A combined photochemistry/aerosol dynamics model: model development and a study of new particle formation

A new coupled photochemistry/aerosol model has been developed for studies of multicomponent aerosol dynamics. The photochemistry module, based on the reaction mechanisms of OSLO-CTM2 global model, has been extended with a simple oxidation scheme of volatile organic gases to produce condensable products. The aerosol module is a further development of UHMA (University of Helsinki Multicomponent Aerosol model) which incorporates all major aerosol dynamic processes with a special focus on new particle formation and growth. The new model explicitly incorporates production and loss terms from photochemistry into the calculation of condensation fluxes and nucleation rates, making the two models completely interactive. In this paper, we apply the new model to investigate the role of photochemistry and biogenic emissions in new aerosol formation and growth. Although using a simplified photochemistry scheme, the model allows for analysis of the evolution of chemical composition of an air mass simultaneously with the aerosol size distribution. The simulation results suggest a classification of air masses which goes beyond describing the air as \clean\ or \polluted\.