Mobility distribution of acetone cluster ions

Mobility distributions of both positive and negative cluster ions in acetone rich air were studied. A bipolar radioactive charger was used to irradiate air which contained acetone vapour within a wide range of concentrations ( 100–300,000 ppm). The ion mobility distributions were measured with two instruments: a differential mobility analyzer (DMA) and and a time-of-flight mobility spectrometer (TOF). Low concentrations of acetone (100–1000 ppm) appeared to set the mean mobility of the positive ions to 1.3–1.5 cm2V−1s−1 and also to increase the unimodality of the spectrum, whereas the negative ion spectrum was affected only by acetone concentrations higher than some 1000 ppmʹs. With even higher acetone vapour concentrations the mobilities of both the positive and negative ions were seen to decrease, apparently due to increased vapour molecule attachment onto the ions, and approaching each other, indicative of a thermodynamical and polarity-insensitive behaviour. The concentration range of acetone was, thus, concluded to consist of two separate ranges. The “low concentration range” is dominated by ion-molecule reactions of acetone and other impurities. At the “high concentration range” the ions were seen to behave like “pre-nucleation embryos” as expected according to ion-induced nucleation theory. At high concentrations, the large acetone cluster ions may already be considered close to charged ultrafine aerosol particles.