Aerosol Condensation Research Articles

Counting Efficiency of an Improved 30-Å Condensation Nucleus Counter

The ultrafine aerosol condensation nucleus counter (UFACNC) developed by Stolzenburg and McMurry (1990) has been improved. This new instrument can detect particles as small as 30 Å in nitrogen at a sample flow rate of 2.5 cm3·s−1. This flow rate is five times that of the earlier design with the same particle counting efficiency. An increased sample flow rate causes an increase in the arrival rate of detected particles. This improves accuracy in estimating the true particle concentration in ultraclean gas systems. In order to demonstrate its high accuracy the new UFACNC was used to measure the concentration of contaminant particles in ultrafiltered nitrogen. This represents the first accurate determination of ultrafine contamination levels in large ultraclean gas systems. The nitrogen was found to contain as few as 1.28 × 10−4 particles/cm3 (3.62/ft3). The higher volumetric flow rate of the new counter reduced the sample standard deviation of the measured count rate. Therefore, in the case of a low particulate count rate, the difference between the measured and instrument noise background count rates could be more accurately resolved. In addition, the new UFACNC was used to demonstrate a new technique for size classifying ultrafine contaminant particles using only a UFACNC, i.e., without a separate size classifier. The results demonstrate that ultrafine particle size distributions can be obtained directly from a stand-alone UFACNC

Conditions for monodispersity of heterogeneous condensation aerosols using dimensionless groups

Correlations between dimensionless groups describing the conditions of convective heat transfer in a condenser are developed from the operating parameters of heterogeneous condensation aerosol generators found in the literature. Seventeen different condenser systems were analyzed including seven different condensate materials and three different carrier gases. Almost all the condensers were operated within the region of laminar mixed convection for optimized monodispersity. The correlations are useful for condenser design and system optimization for monodisperse continuous flow condensation aerosol generators.

Simulation of multicomponent aerosol condensation by the moving sectional method

The moving sectional method is extended to simulate multicomponent aerosol dynamics resulting from condensation/evaporation processes. This method uses a Lagrangian approach in which section boundaries and component masses in a section vary according to the characteristics of condensation/evaporation rates while conserving number concentration in a section throughout the simulation. Simulation of model problems for which new analytical solutions have been obtained shows excellent agreement with the analytical solutions. Limitations and applicability of the sectional method are discussed.

Dynamic Behavior of Condensation and Evaporation of Polydisperse Volatile Aerosols

It is demonstrated that, at room temperature and relatively low supersaturation, Stefan flow is relatively unimportant for condensational growth and evaporation of polydisperse volatile aerosols. Nevertheless, the latent heat effect on droplet temperature must be calculated as the use of isothermal condition may overpredict the growth or evaporation process by orders of magnitude in total aerosol mass. For nonisothermal Ostwald ripening process, the latent heat involved in the condensation/evaporation process creates a time delay effect on the asymptotic behavior of aerosol. Yet the aerosol system behaves according to the classical theory of Lifshitz, Slyozov, and Wagner.

Studies on sampling methods for PCDDs and PCDFs in stack emission

Polychlorinated dibenzo- p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in stack emission exist both as vapor and sorbed onto the surface of particles. The partition between the two phases is of a dynamic nature and changes rapidly with temperature. When monitoring stack emissions, a sampling method must be used that efficiently collects both phases as well as any condensation aerosol formed in the sampling train. In this study, the two sampling methods used in Sweden, the cooled probe polyurethane foam plug (PUFP) sampling train and the sampling train recommended by the Swedish Environmental Protection Board has been tested for particle collection ability. The results show that an aerosol filter has to be introduced after the condensor in order to collect particle bound PCDDs and PCDFs efficiently.

Experimental Study of Submicrometer and Ultrafine Particle Penetration and Pressure Drop for High Efficiency Filters

The fractional penetration of submicrometer particles through five high-efficiency glass fiber filters, one composite fiber filter, and two membrane filters was measured for particles of 0.004–0.42-μm diameter at filter face velocities ranging from 0.5 to 20 cm/s. The glass fiber filters all had approximately the same thickness and weight per unit area, and were rated 93% to 99.999% efficient using the conventional 0.3-μm dioctyl phthalate (DOP) test. The challenge aerosols were electrically classified monodisperse DOP in the diameter range of 0.032–0.42 μm, and polydisperse silver condensation aerosols having diameters of ∼ 0.004–0.01 μm. Aerosol penetration through these media was found to be generally consistent with current theory for collection by diffusion and interception over the particle size and velocity range studied. Using a filter figure of merit calculated for penetration by 0.1- and 0.3-μm particles to facilitate comparison, all of the filters except one tetrafluoroethylene membrane filter ...

