Nonspherical Dust Particles Research Articles

Aerosol optical and microphysical properties as derived from collocated measurements using polarization lidar and direct sampling

Collocated and simultaneous measurements of aerosols near the ground were conducted using a lidar and aerosol sampler at Tsukuba, Japan, to clarify the relationship between lidar-derived optical properties and in-situ microphysical properties. The total linear particle depolarization ratio (δp) ranged from 14% to 18% when nonspherical mineral dust particles were predominant in the supermicrometer range on May 7–8, 2008, whereas it ranged from 6% to 7% when spherical sea-salt particles were predominant in that range on September 3–4, 2008. Sulfates and nitrates were predominant in the submicrometer range for these two periods. Water-dialysis analysis on May 6–7 indicated that 29% of the coarse particles were water insoluble, whereas 70% were water soluble or nearly soluble on September 3–4. The ratio of dry mass concentration to the backscattering coefficient (M/βp) was 34–39 g m−2 sr on May 7–8 and 6.2–6.3 g m−2 sr on September 3–4. Our results provide evidence that lidar-derived βp and δp capture the aerosol mass concentration and relative abundance of the spherical and nonspherical particles although the microphysical properties vary significantly for individual particles.

Depolarization of millimeter wave due to non-spherical dust particles in storms

In the present paper theoretical investigation has been carried out to estimate the depolarization of millimeter wave by non-spherical dust particles present in sand and dust storms. For this purpose ellipsoidal shape of dust particles is considered. The concept of differential attenuation has been utilized to quantity the depolarization. It has been found that in general depolarization angle, depolarization and loss due to depolarization produced by ellipsoidal dust particles depend heavily on frequency and optical visibility (particle density). The values of depolarization and loss due to depolarization increase with increasing frequency and decrease with increasing visibility. It has been also observed that the values of differential attenuation increase with increasing frequency for particular visibility and thus depends directly on frequency. Similar experimental observation have been made by Cox Arnold and Hoffman in ice and rain at 19 and 28 GHz. Also the reported values by several workers are between 19.4 and 77.5 dB at 10 GHz frequency for 1 km path length. Basir et al., XPD = 77.5 dB, Ghobrial et al., XPD = 30 dB, McEwan et al., XPD = 19.4 dB for 1 km path length and 15 m visibility at 10 GHz.

OPTICAL PROPERTIES OF NONSPHERICAL ATMOSPHERIC PARTICLES AND RELEVANT APPLICATIONS

Recent progress in the study of the single-scattering properties of nonspherical ice crystals within cirrus clouds and nonspherical dust particles is reviewed. We have been using the finite-difference time domain (FDTD) method, the discrete dipole approximation (DDA), and an improved geometric optics method (IGOM) to compute the single-scattering properties of nonspherical particles. We have incorporated the so-called edge effect associated with the surface wave into the IGOM extinction and absorption efficiencies. The simulation results in the solar and thermal infrared spectral regimes are presented. Furthermore, the impacts of particle nonsphericity on downstream remote sensing implementations and radiative transfer simulations involving ice clouds and dust aerosols are also summarized.

Application of randomly oriented spheroids for retrieval of dust particle parameters from multiwavelength lidar measurements

