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. The water uptake at sub-and super-saturations is parameterized by the hygroscopicity parameter, kappa (c.f. Petters and Kreidenweis, 2007). For the wood burns, kappa is low, generally around 0.06. The main emphasis of this study is a comparison of kappa derived from measurements at sub-and supersaturated conditions (kappa(G) and kappa(CCN)), in order to see whether the water uptake at 85% RH can predict the CCN properties of the biomass burning particles. Differences in kappa(G) and kappa(CCN) can arise through solution non-idealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that kappa(G) and kappa(CCN) agree within experimental uncertainties (of around 30 %) for particle sizes of 100 and 150 nm; only for 50 nm particles is kappa(CCN) larger than kappa(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 water-soluble organic carbon (WSOC) fraction can be represented by a kappa(WSOC) value of approximately 0.2. The effective hygroscopicity of a typical wood burning particle can therefore be represented by a linear mixture of an inorganic component with kappa congruent to 0.6, a WSOC component with kappa congruent to 0.2, and an insoluble component with kappa = 0.