Properties Of Dust Particles Research Articles

Broadband imaging and narrowband polarimetry of comet 73P/Schwassmann-Wachmann 3, components B and C, on 3, 4, 8, and 9 May 2006

Context. The Jupiter family comet 73P/Schwassmann-Wachmann 3 (SW3) split into several pieces in 1995. Some of the original fragments were observed during the next apparition of the comet in 1999−2001. The last return of the comet in 2005−2006 was accompanied by tremendous further splitting of some SW3 components – in particular component B – into a large number of subfragments. Aims. We present observations of components B and C during their closest approach to Earth in the first half of May 2006. These results aim at characterizing the properties of dust particles released from the fragments of comet SW3 and at identifying dusty and gaseous structures in the comae and tail regions of components B and C, which could be useful for conclusions about the presence of active regions and break-up events of the components. Methods. We used narrowband imaging polarimetry and an analysis of the broadband surface brightness for the characterization of the dust comae of the SW3 components. Coma and tail structures in components B and C were enhanced by numerical methods. Results. The degree of linear polarization of components B and C follows the empirical phase-angle dependence as derived from other comets. Fragment B was found in outburst on May 8. The brightness outburst was accompanied by changes in the shape of the coma. During the outburst, the spatial distribution of the linear polarization showed some peculiarities that had disappeared again on May 9. Arc-like signatures of a temporary nature, typical of fragmentation events, are seen in broadband filter images of component B on 3, 8, and 9 May 2006. The secondary dust tail, seen in component B on 3 and 4 May 2006, marks the synchrone of an earlier splitting event around 25 April 2006. Fan-like coma structures are found in the sunward coma hemisphere of components B and C that might be related to localized enhanced activity on the rotating nuclei.

Columnar Asian dust particle properties observed by sun/sky radiometers from 2000 to 2006 in Korea

Multi-year AERONET sun/sky radiometer data (2000–2006) at Gosan and Anmyon in Korea were analyzed to characterize the optical, radiative, and physical properties of Asian dust particles. Approximately, 80% of the dust storms were reported in March and April, and about 10–15% of the Asian dust events were observed during the wintertime. Similarities in the aerosol optical properties during the spring Asian dust period (SPR-AD) were found between the Gosan and Anmyon sites. The aerosol optical depth (Ångström exponent) during SPR-AD was about 40–44% higher (39–56% lower) than the optical depths reported during the spring non-Asian dust period (SPR-NAD). There was no significant difference in single scattering albedo between SPR-AD and SPR-NAD. At the Gosan and Anmyon sites, approximately 22% (29%) and 16% (16%) of direct and almucantar (the second value in parentheses) measurement data during SPR-NAD, respectively, were classified as SPR-NAD Coarse case (SPR-NAD-C), which indicates that the Asian dust storms were elevated and thus were not observed at the surface station. The aerosol properties between SPR-AD and SPR-NAD-C are almost identical. The AOD of SPR-AD is similar to that of the non-Asian dust period in May and June (MJ-NAD). However, the Ångström exponent is about two times lower than that of MJ-NAD. Aerosol volume size distributions illustrate clearly that the coarse-mode particles are dominant during SPR-AD, but that the accumulation-mode particles dominate during MJ-NAD. The largest aerosol radiative forcing and forcing efficiency were apparent during SPR-AD and SPR-NAD-C.

Modeling of the scattering and radiative properties of nonspherical dust-like aerosols

The optical and radiative properties of dust particles in solar and thermal infrared regions are investigated. Dust particles are assumed to be spheres and spheroids for a comparison aimed at understanding the nonsphericity effect of these particles on the radiation at the top of a dusty atmosphere. The classical Lorenz–Mie theory is employed to compute the optical properties of spherical dust particles. To compute the single-scattering properties of spheroidal dust particles, a combination of the T-matrix method and an approximate method is used in the present study. In the approximate method, applicable to large particles, the geometric optics method is applied to the computation of the scattering phase matrix. A combination of the solution from the geometric optics method and the contribution of the so-called edge effect is used to compute the extinction efficiency of a spheroidal particle whose absorption efficiency is computed by adding the so-called above- and below-edge effect (a term from the well-known complex angular momentum theory) to the geometric optics result. Numerical results show that the results from the T-matrix method and the present approximate approach converge at a size parameter of 50 for computing the integrated scattering properties (i.e., the extinction efficiency, single-scattering albedo, and asymmetry factor). Additionally, the phase functions computed from the two methods are quite similar for size parameters larger than 40 although some considerable differences may still be noticed for other phase matrix elements. Furthermore, the effect of surface roughness on the single-scattering properties of spheroidal particles is discussed. The present radiative transfer simulations illustrate the nonsphericity effect of dust particles is significant at short wavelengths, however, not at the thermal infrared wavelengths.