Multiwavelength (MW) Raman lidars have demonstrated their potential to profile particle parameters; however, until now, the physical models used in retrieval algorithms for processing MW lidar data have been predominantly based on the Mie theory. This approach is applicable to the modeling of light scattering by spherically symmetric particles only and does not adequately reproduce the scattering by generally nonspherical desert dust particles. Here we present an algorithm based on a model of randomly oriented spheroids for the inversion of multiwavelength lidar data. The aerosols are modeled as a mixture of two aerosol components: one composed only of spherical and the second composed of nonspherical particles. The nonspherical component is an ensemble of randomly oriented spheroids with size‐independent shape distribution. This approach has been integrated into an algorithm retrieving aerosol properties from the observations with a Raman lidar based on a tripled Nd:YAG laser. Such a lidar provides three backscattering coefficients, two extinction coefficients, and the particle depolarization ratio at a single or multiple wavelengths. Simulations were performed for a bimodal particle size distribution typical of desert dust particles. The uncertainty of the retrieved particle surface, volume concentration, and effective radius for 10% measurement errors is estimated to be below 30%. We show that if the effect of particle nonsphericity is not accounted for, the errors in the retrieved aerosol parameters increase notably. The algorithm was tested with experimental data from a Saharan dust outbreak episode, measured with the BASIL multiwavelength Raman lidar in August 2007. The vertical profiles of particle parameters as well as the particle size distributions at different heights were retrieved. It was shown that the algorithm developed provided substantially reasonable results consistent with the available independent information about the observed aerosol event.

Nonspherical zodiacal dust particles driven by radiation pressure

The evolution of the orbit of a nonspherical dust particle under the action of solar electromagnetic radiation is presented here. We consider the equation of motion of uncharged, arbitrarily shaped dust particles. The size of particles is up to tens of microns. For large particles the relevant nonradial components of radiation pressure are taken from experimental data. Detailed numerical solution of the vectorial components of cross sections for micron-sized dust grains is based on the DDA method. The time it takes for a particle to spiral into the Sun or another central star is proportional to r eff α , where r eff is the effective radius of the particle. α equals 1 for spherical particles (the Poynting–Robertson effect) and α is < 1 for nonspherical grains. There is no systematic decrease or increase of the semi-major axis and eccentricity of the nonspherical particle.

Analysis of Optical Properties of Saharan Dust Derived From Dual-Wavelength Aerosol Retrievals From CALIPSO Observations

Since its first collection of data in June of 2006, the Cloud and Aerosol LIdar with Orthogonal Polarization (CALIOP) lidar instrument aboard the CALIPSO satellite has observed numerous Saharan dust events over West Africa and the Atlantic, predominantly from late May through early September. Due to CALIOP's sensitivity to polarization at 532 nm, the depolarization arising from scattering from the nonspherical dust particles serves as an independent means of discrimination between dust and other aerosol species. In addition to being an important and frequently observed class of aerosol, the Saharan dust provides an excellent basis for the evaluation of elastic-scatter lidar retrieval techniques. The Constrained Ratio Aerosol Model-fit (CRAM) method is one such technique, which applies aerosol modeling constraints in order to limit the ambiguity inherent in aerosol retrievals from elastic-scatter lidar. This letter, in addition to investigating the overall scattering properties of Saharan dust, highlights various retrieval approaches useful in determining the scattering properties of aerosols, as observed by the CALIOP instrument. Herein, we attempt to arrive at an improvement in the dust aerosol model used within CRAM.

Light scattering and absorption properties of dust particles retrieved from satellite measurements

We use the radiative transfer model and chemistry transport model to improve our retrievals of dust optical properties from satellite measurements. The optical depth and absorbing optical depth of mineral dust can be obtained from our improved retrieval algorithm. We find the nonsphericity and absorption of dust particles strongly affect the scattering signatures such as phase function and polarization at the ultraviolet wavelengths. From our retrieval results, we find the high levels of dust concentration occurred over most desert regions such as Saharan and Gobi deserts. The dust absorption is found to be sensitive to mineral chemical composition, particularly the fraction of strongly absorbing dust particles. The enhancement of polarization at the scattering angles exceeding 120 ∘ is found for the nonspherical dust particles. If the polarization is neglected in the radiative transfer calculation, a maximum 50 percent error is introduced for the case of forward scattering and 25 percent error for the case of backscattering. We suggest that the application of polarimeter at the ultraviolet wavelengths has the great potential to improve the satellite retrievals of dust properties. Using refined optical model and radiative transfer model to calculate the solar radiative forcing of dust aerosols can reduce the uncertainties in aerosol radiative forcing assessment.

