Extended Data Fig. 6. Particle-nucleation rates at 2.5 nm versus vapour concentrations.

Nucleation rates at 2.5 nm, J_2.5, versus IP_0N (a,b), HIO_x + H₂SO₄ (c) and the product (HIO_x + H₂SO₄) × IP_0N (d). Panel a and b show experiments without and with added NO_x. Panel c shows all experiments and panel d shows experiment with acids, both with and without NO_x. IP_0N excludes nitrogen-containing isoprene-oxygenated molecules. The solid lines in panel c show the nucleation rates at 1.7 nm expected for H₂SO₄ with 4 pptv NH₃ at 60% RH (ref. ⁴⁴). The dashed and solid lines in panel d represent fits to the equation 10^{a×log10(x)+b}, in which, for the dashed line, a = 1.167 and b = −14.743, and for the solid line, a = 1.669 and b = −22.707. Panels a and b show that both total acid (HIO_x + H₂SO₄) and IP_0N contribute to the nucleation rate. The experimental conditions are: isoprene = 0.04–1.50 ppbv (0.1–4.2 × 10¹⁰ cm⁻³), O₃ = 1–590 ppbv (3.7 × 10¹⁰ to 1.8 × 10¹³ cm⁻³), I₂ = 0–7.5 × 10⁷ cm⁻³, SO₂ = 0–4.6 × 10⁹ cm⁻³, OH = 0.11–6.90 × 10⁷ cm⁻³, HO₂ = 0.6–18.0 × 10⁸ cm⁻³, HO₂/OH ratio = 11–118, NO = 0–0.22 ppbv, NO₂ = 0–0.77 ppbv, RH = 29–70% and temperature = −30 °C and −50 °C. All experiments are carried out under galactic cosmic ray ionization (ambient conditions). The error bars represent the standard deviation of the measurement at steady state.