Effect of particle phase oligomer formation on aerosol growth

We study theoretically the effect of oligomer formation on secondary organic aerosol (SOA) growth following ozonolysis of α-pinene. Our goal is to show qualitatively, using a simplified condensational growth model, that the formation of involatile oligomers can induce the condensation growth of aerosols even if the secondary organic species formed in the oxidation reactions are rather volatile and therefore poorly condensable. The closed system studied consists of a single effective gas-phase oxidation product, and a seed aerosol population consisting of monodisperse ammonium sulphate particles. The oligomer formation is described as an effective dimerisation reaction between the monomers in the aerosol phase. By varying the dimerisation constant until our model results match the results from earlier smog-chamber studies, we are able to make order-of-magnitude estimations of the unknown dimerisation rates needed to induce the observed SOA growth. We also make model simulations using atmospheric α-pinene and ozone concentrations, and find that the effective dimerisation rate inducing SOA growth in the atmosphere has to be considerably higher than that causing SOA growth in smog-chamber conditions.