Vertical distribution and optical properties of aerosols observed over Japan during the Atmospheric Brown Clouds–East Asia Regional Experiment 2005

Lidar and sky radiometer systems at Sapporo, Toyama, and Nagasaki, Japan, observed the vertical distributions and optical properties of (nonspherical) dust and spherical aerosol particles from March to May 2005 as part of the Atmospheric Brown Clouds–East Asia Regional Experiment 2005 (ABC‐EAREX2005). Sky radiometer observations suggest that single scattering albedo at Nagasaki was smaller than that at Toyama and Sapporo. Relationships between the single scattering albedo and Ångstrom exponent suggest that aerosol particles observed at Toyama in March differed from those observed in April and May. In contrast, at Nagasaki, there was no obvious difference in aerosol particles among the 3 months. Aerosol optical thicknesses observed by the sky radiometer resemble the aerosol optical thicknesses observed by lidar and simulated by the Chemical Weather Forecasting System (CFORS) model. A new parameter that describes the aerosol vertical distribution, Hm, is the modified scale height of the extinction coefficient. Hm can be used as an index of the vertical aerosol extent even if the detailed structure of the vertical profile cannot be shown. Hm observed by lidar was consistent with Hm simulated by the CFORS. Relationships between the aerosol optical thickness and Hm obtained by lidar measurements and CFORS simulations suggest that dust aerosol particles are generally transported over Japan at higher altitudes than are spherical aerosol particles.

Evaluation of Moderate‐Resolution Imaging Spectroradiometer (MODIS) Collection 004 (C004) aerosol retrievals at Kanpur, Indo‐Gangetic Basin

We have compared the spectral aerosol optical depth (AOD, τλ) and aerosol fine mode fraction (AFMF) of Collection 004 (C004) derived from Moderate‐Resolution Imaging Spectroradiometer (MODIS) on board National Aeronautics and Space Administration's (NASA) Terra and Aqua platforms with that obtained from Aerosol Robotic Network (AERONET) at Kanpur (26.45°N, 80.35°E), India for the period 2001–2005. The spatially‐averaged (0.5° × 0.5° centered at AERONET sunphotometer) MODIS Level‐2 aerosol parameters (10 km at nadir) were compared with the temporally averaged AERONET‐measured AOD (within ± 30 minutes of MODIS overpass). We found that MODIS systematically overestimated AOD during the pre‐monsoon season (March to June, known to be influenced by dust aerosols). The errors in AOD at 0.66 μm were correlated with the apparent reflectance at 2.1 μm (ρ*2.1) which MODIS C004 uses to estimate the surface reflectance in the visible channels (ρ0.47 = ρ*2.1/4, ρ0.66 = ρ*2.1/2). The large errors in AOD (Δτ0.66 &gt; 0.3) are found to be associated with the higher values of ρ*2.1 (0.18 to 0.25), where the uncertainty in the ratios of reflectance is large (Δρ0.66 ± 0.04, Δρ0.47 ± 0.02). This could have resulted in lower surface reflectance, higher aerosol path radiance and thus lead to overestimation in AOD. While MODIS‐derived AFMF has binary distribution (1 or 0) with too low (AFMF &lt; 0.2) during dust‐loading period, and ∼1 for the rest of the retrievals, AERONET showed range of values (0.4 to 0.9). The errors in τ0.66 were also high in the scattering angle range 110°–140°, where the optical effects of nonspherical dust particles are different from that of spherical particles.

