Atmos. Chem. Phys. Discuss., 9, 1669-1702, 2009
www.atmos-chem-phys-discuss.net/9/1669/2009/
doi:10.5194/acpd-9-1669-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Increased cloud activation potential of secondary organic aerosol for atmospheric mass loadings
S. M. King1, T. Rosenoern1, J. E. Shilling1,*, Q. Chen1, and S. T. Martin1
1School of Engineering and Applied Sciences & Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
*now at: Pacific Northwest National Laboratory, Atmospheric Sciences and Global Change Division, Richland, WA 99352, USA

Abstract. The effect of organic particle mass loading from 1 to ≥100 μg m−3 on the cloud condensation nuclei (CCN) properties of mixed organic-sulfate particles was investigated in the Harvard Environmental Chamber. Mixed particles were produced by the condensation of organic molecules onto ammonium sulfate particles during the dark ozonolysis of α-pinene. A continuous-flow mode of the chamber provided stable conditions over long time periods, allowing for signal integration and hence increased measurement precision at low organic mass loadings representative of atmospheric conditions. CCN activity was measured at eight mass loadings for 80- and 100-nm particles grown on 50-nm sulfate seeds. A two-component (organic/sulfate) K√∂hler model, which included the particle heterogeneity arising from DMA size selection and from organic volume fraction for the selected 80- and 100-nm particles, was used to predict CCN activity. For organic mass loadings of 2.9 μg m−3 and greater, the observed activation curves were well predicted using a single set of physicochemical parameters for the organic component. For mass loadings of 1.74 μg m−3 and less, the observed CCN activity increased beyond predicted values using the same parameters, implying changed physicochemical properties of the organic component. Of possible changes in surface tension, effective molecular weight, and effective density, a sensitivity analysis implicated a decrease of up to 10% in surface tension at low mass loadings as the plausible dominant mechanism for the observed increase in CCN activity.

Citation: King, S. M., Rosenoern, T., Shilling, J. E., Chen, Q., and Martin, S. T.: Increased cloud activation potential of secondary organic aerosol for atmospheric mass loadings, Atmos. Chem. Phys. Discuss., 9, 1669-1702, doi:10.5194/acpd-9-1669-2009, 2009.
 
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