This work concentrates on the simultaneous mobility and mass measurement of negative ions generated by the ionizing radiation in a ~(241)Am aerosol charger in N2 (5.0), a l:l-mixture of N2 and synthetic air, pure synthetic air (5.0), and filtered laboratory air at ~30 relative humidity. Therefore, a high-resolution mobility analyzer (UDMA) and an atmospheric pressure interface time-of-Hight mass spectrometer (APi-TOF) were operated in series. Experiments with N2 as carrier gas showed a dominating signal at an electrical mobility of 2.09 cm~2/Vs with 90 of the ions being nitrate based. The ion composition was altered after a baking-out to a spectrum with three strong mobility-peaks at Z1 = 2.34 cm~2/Vs, Z2 = 1.42 cm~2/Vs, Z3 = 1.08 cm~2/Vs and a higher diversity of ions in the corresponding mass spectra. The carrier gas was gradually changed from N2 (5.0) to a l:l-mixture of N2 with synthetic air and pure synthetic air (5.0), having only a minor effect on the overall pattern of the ion spectrum. Using room air leads to a domination of the nitrate based ions. The mobility-dependent transmission efficiency of the UDMA was modeled using an empirical, laminar diffusion deposition model. The data were further compared to an empirical mass-mobility relationship to evaluate the fragmentation of the ion clusters in the inlet of the mass spectrometer. This study suggests that the nitrate ion, NO3-, is found to be the dominant ion species produced in an aerosol charger, and that it may be mostly responsible for the charging of aerosol particles in negative polarity.
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