Effect of radiation on dust particles in orbital resonances

The effect of electromagnetic radiation on the dynamics of arbitrarily shaped cosmic dust particles is investigated. The paper concentrates on the motion of dust grains near commensurability resonances with a planet—mean-motion resonances—and possible capture of the grains in the resonances. A particle is in resonance with a planet when the ratio of the mean motions of the two objects is a ratio of two small integers. The most fundamental properties of the orbital evolution of spherical dust particles in the mean-motion resonances are shortly rederived: the solar wind effect is also included and the existing result is improved. The results for spherical particles are compared with the detailed numerical calculations for nonspherical particles. It is shown that the fundamental results valid for spherical grains do not hold, in general, for nonspherical particles. While spherical particles are always characterized by the secular decrease of the semi-major axes near mean-motion resonances, this may not be true for nonspherical particles. Nonspherical grains may exhibit an increase of the semi-major axes before capturing in the mean-motion resonances. This is caused by the effect of electromagnetic radiation on nonspherical dust grains. The eccentricities of spherical particles in the exterior resonances approach a limiting value, but nonspherical grains may not follow this behaviour. The interior resonances are characterized by a systematic decrease of eccentricity for spheres, but various behaviours exist in the case of irregularly shaped particles. The motion of a nonspherical dust particle under the action of electromagnetic radiation may be characterized by a small change of the semi-major axis during a long-time interval, but the particle is not captured in any mean-motion resonance. This kind of motion does not exist for spherical grains.

Motion of dust near exterior resonances with planet

Motion of non-spherical cosmic dust particle under the action of electromagnetic radiation and gravitational forces of central star and a planet orbiting the star is investigated. The contribution concentrates on the problem if the particle can be trapped in commensurability resonances with the planet (a particle is in resonance with a planet when the ratio of their orbital periods is the ratio of two small integers). Trapping in the circular restricted three-body problem (planetary orbit is circle) with electromagnetic radiation coming from the star is considered. The effect of radiation plays an important role as for the possible capture in exterior resonances (particle's orbital period is larger than that for the planet): particle's optical properties not only define positions of the resonances, but also the capture times and evolution of orbital elements. Results for spherical and non-spherical particles are compared: physical difference between possibilities for resonant captures for spherical and non-spherical dust particles is pointed out.We concentrate on possible trapping of the grain in commensurability resonances – mean motion exterior resonances – with planet Neptune. The orbital evolution of non-spherical dust grains of radius ≈ 2 micrometers is numerically calculated.Capture of spherical grain in a resonance is characterized by decrease of semimajor axis outside the resonance. Non-spherical grain can exhibit also an increase of semimajor axis outside the resonance. This is the first case when a physically justified force – force given by interaction between electromagnetic radiation and non-spherical dust particle – can generate such a behaviour.

Nonspherical dust in exterior resonances with Neptune

Nonspherical dust grains orbiting the Sun are influenced by solar electromagnetic radiation. Interaction of electromagnetic radiation with nonspherical grains is complex and analogy with spherical grains may not be physically justified. As a consequence, equation of motion for nonspherical grain is more general than the equation of motion for spherical particle. Application of this more general interaction to possible trapping of the grain in resonances with planet Neptune is investigated. Approximation of the planar circular restricted three-body problem with action of solar electromagnetic radiation on nonspherical grain is used.The orbital evolution of nonspherical dust grains of radius 2 micrometers is numerically calculated. Attention is payed to exterior resonances with the Neptune, and the numerical experiments are concentrated on their possible high capture efficiency. Physical difference between possibilities for resonant captures for spherical and nonspherical dust particles is pointed out.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Extinction versus backscatter relationships for lidar applications at 351 nm: maritime and desert aerosol simulations and comparison with observations

Functional relationships linking at λ 0=351 nm aerosol extinction α λ 0 aer and backscatter coefficient β λ 0 aer of maritime and desert type aerosols are determined to allow for inversion of the single-wavelength lidar signals. Such relationships are derived as mean behavior of 20,000 extinction versus backscatter computations, performed for aerosol size distributions and compositions whose describing parameters are randomly chosen within the naturally observed variability. For desert-type aerosols, the effect of the particle non-sphericity is considered and it is shown that the extinction to backscatter ratio of non-spherical dust particles can be up to 60% larger than the values obtained for spherical particles. Aerosol extinction and backscatter coefficient profiles obtained inverting the single-wavelength lidar signal with the modeled relationships are then compared to the same profiles measured by a combined elastic-Raman lidar operating at 351 nm. Analytical back trajectories and satellite images are used to characterize advection patterns during lidar measurements and to properly choose the modeled functional relationship. A good accordance between the two techniques is found for advection patterns over the lidar site typical of maritime and dust conditions. Maximum differences between the model-based α λ 0 aer and β λ 0 aer vertical profiles and the corresponding ones measured by the combined elastic-Raman lidar technique are of 30% and 40% in maritime and desert dust conditions, respectively. The comparison of elastic-Raman lidar measurements and model-based results also reveals that particle non-sphericity must be taken into account when mineral dust-type aerosols are directly advected over the measurement site.

