Secondary organic aerosols are believed to make an important contribution to the atmospheric radiation budget, though the exact mechanisms responsible for their formation are still uncertain. We compute a remarkably large contribution of hydroperoxides from the oxidation of biogenic monoterpenes. Although currently neglected in aerosol models, the hydroperoxides lead to 63% of our calculated global secondary organic aerosol formation, whereas carboxylic acids, usually considered as the dominant secondary organic aerosol precursors, were responsible for only 26%. Detailed studies are needed to determine the implications of this aerosol formation pathway for the total atmospheric aerosol burden and its impacts on climate.