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Water uptake by biomass burning aerosol at sub-and supersaturated conditions: Closure studies and implications for the role of organics

by U. Dusek, G. P. Frank, A. Massling, K. Zeromskiene, Y. Iinuma, O. Schmid, G. Helas, T. Hennig, A. Wiedensohler, M. O. Andreae show all authors
Atmospheric Chemistry and Physics ()

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 (+�). 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 +� (another widely used parameterization; cf. Petters and Kreidenweis, 2007) of around 0.07. The main emphasis of this study is a comparison of +� derived from measurements at sub- and supersaturated conditions +�G and +�CCN), in order to see whether the water uptake at 85% RH can predict the CCN properties of the biomass burning particles. Differences in +�G and +�CCN can arise through solution non-idealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that +�G and +�CCN agree within experimental uncertainties (of around 30%) for particle sizes of 100 and 150 nm; only for 50 nm particles is +�CCN larger than +�G 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 +� = 0.15-0.2. -� Author(s) 2008

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