Aerosol Discrimination Research Articles

Electrodynamic radioactivity detector for microparticles

A new technique for the measurement of the radioactive decay of single microparticles has been demonstrated. Although the experiments were made with droplets of order 20 μm in diameter, microparticles in the range 0.1–100 μm can be accommodated. An electrodynamic balance and combination light-scattering photometer were used to measure the charge-loss rate and size of a charged microsphere suspended in a laser beam by superposed ac and dc electrical fields. The charged particle undergoes charge loss in the partially ionized gas atmosphere which results from radioactive decay of 14C-tagged compounds, and the rate of charge loss is proportional to the rate of decay here. The charge on a particle was determined by measuring the dc voltage necessary to stably suspend the particle against gravity while simultaneously determining the droplet size by light-scattering techniques. The parameters which affect the operation of the electrodynamic balance as a radioactivity detector are examined, and the limits of its sensitivity are explored. Radioactivity levels as low as 120 pCi have been measured, and it appears that by reducing the background contamination inside our balance activity levels on the order of 10 pCi can be detected. This new technique has application in the measurement of activity levels and source discrimination of natural and man-made aerosols and smokes and is also useful for studies involving specifically labeled radio-chemical probes.

The application of mineral magnetic methods to atmospheric aerosol discrimination

The magnetic mineral and grain size compositions of atmospheric dust samples collected from the marine aerosols of the North Atlantic Ocean, and the Arabian Sea and from a sampling station on the island of Barbados have been defined using a range of mineral magnetic measurements. The magnetic mineral content or magnetic signature of the soil-sized (predominantly > 5 μm) dust material has been obtained by measuring each sample's response to a number of laboratory-imparted low field and remanent magnetisations. Variations in magnetic signature between groups of dusts have been established using the statistical procedure discriminant analysis. Dissimilarities in magnetic signature suggest that atmospheric dust samples derived from different source areas vary significantly in terms of mineral magnetic composition. In addition it appears that magnetic signatures are often strongly influenced by components that are urban/industrial in origin. The influence of anthropogenic particles on magnetic signatures being confirmed by an examination of the strongly magnetic phases of selected samples under the scanning electron microscope. The existence of differences in the magnetic mineral composition of atmospheric dust samples would suggest that the analysis of magnetic signatures could be applied to such research areas as the study of the long range transport of natural and polluting aerosols.