Efficiency of the deposition mode ice nucleation on mineral dust particles

Abstract. The deposition mode ice nucleation efficiency of various dust aerosols was investigated at cirrus cloud temperatures between 196 and 223 K using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). Arizona test dust (ATD) as a reference material and two dust samples from the Takla Makan desert in Asia (AD1) and the Sahara (SD2) were used for the experiments at simulated cloud conditions. The dust particle sizes were almost lognormally distributed with mode diameters between 0.3 and 0.5 μm and geometric standard deviations between 1.6 and 1.9. Deposition ice nucleation was most efficient on ATD particles with ice-active particle fractions of about 0.6 and 0.8 at an ice saturation ratio Si<1.15 and temperatures of 223 and 209 K, respectively. No significant change of the ice nucleation efficiency was found in up to three subsequent cycles of ice activation and evaporation with the same ATD aerosol. This indicates that the phenomenon of preactivation does not apply to ATD particles. The desert dust samples SD2 and AD1 showed a significantly lower fraction of active deposition nuclei, about 0.25 at 223 K and Si<1.35. For all samples the ice activated aerosol fraction could be approximated by an exponential equation as function of Si. This indicates that deposition ice nucleation on mineral particles may not be treated in the same stochastic sense as homogeneous freezing. The suggested formulation of ice activation spectra may be used to calculate the formation rate of ice crystals in models, if the number concentration of dust particles is known. More experimental work is needed to quantify the variability of the ice activation spectra as function of the temperature and dust particle properties.

Saharan dust particle properties over the central Mediterranean

AERONET sun photometer measurements performed at five different sites of the Central Mediterranean during strong Saharan dust outbreaks are used to characterize optical and physical properties of dust particles. Sun photometer retrievals are combined at two of the five sites with lidar observations. It is shown that at low aerosol optical depths (AODs) the dust particle properties are quite dependent on dust load and monitoring site location. Differences on retrieved particle properties reduce with increasing dust load. AERONET retrievals at AOD (440 nm) ≥ 0.6 are then used in this paper to characterize dust particles over the central Mediterranean basin leading to columnar averaged values of the real refractive index < n > = 1.5 ± 0.1, the imaginary refractive index < k > = 0.004 ± 0.002, the single scattering albedo < SSA > = 0.89 ± 0.03, and the Angstrom exponent < Å > = 0.2 ± 0.1. It is shown that Å represents the best marker to trace the temporal evolution of dust events. Volume size distributions show a dominant coarse mode peaking at 1.7–3 μm. In particular, the average coarse mode that is centred at ≅ 2.2 μm at Lampedusa, which is ∼200 km away from the northwest Africa coast, gets peaked at ≅ 1.7 μm at Lecce, which is ∼800 km away. Lidar retrievals are used to characterize the vertical distribution of dust particles by the vertical profiles of the backscatter and extinction coefficients, the lidar ratio, and the depolarization ratio. Lidar retrievals show that over the Mediterranean basin, dust layers generally extend from 1 up to 6 km and that their vertical distribution can significantly change within a few hours. It is also shown that at high values of the AODs dust particles are characterized by lidar ratios spanning the 50–70 sr range and depolarization ratios larger than 30%.

Properties of Dust Particles Sampled from Windboxes of an Iron Ore Sintering Plant: Surface Structures of Unburned Carbon

Aiming to understand the formation mechanism of dioxins in the iron ore sintering process, dust samples obtained from some windboxes of a commercial iron ore sintering plant have been characterized with a powder X-ray diffraction (XRD), by the transmission electron microscope (TEM) equipped with an electron energy loss spectroscopy (EELS), and by the temperature-programmed desorption (TPD) and temperature- programmed oxidation (TPO) techniques. The elemental and XRD analyses reveal that the content of the Cl present in the samples ranges from 0.075 mass%-dry to 5.1 mass%-dry and tends to be higher at smaller dust particles, and that some of the Cl exists as KCl with the average crystalline size between 40 nm and 50 nm. Dust samples also contain a significant amount of unburned carbon, and the smallest dust particles, <500 μm, show the highest C contents in many cases and consist partly of C, K, and Cl elements. The TPD and TPO experiments exhibit that the dust samples have several types of oxygen functional forms on the carbon surface, and the proportion of carboxyl and lactone/acid anhydride groups, which can be partly decomposed into CO2 and carbon active sites at 150 to 500°C, tends to be larger at smaller dust particles. On the basis of the above results, the formation mechanism of chlorinated organic compounds including dioxins is discussed in term of interactions among HCl (and/or Cl2), metallic chlorides, and carbon active sites.

