In a forested near-urban location in central Germany, the CCN efficiency of particles smaller than 100 nm decreases significantly during periods of new particle formation. This results in an increase of average activation diameters, ranging from 5 to 8% at supersaturations of 0.33% and 0.74%, respectively. At the same time, the organic mass fraction in the sub-100-nm size range increases from approximately 2/3 to 3/4. This provides evidence that secondary organic aerosol (SOA) components are involved in the growth of new particles to larger sizes, and that the reduced CCN efficiency of small particles is caused by the low hygroscopicity of the condensing material. The observed dependence of particle hygroscopicity (κ) on chemical composition can be parameterized as a function of organic and inorganic mass fractions (forg, finorg) determined by aerosol mass spectrometry: κ = κorg forg + κinorg finorg. The obtained value of κorg ≈ 0.1 is characteristic for SOA, and κinorg ≈ 0.7 is consistent with the observed mix of ammonium, sulfate and nitrate ions.
 During the Feldberg Aerosol Characterization Experiment in June–July 2005 (FACE-2005) we measured CCN efficiencies, chemical composition and number size distributions of aerosol particles at the Taunus Observatory field site. The site is on a small mountain at ∼900 m asl in a forested area. A rural, mountainous area extends to the north, west and east of the field site, whereas roughly 30–50 km to the south and southeast the Rhine-Main metropolitan area is located, including the cities Frankfurt, Mainz, Wiesbaden, and Darmstadt. This enabled us to measure diverse aerosol types, ranging from urban plumes to aged rural aerosol.
 Size resolved CCN spectra were measured by a differential mobility analyzer (DMA) in front of a Droplet Measurement Technologies (DMT) CCN counter [Roberts and Nenes, 2005]. The DMA selected particles within a narrow size range, which were passed to the CCN counter and a condensation particle counter (CPC, TSI 3762) measuring in parallel. While the CCN counter measured the particles that acted as CCN (NCCN), the CPC measured the number concentration of total condensation nuclei (NCN) larger than approximately 10 nm. CCN efficiency spectra (defined as NCCN/NCN vs. particle diameter d) were recorded at several supersaturations (S) between 0.07% and 0.74% (relative uncertainty < 5%). The CCN spectra were corrected for multiply charged particles according to Frank et al.  and for the broadening of the CCN spectra caused by the DMA transfer function according to Rose et al. [2008a]. The CCN counter was calibrated before and after FACE-2005 using ammonium sulfate particles. Critical diameters for ammonium sulfate were calculated using the Köhler equation with a variable van't Hoff factor [Frank et al., 2007]. The calibration after the experiment was made at a similar altitude as the field site (p ∼ 900 hPa). This calibration was therefore used for calculating the effective S from the temperature gradient in the instrument. Since the ambient temperature affects S in the instrument, the temperature difference between calibration and measurement location (on average 4°C) was taken into account according to Rose et al. [2008a, Figure 9].
 Particle number size distributions in the range 10–900 nm were measured using a Grimm SMPS system [Winklmayr et al., 1991]. Size resolved chemical composition was measured using a Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS) [Drewnick et al., 2005]. A particle density of 1.6 g cm−3 was used to convert the vacuum aerodynamic diameters of the ToF-AMS to mobility equivalent diameters used in CCN and size distribution measurements. From here on, ‘particle diameter’ (d) refers to the mobility equivalent diameter. The ToF-AMS transmits particles in the size range from ∼40–50 nm up to ∼400 nm with 100% efficiency. Particles with d < 20–25 nm are not transmitted at all.
