We suggest a chemical model for the composition, structure, and atmospheric processing of organic aerosols. This model is stimulated by recent field measurements showing that organic compounds are a significant component of atmospheric aerosols. The proposed model organic aerosol is an inverted micelle consisting of an aqueous core that is encapsulated in an inert, hydrophobic organic monolayer. The organic materials that coat the aerosol particles are surfactants of biological origin. We propose a chemical mechanism by which the organic surface layer will be processed by reactions with atmospheric radicals. The net result of an organic aerosol being exposed to an oxidizing atmosphere is the transformation of an inert hydrophobic film to a reactive, optically active hydrophilic layer. Consequently, processed organic aerosols can grow by water accretion and form cloud condensation nuclei, influencing atmospheric radiative transfer. Radiative transfer may be affected directly by the chromophores left on the surface of the aerosol after chemical transformation. The chemical model yields certain predictions which are testable by observations. Among them is a curve of the percent organic material as a function of particle diameter which predicts that a high fraction of the mass of the upper tropospheric aerosol will be organic. Atmospheric processing of organic aerosols will lead to the release of small organic fragments into the troposphere which will play a subsequent role in homogeneous chemistry. Organic aerosols are likely to act as a transport vehicle of organics and other water insoluble compounds into the atmosphere. We speculate that biomass burning will produce a similar coating of surfactants derived from land sources. Finally, it is pointed out that the radical-induced transformation of the surface layer of aerosol particles from hydrophobic to hydrophilic offers an additional means by which the biosphere, through atmospheric chemistry, can affect the radiative balance. ¿ 1999 American Geophysical Union