The use of the finite-element method for calculating the photophoresis velocity of large aerosol particles

The finite-element method has been employed to calculate the photophoresis velocity of solid aerosol particles, the sizes of which are much larger than the mean free path of molecules in a gas. The thermal electromagnetic radiation from the particle surface and the temperature dependences of the density, viscosity, and thermal conductivity of the gaseous medium and particle material have been taken into account. The photophoresis velocity has been numerically calculated for a number of axially symmetric particles moving along their rotation axes. Cylindrical particles, particles having a shape resulting from rhomb rotation around one of its diagonals, and spheroidal particles have been considered.     

The use of the finite element method for calculating the thermophoresis velocity of large aerosol particles

The finite element method has been employed to calculate the thermophoresis velocity of solid aerosol particles, the sizes of which are much larger than the mean free path of molecules in a gas. The thermophoretic velocities of axially symmetric particles moving along their rotation axes have been numerically calculated. Cylindrical particles, particles having a shape resulting from rhomb rotation around one of its diagonals, and spheroidal particles have been considered. The data obtained for spheroidal particles have been compared with the available results of analytical calculations.     

The photolysis of 3,4-dimethylcyclopentanone

The photolysis of trans-3,4-dimethylcyclopentanone has been studied in the gas phase, principally at 313 nm. However, a few experiments have also been performed using laser sources at 308 and 325 nm. Additionally, experiments were also carried out using the cis isomer. The major products produced by all three wavelengths and in the temperature range 100 to 150°C were propene, 1,2-dimethylcyclobutane, carbon monoxide, and 3,4-dimethylpent-4-en-al. The formation of the cyclobutane was stereospecific and the effects of temperature, pressure, and wavelength on the relative product yields could be rationalized in terms of a mechanism involving the formation of a vibrationally excited triplet state which could yield both hydrocarbons and the aldehyde and a nonexcited triplet yielding only the aldehyde. Some high intensity experiments with an exciplex laser at 308 nm gave results which could be due to the occurrence of some two photon absorption by the cyclopentanone or absorption by an excited intermediate. The results are compared with those previously reported for other substituted cyclopentanones.     

气溶胶粒子通过填充柱的保留时间分布测定

采用亚微米单分散聚苯乙烯球形硬气溶胶粒子和脉冲进样技术,测定了气溶胶粒子通过无规则石英砂填充柱的保留时间分布,从保留时间分布曲线得到了气溶胶粒子在填充柱中的平均保留时间和穿透率.研究了平均保留时间和穿透率与流体流速、填充柱的长度、填料粒度和气溶胶粒子大小之间的关系.研究发现,流速越大,保留时间分布曲线越尖锐,流速越小,保留时间分布曲线越平坦;气溶胶粒子的穿透率随着柱长的增加而降低,随流速、气溶胶粒子粒径和石英砂颗粒大小的减小而减小;平均保留时间随柱长增加而增大,随流速增大而减小,随气溶胶粒子粒径减小而减小,而与石英砂颗粒大小几乎无关.     

Risk assessment of TiO2 photocatalyst by individual micrometer-size particle analysis with on-site combination of SEM-EDX and SR-XANES microscope

Applications of synchrotron radiation X-ray fluorescence (SR-XRF) microscopy combined with scanning electron microscopy (SEM) are reported. Electron beam excited and synchrotron radiation induced X-ray emission spectra of the same yellow sand single particles are reported and compared. The Ti-K edge absorption fine structure of single microparticles of TiO₂ (rutile, anatase, and a photocatalyst aerosol) are recorded by using monochromatic synchrotron radiation of tunable energy. It is shown that the discrimination between rutile and anatase is possible. Based on the single particle speciation, the toxicity of photocatalyst aerosol powder is discussed.     

Chemical reactivity of two-photon excited trimethylsilylacetylene in aerosol particle formation with acrolein

Under irradiation with N₂ laser light, a gaseous mixture of trimethylsilylacetylene (ethynyltrimethylsilane) (TMeSiA) and acrolein (AC) produced sedimentary aerosol particles with a mean diameter of 1.0 μm. Nucleation process of the aerosol particles was studied by measuring monitor (He–Ne laser) light intensity scattered by the aerosol particles as formed under N₂ laser light irradiation. With increasing partial pressure of TMeSiA, the nucleation reaction of aerosol particles was accelerated due to additional generation of reactive species from TMeSiA molecules by a two-photon process. FT-IR spectra of the sedimentary particles showed that TMeSiA molecules were incorporated into polymerization reaction of AC by forming –Si–O–C– bond from R(CH₃)₂Si radicals. Two-photon processes of both AC and TMeSiA molecules under N₂ laser light irradiation were briefly discussed.     

