On Dry Deposition Modelling of Atmospheric Pollutants on Vegetation at the Microscale: Application to the Impact of Street Vegetation on Air Quality

A Reynolds-averaged Navier-Stokes model is used to investigate the impact of urban canopy vegetation on air quality, with particular emphasis on the comparison between the positive effect induced by deposition versus the negative effect due to a reduction of ventilation. With this aim, a series of simulations over a simplified urban geometry with different vegetation designs are carried out. The problem is tackled at two scales. From the mesoscale point of view, the relevant variable is the total deposition flux of pollutant as a function of the pollutant concentration above the canopy (e.g. the "mesoscale" deposition velocity). This is assessed within the Monin-Obukov similarity theory framework, and a modification of the classical formulation is proposed based on the numerical results. At the microscale, the distribution of concentration within the urban canopy is investigated for the different configurations. The main conclusion is that the height of the vegetation and the magnitude of the microscale deposition velocity are key parameters that determine which of the two effects (deposition or reduction of ventilation) prevails.