Atmos. Chem. Phys. Discuss., 10, 29853-29895, 2010
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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.
Water uptake of biomass burning aerosol at sub- and supersaturated conditions: closure studies and implications for the role of organics
U. Dusek1,*, G. P. Frank1,******, A. Massling2,***, K. Zeromskiene2,*****, Y. Iinuma2, O. Schmid1,**, G. Helas1, T. Hennig2,****, A. Wiedensohler2, and M. O. Andreae1
1Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany
2Leibnitz-Institute for Tropospheric Research, Leipzig, Germany
*now at: Institute for Marine and Atmospheric Sciences Utrecht, Utrecht University, Utrecht, The Netherlands
**now at: Comprehensive Pneumology Center, Institute of Lung Biology and Disease, Helmholtz Zentrum München, Germany
***now at: National Environmental Research Institute, Aarhus University, Roskilde, Denmark
****now at: Department for Applied Environmental Science (ITM), Stockholm University, Stockholm, Sweden
*****now at: Dillon Consulting Ltd., Toronto, Canada
******now at: Department of Physics, Lund University, Lund, Sweden

Abstract. We investigate the CCN activity of freshly emitted biomass burning particles and their hygroscopic growth at a relative humidity (RH) of 85%. The particles were produced in the Mainz combustion laboratory by controlled burning of various wood types, peat and grass. The water uptake at sub- and supersaturations is parameterized by deriving a soluble volume fraction (ε). It is defined as the volume fraction of ammonium sulfate in the total aerosol material, which would be sufficient to explain the observed water uptake. For the wood burns, soluble volume fractions are low, generally around 0.11. This translates to a hygroscopicity parameter κ (another widely used parameterization; cf. Petters and Kreidenweis, 2007) of around 0.07. The main emphasis of this study is a comparison of ε derived from measurements at sub- and supersaturated conditions εG and εCCN), in order to see whether the water uptake at 85% RH can predict the CCN properties of the biomass burning particles. Differences in εG and εCCN can arise through solution non-idealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that εG and εCCN agree within experimental uncertainties (of around 30%) for particle sizes of 100 and 150 nm; only for 50 nm particles is εCCN larger than εG by a factor of 2. The magnitude of this difference and its dependence on particle size is consistent with the presence of surface active organic compounds. These compounds mainly facilitate the CCN activation of small particles, which form the most concentrated solution droplets at the point of activation. The 50 nm particles, however, are only activated at supersaturations higher than 1% and are therefore of minor importance as CCN in ambient clouds. By comparison with the actual chemical composition of the biomass burning particles, we estimate that the hygroscopicity of the organic fraction is roughly 1/3 that of ammonium sulfate and can be represented by κ = 0.15–0.2.

Citation: Dusek, U., Frank, G. P., Massling, A., Zeromskiene, K., Iinuma, Y., Schmid, O., Helas, G., Hennig, T., Wiedensohler, A., and Andreae, M. O.: Water uptake of biomass burning aerosol at sub- and supersaturated conditions: closure studies and implications for the role of organics, Atmos. Chem. Phys. Discuss., 10, 29853-29895, doi:10.5194/acpd-10-29853-2010, 2010.
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