2.2. Parameterization of CCN Spectra
 A cumulative Gaussian distribution function was fitted to each CCN spectrum to derive a midpoint activation diameter (da) of the ambient aerosol, defined as the diameter at which the cumulative Gaussian function reaches half its maximum height. The activation diameter depends on S and the chemical composition and surface tension of the droplet at the point of activation. To eliminate the dependence on S, da can be converted into an effective hygroscopicity parameter κ [Petters and Kreidenweis, 2007; Gunthe et al., 2009]:
where σw is the surface tension of water, Mw and ρw the molecular weight and density of water, R the universal gas constant, and T the ambient temperature. κ is a measure of the hygroscopicity of the aerosol particle, and typical κ values for atmospheric aerosol constituents are: 0.1 for secondary organics, 0.2 for levoglucosan, 0.6 for ammonium sulfate, 0.7 for ammonium nitrate, and 0.9 for sulfuric acid [Petters and Kreidenweis, 2007].
 During FACE-2005 a strong increase of the particle concentration in the 10–20 nm size range occurred almost every day. These new particles rarely grew to sizes larger than 70 nm. New particle formation (NPF) events are defined as time periods when the size distribution exhibits either an absolute maximum or a clear relative maximum in the size range <30 nm. A summary of all time periods with NPF can be found in Text S1 of the auxiliary material.
 Aerosol properties differ significantly between time periods with and without NPF (Figure 1). During NPF events, number concentrations in the size range <40 nm are strongly enhanced (Figure 1a). On average, number concentrations in the 50–100 nm size range are somewhat reduced, consistent with the fact that NPF is facilitated when the surface area of preexisting particles is reduced.
Figure 1b shows that the organic mass fraction of particles with d < 100 nm is enhanced during NPF, increasing from approximately 2/3 to 3/4. Figure 1c shows that CCN efficiency spectra of large particles are comparable during and outside of NPF events. Smaller particles (activated at higher S) show reduced CCN efficiency during NPF, consistent with the enhanced organic mass fraction. The fact that only the CCN efficiency of small particles is significantly changed suggests that condensing organic compounds are involved in the growth of new particles. Condensing gases change the chemical composition of smaller particles much more drastically than that of larger particles.
 To study the changes in chemical composition and CCN activity during NPF in more detail, time periods were selected using more stringent criteria: We chose NPF events with an adjacent time period without NPF, but otherwise unchanged air mass back trajectories, local wind direction and meteorological conditions, such as rainfall or fog. Differences in aerosol properties between these adjacent time periods should therefore not be caused by large-scale changes in aerosol type or local pollution and weather conditions. Nevertheless, it is possible that changes in aerosol properties were caused by changes in weather conditions or pollution events further upstream from the sampling site. Six time periods remained, encompassing at least one NPF event and an adjacent non-event. If two NPF events were adjacent to a non-event, both are evaluated.
 At the highest S of 0.74% (Figure 2) and 0.49%, da are significantly larger during NPF events than before or after (at the 95% confidence level). For S = 0.33%, da also are larger during NPF events (except in one case), although not significant at the 95% confidence level. However, at even lower S (da > 100 nm) there is no consistent increase in da during NPF events (see Text S2 for da and κ values for all supersaturations). The lowered CCN efficiency for particles <100 nm can therefore not only be observed in the grand average (Figure 1), but also in individual events, as long as meteorological and air mass conditions are comparable.
