Dust aerosol forward scattering effects on ground-based aerosol optical depth retrievals

Monte Carlo radiative transfer calculations are performed to examine the forward scattering effects on retrievals of dust aerosol optical depth (AOD) from ground-based instruments. We consider dust aerosols with different AOD, effective radius and imaginary refractive index at 0.5 μm wavelength. The shape of dust aerosols is assumed to be spheroids and the equivalent spheres that preserve both volume and projected area (V/P) are also considered. The single-scattering albedos and asymmetry factors of spheroids and V/P-equivalent spheres have small differences, but the scattering phase functions are very different for the scattering angle range ∼90-180°. The relative errors of retrieved AOD caused by forward scattering effects due to the differences between the single-scattering properties of spheroids and spheres are similar. It is shown that at solar zenith angle (SZA) smaller than ∼70° the effect of the forward scattering is generally small although the relative errors in retrieved AOD can be as large as −10% when r_e=2. However, the largest relative errors, which can reach −40%, appear at high SZA (>∼70°) with AOD larger than 1. This is not caused by the increase of forward scattering intensity, but is due to the strong attenuation of solar direct beam.     

Thermophoretically dominated aerosol coagulation

A theory of aerosol coagulation due to size-dependent thermophoresis is presented. This previously overlooked effect is important when local temperature gradients are large, the sol population is composed of particles of much greater thermal conductivity than the carrier gas, with mean diameters much greater than the prevailing gas mean free path, and an adequate "spread" in sizes (as in metallurgical mists or fumes). We illustrate this via a population-balance analysis of the evolution of an initially log-normal distribution when this mechanism dominates ordinary Brownian diffusion.     

基于气溶胶光学厚度反演大气气溶胶尺度分布

介绍一种基于气溶胶光学厚度测量反演大气气溶胶尺度分布的新方法. 从实测气溶胶光学厚度出发，依据严格的Mie散射理论，将气溶胶尺度分布函数离散，采用线性回归法确定气溶胶尺度分布. 还通过对多重共线性的讨论，确定了用于反演气溶胶尺度分布的光谱波段.     

Nucleation of atmospheric aerosol particles

A significant fraction of the total number of particles present in the atmosphere is formed originally by nucleation from the gas phase. Binary nucleation of sulphuric acid and water, ternary nucleation of sulphuric acid, water and ammonia and ion-induced nucleation are thought to be the most important aerosol nucleation processes in the atmosphere. Within the last two decades, instrumentation to observe and characterize nucleation has improved greatly and numerous observations of nucleation have been made including quantification of the nucleation rate, characterization of the growth process and first chemical characterizations of the freshly formed particles. Nucleation has been observed at many different places in the atmosphere: in the boundary layer, in the free troposphere, in remote locations, in coastal areas, in boreal forests as well as urban areas and pollution plumes. In most cases gaseous sulphuric acid is assumed to be the key precursor gas. After nucleation, other supersaturated substances, especially low vapour pressure organics often take part in the subsequent aerosol growth. Iodine oxides seem to be responsible for nucleation observed in some coastal areas.Recent advances in modelling allow for a kinetic treatment of the nucleation process based on measured thermochemical data for the cluster formation. Considerable improvement over the classical nucleation treatment is expected from this approach.A detailed understanding of atmospheric aerosol nucleation processes is needed as the freshly formed particles directly influence the number concentration and size distribution of the atmospheric aerosol. The formation of clouds and precipitation is affected and influences on climate are anticipated. Anthropogenic emissions influence atmospheric aerosol nucleation processes considerably.Despite the comprehensive research efforts, substantial inconsistencies remain and conflicting results of laboratory studies, model studies as well as atmospheric observations persist. Several key questions about the predictability of atmospheric nucleation in general, about the substances, that take part in nucleation and subsequent growth and about the size and composition of the critical cluster, have not been resolved so far. To cite this article: J. Curtius, C. R. Physique 7 (2006).     

Design and evaluation of a unipolar aerosol charger to generate highly charged micron-sized aerosol particles

In this study, a unipolar charger for generating highly charged microparticles was designed and its performance was evaluated both theoretically and experimentally. The measured particle charge number and corona current of the charger were in good agreement with the theoretical results from FLUENT. The experimentally determined average particle charge number of 1 μm PSL under an applied voltage of 8 kV was 128, which agreed well with the theoretically predicted and simulated values of 118 and 121, respectively. Computational calculations revealed the average charge of 10 μm particles to be 7560 at an applied voltage of 8 kV.     

Evaluation of an electrical aerosol detector (EAD) for the aerosol integral parameter measurement

The performance of an electrical aerosol detector (EAD; TSI Model 3070A) was experimentally evaluated for measuring the integral parameters of particles (i.e., total length concentration of particles, and the total surface area concentrations of particles deposited in a human lung). The EAD consists of a unipolar diffusion charger with an ion trap, and aerosol electrometer. We first evaluated the performance of the EAD charger. Both polydisperse and monodisperse particles of Ag, NaCl, and oleic acid (with the dielectric constants of infinite, 6.1 and 2.5) were then generated to evaluate the particle material effect on the EAD readout.     