Pool of dust particles over the Asian continent: balloon-borne optical particle counter and ground-based lidar measurements at Dunhuang, China.

Measurements of aerosols were made in 2001 and 2002 at Dunhuang (40 degrees 00'N, 94 degrees 30'E), China to understand the nature of atmospheric particles over the desert areas in the Asian continent. Balloon-borne measurements with an optical particle counter suggested that particle size and concentration had noticeable peaks in super micron size range not only in the boundary mixing layer but also in the free troposphere. Super-micron particle concentration largely decreased in the mid tropopause (from 5 to 10 km; above sea level, a.s.l.). Lidar measurements made during August 2002 at Dunhuang suggested the possibility that mixing of dust particles occurred from near the ground to about 6 km even under calm weather conditions, and a large depolarization ratio of particulate matter was found in the aerosol layer. The top of the aerosol layer was found at heights of nearly 6 km (a.s.l.). It is strongly suggested that nonspherical dust particles (Kosa particles) frequently diffused in the free atmosphere over the Taklamakan desert through small-scale turbulences and are possible sources of dust particles of weak Kosa events that have been identified in the free troposphere not only in spring but also in summer over Japanese archipelago. Electron microscopic experiments of the particles collected in the free troposphere confirmed that coarse and nonspherical particles observed by the mineral particle were major components of coarse mode (diameter larger than 1 microm) below about 5 km over Dunhuang, China.

Motion of nonspherical dust particle under the action of electromagnetic radiation

Equation of motion of realistically shaped particle in the circumstellar dust shell is derived under the action of electromagnetic radiation including the gravity of central body. The effect is considered to the accuracy v → /c , where v → is particle's velocity in a given inertial frame of reference and c is the speed of light. Equation of motion is expressed in terms of particle's optical properties, standardly used in optics for stationary particles. Application to nonspherical dust particle in the Solar System with initial orbital elements identical to those of comet Encke is presented as an example. It is shown that the motion of nonspherical submicron- and small micron-sized particle may significantly differ from the motion for spherical particle of an identical volume.

Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements

Sensitivity studies are conducted regarding aerosol optical property retrieval from radiances measured by ground‐based Sun‐sky scanning radiometers of the Aerosol Robotic Network (AERONET). These studies focus on testing a new inversion concept for simultaneously retrieving aerosol size distribution, complex refractive index, and single‐scattering albedo from spectral measurements of direct and diffuse radiation. The perturbations of the inversion resulting from random errors, instrumental offsets, and known uncertainties in the atmospheric radiation model are analyzed. Sun or sky channel miscalibration, inaccurate azimuth angle pointing during sky radiance measurements, and inaccuracy in accounting for surface reflectance are considered as error sources. The effects of these errors on the characterization of three typical and optically distinct aerosols with bimodal size distributions (weakly absorbing water‐soluble aerosol, absorbing biomass‐burning aerosol, and desert dust) are considered. The aerosol particles are assumed in the retrieval to be polydispersed homogeneous spheres with the same complex refractive index. Therefore we also examined how inversions with such an assumption bias the retrievals in the case of nonspherical dust aerosols and in the case of externally or internally mixed spherical particles with different refractive indices. The analysis shows successful retrieval of all aerosol characteristics (size distribution, complex refractive index, and single‐scattering albedo), provided the inversion includes the data combination of spectral optical depth together with sky radiances in the full solar almucantar (with angular coverage of scattering angles up to 100° or more). The retrieval accuracy is acceptable for most remote sensing applications even in the presence of rather strong systematic or random uncertainties in the measurements. The major limitations relate to the characterization of low optical depth situations for all aerosol types, where high relative errors may occur in the direct radiation measurements of aerosol optical depth. Also, the results of tests indicate that a decrease of angular coverage of scattering (scattering angles of 75° or less) in the sky radiance results in the loss of practical information about refractive index. Accurate azimuth angle pointing is critical for the characterization of dust. Scattering by nonspherical dust particles requires special analysis, whereby approximation of the aerosol by spheres allows us to derive single‐scattering albedo by inverting spectral optical depth together with sky radiances in the full solar almucantar. Inverting sky radiances measured in the first 40° scattering angle only, where nonspherical effects are minor, results in accurate retrievals of aerosol size distributions of nonspherical particles.

