The nucleation rates (J cm⁻³s⁻¹) are measured under neutral (Jn), galactic cosmic ray (Jgr), or charged pion beam (Jch) conditions. For Jgr, the chamber is irradiated by GCRs and a small component of penetrating beam muons, while for Jch, the pion beam is set to a time-averaged rate of (5-6)×10⁴ s⁻¹. Neutral nucleation rates are measured without any beam and with the high voltage (HV) of the clearing field electrodes set to ±30 kV, excluding ion-induced nucleation. Both Jgr and Jch include ion-induced and neutral nucleation rates, while Jn measures only neutral nucleation. Ion pair concentrations are around 400 cm⁻³ for Jgr and 3000 cm⁻³ for Jch. Radon decay contributes more ionization than cosmic rays in continental boundary layers but not over oceans or above the boundary layer. The CLOUD chamber does not have radon decay as the air is prepared from cryogenic liquid nitrogen and oxygen. Figure S2 shows a typical measurement sequence. The experimental conditions are established, and the chamber is cleared of pre-existing aerosols. High voltage is applied to sweep ions from the chamber. The UV system is opened to establish a chosen [H2SO4] in the chamber. Particles appear in aerosol counters after a time delay depending on growth rate and detection size threshold. Nucleation rates are derived from the formation rates measured in the particle counters. When Jn is measured, the clearing field is turned off, allowing GCRs to generate ion pairs. Larger cluster ions appear and build up over time, leading to an increase in nucleation rate due to ion-induced nucleation. The CERN pion beam is then turned on, causing a further sharp increase in nucleation rate corresponding to Jch. At low NH3 concentrations, ion-induced nucleation involves negatively-charged clusters. The run ends by closing the UV shutter, turning on the clearing field HV, and starting to clear the chamber. Supplementary references are provided.The nucleation rates (J cm⁻³s⁻¹) are measured under neutral (Jn), galactic cosmic ray (Jgr), or charged pion beam (Jch) conditions. For Jgr, the chamber is irradiated by GCRs and a small component of penetrating beam muons, while for Jch, the pion beam is set to a time-averaged rate of (5-6)×10⁴ s⁻¹. Neutral nucleation rates are measured without any beam and with the high voltage (HV) of the clearing field electrodes set to ±30 kV, excluding ion-induced nucleation. Both Jgr and Jch include ion-induced and neutral nucleation rates, while Jn measures only neutral nucleation. Ion pair concentrations are around 400 cm⁻³ for Jgr and 3000 cm⁻³ for Jch. Radon decay contributes more ionization than cosmic rays in continental boundary layers but not over oceans or above the boundary layer. The CLOUD chamber does not have radon decay as the air is prepared from cryogenic liquid nitrogen and oxygen. Figure S2 shows a typical measurement sequence. The experimental conditions are established, and the chamber is cleared of pre-existing aerosols. High voltage is applied to sweep ions from the chamber. The UV system is opened to establish a chosen [H2SO4] in the chamber. Particles appear in aerosol counters after a time delay depending on growth rate and detection size threshold. Nucleation rates are derived from the formation rates measured in the particle counters. When Jn is measured, the clearing field is turned off, allowing GCRs to generate ion pairs. Larger cluster ions appear and build up over time, leading to an increase in nucleation rate due to ion-induced nucleation. The CERN pion beam is then turned on, causing a further sharp increase in nucleation rate corresponding to Jch. At low NH3 concentrations, ion-induced nucleation involves negatively-charged clusters. The run ends by closing the UV shutter, turning on the clearing field HV, and starting to clear the chamber. Supplementary references are provided.