The ratio of the extinction coefficient to the mass of atmospheric aerosol particles as a function of the relative humidity

The ratio R_K of the extinction coefficient of aerosol particles in cm ¹, at the wavelength of light λ = 0·55 μm, and the mass of atmospheric aerosol particles in gem³ has been computed as function of the relative humidity for six types of continental and maritime aerosols. With the mean value R_K = 10⁴ cm²/g for all aerosol types being assumed to be independent of relative humidity, only the order of magnitude of the aerosol mass can be determined from visibility observation or measurement of the extinction coefficient of the aerosol particles.     

Measurements of the total and regional deposition of inhaled particles in the human respiratory tract

The total and regional deposition of monodisperse aerosols in the human respiratory tract has been measured in 12 healthy subjects breathing through the mouth. Radioactively labelled polystyrene particles in the aerodynamic diameter range 3.5–10.0 μm were employed. The total deposition results are similar to those reported by Stahlhofen et al. (1980), showing only a slight progressive increase with particle size, from a mean fraction of 0.79 of the inhaled aerosol at 3.5 μm, to 0.88 for 10 μm particles. The extrathoracic airways show a very marked deposition at all sizes, predominantly in the throat. The throat values rise rapidly from a mean of 0.09 at 3.5 μm to 0.36 at 10 μm particle diameter. Two intrathoracic fractions were also obtained by the widely accepted method of measuring the relative amounts of activity cleared from the thorax as a function of time. Alveolar deposition was apparently still some 0.06 of the inhaled aerosol at 10 μm particle diameter. Tracheo-bronchial deposition showed little change at any particle size except at 3.5 μm, when it was 0.24 of the inhaled aerosol.     

Measurement of the droplet size distribution of fog using multi-wavelength lidar: A feasibility study

The extinction and backscatter coefficients of an aerosol are determined in part by the aerosol size distribution. If specified at more than one wavelength these optical data can be used to infer the aerosol size distribution if the relationship between the optical data and the size distribution is well defined. In the case of fog the index of refraction is known and the fog droplets are spherical so that the relationship between the optical data and the droplet distribution is given by Mie theory. The inversion procedure of Twomey (1977) is adapted to derive the size distribution of an example fog using the extinction and backscatter coefficients at wavelengths between 0.53 μm and 10.6 μm. Such measurements could be made using a multi-wavelength lidar. The accuracy of the solution as a function of intrinsic inversion error and measurement error is discussed.     

Vertical distribution of aerosol particles near the air-sea interface in coastal zone

Measurements of vertical aerosol concentration profiles in the Mediterranean coastal zone are presented in the size band 0.1 to 20 μm, with focus on aerosol particles smaller than 10 μm. The results show that the profile shape depends mainly on turbulent processes induced by the wind rather thanthe whitecap production of freshly generated aerosols from the wind-wave interaction. Therefore, the data recorded during weak and moderate wind speeds show the prevalence of turbulent mixing processes induced by the wind for profiles in the size range 0.1–1 μm. For the size range 1–10 μm, in addition to the effect of wind velocity and direction (i.e. the coastal influence), we also noted the importance of the relative humidity gradient (very close to the interface). Good agreement was found between vertical concentration profiles recorded for winds of marine origin and the marine mixed layer model (Davidson and Shutz, 1983). However, we show that the model cannot be used for winds of continental origin. For wind speeds above >9 ms⁻¹, a maximum occurs in the concentration profiles (near 2–3 m height) for all particles larger than 1 μm, which confirms the predominant influence of an eddy which appears in the lee of the crest, referred to as the “wave rotor” model (De Leeuw, 1986).     

MEASUREMENTS OF THE NEAR INFRARED OPTICAL PROPERTIES OF COAL SLAGS

The spectral complex refractive index has been measured over the wavelength range of 1 μm to 12 μm for three coal ash samples. The measurements were conducted by making spectral transmittance and near-normal reflectance measurements on thin, homogeneous slag wafers prepared from the ash samples. The imaginary part of the refractive index k exhibits a large peak near 9 μm, attributable to Si-O vibrational absorption. Three smaller peaks in the k spectra were observed at 1·2μm, 1·8 μm, and 2·8 μm, which have been identified as due to absorption by the ferrous iron ion Fe²⁺ and the hydroxyl group OH. The dependence of the ash optical properties on ferrous iron content in the wavelength range of 1 μm to 5 μm suggests that both the total iron content of the ash and the ash oxidation state may affect the contribution of ash to radiative heat transfer in coal combustion systems.     

