Field measurement of positive ion mobility and mass spectra at a Colorado site in winter

Mobility and mass spectra of positive ions were measured near ground level in winter time. The mobility distribution of positive ions generated from ambient air and aged in the 0.1‐s range is strongly dependent on air temperature. At low temperature, two peaks were observed in the mobility range > 1.5 cm2 V−1 s−1 and interpreted as H3O+ and NH4+ clusters. Ions with mobilities < 1.5 cm2V−1 s−1 became dominant with increasing temperature and ion age. It is suggested that the neutral species responsible for these ions have soil and vegetation sources, the emissions from which are temperature dependent. Mass spectra were obtained via proton transfer reactions to trace gases with proton affinities > 185.6 kcal/mol (from ethanolH+) or > 194.1 kcal/mol (from acetoneH+). The mass spectra contain NH4+ clusters and numerous unidentified peaks extending to the highest mass scanned, 450 amu. Ions in the high mass range > 200 amu also show temperature dependence similar to ions with lower mobilities.