Modelling of the influence of aerosol processes for the dispersion of vehicular exhaust plumes in street environment

We have analysed the influence of various aerosol dynamical processes and plume dilution on the properties of vehicular exhaust particulate matter. A monodisperse aerosol process model MONO32 with four size modes was applied for evaluating the number concentration, size distribution and chemical composition of the particles during a time period of 25 s, after the particles were emitted from the exhaust pipe. The model takes into account aerosol dynamics and gas-phase chemistry including emissions of gases and particles, chemical reactions in the gas phase, dry deposition of particles and gases, homogenous binary H2SO4–H2O or ternary H2SO4–H2O–NH3 nucleation, multicomponent condensation of H2SO4, H2O and some organic vapour onto the particles, inter- and intra-mode coagulation of particles, and plume dilution with the background air. The dilution parameters are calculated by a simple plume model. Numerical computations were performed assuming specific vehicular exhausts under an urban environment, characteristic for the Helsinki Metropolitan Area, in summer. The results showed that condensation of an insoluble organic vapour is important under the selected conditions, if its concentration exceeds a threshold value of 1010 or 1011 cm–3 for the Aitken and accumulation mode particles, respectively. The condensation or evaporation of water can also be an important process; however, its influence is strongly dependent on the hygroscopicity of particles. The effect of coagulation is substantial only, if the dilution of the exhaust plume is neglected. After the computational time of 25 s, the particulate population reaches so-called quasi-equilibrium state, i.e., most of the particulate matter transformation processes have already taken place within this time period.