Ambient gas/particle partitioning. 1. Sorption mechanisms of apolar, polar, and ionizable organic compounds

There remain several ambiguities in the literature regarding the dominating sorption mechanisms involved in gas/particle partitioning, particularly for polar and ionizable compounds. The various hypothetical mechanisms would depend differently on relative humidity (RH) and the presence of various aerosol components. Thus, in order to resolve these ambiguities, here we measured the RH-dependency of gas/particle partitioning constants, Kip, for four diverse aerosol samples and a large set of chemicals covering apolar, polar, and ionizable organic compounds. In addition, we also removed the water-soluble components from two ambient particle samples to study how their presence influences sorption behavior. The measured Kip values collectively indicate that a dual-phase sorption mechanism is occurring, in which organic compounds partition into a RH-independent water-insoluble organic matter phase and additionally into a RH-dependent mixed-aqueous phase. All Kip values could be successfully fitted to a RH-dependent dual-phase sorption model. The trends in Kip data further support findings that the sorption behavior of ambient aerosol samples is different from raw mineral surfaces and soot.