Atmospheric new particle formation is generally thought to occur due to homogeneous or ion-induced nucleation of sulphuric acid. We compare ambient nucleation rates with laboratory data from nucleation experiments involving either sulphuric acid or oxidized SO<sub>2</sub>. Atmospheric nucleation occurs at H<sub>2</sub>SO<sub>4</sub> concentrations 2–4 orders of magnitude lower than binary or ternary nucleation rates of H<sub>2</sub>SO<sub>4</sub> produced from a liquid reservoir, and atmospheric H<sub>2</sub>SO<sub>4</sub> concentrations are very well replicated in the SO<sub>2</sub> oxidation experiments. We hypothesize these features to be due to the formation of free HSO<sub>5</sub> radicals in pace with H<sub>2</sub>SO<sub>4</sub> during the SO<sub>2</sub> oxidation. We suggest that at temperatures above ~250 K these radicals produce nuclei of new aerosols much more efficiently than H<sub>2</sub>SO<sub>4</sub>. These nuclei are activated to further growth by H<sub>2</sub>SO<sub>4</sub> and possibly other trace species. However, at lower temperatures the atmospheric relative acidity is high enough for the H<sub>2</sub>SO<sub>4</sub>–H<sub>2</sub>O nucleation to dominate.