First Differential Mobility Analysis -DMA- Measurements of Air Ions Produced by Radioactive Source and Corona

A recently developed Differential Mobility Analyzer (DMA) of high resolving power (50 for ions of mobility equivalent diameter ~1 nm) has been used to measure the mobility distribution of laboratory air ions generated by ²⁴¹Am and corona discharge. In the case of ²⁴¹Am, it has been possible to follow the changes in mobility during the early stages of ion aging: within the time interval between 10 and 30 ms after ion formation, the mean reduced mobility of positive ions decreased from 1.30 to 1.15 cm²/Vs, and that of negative ions from 2.0 to 1.75 cm²/Vs. The mobility distribution of air ions at the outlet of a corona ionizer strongly depends on the corona voltage, mainly due to the mobility-dependent extent of electrostatic losses within the ionizer. The mean reduced mobility of corona ions ranged between 1.01 and 1.20 cm²/Vs (positive) and 1.76 and 1.96 cm²/Vs (negative).