Fig. 2. Particle-nucleation rates from IP-OOM at −30 °C and −50 °C, with variable NO_x.

a–d, Nucleation rates at 1.7 nm, J_1.7, versus IP_0N without NO_x present (a), IP_0N with NO_x (b), HIO_x + H₂SO₄ with and without NO_x (c) and the product (HIO_x + H₂SO₄) × IP_0N with and without NO_x (d). Measurements without NO_x are indicated by diamonds (without acids) and circles (with acids). Measurements with NO_x are indicated by triangles (without acids) and squares (with acids). Hollow symbols indicate −30 °C and solid symbols indicate −50 °C. In c, nucleation rates measured at contaminant acid concentrations are assigned the acid limit of detection (around 2 × 10⁴ cm⁻³). The solid lines in c show the nucleation rates expected for H₂SO₄ with 4 pptv NH₃ at −30 °C (red) and −50 °C (blue), both at 60% RH (ref. ⁴⁴). The dashed and solid lines in d represent fits to the equation 10^{a×log10(x)+b}, in which, for the dashed line, a = 1.241 and b = −15.065, and for the solid line, a = 1.505 and b = −19.948. Panels a–c show that both IP_0N and total acid (HIO_x + H₂SO₄) contribute to the nucleation rate. Panel d indicates that it is the product of IP_0N and total acid (that is, the dimer formation rate) that best describes the nucleation rate, J_1.7, as the data points cluster into two groups primarily characterized by temperature alone. IP_1-2N also contribute to particle nucleation but they are less effective than IP_0N (Extended Data Fig. 5). 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 = 0.6–17.0 × 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. The error bars represent the standard deviation of the measurement at steady state. All measurements are made under galactic cosmic ray conditions (natural ionization amounts).