Developments in 30 angstrom particle detection for ultraclean gas systems

The ultrafine aerosol condensation nucleus counter (UFACNC) developed by Stolzenburg and McMurry, 1990 has been improved. The improved instrument can detect particles as small as 30 Angstroms in nitrogen at a sample flow rate of 2.5 cm 3.sec. −1. This flow rate is five times that of the earlier design. The increased sample flow rate provides an increase in the arrival rate of detected particles. The result is an improved accuracy in estimating the true particle concentration in ultraclean gas systems. The improved UFACNC was used to measure the concentration of ultrafine contaminant particles in two ultraclean nitrogen systems. The nitrogen was found to contain as few as 1.28 × 10 −4 particles per cm 3 (3.62 per cubic foot). The higher flow rate of the improved counter resulted in a relatively small standard deviation of the measured particle concentration. Therefore, the small difference between the process and instrument noise background count rates was accurately resolved. The improved UFACNC was also used to demonstrate a new technique for size classifying ultrafine particles using only a UFACNC, i.e., without a separate size classifier. The results demonstrate that particle size distributions can be obtained directly from a stand-alone UFACNC.

A monitor for atmospheric sulphur dioxide, based on enrichment of SO2 by a polydispersive water aerosol

A monitor for continuous analysis of sulphur dioxide in the atmosphere at the ppb v/v level (1 ppb v/v = 2.62 μg/m3 SO2) is described. The apparatus operates on the principle of equilibrium accumulation of sulphur dioxide from the air by a polydispersive water aerosol which continuously transfers SO2 from an air-flow of l/min into microlitre volumes of water condensate. High sensitivity (1 ppb v/v), low relative error (± 5% at 4 ppb v/v SO2), high selectivity (CO2 does not interfere at a concentration of 2 × 103 ppm v/v, interference by NOx and H2S is acceptably low), and low response delay (10 s) are provided by a compact coupling of the enrichment procedure with conductivity detection of SO2 in the film of water aerosol condensate formed directly on the wire-gauze sensor. The reliability of the method has been studied under simulated conditions, with spectrophotometric method as reference. The analyser is computer-controlled, and the detector response is processed on-line and displayed (as μg/m3) in real-time on a screen or is transmitted telemetrically to a control centre. It is portable and suitable for use in both stationary and moving locations.

Wall and aerosol condensation during cooled laminar tube flows

The equations for heat and mass transfer in laminar flow are presented and their solutions are considered in the limit of low vapor density and constant thermodynamic properties. Simple ways of determining the maximum and minimum possible wall condensation are discussed. Numerical calculations between these limits are performed by orthogonal collocation. For dibutyl phthalate vapor in air, latent heat release is negligible and most of the vapor condenses das aerosol. New results are presented for water vapor, including the effects of latent heat. Water aerosol growth is inhibited due to latent heat release and most of the vapor condenses on the wall. Also, water droplets near the tube axis may evaporate, due to the large diffusion rate of water molecules to the wall.

Change in concentrations of cloud condensation nuclei and aerosol particles during the dissipation of fog.

Measurements on the number concentrations of cloud condensation nuclei (CCN) and aerosol particles were carried out at Tsukuba during the dissipation stage of fog. Number concentrations of CCN in the supersaturation range of 0.24-1.24% and submicron aerosol particles with radii between 0.05 and 0.25 μm tended to increase during the dissipation of fog. Chemical elements in individual aerosol particles collected during the dissipation of fog were examined by using an electron microscope equipped with an energy-dispersive X-ray analyzer (EDX). The results of quantitative analysis indicated that sulfur was dominant in the submicrometer particles. Estimated radius growth rates of aerosol particles during the dissipation showed increase in proportion to aerosol radius, suggesting the importance of chemical reactions in fog droplets. The elemental composition of the residues of individual fog droplets was also examined with the EDX, together with that of large particles after the dissipation of fog. It was found from the quantitative analysis that the weight ratios of S/Fe and S/K in large particles were 3.5 and 5.3 times greater than those of fog droplet residues, respectively. It is suggested that SO2 oxidation took place in fog droplets. And evaporation of fog droplets will contribute to the increase in the concentrations of large particles and CCN in the lower atmosphere near the ground.

