Articles | Volume 11, issue 7
Research article
24 Jul 2018
Research article |  | 24 Jul 2018

Determining the link between hygroscopicity and composition for semi-volatile aerosol species

Joel Alroe, Luke T. Cravigan, Marc D. Mallet, Zoran D. Ristovski, Branka Miljevic, Chiemeriwo G. Osuagwu, and Graham R. Johnson

Abstract. Internally and externally mixed aerosols present significant challenges in assessing the hygroscopicity of each aerosol component. This study presents a new sampling technique which uses differences in volatility to separate mixtures and directly examine their respective composition and hygroscopic contribution. A shared thermodenuder and unheated bypass line are continuously cycled between an aerosol mass spectrometer and a volatility and hygroscopicity tandem differential mobility analyser, allowing real-time comparative analysis of heated and unheated aerosol properties. Measurements have been taken of both chamber-generated secondary organic aerosol and coastal marine aerosol at Cape Grim, Australia, to investigate system performance under diverse conditions. Despite rapidly changing aerosol properties and the need to restrict analysis to a narrow size range, the former experiment separated the hygroscopic influences of ammonium sulfate and two distinct organic components with similar oxygen to carbon ratios but different volatilities. Analysis of the marine aerosol revealed an external mixture of non-sea-salt sulfates and sea spray aerosol, which likely shared similar volatile fractions composed of sulfuric acid and a non-hygroscopic organic component.

Short summary
This study describes a new volatility-based method to directly examine the composition and corresponding hygroscopic contribution of mixed aerosol components. Measurements of chamber-generated secondary organic aerosol and coastal marine aerosol demonstrated effective separation of both internal and external mixtures. In each case, the findings enabled composition-based models to reliably reproduce observed particle hygroscopicities.