F.G. Pohl

The size distribution of the urban aerosol in Vienna

Abstract Two instruments have been developed recently which permit to measure the size distribution of urban aerosols in the size range from 0.002 to 16 μm, a special cloud chamber and a low pressure cascade impactor. The cloud chamber is a computer-controlled condensation nuclei counter which measures aerosols in the range between 0.002 and 0.1 μm by exposing the aerosol stepwise to well-defined supersaturations; the instrument is capable of measuring the nucleation mode. The cascade impactor samples the aerosol through nine stages. The stages for the smaller sizes operate between pressure considerably below atmospheric pressure and permit the extension of the size range down to 0.06 μm providing in this way a sufficient overlap to the cloud chamber. The impactor measures the accumulation mode completely. Both instruments have been applied to the Viennese urban aerosol and the accumulation mode as well as the nucleation mode were measured.

Heterogeneous Nucleation of Water Vapor on Monodispersed Ag and NaCl Particles with Diameters Between 6 and 18 nm

During the 1980 workshop of the working group on ultrafine aerosols (WUFA), the heterogeneous nucleation of supersaturated water vapor on well-defined aerosols was investigated. Ultrafine Ag and NaCl aerosols were generated using a constant temperature tube furnace. By means of electrostatic classification of the polydispersed primary aerosol, monodispersed fractions with electrical mobility particle diameters of 6, 8, 12, and 18 nm were obtained. After passing through a humidifier, the vapor-saturated monodispersed aerosol entered a process-controlled expansion chamber, the size-analyzing nuclei counter (SANC)). Supersaturation was then achieved by a fast adiabatic expansion, and heterogeneous nucleation was initiated by aerosol particles acting as condensation nuclei. The number concentrations of the droplets growing in the expansion chamber were measured for stepwise increasing supersaturations. At well-defined “critical” saturation ratios, the onset of heterogeneous nucleation was observed. For the in...

Measurements of Kelvin-equivalent size distributions of well-defined aerosols with particle diameters > 13 nm

During the 1979 workshop of the working group on ultrafine aerosols, different experimental techniques for measuring the number concentration and size of ultrafine aerosol particles were compared. In the present paper we report on a comparison of different particle size measuring techniques for ultrafine aerosols. Well-defined monodisperse aerosols with electrical mobility particle diameters ranging from 13 to 100 nm were generated using an electrical aerosol classifier. Kelvin-equivalent size distributions of these aerosols were determined by means of a process-controlled expansion chamber, the size-analyzing nuclei counter (SANC). To this end the considered aerosol was humidified and the number concentration of the droplets growing in the expansion chamber was measured for stepwise increase in supersaturation. At a quite well defined critical supersaturation, a significant increase in the measured droplet concentration, and thus the onset of heterogeneous nucleation, was observed. By means of the Kelvin...

Intercomparison of different “absolute” instruments for measurement of aerosol number concentration

Abstract During the 1979 workshop of the working group on ultrafine aerosols (WUFA) an intercomparison of different instruments for measurement of aerosol number concentrations was performed. Each of these instruments (TSI-aerosol electrometer, TSI-condensation nuclei counter, Jaenicke-condensation nuclei counter, size analyzing nuclei counter SANC) can be regarded as “absolute”, because they do not depend on empirical calibration relative to external reference standards. Number concentrations were measured for monodispersed NaCl-aerosols with a mean particle diameter of 56 nm, generated by means of a collision atomizer and an electrostatic aerosol classifier. The transmission losses in the SANC-humidifier were determined quantitatively and the SANC-measurements were properly corrected. Measurements were performed over a concentration range from about 2.5 × 10 2 to 3.5 × 10 5 cm –3 . Considering the differences between the various applied measuring techniques, fair agreement of the obtained concentration data was observed. The TSI-condensation nuclei counter agrees closely with the TSI-aerosol electrometer, however, it can be regarded as an “absolute” instrument only for concentrations below 10 3 cm −3 . For low concentrations the Jaenicke-condensation nuclei counter tends to agree with the average concentration, obtained from all instruments used, above 10 4 cm −3 it agrees closely with the SANC. The readings of TSI-aerosol electrometer and SANC are quite linearly related over the whole concentration range, the SANC being low by a factor of about 0.59. Thus different measuring techniques, based on completely different principles, yield comparable aerosol number concentrations and accordingly condensation nuclei counters are truly aerosol counters.

The interdependence of droplet growth and concentration. I. Theory of droplet growth and applications on condensation nuclei counters

Abstract This paper reports theoretical investigations on droplet growth in monodisperse droplet ensembles. Computations are based on diffusion theory of single droplet growth. The mutual influence of growing droplets is taken into account by introducing slowly varying gas temperature and vapor pressure. The time course of droplet radius, gas temperature, and saturation ratio is evaluated numerically. As expected, stoppage of droplet growth at a terminal size is predicted dependent on droplet concentration and initial supersaturation. Gas temperature rises and saturation ratio decreases during droplet growth due to vapor consumption and heat production by the growing droplets. Based on computed droplet growth curves the response of condensation nuclei counters is investigated. Conditions for minimal counting losses and linear response are given.

Observation of resonances for elastic light scattering by spherical particles

Abstract This paper reports theoretical and experimental investigations of resonances for light scattering by spherical particles. Numerical calculations of the scattering intensity as a function of the particle size parameter α for various different scattering angles θ were performed by means of Mie theory. For forward scattering angles the obtained theoretical intensity vs α curves exhibit a series of main oscillations. For sufficiently large α values usually a ripple structure is superimposed analogous to the case of the well-known Mie-extinction curve. The main oscillations are found to be connected with diffraction by the sphere, their period decreases with increasing forward scattering angle. The ripple structure is caused by electric and magnetic partial wave resonances, which occur at certain α values. Accordingly a resonance peak corresponding to a particular partial wave is observed at the same a value for arbitrary scattering angles. It is found that the resonance peaks are considerably reduced for absorbing particles even at weak absorptions. Experimental observations of partial-wave resonances were performed by monitoring the light flux, scattered by growing water droplets under various different scattering angles. Excellent agreement of the measured scattered light flux vs time curves with the corresponding theoretical scattered light flux vs α curves was obtained. A unique correspondence of experimental light scattering peaks and the first-order resonances of particular partial waves was established. Thus the existence of partial-wave resonances is quantitatively verified by light scattering experiments. Some applications of physical interest are discussed.

Letter to the Editor: Reply to Comments on the Paper “Heterogeneous Nucleation of Water Vapor on Monodispersed Ag and NaCl Particles with Diameters between 6 and 18 nm”

Please click here to find the Letter to the Editor to which this Response refers: http://dx.doi.org/10.1080/02786828608959113