Atmos. Chem. Phys. Discuss., 12, 24677-24733, 2012
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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.
Analysis of CCN activity of Arctic aerosol and Canadian biomass burning during summer 2008
T. L. Lathem1, A. J. Beyersdorf2, K. L. Thornhill2,3, E. L. Winstead2,3, M. J. Cubison4,*, A. Hecobian1,**, J. L. Jimenez4, R. J. Weber1, B. E. Anderson2, and A. Nenes1,5
1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA
2NASA Langley Research Center, Hampton, Virginia, USA
3Science Systems and Applications, Inc., Hampton, Virginia, USA
4Cooperative Institute for Research in Environmental Sciences (CIRES) and Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado, USA
5School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
*now at: Tofwerk AG, Thun, Switzerland
**now at: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA

Abstract. The NASA DC-8 aircraft characterized the aerosol properties, chemical composition, and cloud condensation nuclei (CCN) concentrations of the summertime Arctic during the 2008 NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. Air masses characteristic of fresh and aged biomass burning, boreal forest, Arctic background, and anthropogenic industrial pollution were sampled. Observations were spatially extensive (50–85° N and 40–130° W) and exhibit significant variability in aerosol and CCN concentrations. The chemical composition was dominated by highly oxidized organics (66–94% by volume), more than half of which was water-soluble. The aerosol hygroscopicity parameter, κ, ranged between κ = 0.1–0.32 for all air mass types. Industrial pollution had the lowest κ of 0.08 ± 0.01, while the Arctic background had the highest and most variable κ of 0.32 ± 0.21, resulting from a lower and more variable organic fraction. Both fresh and aged (long-range transported) biomass burning air masses exhibited remarkably similar κ (0.18 ± 0.13), consistent with observed rapid chemical and physical aging of smoke emissions in the atmosphere, even in the vicinity of fresh fires. The organic hygroscopicity (κorg) was parameterized by the volume fraction of water-soluble organic matter (ϵWSOM), with a κ = 0.12, such that κorg = 0.12ϵWSOM. Assuming bulk (size-independent) composition and including the κorg parameterization enabled CCN predictions to within 30% accuracy for nearly all environments sampled. The only exception was for industrial pollution from Canadian oil sands exploration, where an external mixture and size-dependent composition was required. Aerosol mixing state assumptions (internal vs. external) in all other environments did not significantly affect CCN predictions; however, the external mixing assumption provided the best results, even though the available observations could not determine the true degree of external mixing. No correlation was observed between κorg and O : C. A novel correction of the CCN instrument supersaturation for water vapor depletion, resulting from high concentrations of CCN, was also employed. This correction was especially important for fresh biomass burning plumes where concentrations exceeded 1.5 × 104 cm−3 and introduced supersaturation depletions of ≥25%. Not accounting for supersaturation depletion in these high concentration environments would therefore bias CCN closure and inferred κ by up to 50%.

Citation: Lathem, T. L., Beyersdorf, A. J., Thornhill, K. L., Winstead, E. L., Cubison, M. J., Hecobian, A., Jimenez, J. L., Weber, R. J., Anderson, B. E., and Nenes, A.: Analysis of CCN activity of Arctic aerosol and Canadian biomass burning during summer 2008, Atmos. Chem. Phys. Discuss., 12, 24677-24733, doi:10.5194/acpd-12-24677-2012, 2012.
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