Distinction of Coal Dust Particles from Liquid Droplets by Variations in Azimuthal Light Scattering

Abstract An experimental study was performed to determine the feasibility of optically distinguishing coal particles from water droplets for the purpose of reducing the mass concentration artifact from water sprays and steam reported for optical dust monitors. A single-particle multiangle optical detector was used to measure the scattering of monochromatic, circularly polarized light by mixtures of nonspherical bituminous coal dust particles and 0.5 μm dioctyl sebacate (DOS) droplets, which were used for modeling small, spherical water droplets. A total of 20 different mixtures with known coal number fractions ranging from 6 to 90 percent were measured by the detector using two data acquisition methods known as peak-detection and digital signal evaluation (DSE). Variability in azimuthal scattering by single particles, which is an indicator of the degree of sphericity, was used to discern the fraction of coal particles present in each aerosol mixture. Number concentrations of coal and DOS aerosols were mea...

Radiation forces and nonspherical dust particles

Action of electromagnetic radiation on nonspherical dust particles is discussed. It is stressed that the radiation pressure coefficientQ PR cannot be considered to be a scalar quantity, as it is used in all calculations for dynamical studies of interplanetary dust particles. Also the equation = (A - area of the particle) holds only for perfectly absorbing convex dust particle (Q PR = 1) and not even one of these two properties holds for interplanetary dust particles. Plane mirror is discussed in detail - all calculations can be done in this simple case.

Comet ejection and dynamics of nonspherical dust particles and meteoroids

This paper generalizes the formalism for calculating the ejection velocity of meteoroids and dust from comets and the forces to which such objects are subject in interplanetary space, including the dust tail of comets. It is found that spheres have the smallest cross section of any geometrical figures of the same valume averaged over random orientations, so for a fixed volume and mass, both the ejection velocity and beta reaches a minimum for bodies of spherical shapes. Flakes in random orientation are ejected near 70 percent of the highest ejection velocity for any orientation. Needles in random orientation escape a comet at nearly 90 percent of their maximum velocities. Randomly oriented cylinders of finite thickness escape at lower velocities that are slightly closer to their maximum velocities. The average beta acting on spin-aligned, perfectly absorbing needles is more than half that acting on a sphere of the same material and radius.

Evidence for Fragmentation of Strongly Nonspherical Dust Particles in the Tail of Comet West 1976 VI

Following Sekanina's (1976a,b) suggestion that striae (also known as “synchronic” or pseudosynchronic bands) observed in the dust tails of several comets could be products of fragmentation of friable dust particles ejected from the nucleus, we have completed an investigation of these structures in Comet West 1976 VI (Sekanina and Farrell 1979, detailed paper in preparation), which shows that the dynamical solutions, provided by the fragmentation model on the assumption that solar attraction and radiation pressure are the only forces involved, are indeed in agreement with the observed motions of 16 striae through the tail over a time interval of more than three days. The match is so good that the probability of its being a coincidence of some sort must be virtually nil.