Tropospheric aerosol can affect climate and the chemistry of the atmosphere. Organic particulates form a significant fraction of the atmospheric suspended matter over forested areas and may originate to a large extent from the oxidation of natural hydrocarbons. A three-dimensional global model of the troposphere is used to evaluate the contribution to the global organic aerosol (OA) source of the secondary organic aerosol (SOA) derived from the ozonolysis of biogenic volatile organic compounds (BVOC) and its evolution since preindustrial times. BVOC have been represented by a mixture of α- and β-pinenes, and their aerosol-forming parameters and chemical reactivities versus O3, OH, and NO3 have been estimated using laboratory information. An important factor in SOA formation is the deposition of condensable aerosol oxidation products onto preexisting organic aerosol, and this has been taken into account. The thus-calculated source of SOA is evaluated to have increased from 17--28 Tg/yr in preindustrial times to 61--79 Tg/yr at present. This threefold to fourfold enhancement of the formation of organic aerosol from natural BVOC is attributed to an increase in ozone and organic aerosol from anthropogenic sources. The main uncertainties involved in our calculations are related to the composition of BVOC emissions and the details of their aerosol formation capabilities. ¿ 2000 American Geophysical Union<