Atmospheric processing of organic aerosols

We suggest a chemical model for the composition, structure, and atmospheric processing of organic aerosols. This model is stimulated by recent field m easurements showing that organic compounds are a significant component of a tmospheric aerosols. The proposed model organic aerosol is an "inverted mic elle" consisting of an aqueous core that is encapsulated in an inert, hydro phobic organic monolayer. The organic materials that coat the aerosol parti cles are surfactants of biological origin. We propose a chemical mechanism by which the organic surface layer will be processed by reactions with atmo spheric 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 org anic aerosols can grow by water accretion and form cloud condensation nucle i, influencing atmospheric radiative transfer. Radiative transfer may be af fected directly by the chromophores left on the surface of the aerosol afte r chemical transformation. The chemical model yields certain predictions wh ich are testable by observations. Among them is a curve of the percent orga nic material as a function of particle diameter which predicts that a high fraction of the mass of the upper tropospheric aerosol will be organic. Atm ospheric processing of organic aerosols will lead to the release of small o rganic fragments into the troposphere which will play a subsequent role in homogeneous chemistry. Organic aerosols are likely to act as a transport ve hicle of organics and other water insoluble compounds into the atmosphere. We speculate that biomass burning will produce a similar coating of surfact ants derived from land sources. Finally, it is pointed out that the radical -induced transformation of the surface layer of aerosol particles from hydr ophobic to hydrophilic offers an additional means by which the biosphere, t hrough atmospheric chemistry, can affect the radiative balance.