[1] An atmospheric HULIS (humic-like substance) sample dissolved in water was used to generate particles with different dry diameters. A HHTDMA (High Humidity Tandem Differential Mobility Analyzer) and LACIS (Leipzig Aerosol Cloud Interaction Simulator) were used to measure hygroscopic growth of the HULIS particles. LACIS also was used to measure the critical supersaturation for the activation of HULIS particles with dry diameters of 50, 75, 100, and 125 nm. Simple Köhler theory was used to simulate the measured hygroscopic growth factors. For this, a new technique was used, where the ionic density (r ion ) was defined as a combination of the HULIS physical properties for which values could not be reliably determined. By adjusting r ion in the Köhler equation, modeled hygroscopic growth could be brought into agreement with the measurements, even without the explicit knowledge of the different HULIS properties. It was demonstrated that the values of r ion determined with our procedure can be reproduced from combinations of physically realistic values of the physical properties represented by r ion . Adjustments of the ionic density were done for two different surface tensions, that of water as the upper limit, and the lowest value that had been measured for this HULIS sample (published previously) as the lower limit. The two adjusted values of r ion were used in the Köhler model to derive critical super-saturations. For more dilute droplets, measured and modeled critical supersaturations were in agreement for both values of the surface tension, whereas for the less dilute solutions, agreement only could be achieved when a lowering of the surface tension due to HULIS was taken into account.

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 supersaturations is parameterized by the hygroscopicity parameter, κ (c.f. Petters and Kreidenweis, 2007). For the wood burns, κ is low, generally around 0.06. The main emphasis of this study is a comparison of κ derived from measurements at sub-and supersaturated conditions (κ G and κ CCN ), in order to see whether the water uptake at 85 % RH can predict the CCN properties of the biomass burning particles. Differences in κ G and κ CCN can arise through solution nonidealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that κ G and κ CCN agree within experimental uncertainties (of around 30 %) for particle sizes of 100 and 150 nm; only for 50 nm particles is κ CCN larger than κ 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 concentratedCorrespondence to: U. Dusek (u.dusek@uu.nl) 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 κ 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 κ ∼ = 0.6, a WSOC component with κ ∼ = 0.2, and an insoluble component with κ = 0.

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Abstract. The hygroscopic growth and activation of two HULIS (HUmic LIke Substance) and one Aerosol-WaterExtract sample, prepared from urban-type aerosol, were investigated. All samples were extracted from filters, redissolved in water and atomized for the investigations presented here. The hygroscopic growth measurements were done using LACIS (Leipzig Aerosol Cloud Interaction Simulator) together with a HH-TDMA (High Humidity Tandem Differential Mobility Analyzer). Hygroscopic growth was determined for relative humidities (RHs) up to 99.75%. The critical diameters for activation were measured for supersaturations between 0.2 and 1%. All three samples showed a similar hygroscopic growth behavior, and the two HULIS samples also were similar in their activation behavior, while the Aerosol-Water-Extract turned out to be more CCN active than the HULIS samples. The experimental data was used to derive parameterizations for the hygroscopic growth and activation of HULIS particles. The concept of ρ ion (Wex et al., 2007a) and the Szyszkowski-equation (Szyszkowski, 1908;Facchini et al., 1999) were used for parameterizing the Raoult and the Kelvin (surface tension) terms of the Köhler equation, respectively. This concept proved to be very successful for the HULIS samples in the saturation range from RHs larger than 98% up to activation. It was also shown to work well with data on HULIS taken from literature. Here, different atmospheric life-times and/or different sources for the different samples showed up in different coefficients for the parameterization. However, the parameterization did not work out well for the Aerosol-Water-Extract.