Interactions between atmospheric ultrafine particles and secondary organic aerosol mass : a model study

We use dynamic mass-balance equations to treat the role of organic composition in driving net condensation to atmospheric particles. We consider the growth of newly formed nanoparticles that may have very different composition (volatility) than the pre-existing aerosol. Production of vapors much less volatile than the background aerosol enhances the growth of nanoparticles by the re-partitioning of semi-volatile vapors to the growing nanoparticles. In contrast, production of more-volatile vapors suppresses the growth of nanoparticles because the background aerosol draws down the semi-volatile gas-phase concentrations. The background aerosol thus serves as either a source or a sink of additional organic material. For fresh nanoparticles, the implication is that net condensation depends significantly on any imbalance between the produced organic vapors and their equilibrium (vapor) distribution over the background aerosol. This phenomenon may be important during nucleation and growth events, where relatively rapid growth of newly formed particles is difficult to explain.