Similarity and diversity in photometric and polarimetric opposition effects of small Solar System bodies

The phase-angle dependences of brightness and polarization of light scattered by atmosphereless Solar System bodies (satellites, asteroids, planetary rings) as well as comets and zodiacal light are analysed on the basis of available ground-based and spacecraft observations. We study similarity and diversity in the photometric and polarimetric opposition effects. The similarity of the brightness and polarization phase functions for polydisperse dust media (cometary and interplanetary dust) and for atmosphereless bodies at small phase angles gives grounds to state that at least some physical properties of dust particles and light-scattering mechanisms should be similar for the two classes of objects. The aggregate structure of particles in different objects can be precisely the property that determines the photometric and polarimetric effects observed in the opposition region. Differences observed in opposition effects for different objects are likely caused by different physical properties (composition, sizes, structure, density) of the scattering particles and, therefore, by different relative contributions of the light-scattering mechanisms. However, the relationship between the photometric opposition effect, spike effect, negative polarization branch, and polarization opposition effect for different bodies is ambiguous.

Sensitivity Experiments on the Effects of Optical Properties of Dust Aerosols on Their Radiative Forcing under Clear Sky Condition

There are many uncertainties in the quantitative assessment of radiative effects due to atmospheric dust aerosol and the optical properties of dust particles contribute large to them. Numerical sensitivity experiments to evaluate the impacts of optical characteristics on the radiative forcing have been performed in this study. The experiments involve the effects of refractive indices, single scattering albedo, asymmetry factor and optical depth of the dust particles. An updated data set of refractive indices representing East Asian dust and the data set recommended by World Meteorology Organization (WMO) are used in our calculations for comparison. A k-distribution model for solar and thermal radiation transfer is employed in the calculation of radiative forcing. It is found that comparing with the WMO model, East Asian dust model has stronger scattering and weaker absorption at solar regime, which leads to higher negative forcing at the top of atmosphere (TOA) in this study. The more important is the signs of radiative forcing at TOA over certain regions could be reversed for the two dust models, which emphasizes the essentiality of accurate measurements of optical properties of dust aerosols for quantitatively estimating their radiative forcing.

Light-scattering models applied to circumstellar dust properties

Radiation pressure force, Poynting–Robertson effect, and collisions are important to determine the size distribution of dust in circumstellar debris disks with the two former parameters depending on the light-scattering properties of grains. We here present Mie and discrete-dipole approximation (DDA) calculations to describe the optical properties of dust particles around β Pictoris, Vega, and Fomalhaut in order to study the influence of the radiation pressure force. We find that the differences between Mie and DDA calculations are lower than 30% for all porosities. Therefore, Mie calculations can be used to determine the cut-off limits which contribute to the size distribution for the different systems.

Evaluation of Dust Particle Properties and Particulate Contamination in a PECVD Reactor by Visualization Measurements

Evaluation of Dust Particle Properties and Particulate Contamination in a PECVD Reactor by Visualization Measurements

Annual variations of physical properties of desert dust over Israel

The annual variation of the desert dust aerosol loading above the eastern Mediterranean is studied. Three periods are identified, March–May, July–August, and September–November, for which the properties of dust particles are distinctly different. The dust layers are at higher altitudes and consist of larger particles in summer and autumn than in spring. The real part of the refractive index of the particles is the same for summer and autumn periods and exceeds the real part of the refractive index measured during the spring. The imaginary part of the refractive index is negligible both in spring and in summer, whereas the imaginary refractive index becomes significant in September–November, indicating the presence of absorbing aerosols. The difference is attributed to different sources and desert dust trajectories in these periods. In spring the desert aerosol from the source in Chad is transported to the eastern Mediterranean predominantly along the North African coast. This is the motion associated with Sharav cyclones. The aerosols come to the eastern Mediterranean via Egypt from the sources near the Red Sea in July–August. In autumn the dust arrives to the eastern Mediterranean from the Libyan coast.

