Séminaire LATMOS-OVSQ (Amphithéâtre Gérard Mégie, Guyancourt), mardi 22 octobre à 11h. Professeur Hanna Vehkamäki, Department of Physics, University of Helsinki, Finlande: "Uncovering the secrets of atmospheric nanoparticles". Atmospheric aerosol particles affect human health, visibility and climate. Cloud formation in our atmosphere always involves particles as cloud condensation nuclei. The direct scattering effect together with the influence via clouds constitute the largest uncertainty in predicting future climate. Not all the particles are emitted in the air in solid or liquid form: it is estimated that approximately half of modern-day cloud condensation nuclei originate from atmospheric nucleation born in the air from condensable vapours. In many atmospheric locations, the strongest candidate for a particle forming vapour is sulphuric acid, but other vapours are needed to assist the process. We have used first principles computational chemistry method to study the stability of charged and neutral sulphuric acid clusters containing ammonia and dimethylamine. Using the computed cluster properties in a cluster size distribution dynamics model we are able to replicate the nucleation rates observed in the CLOUD chamber in CERN. After validation, our model is now being used to assess the how the description of particle formation affects parameters of climatic importance, such as the number of cloud condensation nuclei.