Surface reactions of SO 2 on metallic particles

In this note the particle surface transformations are examined, using the results obtained by electronic spectroscopy measurements. The tests concerned the preparation of condensation aerosols from metallic (Zn, Cd) vapours in N 2 flow (or N 2 and O 2) so as to obtain clean surfaces immediately brought in contact with the gas. On the surface, new sulphurated compounds and physically adsorbed gas layers were generated and revealed by degassing, using X-ray photoelectron spectroscopy (XPS-ESCA). The determination of the sulphur chemical state on the surface suggests a model of the particles composed of a metallic core and a layer, which depends on the particles' reactivity to the gas. These transformations can be responsible for behaviour in the atmosphere and irritant properties, and for dangerous effects for human health.

Theory of surface deposition from a unary dilute vapor-containing stream allowing for condensation within the laminar boundary layer

Deposition rates on targets cooled far below the dew point of undersaturated mainstreams have often been found to be surprisingly low and surface temperature dependent. A rational yet tractable theory to account for these observations is formulated and exploited in particular cases of current practical interest—e.g. the deposition of trace alkali sulfate vapors present in combustion products. The present physico-chemical model is based on the formation of a condensate aerosol near the deposition surface, with the resulting droplets (or particles) collected by the mechanism of thermophoresis [shown to be dominant, but previously neglected in related two-phase boundary layer (BL) analyses]. The vapor, assumed here to be in local equilibrium with the aerosol phase, is collected by the familiar mechanism of Fick (concentration) diffusion across the prevailing laminar BL(LBL), but the overly restrictive assumption D v ≌ α n (unity Lewis number) is not made. As by-products of the calculation of the total (aerosol + vapor) deposition rate the position of nucleation onset, as well as the structure of the LBL on either side of this “fog-locus”, are obtained. Encouraging agreement with limited available data on Na 2SO 4 deposition is obtained by assuming that the thermophoretic diffusivity of the resulting aerosol phase is about one decade smaller than the momentum diffusivity of the host combustion products.

Equilibrium theory of surface deposition from particle-laden dilute, saturated vapor containing laminar boundary layers

Mass deposition rates on cooled solid surfaces exposed to saturated mainstreams containing a dispersed condensate aerosol are influenced by both the thermophoretic capture of such particles and the inevitable scavenging of vapor that takes place during their transit through the thermal boundary layer (BL). A tractable asymptotic theory is presented which allows total mass deposition rates to be predicted from such coupled two-phase boundary layers on the realizable assumptions that the dispersed condensate: (i) is captured predominantly as a result of thermophoresis down the prevailing temperature gradient, and (ii) is able to maintain the condition of local vapor—condensate phase equilibrium (LVCE) throughout the thermal BL. It is shown that under such circumstances the total mass deposition rate can be conveniently split into three additive contributions, the simplest being the vapor contribution, which can be computed (based on LVCE) without a knowledge of the condensate contributions. The latter can themselves be split into two calculable portions—one associated with the deposition rate of the mainstream particles computed in the (hypothetical) absence of vapor scavenging, and one associated with the amount of condensate scavenged from the vapor during the process of particle thermophoresis across the thermal BL. The present formulation applies to carrier gas—vapor—condensate systems of any Prandtl number ( Pr), vapor Schmidt number, dilute condensate mass fraction, dimensionless condensate particle thermophoretic diffusivity, and heat of vaporization. Illustrative numerical results are presented here for high Reynolds number, Pr = 0.7 (air-like) streams in forward stagnation-point flow (two-dimensional and axisymmetric) or flat-plate flow, for absolute surface temperatures in the range 0.5 ⩽ T w / T ∞ ⩽ 1.0, over the interesting ranges of dimensionless thermophoretic diffusivities (α TD c / v) and latent heats [Λ/( RT ∞)].

