Extended Data Figure 5. Ion-induced nucleation event without H₂SO₄, measured in the NAIS.

a, b, Example of a nucleation event showing the growth versus time of positive (a) and negative (b) charged particles at 530 p.p.t.v. α-pinene, 35 p.p.b.v. O₃, zero H₂ or HONO, 3.4 × 10⁷ cm⁻³ HOM, 38% relative humidity, 278 K and [H₂SO₄] < 5 × 10⁴ cm⁻³. The colour scale shows the concentration of ions and charged particles. The clearing field high voltage was turned off at 06:48, marking the start of GCR ionization conditions in the chamber, and the α-pinene flow into the chamber was stopped at 10:52. Ion-induced nucleation can be seen for positive and negative charged particles, followed by rapid growth to sizes above 10 nm. Ion–ion recombination progressively neutralizes the charged particles as they grow, but some reappear at larger sizes, owing to diffusion charging.