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 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. ¿ 1999 American Geophysical Union