Simulations of vehicle-induced mixing and near-road aerosol microphysics using computational fluid dynamics

Abstract

Understanding the fate of ultrafine particles (UFP), especially from combustion sources, is essential to assess their impact on health and climate. Here, we present simulations of the behavior of UFP in the near-roadway environment (up to 300 m downwind) based on a model with coupled computational fluid dynamics (CFD) and aerosol microphysics. It is found that vehicle-induced mixing (VIM) caused by the combined effect of vehicle wake formation and production of turbulent kinetic energy plays an important role in downwind dilution of pollutants. Various methodologies for simulating VIM are explored, and a computationally efficient approach based on an effective roughness length for vehicle-induced mixing is proposed. Whereas the coagulation behavior of ultrafine particles is relatively well understood, condensation and/or evaporation can have equally large or larger impacts on the number and sizes of particles downwind of a roadway. Through a set of sensitivity simulations, we show that the particle losses are potentially significant via evaporation but depend strongly on several parameters or processes that are poorly understood and difficult to fully constrain for on-road traffic using measurements: the volatility distribution of organic species and gas-phase concentrations of semi-volatile organics.