Hygroscopicity frequency distributions of secondary organic aerosols

Secondary organic compounds are an important component of ambient aerosol and potentially lower the supersaturation that is required for individual particles to serve as cloud condensation nuclei (CCN). Secondary organic aerosol (SOA) formed from the oxidation of a single precursor can be composed of many different compounds and their overall CCN efficiency has been reported for many different SOA systems. An aerosol's CCN efficiency can be described by a single hygroscopicity parameter, kappa. However, this kappa comprises an unknown distribution of underlying kappa-values resulting from each individual compound in the SOA mixture. Here we report on a new technique for characterizing this distribution of kappa. Precursor hydrocarbons were oxidized in an environmental chamber to form SOA, which was collected on filters and extracted using ethyl acetate. Extracts were then fractionated by reversed-phase high-performance liquid chromatography using gradient elution with acetonitrile and water. The eluate was atomized, the solvent was removed by evaporation, and the residual aerosol particles were analyzed as a function of retention time using scanning flow CCN analysis. Kappa-values generally decreased with component retention time, consistent with expected decreasing polarity. Averaged SOA kappa-values reconstructed by integrating over the chromatogram agreed well with values measured for SOA sampled directly from the environmental chamber, suggesting that kappa for SOA represents the volume-weighted average of the constituent compounds' kappa-values. We anticipate that our measured hygroscopicity distributions will serve as validation points for mechanistic models that seek to predict the generation and evolution of organic aerosol composition and properties in the atmosphere.