Vibrational-translational/rotational and vibrational-vibrational processes in methane/inert-gas mixtures: Optoacoustic phase measurements

We consider the model with kinetic excitation into the quasicontinuum (KEQ) for resonant polyatomic molecules which absorb laser radiation and are surrounded by buffer molecules. KEQ takes place when the resonant molecules in the lower part of the energy spectrum interact weakly with the laser radiation, but the molecules in the quasicontinuum are rapidly excited to still higher energy and dissociate. Under these conditions the collisions of the resonant and buffer molecules lead to excitation of resonant molecules into the quasicontinuum because the population of the quasicontinuum is much less than its thermodynamical equilibrium value. It is found, that the smaller the V-T relaxation time τ_VT, the larger the rate of KEQ and the dissociation rate (if only τ_VT is not too small). Thus, if we change the experimental conditions and decrease τ_VT (for instance, by passing from the heavy buffer gas Xe to the light buffer gas He), for some resonant molecules we may observe that the probability of dissociation increases.     

Ultraviolet photolytic vapor generation from particulate ensembles for detection of malathion residues in aerosols

A method to selectively generate vapor signatures from malathion entrained within matrices of surface-impacted aerosol particles has been demonstrated. The method uses ultraviolet radiation (172 or 222 nm) from a continuous wave discharge lamp to photodissociate malathion molecules collected within and on surface-impacted particles, followed by detection via ion mobility spectrometry (IMS). Since surface heating does not occur, only those molecules whose photofragments exhibit high vapor pressure are introduced into the IMS instrument and then only those exhibiting high proton affinity are subsequently detected. This process produces less signal clutter than in pyrolysis-IMS, where the background aerosol is pyrolyzed along with the sample. Quantities of malathion as small as 50 ng could be detected when the malathion was entrained on a clean surface, and as small as 100 ng when co-entrained on a surface with much larger quantities of background aerosols such as diesel soot, road dust, Bacillus globigii, albumin, and cotton lint. This sensitivity indicates that, when combined with a particle collector as an effective pre-concentrator, detection of malathion aerosol concentrations of <0.01 mg/m³ will be possible. Since malathion can be viewed as a model compound, this technique is also extendable to the detection of organophosphate war chemicals.     

Cluster size effect on electron-induced luminescence

As a result of combined measurements of radiation intensity, electronically excited particles density and the density of mass-separated ions formed by a collision of electrons with CO₂, N₂O, H₂O molecules and clusters in intersecting beams it has been found that the process of ejection of electronically excited particles from clusters forms a main contribution to electron-induced luminescence in a mass-spectrum of molecular, fragmented and microcluster ions. The value of this contribution depends on the cluster sizeN. The enhancement of electron-induced luminescence by small clusters (N ? 15) has been found.     

Nanoparticle formation using a plasma expansion process

We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.     

Relaxation of highly vibrationally excited molecules

The rate constant for V-V relaxation of diatomic homonuclear molecules is determined from collisions of unexcited molecules with molecules near the dissociation threshold. It is shown that a quasi-resonant transition through several levels dominates in this process so that the energy difference between the initial and final states of the excited molecule is approximately equal to the transition energy from the zero level to the first one. The relaxation kinetics of excited molecules is studied. Absorption of IR radiation by polyatomic molecules is discussed taking into account collisions. A criterion for the negligibility of energy loss is obtained, and the dissociation rate of molecules under the action of IR laser irradiation found. The computational results are compared with experimental data. A self-consistent procedure is formulated for a gas irradiated by a quasi-continuously operating IR laser, in order to determine simultaneously the dissociation rate, dissipation energy flux and temperature. The existence of an optimum radiation region for dissociation is found.     

