possible. Laboratory experiments with particles produced by an arc discharge in air between iron electrodes have quantitatively shown magneto-photophoretic levitation. Using these results, we show that magneto-photophoresis can be an essential factor for the formation and persistence of particle layers. Under conditions of illumination by the sun, magneto-photophoretic levitation takes place with particles having sizes around and below 0.1 μm whereas gravito-photophoresis occurs for particle sizes around 1 μm.

The extinction coefficient of the atmospheric aerosol has been determined in remote locations and the following results were obtained. 1. (a) Europe: on days with excellent (European) visibility the extinction coefficient usually has a value of 0.03 km −1 , on average days values around 0.1 km −1 are typical. It is interesting to note, that these values were found both in many location of the Austrian Alps and at a rural site 70 km south of the heavy industrialized Ruhr district in Germany. This indicates that the aerosol found at remote sites in Europe apparently is the average pollution spread over a large area. 2. (b) North America: spectral extinction coefficients were measured at a rural site north of Denver, Colorado, and at a site in the desert of northern Arizona in the summer of 1979. Usually the extinction coefficients had values between 0.016 and 0.04 km −1 . At the rural site in Colorado an influence of the urban plume of Denver could be noticed, since in most cases the values for sight paths towards the town were lowest, whereas in the desert site in Arizona the same extinction coefficient was measured in all directions. A severe smog episode in Los Angeles (more than 700 km to the west) caused an increase of the extinction coefficient by more than a factor of two at this remote location. 3. (c) South America: in the Cordillera de los Andes near Santiago de Chile extinction coefficients have been determined at elevations above 3000 m. Values increasing from 0.018 km −1 in the morning to 0.15 km −1 in the afternoon have been found. This mainly depends on the height of the haze layer, which usually increases from below 2000 m in the morning to above 4000 m in the afternoon. The low extinction coefficients in the morning still are very close to the background value measured at the Easter Island in the South Pacific. From this it can be concluded that an unpolluted atmosphere only exists on rare occasions. The usual background at least in Europe is a more or less diluted average pollution.

The coarse mode of the atmospheric aerosol, containing mostly particles larger than 1 μm in diameter, can conveniently be measured by means of an optical-forward-scattering spectrometer probe mounted on an airplane. Although the instrument is able to count single particles, at least 10,000 particles have to pass the sensitive volume in order to reduce errors due to statistical fluctuations of the counts, especially in the bins of the larger particles. The fitting of a lognormal curve to the measured particle counts is possible by means of a least-squares technique, described in this paper. The quality of the fit can be examined by determination of the best fit for the particle number distribution and the particle volume distribution, and an intercomparison of the two. Data for atmospheric samples show geometric mean diameters for the number size distribution between 1 and 2.5 μm and standard deviations between 1.7 and 2.5. Aerosols dominated by one source have a small standard deviation. Standard deviatio...