Articles | Volume 10, issue 8
Atmos. Chem. Phys., 10, 3953–3964, 2010
Atmos. Chem. Phys., 10, 3953–3964, 2010

  27 Apr 2010

27 Apr 2010

Cloud droplet activation of mixed organic-sulfate particles produced by the photooxidation of isoprene

S. M. King1,2, T. Rosenoern1,2, J. E. Shilling1,3, Q. Chen1, Z. Wang1,4, G. Biskos5,6, K. A. McKinney7, U. Pöschl8, and S. T. Martin1,9 S. M. King et al.
  • 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

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.

Final-revised paper