SIMULATIONS OF STREET-CANYON AIR-QUALITY USING FLUID DYNAMICS AND AEROSOL MODELLING

High concentrations of nitrogen dioxide and particulate matter are often observed locally in streets. Because of the spatial resolution limit, regional-scale chemical transport models cannot reproduce these high concentrations. Traditional local-scale methods such as computational fluid dynamics (CFD) often neglect chemical reactions and aerosol dynamics, which leads to inaccuracy in the simulation of local air quality. In this study, 2D CFD simulations performed by Code_Saturne and OpenFOAM and coupled with the chemical aerosol module SSH-Aerosol are used to model pollutant dispersion, chemical reactions and aerosol dynamics during a period of 12 hours (from 4 a.m. to 4 p.m., local time, GMT+2h) in a street of Greater Paris. For both CFD codes, the setup is validated by comparing the simulated NO2 and PM10 concentrations with measurements. The impact of turbulence model and coupling strategy on reactive and non-reactive pollutant concentrations is assessed by comparing the concentrations simulated by two codes. This comparison of the CFD tools provides a qualitative estimation of the uncertainty associated with the modelling of the atmospheric flow and of the coupling between dispersion, chemistry and aerosol dynamics. In order to understand the impact of chemical processes on aerosol formation, sensitivity tests concerning gas chemistry and aerosol dynamics are conducted. A non-neglectable under-estimation of some pollutant concentrations is observed when gas chemistry and aerosol dynamics are not taken into account. Gas chemistry significantly increases NO2 concentrations in the street, which is underestimated by 41% on average when gas chemistry is not considered. Although the impact of gas chemistry on inorganic and organic condensables is limited, inorganic and organic aerosol concentrations in the street are largely impacted by aerosol dynamics. For inorganic aerosols the concentrations increase because of the formation of ammonium nitrate, partly due to the ammonia emission by traffic and partly due to the lack of thermodynamic equilibrium between gas and aerosols in the background regional concentration with high concentrations of nitric acid. For organic aerosols, the concentrations are strongly influenced by the increase of inorganic aerosols.