Preliminary results of a non-spherical aerosol method for the retrieval of the atmospheric aerosol optical properties

There is experimental evidence that the non-sphericity of certain atmospheric particles can cause scattering properties different from those predicted by standard Mie theory. Numerous studies indicate the need to consider the presence of non-spherical particles in modeling the optical properties of atmospheric aerosols. On the other hand, natural aerosols show a great variety of shapes, making difficult a realistic choice of a particle shape (or shape mixture) model. In this paper, we test a parameterization of the particle shape in the retrieval of size distribution, phase function, single scattering albedo and asymmetry parameter from direct and sky-radiance measurements. For this purpose we have substituted the Kernel based on the Mie theory included in the model SKYRAD.PACK by one derived for non-spherical particles. The method is applied under different atmospheric conditions, including Saharan dust outbreak, polluted and local mineral episodes. We compare the results with those obtained by the well known spheroids algorithm used in the AERONET network.     

Toward unified satellite climatology of aerosol properties: Direct comparisons of advanced level 2 aerosol products

The development of a unified satellite climatology of aerosol properties requires accurate quantification and deep understanding of the underlying factors contributing to discrepancies between individual satellite products. In this paper we compare the most recent level 2 results obtained for coincident pixels viewed at essentially the same time by the Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) instruments flown on the EOS Terra platform. This strategy eliminates potential sampling effects and provides a virtually direct comparison of spatially and temporally collocated MODIS and MISR retrievals. We show that the MODIS and MISR Ångström exponent datasets reveal essentially no correlation. Although the corresponding aerosol optical thickness (AOT) datasets can agree worse than expected over the oceans, still the agreement is often satisfactory. However, the agreement over the land is often poor or even unacceptable. Of the collocated pixels for which there is a MODIS aerosol retrieval, only ∼40% or fewer pixels have a MISR aerosol retrieval, and vice versa. These findings further illustrate the complexity of the problem of aerosol retrievals from satellite observations and indicate that the creation of a meaningful unified MODIS-MISR aerosol climatology will be a nontrivial task.     

Dissipative instability in an aerosol plasma

Dissipative instability in a weakly ionized aerosol plasma has been studied with allowance for a finite electric conductivity of the medium, electron and ion diffusion, and friction of the aerosol component against a neutral gas. Instability is caused by the relative drift of the aerosol and ion components. Estimates of the basic parameters of instability (threshold, characteristic wavelengths, and increments) in experiments with dust crystals indicate that this instability can be an important additional factor upon the formation of regular structures in an aerosol plasma.     

Electrostatic characterisation of corona-wire aerosol chargers

This paper investigates the electrostatic properties of aerosol corona-wire diffusion chargers. The most important parameters that determine charging levels of particles at the outlet of such devices are the ion concentration, $N_{\mathrm{i}}$, and the mean residence time of the aerosol in the charging zone, t. A method to determine the exact $N_{\mathrm{i}}t$ product is presented and used for the characterization of a Hewitt-type corona charger. It is shown that ignoring the space charge effect leads to significant errors when the ion concentration is greater than $5\times {10}^{13}\mathrm{ions}/{\mathrm{m}}^3$. A numerical model developed to describe the performance of the device in more detail is used to predict the behavior of the charger when some geometric characteristics are changed. Estimations of ion penetration levels through the inner electrode based on the numerical model show better agreement with the experimental results at sub-atmospheric pressures. Finally, calculations of the average $N_{\mathrm{i}}t$ product appear to increase with pressure despite the fact that the ionic concentration is significantly lower.     

Discrete-element modeling of particulate aerosol flows

A multiple-time step computational approach is presented for efficient discrete-element modeling of aerosol flows containing adhesive solid particles. Adhesive aerosol particulates are found in numerous dust and smoke contamination problems, including smoke particle transport in the lungs, particle clogging of heat exchangers in construction vehicles, industrial nanoparticle transport and filtration systems, and dust fouling of electronic systems and MEMS components. Dust fouling of equipment is of particular concern for potential human occupation on dusty planets, such as Mars. The discrete-element method presented in this paper can be used for prediction of aggregate structure and breakup, for prediction of the effect of aggregate formation on the bulk fluid flow, and for prediction of the effects of small-scale flow features (e.g., due to surface roughness or MEMS patterning) on the aggregate formation. After presentation of the overall computational structure, the forces and torques acting on the particles resulting from fluid motion, particle–particle collision, and adhesion under van der Waals forces are reviewed. The effect of various parameters of normal collision and adhesion of two particles are examined in detail. The method is then used to examine aggregate formation and particle clogging in pipe and channel flow.     

Environmental implications of acoustic aerosol agglomeration

An overview is presented of acoustically induced agglomeration of fine particulates, potential industrial applications of the technology, and its environmental implications in terms of fine particle pollutants. Adverse health effects due to exposure to fine aerosols are discussed as well as recent legislation to reduce the output of such emissions. Based on this, the need for new, more efficient particle filtration technologies is demonstrated. It is shown that acoustic aerosol preconditioning meets all the requirements to reduce the fine particle output of conventional filter systems. The results of laboratory scale experiments are presented to illustrate the underlying mechanisms of the acoustic agglomeration process, while data from pilot scale testing are shown to prove the effectiveness of acoustic agglomeration systems in reducing the fine particle content of aerosols. Other filtration technologies are compared with acoustic agglomeration equipment.     

Semiconductor nanostructures enabled by aerosol technology

Aerosol technology provides efficient methods for producing nanoparticles with well-controlled composition and size distribution. This review provides an overview of methods and results obtained by using aerosol technology for producing nanostruetures for a variety of applications in semiconductor physics and device technology. Examples are given from： production of metal and metal alloy particles： semiconductor nanoparticles; semiconductor nanowires, grown both in the aerosol phase and on substrates; physics studies based on individual aerosol-generated devices; and large area deviees based on aerosol particles.