A new method to analyze the hygroscopic properties of soot particles from their aging process

Soot particles are considered as important aerosols in the atmosphere due to their potential role as cloud condensation nuclei (CCNs). Freshly released soot is generally considered as hydrophobic. However, the so-called aging process in the atmosphere can modify morphology and surface composition of soot particles and turn them into efficient CCNs. The hygroscopic properties of soot are commonly measured from their activated fraction F_a that is the ratio of the number of nucleated droples on the number of CCNs. For spherical and monodisperse aerosols, the κ-Kohler theory is generally used to obtain the parameter κ which quantifies the hygroscopic properties of aerosol. In this work, a model to obtain κ is proposed by taking into account the size distribution and the morphology of the aerosol particles. The model is first tested on dry ammonium sulfate that is well known to produce quasi-spherical particles. The results of the activation experiments are in good agreement with the predictions of the model. Then, the model is applied to soot particles that are characterized by complex morphology. Soot particles had to be activated by aging with ozone before activation experiments to increase their k into the validity range of the model (i.e. to turn fresh soot to hydrophilic aggregates). The parameters of the determination of κ are soot particle mobility diameter Dm, fractal dimension Df, the primary particle diameter Dpp and the geometric standard deviation of volume equivalent particles of soot.