Differences in the volatility of organic aerosols in unpolluted tropical and polluted continental atmospheres

In this paper we describe experimental results that demonstrate that chemic al and physical properties of the organic aerosol material in tropical trad e winds an drastically different from the organic aerosol component typical ly found in fossil fuel combustion influenced atmospheres. These experiment s 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. Specifical ly, we find that the tropical organic aerosol material is highly volatile. The condensed phase organic carbon (OC) appears to exist in equilibrium wit h its gas-phase counterpart. Its mass concentration drastically decreases i f the concentration of the gas-phase is reduced. The condensed phase OC adj usts quickly to the new equilibrium when the equilibrium at the sampler inl et 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 partitionin g. As a result, analyses based on conventional filter techniques may result in OC mass concentrations that are not representative of actual ambient co ncentrations. In contrast, the anthropogenic aerosol OC is much more stable and not appreciably influenced by changes in gas-phase organic compound co ncentrations.