Theory of surface deposition from a binary dilute vapor-containing stream, allowing for equilibrium condensation within the laminar boundary layer

Deposition rates on solid targets cooled far below the dew point of undersaturated, dilute binary vapor and hot gas mainstreams have recently been found to be reduced and sharply surface temperature dependent compared to their pure vapor deposition counterparts. A rational yet tractable theory to account for such observations is formulated and exploited in particular cases of current practical interest; e.g. the deposition of trace multiple alkali sulfate vapors present in flowing combustion products. Our physicochemical model is based on the formation of a binary solution condensate aerosol near the deposition surface, with the resulting droplets collected by the mechanism of thermophoresis. The binary vapors, assumed here to be in local equilibrium with the solution aerosol phase, are collected by the familiar mechanism of Fick (concentration) diffusion across the prevailing laminar boundary layer (LBL) but we do not make the restrictive assumption that the Fick diffusivities are equal to the energy diffusivity of the carrier gas. As by-products of our calculation of the total (aerosol + vapor) deposition rate we obtain the BL position of condensation onset, as well as the structure of the LBL on either side of this binary “fog-locus”. Illustrative predictions are shown for the effects of Na 2SO 4(g) addition on salt deposition rates from combustion product streams containing a fixed amount of K 2SO 4(g), on the assumption that the resulting Na 2SO 4 + K 2SO 4 droplets form nearly ideal solutions, having droplet thermophoretic diffusivities within two decades of the host gas momentum diffusivity, v. With systematic corrections for the effects of (a) alkali salt vapor dissociation/equilibrium chemical reaction with the hydrocarbon combustion products and (b) slight solution non-ideality, the present theoretical formalism should be useful in accounting for binary nucleation onset effects observed in recent deposition rate experiments using alkali-seeded atmospheric pressure flat flames.

An experimental and theoretical investigation of the transition phenomenon in fuel spray deflagration: 1. The experiment

An experimental verification of the transition from a heterogeneous controlled to a homogeneous gas phase combustion of a monodisperse tetralin-air spray is presented. The fuel sprays are produced by a condensation aerosol generator. The flame tube method, together with multiple-exposure, high speed photography and an in situ electro-optical sensor array, are employed to measure the flame propagation speed. Flat flame fronts are obtained with the aid of suitable acoustic devices. The results show that for relatively large particle sizes, droplets burn in their own flame envelope. As the droplet size is progressively reduced at a constant equivalence ratio, the flame propagation speed first increases to a maximum and then decreases to that of the premixed mixture having the same equivalence ratio. The result is interpreted in terms of the transition from heterogeneous to homogeneous combustion of fuel sprays.

On the prolonged lifetime of the El Chichón sulfuric acid aerosol cloud

The observed decay of the aerosol mixing ratio following the eruption of El Chichón appears to have been 20–30% slower than that following the eruption of Fuego in 197M, even though the sulfuric acid droplets were observed to grow to considerably larger sizes after El Chichón. This suggests the possible presence of a condensation nuclei and sulfuric acid vapor source and continued growth phenomena occurring well after the El Chichón eruption. It is proposed that the source of these nuclei and the associated vapor may be derived from annual evaporation and condensation of aerosol in the high polar regions during stratospheric warming events, with subsequent spreading to lower latitudes.

PIXE analysis of south pole aerosol

The Geophysical Monitoring tor Climatic Change (GMCC) program of the National Oceanic and Atmospheric Administration (NOAA) operates an atmospheric monitoring observatory at Amundsen-Scott Station, South Pole. Long-term measurements of carbon dioxide, ozone, aerosols, and other background pollutants are obtained to understand their possible effects on the earth's climate. The aerosol measurement program consists of the continuous measurement of condensation nucleus (CN) concentration and aerosol scattering extinction coefficient ( σ sp ). During 1982 Nuclepore-filter aerosol samples were taken with 8-h time resolution for analysis by the proton induced X-ray emission (PIXE) technique. A time series of sodium, chlorine, and sulfur concentrations shows that the sulfur and CN records are similar and that the sodium, chlorine, and σ sp records are similar. Episodes of sodium are measured at the ground in the austral winter and are apparently caused by large-scale transport from coastal regions and vertical transport to the surface during times of surface warming and weakening of the surface temperature inversion. These episodes are characterized by increases in sodium concentration, CI/Na ratio, σ sp , and particle size, and decreases in nonseasalt sulfur concentration, suggesting a decrease in atmospheric acidity with a displacement of sulfuric acid aerosol. An analysis of back trajectories suggests transport times of several days from the Antarctic coast.