Negative Polarization of Light Scattered by Cometary Dust and Planetary Regolith: Two Different Mechanisms

strate that the mechanisms of NPB for comets and planetary surfaces can be quite different. In case of comets this can be due to scattering by individual particles, and for planetary regoliths this could be a combination of the “comet” mechanism and the coherent backscatter effect. Light scattering by cometary dust: Individual particles with size compared to the wavelength can in principle produce NPB. This is well known from calculations of scattering properties of spheres and irregular particles by, respectively, the Mie formulas [3] and DDA approach [4]. Spheres as a model of cometary particles are a too rough approximation, since perfect balls, even after particle size averaging, can demonstrate resonant features (both in intensity and in polarization) that are not observed for comets. Results of DDA modeling for irregular particles are rather contradictory. In particular, results obtained in [4] and with the well-known code [5] show bad coincidence. That is why we start DDA modeling using our own codes [6]. Here we simulate scattering properties of dust particles regarding them to scatter independently of each other. Besides we consider that they are randomly shaped and have the following mass distribution: m, where u = 1.5 [7] at a constant density of particles. To calculate scattering properties we used an improved DDA method presented in [6]. To simulate cometary dust particles, a set of 137062 cells is used to form an approximately spherical volume. Then two randomly chosen cells of the volume are marked as seeds of a particle material and two cells are considered as seeds of the empty space. Each cell different from the seed cells is marked as the nearest seed cell. Examples of cross-sections of particles generated are shown on Fig. 3. As can be seen we generate particles with very irregular shapes that are undoubtedly closer to real

The influence of atmospheric dust on limb darkening of M-type Mira variables

We explore the occurrence of dust in M-type Mira atmospheres and its effect on limb darkening under schematic assumptions about dust temperatures and dust particle properties. Dust particles that are thermodynamically coupled to the surrounding gas may form and may affect limb darkening, though only by very little in infrared continuum bandpasses. Dust particles that assume the equilibrium temperature given by the mean intensity of the radiation field only form under rare circumstances. Unexpectedly large or wavelength-dependent infrared continuum radii observed by interferometry are unlikely to be caused by atmospheric dust, except possibly near 1 μm; however, radius measurements may be significantly affected by molecular band contamination.

A Qualitative Study on the Retention of Mercury From SO2-Containing Process Gases on Copper Smelter Dusts

Mercury present in process gases can to some extent be removed in dust filters such as textile bag house filters. The removal efficiency in such filters is dependent on gas composition, gas temperature and properties of dust particles collected in the filter. In the present study, laboratory scale experiments were conducted to study qualitatively the influence of different types of dusts on the retention of mercury in SO2-containing gases. A gas mixture composed of N2, SO2, O2 and Hg was passed through a textile filter covered with dust. Two different types of dust were used in the experiments: one dust containing mainly Cu- and Fe-sulphides and Fe-oxides and one containing mainly Cu-, Fe, and Zn-oxides and -sulphates. The dusts were studied using microprobe equipment after exposure to the mercury-containing gas. Thermodynamic calculations were carried out for comparison with the experimental results. The study shows that mercury is mainly associated with partially roasted chalcopyrite particles.Le mercure présent dans les gaz du procédé peut, jusqu'à un certain degré, être enlevé parles filtres à poussière tels que les filtres textiles de l'unité de filtration. L'efficacité d'enlèvement de ces filtres dépend, par exemple, de la composition et de la température du gaz ainsi que des propriétés des particules de poussière recueillies par le filtre. Dans l'étude présente, on a exécuté des expériences à l'échelle du laboratoire afin d'étudier qualitativement l'influence de différents types de poussière sur la rétention du mercure de gaz contenant du SO2. On a passé un mélange de gaz composé de N2, de SO2, d'O2 et de Hg à travers un filtre textile couvert de poussière. Dans les expériences, on a utilisé deux types différents de poussière; un type de poussière contenait principalement des sulfures de Cu et de Fe ainsi que des oxydes de Fe alors que l'autre type contenait principalement des oxydes et des sulfates de Cu, de Fe et de Zn. On a étudié les poussières à l'aide d'une microsonde après l'exposition au gaz contenant du mercure. On a comparé les calculs thermodynamiques avec les résultats expérimentaux. L'étude montre que le mercure est associé principalement avec des particules de chalcopyrite partiellement grillées.

