In this paper we describe experimental results that demonstrate that chemical and physical properties of the organic aerosol material in tropical trade winds are drastically different from the organic aerosol component typically found in fossil fuel combustion influenced atmospheres. These experiments were performed at a Caribbean location (Cape San Juan, Puerto Rico), and at a semiurban site influenced by common anthropogenic pollution (Berkeley, California). The results show that the volatility of the polluted and the (largely natural) tropical organic aerosols is vastly different. Specifically, we find that the tropical organic aerosol material is highly volatile. The condensed phase organic carbon (OC) appears to exist in equilibrium with its gas-phase counterpart. Its mass concentration drastically decreases if the concentration of the gas-phase is reduced. The condensed phase OC adjusts quickly to the new equilibrium when the equilibrium at the sampler inlet is perturbed. Consequently, the mass concentration of condensed phase OC in tropical aerosols may be governed by both natural and sampling induced changes in temperature and pressure that alter the gas-particle partitioning. As a result, analyses based on conventional filter techniques may result in OC mass concentrations that are not representative of actual ambient concentrations. In contrast, the anthropogenic aerosol OC is much more stable and not appreciably influenced by changes in gas-phase organic compound concentrations. ¿ 2000 American Geophysical Union