Hygroscopicity frequency distributions of secondary organic aerosols

Description

Secondary organic compounds contribute to ambient aerosol and lower the supersaturation that is required for individual particles to serve as cloud condensation nuclei (CCN). Secondary organic aerosol (SOA) formed from a single reaction can be composed of hundreds of different compounds, and their overall CCN efficiency has been reported for many different SOA systems. The relative contribution of different compounds to the overall CCN efficiency can be described by a single hygroscopicity parameter, kappa. However, this kappa comprises an unknown distribution of underlying kappas. Here we report on a new technique characterizing this distribution. Precursor compounds were oxidized in an environmental chamber to form SOA, collected on filters, extracted using a mixture of acetonitrile and water, passed through a high-pressure liquid chromatography column, and measured as a function of retention time using scanning flow CCN analysis. Kappa generally decreased with retention time, reflecting in part the sorting of the products by polarity. Overall kappa values reconstructed by integrating over the chromatogram agreed well with online measurements from the environmental chamber, suggesting that kappa for SOA represents the volume-weighted average of the constituent compounds’ kappa values. These distributions show that many SOAs consist of a continuum of products with 0 < kappa < 0.3, rather than a pseudo-binary mixture of water-soluble and water-insoluble compounds. We anticipate that our measured hygroscopicity distributions will serve as validation points for mechanistic models that treat the generation and evolution of organic aerosols in the atmosphere.