Klaus Willeke

Atmospheric Aerosols: Size Distribution Interpretation

The body of information presented in this paper is directed towards the scientist or engineer who measures and interprets the physical characteristics of ambient air. This paper interprets measurements made with the Minnesota Aerosol Analyzing System (MAAS) in Denver, near Denver, and near Ft. Collins, Colo., and examines the measurements in the light of what has been learned from several thousand size distributions measured in the other parts of the United States. The origin and physical change of atmospheric particulates is examined through interpretation of modes occurring in the size distributions. The mode coined “AHken Nuclei Range” mode is a measure of aerosol generation, while aged aerosols show up in the “Accumulation Range” mode. Windblown dusts, sea sprays, and mechanically produced particles such as fly ash generate a “Coarse Particle Range” mode. The air environment is categorized into several types of background and urban aerosols whose physical characteristics are described by a limited num...

Characterization of California aerosols—I. Size distributions of freeway aerosol☆

Aerosol along the Harbor Freeway in Los Angeles, California, was sampled and measured with the Air Resources Board Mobile Air Pollution Laboratory. Simultaneously with the taking of filter and impactor samples for chemical analysis, the aerosol particle size distribution was measured with four continuous instruments over the particle size range from approx. 0.003–40 μm. From comparisons of measurements when the wind was directly from the freeway with measurements when the wind was blowing toward the freeway, it was possible to calculate by difference the direct contribution of the freeway traffic to the aerosol mixture. It was found that morning rush hour traffic contributes about 17.1 μm3 cm−3 to the aerosol volume, predominantly in the particle size range below 0.15 μm. The freeway aerosol size distribution exhibits a typical strong combustion mode at about 0.02 μm particle size.

Performance of the slotted impactor.

Slotted impactors perform with a sharpness of cut close to the theoretically predicted value, if used under the conditions of the numerical model. Sideways flow entrance and impaction onto a fibrous surface may change the collection efficiency considerably.

Temperature dependence of particle slip in a gaseous medium

Abstract An expression for the gas temperature dependence of the particle slip correction is derived from kinetic theory considerations and is related to experimental measurements of the gas viscosity. The expression is simplified for air in the temperature range from 200 to 1000 K. The temperature dependence is most significant for fine particles in the free molecular regime for which the slip correction is found to increase with gas temperature more rapidly than the absolute gas temperature. The correction may be applied at all Knudsen numbers, including the free molecular regime. It is demonstrated how the temperature-dependent slip correction may be applied to the calculation of particle drift and collection efficiency in hot electrostatic precipitators, and to the calculation of the size of collected particles in inertial classification devices.

Size distributions of Denver aerosols—a comparison of two sites

Abstract In the Fall of 1971, a 35-ft trailer was equipped with the Minnesota Aerosol Analyzing System and hauled to Denver, Colorado, for field measurements of the atmospheric aerosol size distributions at two different sites. First, the trailer was located in a city environment close to a variety of anthropogenic activities. It was then moved to an open field north of Denver with very few anthropogenic activities close to the trailer, but a large industrial complex and downtown Denver were several kilometers upwind of the site. A heavy pollution episode with Denvers famous “brown cloud” was monitored during this time. A great variety of size distributions was measured and analyzed. Depending on the type of emission source close to the measurement site, the ratio of submicron “fine” particles to the total of “fine” and “coarse” particles varied from 1 to 73 per cent on a volume (or mass) basis. Surface distribution plots revealed two distinct modes in the “fine” particle range below partical dia. of 1 μm. A “Transient Nuclei Range Mode” at about 0.01 μm was particularly prominent when fresh aerosols were emitted from automobiles on a freeway, and an “Accumulation Range Mode” at about 0.2 μm dominated during a pollution episode when measurements were made several kilometers downwind of the industrial complex. Increase in particle surface in the visible range was directly related to visibility degradation. The steepness of the negative slope of the size distribution curve in the visible range may partially explain the color of Denvers “brown cloud”. High humidity appears to accelerate the coagulation of automobile-generated particles.

Dispersion characteristics of a fluidized bed

Abstract Several common dusts were injected into a fluidized bed of spherical, inert and electrically conducting bronze spheres, about 100 μm in size. The fluidized bed deagglomerated the dust and dispersed most of it out of the bed during the first minute. Comparisons of size distributions measured microscopically from dry samples of aerosols above the bed with size distributions of the feed dust dispersed in liquid showed that for fly ash, coal dust and classified air cleaner test dust, the bed dispersion was complete. However, the measured size distributions showed some dust classification for intermittent dust injections.

The influence of flow entry and collecting surface on the impaction efficiency of inertial impactors

Abstract The theoretical impaction efficiency curves for round and rectangular impactors are approximately the same when plotted as a function of modified Stokes number, defined as the ratio of aerodynamic stopping distance to impact distance I, where I is the distance from the impact surface to the point at which the free streamline, emanating from the jet orifice, has deflected from a straight line by 5–10% relative to the half-width of the throat. This distance is also equal to half the hydraulic diameter of the throat. Experiments confirm the prediction when tested under the conditions of the numerical model. However, lateral flow entrance into a cascade stage and impaction onto glass fiber paper may change the impaction efficiency considerably, resulting in a shift of the impaction efficiency curve with respect to Stokes number and in a considerable decrease in the sharpness of cut.

Comparison of volume and mass distributions for denver aerosols

Abstract Using data of the Denver 1971 aerosol characterization study, volume concentration distributions calculated from size distribution measurements by the Minnesota Aerosol Analyzing System (MAAS) are compared to mass concentration distributions measured by 8-stage Andersen impactors. In this study, the upper mode was confirmed by the impactor measurements. The lower mode was not confirmed, probably due to poor resolution resulting from impaction onto the glass fiber filters used as impaction surfaces. However, the total mass in the lower impactor mode was in agreement with the mass inferred from the volume of the lower MAAS mode. In the submicron range, estimates of densities of 1.6–1.8 g cm −3 are inferred for background aerosols and 1.1–1.5 g cm −3 for pollution aerosols.

Extinction coefficients for multimodal atmospheric particle size distributions

Abstract A chart was developed which gives the volume-normalized extinction coefficient for log-normally modelled, multimodal particle volume distributions. The real part of the particle refractive index ranges from 1.30 to 1.70. Two adjustment charts modify the main chart for an imaginary part of the refractive index different from 0.02, and for a geometric standard deviation of the modal volume distribution, different from 2.0. The charts show that particles in the accumulation range, which has a mode in the 0.1 to 1 μm diameter size interval, are optically the most active. Exact knowledge of the particle refractive index is imperative for fine particles, but of small importance for coarse particles.