Fig. 1. Example new particle formation experiment from IP-OOM at −50 °C, without NO_x.

a–c, Evolution of vapour concentrations (a), particle-nucleation rates at 1.7 nm, J_1.7 (b), and naturally charged negative particle number size distribution (dN/dlogD_p) and growth rates measured between 3.2 and 8.0 nm, GR_3.2-8 (nm h⁻¹), for total (naturally charged + neutral) particles (c). The black lines in c depict the linear fits of 50% appearance time of particles between 3.2 and 8.0 nm. The vertical dashed lines and labels indicate the start of a new stage, at which the experimental conditions were adjusted. Trace sulfuric and iodic acid contaminants are present at the start of the run at concentrations of 1–5 × 10⁴ cm⁻³. Sulfur dioxide is injected and progressively increased during stages 5–8, which produces steps in H₂SO₄. The dotted black curve in b shows the expected H₂SO₄–NH₃ nucleation rate, conservatively assuming that NH₃ is present at the 4 pptv limit of detection. Stages 1 and 3 are, respectively, under neutral (ion-free) and beam (ion-enhanced) conditions, whereas all of the other stages are under galactic cosmic ray (GCR; natural ion concentrations) conditions. The experimental conditions are: isoprene = 0.20–0.41 ppbv (6.2–13.0 × 10⁹ cm⁻³), O₃ = 84–96 ppbv (2.6–3.0 × 10¹² cm⁻³), I₂ = 3.7–23.0 × 10⁵ cm⁻³, SO₂ = 0–4 × 10⁹ cm⁻³, OH = 1–3 × 10⁶ cm⁻³, HO₂ = 4.7–6.0 × 10⁷ cm⁻³, HO₂/OH ratio = 18–37, RH = 62%, NO less than limit of detection (7 pptv) and temperature = −49 °C.