Infrared optical constants of crystalline sodium chloride dihydrate: application to study the crystallization of aqueous sodium chloride solution droplets at low temperatures

Complex refractive indices of sodium chloride dihydrate, NaCl·2H(2)O, have been retrieved in the 6000-800 cm(-1) wavenumber regime from the infrared extinction spectra of crystallized aqueous NaCl solution droplets. The data set is valid in the temperature range from 235 to 216 K and was inferred from crystallization experiments with airborne particles performed in the large coolable aerosol and cloud chamber AIDA at the Karlsruhe Institute of Technology. The retrieval concept was based on the Kramers-Kronig relationship for a complex function of the optical constants n and k whose imaginary part is proportional to the optical depth of a small particle absorption spectrum in the Rayleigh approximation. The appropriate proportionality factor was inferred from a fitting algorithm applied to the extinction spectra of about 1 μm sized particles, which, apart from absorption, also featured a pronounced scattering contribution. NaCl·2H(2)O is the thermodynamically stable crystalline solid in the sodium chloride-water system below the peritectic at 273.3 K; above 273.3 K, the anhydrous NaCl is more stable. In contrast to anhydrous NaCl crystals, the dihydrate particles reveal prominent absorption signatures at mid-infrared wavelengths due to the hydration water molecules. Formation of NaCl·2H(2)O was only detected at temperatures clearly below the peritectic and was first evidenced in a crystallization experiment conducted at 235 K. We have employed the retrieved refractive indices of NaCl·2H(2)O to quantify the temperature dependent partitioning between anhydrous and dihydrate NaCl particles upon crystallization of aqueous NaCl solution droplets. It was found that the temperature range from 235 to 216 K represents the transition regime where the composition of the crystallized particle ensemble changes from almost only NaCl to almost only NaCl·2H(2)O particles. Compared to the findings on the NaCl/NaCl·2H(2)O partitioning from a recent study conducted with micron-sized NaCl particles deposited onto a surface, the transition regime from NaCl to NaCl·2H(2)O is shifted by about 13 K to lower temperatures in our study. This is obviously related to the different experimental conditions of the two studies. The partitioning between the two solid phases of NaCl is essential for predicting the deliquescence and ice nucleation behavior of a crystalline aerosol population which is subjected to an increasing relative humidity.     

Kinetic approach to chemical reactions and inelastic transitions in a rarefied gas

A kinetic approach is presented for the analysis of a gas mixture with two kinds of nonconservative interactions. In a bimolecular chemical reaction, mass transfer and energy of chemical link arise, and in inelastic mechanical encounters, molecules get excited or de‐excited due to their quantized structure. Molecules undergo transitions between energy levels also by absorption and emission of photons of the self‐consistent radiation field. From the kinetic Boltzmann‐type equations, the problem of equilibria and of their stability is addressed. A detailed balance principle is proved and a Lyapunov functional is constructed; mass action law and Planck's law of radiation are recovered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Untersuchung der Flammenemission von Metallhalogeniden und deren Nutzung zur Analytik der Halogene

Departing from the until now well known A and B systems of the indium and gallium halide emissions the authors tried to find further spectra of molecule bands in flames. The following band systems were observed: InF¹ (A, B and C systems), GaF¹ (A, B and C systems), A1F¹ (A system), T1C1¹, TlBr¹, TII¹ and the C systems of the GaCl, GaBr, InCl and InBr molecules.From the behaviour of the intensities of excited and nonexcited MeX species and the existence of InF-C bands (excitation energy 5.30 eV) in the hydrogen-nitrogen diffusion flame it could be derived that the excited state results from the reaction of metal and halogen atoms. Furthermore the experimental results of MeX intensity measurements indicate reactions of excited molecules with particles of flame gases. This is especially valid for the A and B systems of the indium and gallium halides (triplet states) which have as a result of the forbidden transitions a relatively long life time. Using other gas mixtures (Hr-air, propane-air) and observation of CuX and alkaline earth halide bands as well gave the possibility to set up a block diagram of optimum conditions for measuring the different MeX emissions.Examination of the analytical performance gave detection limits of 0.7 $\text{mg}\text{l}$ for F^?1 and about 4 $\text{mg}\text{l}$ for Cl^?, Br^? and I^? by means of InX emissions. The limits of the concentration ratios for the determination of traces were 1:640 (Cl^? in HBr) up to 1:60,000 (I^? in HF). The best determinations of other halogens are possible in HC1 and HF matrices.     

