Measurement of Nanometer Aerosols

Abstract

This paper is a review of my work during the past 18 years on nanometer, i.e., submicrometer, aerosols. These aerosols scatter negligible light so they are difficult to study and must be observed by indirect methods such as diffusion batteries and condensation nucleus counters. Several diffusion batteries are described: “cluster tube” batteries, 5.5 km of 1-mm-diam. straight tubing mounted in clusters; collimated holes structures containing 5.1 km of holes, 1/4 mm in diameter; honeycomb structures containing 3.5 km of holes 1/3 mm in diameter; screen batteries containing 55 stainless steel screens in 10 sections; reticulated vitreous carbon batteries containing 60 k interconnected pores per cm5. From theory, the diffusion battery is shown to be only slowly discriminating, so a series of batteries and measurements is required for particle size analysis. Measurements were made with a continuous flow condensation nucleus counter developed to provide the steady flow required by diffusion battery theory. The measurements were found to agree with those made with an electrical aerosol generator. Particle size was analyzed by a “graphical stripping” method developed in this laboratory and by two computer programs described in the literature. Standard sampling methods such as the thermal precipitator and the electrostatic precipitator were tried but found to be inadequate. An induction furnace and a tube furnace were used to generate silver and gold, as well as NaCl aerosol. The furnaces were found to be superior to the more common hot-wire or exploding-wire methods, and heating the dry NaCl was preferred to spray-drying a suspension. Carbon aerosols of a large range of particle size and concentration were conveniently generated by the incomplete combustion of methane. A tube bridge, following Pollak's design, was built and used to test the “intrinsic” calibration of his counter. Good agreement was found with his calibration table up to a concentration of 300 k/cm³. Above that value, however, the tube bridge showed a progressive undercount so that the maximum value of 641 k given in Pollak's table was, according to our measurements, about 1200 k. The temperature drop during adiabatic expansion in the Pollak counter was measured with the aid of a resistance wire 12.7 um in diameter mounted along the axis of the fog tube. It was found both theoretically and experimentally that the dry adiabatic temperature drop is about 16 °C, in agreement with the literature. However, we found that the wet temperature drop is about 8°C, both experimentally and theoretically. It is frequently stated in the literature that the wet and adiabatic temperature drops are the same. The use of the above-described diffusion batteries in the laboratory and field is described. The collimated holes and honeycomb structures are suitable for aerosols of high concentration in the laboratory and uranium mine atmospheres. The carbon batteries are more suitable for radioactive aerosols of low concentration since their flow rate is 280 liters/min.