The cloud condensation nuclei (CCN) properties of ammonium sulfate particles mixed with organic material condensed during the hydroxyl-radical-initiated photo-oxidation of isoprene (C5H8) were investigated in the continuous-flow Harvard Environmental Chamber. The experiments were designed to cover a range of atmospheric conditions, spanning the urban and rural southeastern U.S. during the summer to pristine background conditions of the Amazon Basin’s wet season. The CCN activation curves were measured for organic particle mass concentrations of 0.5 to 10.0 ug m^-3, NOx concentrations from under 0.4 ppbv up to 38 ppbv, particle mobility diameters from 70 to 150 nm, and thermodenuder temperatures from 25°C to 100°C. At 25°C, the observed CCN activation curves were accurately described by a Kohler model with two internally mixed components: ammonium sulfate and secondary organic material. The modeled physicochemical parameters of the organic material were equivalent to an effective hygroscopicity parameter kappa (ORG) of 0.10 +/- 0.03, regardless of the C5H8:NOx concentration ratio for the span of > 200:0.4 to 50:38 (ppbv:ppbv). The volatilization curves (i.e., plots of the residual organic volume fraction against temperature) were also similar for the span of investigated C5H8:NOx ratios, suggesting a broad similarity of particle chemical composition. This suggestion was backed up by limited variance at 25°C among the particle mass spectra. For example, the signal intensity at m/z 44 (which can result from the fragmentation of oxidized molecules believed to affect hygroscopicity and CCN properties) varied weakly from 6–9% across the range of investigated conditions. In contradistinction to the results for 25°C, conditioning up to 100°C in the thermodenuder significantly reduced CCN activity. The altered CCN activity might be explained by chemical reactions (e.g., decomposition or oligomerization) of the secondary organic material at elevated temperatures. The study’s results at 25°C, in conjunction with the results of other chamber and field studies for a diverse range of conditions, suggest that a value of 0.10 +/- 0.05 for kappa (ORG) is representative of both anthropogenic and biogenic secondary organic material. This finding supports the use of kappa (ORG) as a simplified yet accurate general parameter to represent the CCN activation of secondary organic material in large-scale atmospheric and climate models.
