Distribution of natural radioactivity on the environmental submicronic aerosols

Abstract

An electrial ion mobility analyzer has been built for the study of the environmental aerosol particle size spectrum and of the distribution of natural activity on this spectrum in the size range 10-5 cm r > 10-6 cm. The mobility analyzer was carefully tested by a new method to prove the laminarity of flow and the air velocity distribution inside the apparatus. The quantity of charged aerosol particles classified into discrete mobility (or size) groups was estimated by the ?-radioactivity of collected particles tagged before entering the analyzer by ThB ions. The electrical mobility analyzer was then completed by a sonic jet diffusion charger for unipolar aerosol charging by diffusion and the elctrical parameters of this device were tested to ensure reliable operation.The activity size spectra A(r) were transformed to relative particle size distribution curves N(r). The measurements were carried out in Stockholm, Sweden, under varying physical conditions (varying the activity, the residence time, the collector voltage or applying the diffusion charger for unipolar charging of particles). All experiments gave very similar results. The activity curves A(r) of positive and negative particles as a function of radius show a similar profile with a maximum in the region of radius r = 3–6 times 10-6 cm corresponding to Junge's model of aerosol particle distribution. The calculated relative particle size distribution curves N(r) are not always characterized by a peak as expected from Junge's model but are increasing below radii r < 10-6 cm, so that no peak is evident.