Large discrepancies between theoretical and field-determined scavenging coefficients

Below-cloud scavenging coefficients for particles in the diameter range 0.28 < da < 15 μm could be determined as a function of the aerosol diameter d_a in a field experiment. The agreement with state-of-the-art theory is best for particles of about 2 μm diameter. Particles below this size seem to be much more efficiently removed than theory predicts - the experimental values are 10 to 100 times larger for aerosols of da≈0.5 μm. Giant particles of more than 2 μm diameter seem to be less efficiently scavenged than predicted. For 5 μm particles the experimental scavenging coefficients are about 2 to 5 times lower than the theoretical values. Reasons for these deviations are discussed in detail. The dependence on rainfall rate was found to be less pronounced than predicted by theory. The overall aerosol concentration decrease by below-cloud scavenging was found to be 6% (by number) or 21% (by volume), for a mean rainfall rate of 0.85 mm/hr and a mean duration of 170 minutes.     

Response of Knollenberg light-scattering counters to non-spherical doublet polystyrene latex aerosols

Doublet particles of polystyrene, polyvinyltoluene and styrene vinyltoluene latex were measured with the Knollenberg CSASP and ASASP light-scattering aerosol counters. These instruments respond to these non-spherical particles as they do spheres of equal volume and refractive index, providing particles have equivalent radii r 0.4 μm. However, their response to larger nonspherical particles having equivalent radii in the range 0.4 μm r 1.7 μm cannot be approximated by that for spheres of equal volume or equal cross-section. For even larger particles with equivalent radii 1.7 μm r 4 μm the CSASP response is about the same as for spheres of equal cross-section and refractive index. The counting efficiency of the CSASP and ASASP instruments was checked by measurement of monodisperse aerosol particles of known concentration. These instruments measure aerosol concentrations correctly to within a factor 2 for near-micrometer sized particles.     

The growth of ambient aerosols in the conditions of the respiratory system

The effect on ambient aerosols of exposure to the conditions of the respiratory system was determined by sampling simultaneously through two dichotomous samplers in parallel. In one of the samplers the aerosol was brought to 37°C and near 100% relative humidity before passing through the virtual impactor. The results show that, as a result of humidification, about 10–15% by mass of the aerosol which would normally been collected in the fine (less than 2.5 μm aerodynamic diameter) fraction of the sample grows sufficiently when humidified to be collected with the coarse fraction. It is suggested that the direct application to inhalation studies of dichotomous sampler results should be approached with some caution.     

System design and test method for measuring respirator filter efficiency using mycobacterium aerosols

Recently, the protection of health care workers from tuberculosis-containing aerosols has been the subject of considerable debate. An experimental apparatus and test protocol were developed to measure the collection efficiency of surgical mask and respirator filter media using a microbial aerosol challenge. Mycobacterium chelonae (M. chelonae), used as a surrogate for Mycobacterium tuberculosis, was generated from liquid suspension using a Collison nebulizer. Upstream and downstream concentrations of viable aerosol particles were measured using Andersen cascade impactors, while total particle concentrations were measured with an aerodynamic particle sizer (APS). A monodisperse polystyrene latex (PSL) sphere aerosol (0.804 μm) was used in separate experiments to measure filter efficiency; concentrations were determined with the APS. The mycobacterial aerosol ranged in size from 0.65 to 2.2 μm when measured with the cascade impactor. A similar size range was found with the APS, yielding a count median diameter of about 0.8 μm. Samples of the mycobacterial aerosol were collected on glass slides, stained M. chelonae, as determined by environmental scanning electron microscope, were found to be rod shaped with an average length of 2 μm and average width of 0.3 μm. To evaluate the apparatus over a range of filter efficiencies (10–100%), different layers of fiberglass filter paper were tested for penetration using a 0.12 μm dioctyl phthalate (DOP) aerosol measured with a light scattering photometer, in addition to the mycobacterial and PSL aerosols. For the range of efficiencies tested it was shown that filter collection of DOP was linearly related to that of both mycobacterial and PSL sphere aerosols (r² = 0.99), demonstrating that an inert aerosol may be used to predict the collection of biological aerosols by such filter media.     

Transmission and Mechanical Properties of Optical Adhesives

The optical, mechanical and durability performance of selected epoxy, polyester, UV-curable acrylic, cyanoacrylate and silicone adhesives were evaluated and measured for bonding applications of optically transparent glasses in the visible and infra-red regions of the electromagnetic spectra.