Dust resuspension by the flow around an impacting sphere

A rigid body colliding with a layer of dust is capable of resuspending dust through two distinct mechanisms: a ballistic mechanism, where kinetic energy is transferred from the impacting body to dust particles through direct contact, and a hydrodynamic mechanism, where dust particles are resuspended by the flow disturbance generated by the body. In this paper, we study the hydrodynamic resuspension mechanism by considering the flow around a sphere moving either towards or away from a wall. Experiments were performed using a sphere translating at a constant velocity for Reynolds number, Re, in the range 300 to 3500, and at varying angles of approach and departure from a wall. A wider range of Re was investigated by releasing dense rigid spheres above the wall. The high Reynolds number flow past a steadily translating sphere is characterized by a recirculating wake region behind the sphere. When the sphere approaches the wall and stops on making contact with it, the wake vortex which is initially behind the sphere threads over the sphere's surface, generating a secondary vortex ring. The coherent structure, composed of the wake and secondary vortices, strikes the wall and pushes fluid or dust, initially adjacent to the wall, to one side. The resuspension of dust particles of diameter b which are initially at rest on the wall is governed by a particle Shields' parameter, θp, based on the sphere's impact velocity, U: θp = ρfU2/ (ρp−ρf)bg, where ρp and ρf are respectively the density of the dust particles and fluid. The resuspension criterion is a function of particle Reynolds number, Rep, based on the diameter and fall velocity of the dust particles and occurs when θp [ges ] θp,c where θp,c ≈ 3.0 for Rep [gsim ] 1, and θp,c ≈ 5.0/Re1/2p for Rep [lsim ] 1. The geometry of the region of dust resuspended by the sphere was studied as a function of the impact velocity, angle of impact and the properties of the dust particles. When the sphere impacts a thick layer of dust, the volume concentration of resuspended dust is sufficiently high to generate a particle-driven gravity current which transports the dust far from the point of impact. The dynamics of the gravity current were determined as a function of dust particle properties and size of the impacting sphere.A sphere moving impulsively from rest away from a wall is found not to play a significant role in the resuspension of dust; however trailing vorticity generated on the surface of the sphere advects a large volume of fluid away from the wall, which may contain dust already in suspension.

Are there two populations of comets based on polarimetric properties of dust particles?

AbstractA critical analysis of attempts to classify comets on the basis of their polarimetric properties is given. It was shown that a degree of polarization depends strongly on the gas-to-dust ratio of comets and bandpass of filters. At present, there are no rigorous polarimetric evidences for two taxonomic classes of comets.

Scattering Properties of Dust Particles in Weightlessness

AbstractPolarimetric measurements of the light scattered by irregular dust particles are essential to interpret observations of solar system dust in terms of its physical properties. We developed a iight scattering unit to retrieve polarimetric phase curves of dust samples in microgravity conditions. Preliminary results suggest that the values for the maximum polarization are higher under 1 “ g ” than under 0 “ g ” This can be compared to ground-based measurements which exhibit higher values for packed dust than for sifted dust. The unit is operational and is used to help design a related orbital experiment.

Laboratory Measurements of Light Scattering by Dust Particles

AbstractWe have set up an experimental device to optically study the scattering properties of dust particles. Measurements over the 8 — 174° interval of scattering angles are performed on a continuously flowing dust loaded jet illuminated by a polarized red HeNe laser beam. The scattering is averaged over the population of the dust particles in the jet, which can be determined independently, and give the “volume scattering function” for the two directions of polarization directly. While results for spherical particles are in good agreement with Mie theory, those for arbitrary particles show conspicuous deviations.

Temperature measurements of a Martian local dust storm

A technique for estimating the ground and near‐ground atmospheric temperatures within a Martian local dust storm is presented. It is applied to soundings taken by the Viking orbiter infrared thermal mapper (IRTM) instrument at four times‐of‐day for one storm. Essentially, a comparison is made between infrared radiances emerging from the storm interior and those from the region surrounding the storm. Particle extinction properties are assumed to be independent of position in the storm region, and scattering properties must be selected arbitrarily. For the storm studied here, the ground temperature in the interior is at least 6 K cooler, whereas the near‐ground atmospheric temperature may be less than or comparable to, those of the surroundings. The thermal structure of the storm interior did not change measurably between 11.5 and 16.6 hours local time. These observations favor theories of dust storm development in which regional winds rather than local, dust‐driven convection inititate the mobilization of dust from the surface. It is also concluded that the optical properties of dust particles in this local storm differ from those observed by Mariner 9 during the 1971–1972 global dust storm.

Laboratory measurements of light scattering by dust particles

We have set up an experimental device of the nephelometer type to study the scattering properties of dust particles. Optical measurements over the 8 – 174° interval of scattering angles are performed on a continuously flowing dust jet illuminated by a HeNe laser. The detection system uses a pair of photometers equipped with photomultiplier tubes. The first is fixed to provide a reference while the second one is mounted on a rotating table. The measurements are averaged over the population of the dust particles in the jet (which can be determined independently) and give directly its “volume scattering function” for the two directions of polarization.