 We used da at S = 0.74% (40–50 nm), S = 0.49% (53–65 nm), and S = 0.33% (70–85 nm) to calculate volume-averaged hygroscopicity parameters κ, representative for the size range below 100 nm. In Figure 3 these κ values are plotted against the organic mass fraction (forg) of particles smaller than 100 nm. The organic mass fraction was calculated by dividing the average AMS organic mass concentration by the average AMS total mass concentrations for each selected time period (see Table S2). Each data point represents either a NPF event or an adjacent non-event listed in Text S2. On average, the values of κ at a given inorganic mass fraction compare well to earlier measurements at the same location [Dusek et al., 2006]. κ and forg are negatively correlated, indicating that a higher forg leads to reduced CCN activity. If the regression intercept is calculated for finorg = 0 (pure organic aerosol) and forg = 0 (pure inorganic aerosol), we infer values of κinorg = 0.69 ± 0.07 for the inorganic component and κorg = 0.10 ± 0.04 for the organic component (best fit ± standard error). κinorg = 0.69 is in between the values for ammonium sulfate (0.6) and ammonium nitrate (0.7) [Petters and Kreidenweis, 2007], whereas κorg = 0.1 is characteristic of freshly nucleated SOA aerosol in chamber studies [e.g., Petters and Kreidenweis, 2007]. For conditions during our field study we find that in a continental region with frequent NPF, the CCN activity of sub-100-nm aerosol particles can be approximated by a simple two-component model, an organic component with κorg ≈ 0.1 and an inorganic component with κinorg ≈ 0.7. Gunthe et al.  obtained a similar parameterization for pristine Amazonian rainforest aerosols with κorg = 0.09 ± 0.02 and κinorg = 0.63 ± 0.02. The good agreement between the κorg values derived from field measurements in polluted continental and pristine rainforest air, as well as from laboratory experiments of SOA formation, suggests that κorg ≈ 0.1 can be generally applied as an approximate effective hygroscopicity parameter for SOA. The ∼10% higher value of κinorg obtained in this study can be attributed to the higher proportion of ammonium nitrate. For the prediction of CCN concentrations and the modeling of cloud droplet formation, however, such small differences in κ are likely negligible [Rose et al., 2008b; Gunthe et al., 2009; Reutter et al., 2009].
 We observe an increased organic fraction as well as reduced CCN activity for particles <100 nm during time periods that favor NPF. This suggests that condensing organic compounds play a role in the growth of new particles and that the reduced CCN efficiency of the small particles is caused by the low hygroscopicity of the organic material. The κ value of 0.1 derived for the condensing organic material corresponds to typical κ values of fresh SOA observed in chamber studies. The hypothesis that SOA compounds are closely associated with the growth of new aerosol particles has been brought forward for forested remote regions [Tunved et al., 2006] and has been recently substantiated with a comprehensive set of measurements [Laaksonen et al., 2008]. Using a very different set of measurements, we find evidence for the close association of SOA with NPF events also in the relatively polluted, forested Feldberg region. This phenomenon seems therefore not limited to remote boreal forests.
 Because of the reduced CCN efficiency of the newly formed particles, the increase in CCN concentration through NPF may not be as large as inferred from size distribution measurements alone, especially since the presence of a narrow nucleation mode can increase the relative importance of chemical composition. Nevertheless, particle number and size are expected to remain the major predictors for the variability of CCN concentration [Dusek et al., 2006; Rose et al., 2008b; Gunthe et al., 2009]. Reduced hygroscopicity of newly formed particles can be expected where SOA plays a major role in particle growth, e.g., over boreal and tropical forests [Tunved et al., 2006; Gunthe et al., 2009]. In other regions, such as polluted megacities, high proportions of inorganic compounds may enhance the hygroscopicity of CCN [Wiedensohler et al., 2009].
 In a forested, near-urban region in central Germany, the CCN efficiency of particles <100 nm is substantially reduced in time periods with high concentrations of newly formed particles in the size range <30 nm. The CCN efficiency of particles >100 nm remains relatively unaffected. A concurrent increase in organic mass fraction provides evidence that organic compounds contribute to the growth of new particles. From the negative correlation between the hygroscopicity parameter κ and the organic mass fraction of particles <100 nm we infer a value of κorg = 0.1 for the freshly condensed SOA. This value is consistent with other recent field and laboratory studies [Gunthe et al., 2009; Petters and Kreidenweis, 2007, and references therein] and appears well-suited for approximating the hygroscopicity of atmospheric aerosol particles as a function of organic and inorganic mass fraction.
 We thank H. Bingemer from the University of Frankfurt and the Taunus Observatory team for their help. The Max Planck Society and the University of Mainz are acknowledged for funding the FACE-2005 measurement campaign. We thank the University of Frankfurt for access to the facilities at the Taunus Observatory for the duration of the FACE-2005 campaign.