From the initially selected adhesives only the UV-curable modified acrylic, two-component silicone and room temperature cured epoxy, were found to be of high performance characteristics, having good transmission properties and enhanced endurance in a combination of heat and humidity and following thermal cycling.

Sodium chloride substrates served as adherends for the transmission characterization of the optical adhesives, due to their high transmission properties in the 0.4-10 m μ spectral range. A modified lap shear specimen was designed for studying the mechanical properties and failure mechanisms of the adhesives and their durability in a humid and not environment. Finally, a two-piece glass doublet was used for investigating the optomechanical characteristics of the optical adhesive following environmental conditioning and thermal shock cycling.

Due to the inherent C-C bond, polymer adhesives are limited in utility, as far as transparency is concerned, close to 3.5 μm and in most of the 8-12 μm spectral range.     

Large aerosol particles in a convective storm environment

Concentrations of large (d > 10 μm) inorganic and organic particles varied widely at ground level and at subcloud altitudes. Particle concentration decreased at a slower rate aloft than at ground level; as a result, the relative abundance of large aerosol particles aloft tended to increase with increasing particle size. A substantial fraction of organic particles aloft was found to be responsible for this trend.

Curves of freezing frequency indicate the presence of two distinctive sources of freezing nuclei derived from large aerosol particles active in the − 9 to − 12°C and − 19 to − 22°C temperature ranges.     

Note on the movement of insecticide droplets through and out of a crop canopy after emission below canopy level

Coarse aerosol sprays with number median diameters in the range 30–40 μm were emitted at 0.25 m above the ground within the canopy of a cotton crop planted at 0.93 m row separation and standing 1.0 m tall. Movement of spray was monitored by vertical stepped paper tower samplers with narrowest section of width 0.13 cm, giving collection by inertial impaction down to 10 μm in a 1 m/sec wind. Horizontal paper targets were used to sample droplets depositing by sedimentation. Droplet number counts indicated that under conditions of moderate instability and associated convective turbulence, droplets were drawn rapidly out of the crop to a height of 2 m (the upper level sampled). The peak concentration of the spray cloud was centred at a height of between 1.0 and 2.0 m beyond 1.0 m downwind of the emission point in the first test, using droplet spectrum with volume median diameter of 41 μm, and beyond 2.5 m downwind of the emission point in the second test with spectrum of 57 μm volume median diameter. Inertial impaction at the 0.25 and 0.5 m levels was slight, except on the samplers within 0.5 m of the sprayer, and sedimentation was only significant at and below 0.5 m in the first two rows sampled. The moderate wind speed of 2.5 m/sec at canopy level produced marked inertial impaction over the first 4.5 m downwind of the sprayer at the 1.0 m level.     

Aerosol measurement by laser doppler spectroscopy—II. Operational limits, effects of polydispersity, and applications

The theoretical basis and the results of a computer simulation are presented which describe the operational limits of size and concentration for aerosol sizing by laser Doppler spectroscopy LDS,. This analysis suggests that a state of the art LDS system has the capability of sizing 0·03 μm diameter particles when the number concentration is 10⁸ cm⁻³ or greater and 0·2 μm diameter for coocentrations as low as 100 particles cm⁻³. An evaluation of the effect on the laser Doppler spectroscopy measurements of a polydisperse aerosol having a log normal size distribution is presented and methods for combining these measurements with other averaged measurements to determine both count median diameter (CMD) and geometric standard deviation (δ_g) are proposed. For aerosols having log normal distributions with 0·3 < CMD < 3 μm and 1·0 < δ_g < 2·0, laser Doppler spectroscopy is able to measure the surface area median diameter within ± 15 per cent, independent of polydispersity. Applications of LDS to aerosol sizing are evaluated and its advantages and disadvantages relative to other sizing methods are discussed.     

The onset of luminosity in propane/air flames

The onset of luminosity in propane/air flames, enclosed in a furnace operated at two temperature levels, has been examined at four stoichiometric values by determining carbon dioxide, carbon monoxide, oxygen and hydrocarbon concentrations together with temperature, total radiation, and radiation in the 4.3 to 4.5 μm wavelength region at distances along the flames. The data obtained have been processed to give values of total and spectral flame emissivity. The results show that luminous radiation is spectral in nature; the intermediate carbon-forming compounds emit more radiation than the products that are formed. A significant proportion, 10 to 40%, of the radiation is emitted in the 4.3 to 4.5 μm waveband. The concentrations of carbon monoxide, carbon dioxide and oxygen remain essentially constant, and there appears to be no set temperature for the onset of luminosity.     