Particle condensation in evaporative flows

The problem of the steady evolution of a particle-laden vapour, or condensation aerosol, by a source evaporating into an open, unreactive atmosphere is considered. A slip-flow model of energy and mass flow at the surface is combined with a stagnant boundary-layer model of flow in the gaseous continuum to obtain a set of parameters sufficient for the complete solution of the evaporation-condensation problem. These parameters are: the pressure, the source and sink temperatures, the sink concentrations and the flux density of heat or mass from the source. In an initial analysis the curve of vapour saturation ratio (S) against temperature (T) across the layer is obtained and compared to the critical saturation ratio (SR) of nucleation theory to find the conditions necessary for the incipience of particle formation by spontaneous nucleation. Data for zinc distilling into argon are summarized on energy and mass flux diagrams and on a nucleation threshold diagram (the ‘F-diagram’). Possible uses forF-diagrams are envisaged, for example, in the design of manufacturing processes in which particles are produced by aerosol condensation, or, alternatively, where particle-free evaporation is to be achieved. The problem of two-phase flow is tackled numerically. Starting with values at the nucleation threshold as initial conditions, the mass and energy equations are solved by introducing Becker and Doering’s formula for the nucleation rate and the Stefan diffusion model for particle growth to calculate the local depletion of vapour by aerosol condensation. The rise and fall of supersaturation and the evolution of the particle size distribution are revealed as the calculation proceeds along the flow coordinate. Results are given for various combinations of the experimental variables, again with zinc-argon as example. To test predictions in detail, measurements are required on heat or mass transfer rates in suitably designed experiments. Insufficient data are available as yet but a brief overview of reported trends in particle type, size and concentration in metallic condensation aerosols is given and some rationalization attempted. The formation of crystalline particles by the vapour-solid and vapour-liquid-solid routes, which can result in important morphological differences in aerosol precipitates, is shown to depend on several factors, including the mass and energy flux densities at the vapour source.

Aerosol measurements at the South Pole

The Geophysical Monitoring for Climatic Change (GMCC) program of the National Oceanic and Atmospheric Administration (NOAA) operates an atmospheric monitoring observatory at Amundsen-Scott Station, South Pole. Long-term measurements of carbon dioxide, ozone, aerosols, and other background pollutants are obtained to understand their possible effects on the earth's climate. The aerosol measurement program consists of the continuous measurement of condensation nuclei (CN) concentration and aerosol scattering extinction coefficient (σ Sp ) During 1982, Nuclepore-filter aerosol samples were taken for subsequent analysis by the proton-induced X-ray emission (PIXE) technique with 8-h time resolution. A time series of sodium, chlorine, and sulfur concentrations shows that the sulfur and CN records are similar and that the sodium, chlorine, and σ Sp records are similar. Large episodes of sodium are measured at the ground in the austral winter and are apparently caused by large-scale transport from coastal regions and vertical transport to the surface during times of surface warming and weakening of the surface temperature inversion. These episodes are characterized by increases in sodium concentration, Cl/Na ratio, σ Sp , and particle size, and decreases in non-sea-salt sulfur concentration, suggesting a decrease in atmospheric acidity and a displacement of sulfuric acid aerosol. An analysis of back trajectories suggests transport times of several days from the Antarctic coast. A nearly continuous record of South Pole CN measurements from 1974 to the present, and σ Sp , measurements from 1979 to the present, has now been accumulated. The CN data show an annual cycle with a maximum exceeding 100 cm -3 in the austral summer and a minimum of about 10 cm -3 in the winter. The σ Sp data show an annual cycle markedly different from that of CN with a maximum in late winter, a secondary maximum in summer, and a minimum in May. Angstrom exponents calculated from the multiwavelength σ Sp data show a strong annual cycle suggesting larger particles in the winter than in the summer. DOI: 10.1111/j.1600-0889.1986.tb00189.x

Generation of Li Combustion Aerosols for Animal Inhalation Studies

A system was developed for generating Li aerosols to determine the potential health hazards of postulated accidents associated with the use of Li as a fusion reactor blanket or coolant. The aerosol was generated by sweeping Ar through a stainless steel chamber filled with Li metal that was heated inductively to temperatures up to 1300 degrees C. Argon carried the Li vapor into a burning chamber where it was mixed with air. The reaction of Li vapor with air formed an intense white flame that produced typical branched-chain condensation aerosol particles. This system generated well-controlled concentrations up to 2500 mg/m3 for periods of 4 h. The mass median aeordynamic diameter of the aerosol was approximately 0.66 micron with a geometric standard deviation of 1.5. Aerosols could be generated that were greater than 96% Li2O and LiOH, LiOH.H2O, or Li2CO3 by controlling the CO2 and H2O concentrations in the supply air. The system is currently being used to investigate the acute toxicity of Li combustion aerosols in laboratory animals.