Measurement of the effect of indoor atmospheric trace gas on the polonium-218 ion mobility spectrum by alpha-track detection

Abstract

A number of investigators have reported formation of radiolytic ultrafine particles produced by the interaction of ionizing radiation with atmospheric trace gases. Previous studies have suggested that a very high localized concentration of the hydroxyl radical produced by the radiolysis of water can react with atmospheric trace gases such as SO2 and produce lower vapor pressure compounds that can subsequently nucleate. To determine the trace gas and water vapor concentration dependence of the active, positively charged, first decay product of radon, a well-controlled experimental chamber, the Illinois-Clarkson system, was used in this research. The mobility spectrum of the decay products in the range of 0.07-5.0 cm^2/V*sec from the Illinois-Clarkson radon chamber was measured using a specially-designed mobility spectrometer. Measurements were taken for different relative humidities and concentrations of SO2 in purified, compressed air. The mobility of a new-born Po-218 ion measured by the spectrometer was determined to be 1.92 cm^2/V*sec. A comparison of the theoretical and measured alpha track densities was completed to investigate the performance of the ion spectrometer. The resultant mobility spectra at low and high humidities were compared. A Condensation Nuclei Counter (CNC) was used for counting condensation nuclei in each experiment. In the case of low humidities, the reduction of available hydroxyl radical concentration, and hence the neutralization rate, leads to an increase of the shoulder around 1.35 cm^2/V*sec with increasing SO2. This suggests SO2 clustering around the PoO[x]+ ion. For a high humidity condition (5 ppm SO2, 30 % RH), there was clear droplet formation, H2SO4 clustering around the ion and also an increase in the background level between the high mobility peak and the droplet peak. A kinetics study for interpretation of the reaction processes was performed.