Aerosol Catalysis: the Influence of Particle Structure on the Catalytic Activity of Platinum‐Nanoparticle Agglomerates

The structure of nanoparticle agglomerates can have substantial influence on their catalytic activity, as shown here for the oxidation of hydrogen on platinum nanoparticles. The structure of aerosol agglomerates was varied by thermally induced rearrangement of the so‐called primary particles, which were ca. 5 nm in size. In this way, the fraction of outer surface, which is directly accessible for molecules from the gas phase, was varied from a very open agglomerate structure to massive spheres. A Monte‐Carlo (MC) simulation of the surface phenomena was carried out parallel to the experiments, taking into account models for reactions including adsorption, surface diffusion, and desorption. Comparison of the experimental results with these MC simulations indicated that, for gas‐borne nanoparticles, special features appear. For instance, the time scales of experiments and simulations are not identical. This discrepancy can be explained by altered adsorption kinetics on the nanoparticles compared to the kinetics on bulk surfaces, which was introduced into the MC simulation. The assumption of a lower sticking probability for molecules impinging from the gas phase as proposed before in other investigations leads to a shift in the time scale of the MC simulation as well as an increased sticking probability for O‐atoms relative to the H‐atom sticking probability. In addition, the surface‐normalized catalytic activity, given by the turn‐over rate (TOR), is higher for 5‐nm than for 50‐nm particles. Thus, the combination of experiments and simulation may be a useful tool to gain deeper insight into the influence of the properties of catalyst particles on the catalytic activity, whereby the simulation covers the subsecond time range, which is hardly accessible by experimentation.     

Water adsorption around oxalic acid aggregates: a molecular dynamics simulation of water nucleation on organic aerosols

The phase behaviour of binary oxalic acid-water mixtures has been investigated by means of computer simulation techniques. Such mixtures play an important role in atmospheric processes, since the hydrogen bonding ability of oxalic acid molecules allows them to form aerosol particles. Water can in turn be readily adsorbed on the surface of such aerosol particles, which results in the formation of small ice grains. These grains are thus considered to be acting as cloud condensation nuclei, giving rise to the formation of ice clouds.     

Kinetics of supersaturated vapor nucleation on molecular condensation nuclei

The kinetics of nucleation is calculated for a supersaturated vapor containing molecular condensation nuclei, that is, foreign molecules able to induce the formation of viable nuclei of a condensed phase by themselves. In contrast to the previous calculation, the possibility of the escape of molecular condensation nuclei from very small clusters containing a few condensed vapor molecules is taken into account. More exact equations are derived for the rate of steady-state nucleation and the concentration of aerosol particles in a quasisteady-state regime of nucleation. The calculation demonstrates that, at a high probability of the escape of a molecular condensation nucleus, the predominating mechanism of cluster formation is the attachment of a molecular condensation nucleus to a cluster formed from vapor molecules rather than their condensation on the nucleus. At the same time, allowances for the possible escape of molecular condensation nuclei from clusters slightly affect the rate of nucleation and the concentration of aerosol particles being formed.     

Efficiency of inertial deposition of aerosol particles in fibrous filters with regard to particle rebounds from fibers

The inertial deposition of submicron aerosol nanoparticles onto fibers during gas filtration through fine-fiber filters is considered. It is shown that there is critical filtration velocity U* below which the energy loss upon collisions has no influence on the filtration efficiency. Above this critical velocity, the filtration efficiency depends on the mechanism of the inelastic energy loss and can be noticeably lower than the result of its estimation with no allowance for the particle rebound. For a rather dense fibrous medium, when not all particles that have rebounded from a fiber have time to attain the flow velocity before the next collision with another fiber, the filtration efficiency depends on the velocity distribution of the rebounding particles. It is shown that, in this case, the filtration efficiency must increase with the packing density of a filter.     

Gas flow dynamics of an inductively coupled plasma discharge

Temperature and velocity distributions within a low power (0.5–0.75 kW) inductively coupled plasma discharge operating in argon have ben determined for various operating parameters including aerosol gas now rate, plasma gas flow rate, aerosol nozzle diameter, and input power level. All measurements were made without samples. The channel formed in the discharge by the aerosol gas now exhibits temperature and velocity characteristics which distinguishes it from the plasma core and wall regions. Maximum plasma temperature of 8700 ± 300 K and channel gas velocity of 120 $\text{m}\text{s}$ were found.