The hygroscopicity parameter (κ) of ambient organic aerosol at a field site subject to biogenic and anthropogenic influences: Relationship to degree of aerosol oxidation

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Abstract
Cloud condensation nuclei (CCN) concentrations were measured at a rural site in Ontario, Canada during the spring of 2007. The CCN concentrations were compared to values predicted from the aerosol chemical composition and size distribution using κ-Köhler theory. The hygroscopicity of the organic component was characterised by two methods, both of which are based on the aerosol's degree of oxygenation as determined by the mass spectra measured with an Aerodyne aerosol mass spectrometer. The first approach uses a statistical technique, positive matrix factorization (PMF), to separate hygroscopic and non-hygroscopic factors while the second uses the O/C, which is an indication of the aerosol's degree of oxygenation. In both cases, the hygroscopicity parameter (κ) of the organic component is varied so that the predicted and measured CCN concentrations are internally consistent and in good agreement. By focussing on a small number of organic components defined by their composition, we can simplify the estimates needed to describe the aerosol's hygroscopicity. We find that κ of the oxygenated organic component from the PMF analysis is 0.20±0.03 while κ of the entire organic component can be parameterized as κorg=(0.30±0.05)×(O/C).
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