Chemical Aging in Epoxies: A Local Study of the Interphases to Air and to Metals

The aging behavior of an epoxy adhesive (DGEBA and DETA) in the bulk surface region and in the contact region to metal substrates (Au, Cu) is studied locally. Therefore, the chemical depth profile during aging is recorded with FTIR microscopy on sample surfaces prepared with low angle microtomy. Two environmental conditions are applied at 60°C for up to 500 days to separate the role of temperature and humidity in aging.

Quantitative evaluation of the IR spectra provides the following results: in the bulk surface region, three aging mechanisms form a gradient region of more than 200 μm. Their intensity and depth profile depend on the environmental conditions. Epoxy-metal contacts are unaffected under dry conditions. Humidity is needed to form a 50 μm thick region where the copper substrate accelerates aging. Based on the experimental results, chemical aging mechanisms are discussed.     

Aerosol growth studies — IV. Phase transformation of mixed salt aerosols in a moist atmosphere

The phase transformation and subsequent droplet growth of the mixed salt aerosols NaCl—KCl and (NH₄)₂SO₄—H₂SO₄ were investigated in a continuous-flow apparatus at 25 and 30°C as a function of relative humidity. Monodisperse salt aerosols (d = ≈ 0.5 μm, OG = 1.07–1.13) were prepared and mixed with N₂ carrier gas at controlled humidities. The particle-size distribution of the aerosol before and after growth by water vapor condensation was continuously monitored with an optical particle counter. It was found that mixed salt aerosols were characterized by stage-wise growth when the relative humidity in the atmosphere was increased. The onset of growth took place at a specific deliquescence humidity determined by the water activity at the eutonic composition. Thus, mixed NaCl—KCl aerosols deliquesce at 73.8 ± 0.5% r.h. regardless of initial compositions. For sulfate aerosols containing 0.75 to 0.95 mole fraction (NH₄)₂SO₄ (the balance being H₂SO₄), the onset of growth occurs at 69.0 ± 0.5% r.h.. In the composition range of 0.5 to 0.75, a deliquescence humidity of 39.0 ± 0.5% is noted. Below 0.5 mole fraction, however, the mixed-sulfate aerosols are expected to exhibit hygroscopic properties on the basis of thermodynamic considerations.     

Properties of the Stöber centrifugal aerosol spectrometer at a high sampling flow rate: calibration, resolving power and correction factors for particle losses

A centrifugal aerosol spectrometer designed by Stöber and Flachsbart (1969) was calibrated at a high sampling flow rate (1.91. min⁻¹) for the particle size range 0.2–5 μm and the resolving power was determined. Multiplication factors to correct for particle losses in the aerosol inlet, in the edge layers of the foil and through the exhaust of the spiral duct were determined.     

On physical properties of aerosol at Ross Island, Antarctica

We report on the physical characteristics of Aitken nuclei and their relation to air mass type at Ross Island, Antarctica. The size distribution spectrum of the particles was derived with a diffusion battery and non-linear mathematical inversion and is considered to be quite accurate over the particle diameter range 0.01 < d < 0.2 μm.

The Aitken nuclei concentration, n_o, decreased at Ross Island by about an order of magnitude from summer (n_o = 600 cm⁻³) to winter (n_o = 100 cm⁻³). Particle size varied with air mass type: Maritime polar air masses had a geometric mean particle diameter d_g = 0.011 μm, while continental Antarctic, cA, air masses possessed somewhat smaller particles (i.e. d_g 0.005 μm). The anomalously small particles associated with cA air suggests the presence of an upper tropospheric or partly stratospheric origin of nuclei perhaps associated with mixing formed by breaking waves over the Ellsworth Range in Marie Byrd Land.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

Experimental filtration efficiencies for large pore nuclepore filters

Experimental filtration data were collected in an effort to validate an impaction model previously developed and presented. Using a sampler with a 9.5 μm pore diameter Nuclepore filter, collection efficiencies were measured for both liquid and solid aerosols over a size range of 2–9 μm. Data for the liquid aerosol showed good agreement with the impaction model; however, data for the solid aerosol indicated an appreciably lower collection efficiency than predicted by the model. The liquid aerosol data validate the impaction model. The solid aerosol data indicate particle bounce or reintrainment subsequent to impact and underscore particle capture as a problem to be dealt with if the Nuclepore surface is to be used as a size selective filter.