Relating cloud condensation nuclei activity and oxidation level of α-pinene secondary organic aerosols

During a series of smog chamber experiments, the effects of chemical and photochemical aging on the ability of organic aerosols generated from ozonolysis of alpha-pinene to act as cloud condensation nuclei (CCN) were investigated. In particular, the study focused on the relation between oxygenation and the CCN-derived single hygroscopicity parameter kappa for different experimental conditions: varying precursor concentrations (10-40 ppb), different OH sources (photolysis of HONO either with or without the addition of NO or ozonolysis of tetramethylethylene), and exposure to light. Oxygenation was described by the contribution of the aerosol mass spectrometer (AMS) mass fragment m/z 44 to the total organic signal (f(44)) and the oxygen to carbon molar ratio (O/C), likewise determined with AMS. CCN activity, described by the hygroscopicity parameter kappa, was determined with a CCN counter. It was found that f(44) increases with decreasing precursor concentration and with chemical aging, whereas neither of these affects CCN activity. Overall, kappa is largely independent of O/C in the range 0.3 < O/C < 0.6 (0.07 < f(44) < 0.12), although an empirical unweighted least squares fit was determined: kappa = (0.071 +/- 0.02) . (O/C) + (0.0785 +/- 0.009) for particles with diameter in the range 59-200 nm. Growth kinetics of activating secondary organic aerosols were found to be comparable to those of ammonium sulfate and were not influenced by chemical aging.