Extended Data Fig. 2. IP_0N yield from OH reaction with C₅H₁₀O₃.

Measured IP_0N concentration versus reacted C₅H₁₀O₃ (see Methods for details). The dashed lines show the predictions for several IP_0N molar yields; a yield of 100% implies that each OH reaction with C₅H₁₀O₃ produces one IP_0N molecule. The data indicate that the yield is between 20% and 50%, with an overall systematic uncertainty of a factor of two. When NO_x is added to the system, the yield of IP_0N is reduced, owing to NO terminating isoprene peroxy radicals. The ratios of IEPOX and ISOPOOH in C₅H₁₀O₃ are determined from the measured OH concentrations and the fit to the simulation shown in Extended Data Fig. 1b. For simplification, we apply a general reaction-rate coefficient of 10⁻¹⁰ and 10⁻¹¹ cm³ s⁻¹ for the ISOPOOH + OH and IEPOX + OH reactions, respectively³⁵. 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 = 11–117, NO = 0–0.22 ppbv, NO₂ = 0–0.77 ppbv, RH = 29–70% and temperature = −30 °C and −50 °C.