Aerosol formation in the atmosphere is an important process to understand, in that such particles may act as the cloud condensation nuclei responsible for the 'cloud-climate' effect, and could locally be hazardous to health. The number-concentration of total atmospheric aerosols and cloud condensation nuclei is largely contributed by organic aerosols. Much of the organic aerosol is formed from atmospheric gas-to-particle conversion, and the common and widespread non-methane hydrocarbons emitted by vegetation have been investigated as possible precursors. But strong evidence for a quantitative link between biogenic hydrocarbon emission and organic aerosol formation has so far been lacking. Here we present measurements of gaseous and particulate atmospheric species from a forested area to show that some hydrocarbons (for example, terpenes) emitted by vegetation are photo-oxidized to organic acids (for example, pinonic acids), which condense to form organic aerosols. Thus the forests, through their production of large quantities of organic aerosols, could be of considerable significance both for climate (through cloud-condensation-nuclei formation) and for heterogeneous atmospheric chemical processes.