Mechanistic investigations of atmospheric H<sub>2</sub>SO<sub>4</sub> particle formation have been performed in a laboratory study taking either H<sub>2</sub>SO<sub>4</sub> from a liquid reservoir or using the gas-phase reaction of OH radicals with SO<sub>2</sub>. Applying both approaches for H<sub>2</sub>SO<sub>4</sub> generation simultaneously it was found that H<sub>2</sub>SO<sub>4</sub> evaporated from the liquid reservoir acts considerably less effective for the process of particle formation and growth than the products originating from the reaction of OH radicals with SO<sub>2</sub>. Furthermore, for NO<sub>x</sub> concentrations >5×10<sup>11</sup> molecule cm<sup>−3</sup> the formation of new particles from the reaction of OH radicals with SO<sub>2</sub> is inhibited. This suggests that substances other than H<sub>2</sub>SO<sub>4</sub> (potentially products from sulphur-containing peroxy radicals) trigger lower tropospheric new particle formation and growth. The currently accepted mechanism for SO<sub>2</sub> gas-phase oxidation does not consider the formation of such substances. The analysis of new particle formation for different reaction conditions in our experiment suggests that a contribution of impurities to the nucleation process is unlikely.