Due to the exponential increase in aircraft traffic in recent decades, the role of soot particles emitted by aircraft engines on the radiative forcing needs to be addressed, and especially their interaction with clouds has to be better understood and quantified. In this work, we investigate the hygroscopic properties of fresh and aged soot sampled on line in a kerosene flame. The activated fraction (F a) of size selected soot is measured by means of a variable supersaturation condensation nucleus counter at several heights above the burner (HAB), thereby probing soot particles with different residence times in the flame, i.e., different degrees of maturity. In order to simulate atmospheric aging, the activity of soot as cloud condensation nuclei is measured as a function of ozone exposure. We show that fresh soot is hydrophobic (F a $0), while F a increases when soot is exposed to ozone. The measurements depend on the HAB at which soot particles are sampled showing that activation of soot particles is related to their chemical composition. This study brings new results on the link between atmospheric aging of soot and its hygroscopic properties, which is of great interest for understanding the role of soot in the cloud formation.

Combustion and other high-temperature processes frequently result in the emission of aerosols in the form of polydisperse fractal-like aggregates made of condensedphase nanoparticles (soot for instance). If certain conditions are met, the emitted aerosol particles are known to evolve into important cloud condensation nuclei (CCN) in the atmosphere. In this work, the hygroscopic parameter κ of complex morphology aggregates is calculated from the supersaturation-dependent activated fraction F a = F a (SS) in the frame of κ-Köhler theory. The particle size distribution is approximated with the morphology-corrected volume equivalent diameter calculated from the electrical mobility diameter by taking into account the diameter of the primary particle and the fractal dimension of the aggregate experimentally obtained from transmission electron microscopy measurements. Activation experiments are performed in water supersaturation conditions using a commercial CCN-100 condensation nuclei counter. The model is tested in closeto-ideal conditions of size-selected, isolated spherical particles (ammonium sulfate nanoparticles dispersed in nitrogen), then with complex polydisperse fractal-like aggregates (soot particles activated by exposure to ozone with κ as low as 5 × 10 −5 ) that represent realistic anthropogenic emissions in the atmosphere.

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<p><strong>Abstract.</strong> The modern parametrization of the classical theory of nucleation (κ-Kӧhler theory) implicitly assumes a Dirac delta distribution to model the density of ideal spherical point size dry particles and droplets. However, anthropogenic activities like combustion or other high temperature processes frequently result in the emission of aerosols in the form of polydisperse fractal-like aggregates composed of condensed phase nanoparticles (for instance soot). If certain conditions are met, the emitted particles are known to evolve into important cloud condensation nuclei (CCN) in the atmosphere, however their behavior as CCN can deviate significantly from theoretical predictions. In this work, an extension of κ-Kӧhler theory is proposed that takes into account the effect of the size distribution and particle morphology on the activation of the aerosol dry particles. A theoretical and experimental approach are combined to derive the dependence of the activated fraction on supersaturation <i>F</i><sub>a</sub> = <i>F</i><sub>a</sub>(<i>SS</i>) on parameters that describe the size distribution and morphology of the particles like the geometric standard deviation and the fractal dimension of the aggregates. The model is tested on two different aerosols, a simple case of isolated quasi-spherical ammonium sulfate particles generated by atomization, and complex morphology soot aggregates generated by a laboratory diffusion jet flame.</p>