1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
2now at: Department of Chemistry, University of Copenhagen, Copenhagen, Denmark
3now at: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA
4now at: Environmental Research Institute, Shandong University, Shandong, China
5Department of Environment, University of the Aegean, Mytilene, Greece
6Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands
7Department of Chemistry, Amherst College, Amherst, Massachusetts, USA
8Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany
9Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
Received: 25 Nov 2009 – Discussion started: 07 Jan 2010
Abstract. The cloud condensation nuclei (CCN) properties of ammonium sulfate particles mixed with organic material condensed during the hydroxyl-radical-initiated photooxidation of isoprene (C5H8) were investigated in the continuous-flow Harvard Environmental Chamber. CCN activation curves were measured for organic particle mass concentrations of 0.5 to 10.0 μg 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 to 100 °C. At 25 °C, the observed CCN activation curves were accurately described by a Köhler model having two internally mixed components, namely ammonium sulfate and secondary organic material. The modeled physicochemical parameters of the organic material were equivalent to an effective hygroscopicity parameter κORG 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 supported 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 to 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 κORG is representative of both anthropogenic and biogenic secondary organic material. This finding supports the use of κORG as a simplified yet accurate general parameter to represent the CCN activation of secondary organic material in large-scale atmospheric and climate models.
Revised: 22 Mar 2010 – Accepted: 13 Apr 2010 – Published: 27 Apr 2010
King, S. M., Rosenoern, T., Shilling, J. E., Chen, Q., Wang, Z., Biskos, G., McKinney, K. A., Pöschl, U., and Martin, S. T.: Cloud droplet activation of mixed organic-sulfate particles produced by the photooxidation of isoprene, Atmos. Chem. Phys., 10, 3953-3964, doi:10.5194/acp-10-3953-2010, 2010.