The supplementary information provides details on the typical measurement sequence for nucleation rates under different conditions: neutral (Jₙ), galactic cosmic ray (Jₐₐₐ), and charged pion beam (Jₐₐₐ). Neutral nucleation rates are measured without any beam and with high voltage to exclude ion-induced nucleation. Galactic cosmic ray nucleation rates are measured with a beam stopper blocking pions, while charged pion beam nucleation rates are measured with the beam stopper open and a time-averaged pion beam rate. The ion pair concentration varies between 400 cm⁻³ for Jₐₐₐ and 3000 cm⁻³ for Jₐₐₐ. Radon decay contributes significantly to ionization in the boundary layer but not in the CLOUD chamber due to the use of cryogenic liquid nitrogen and oxygen. The measurement sequence involves establishing experimental conditions, clearing the chamber of pre-existing aerosols, and applying high voltage to sweep ions. The run starts with the UV system shutter opening to establish a chosen [H₂SO₄] concentration. Particles appear in the counters after a time delay, and nucleation rates are derived from particle formation rates. After measuring the neutral nucleation rate, the clearing field is turned off to allow GCRs to generate ion pairs, leading to a sharp increase in the nucleation rate due to ion-induced nucleation. The CERN pion beam is then turned on, causing another sharp increase in the nucleation rate. The run ends by closing the UV shutter and starting to clear the chamber for the next run.The supplementary information provides details on the typical measurement sequence for nucleation rates under different conditions: neutral (Jₙ), galactic cosmic ray (Jₐₐₐ), and charged pion beam (Jₐₐₐ). Neutral nucleation rates are measured without any beam and with high voltage to exclude ion-induced nucleation. Galactic cosmic ray nucleation rates are measured with a beam stopper blocking pions, while charged pion beam nucleation rates are measured with the beam stopper open and a time-averaged pion beam rate. The ion pair concentration varies between 400 cm⁻³ for Jₐₐₐ and 3000 cm⁻³ for Jₐₐₐ. Radon decay contributes significantly to ionization in the boundary layer but not in the CLOUD chamber due to the use of cryogenic liquid nitrogen and oxygen. The measurement sequence involves establishing experimental conditions, clearing the chamber of pre-existing aerosols, and applying high voltage to sweep ions. The run starts with the UV system shutter opening to establish a chosen [H₂SO₄] concentration. Particles appear in the counters after a time delay, and nucleation rates are derived from particle formation rates. After measuring the neutral nucleation rate, the clearing field is turned off to allow GCRs to generate ion pairs, leading to a sharp increase in the nucleation rate due to ion-induced nucleation. The CERN pion beam is then turned on, causing another sharp increase in the nucleation rate. The run ends by closing the UV shutter and starting to clear the chamber for the next run.