Including aerosol dynamic processes in LES: evaluation and application

The intricate geometrical nature of urban areas leads to highly complex patterns for turbulent flow and air pollutant dispersion, which can be resolved by high-resolution CFD modelling such as large-eddy simulation (LES). Air pollutants are also transformed by chemical and physical processes, which can be strongly influenced by the local wind conditions, like dry deposition for example. So far, no realistic representation of air pollutants including especially aerosol particles has been implemented in CFD models, not to mention the more computationally demanding LES to limit computational costs. Following the recent development to fill in the aforementioned gaps, we represent here a novel modelling tool for urban air quality that can be used to predict air quality within real urban neighbourhoods where commonly used models typically fail.

The LES-based PALM model system has been extended with the sectional aerosol module SALSA2.0 and a Kinetic PreProcessor -based online chemistry module. The modules are linked together but here the main focus is on the aerosol modelling part. SALSA2.0 resolves the aerosol size distribution and chemical composition utilizing number and mass bins, and includes the aerosol processes of coagulation, condensation, nucleation and dry deposition on both vegetation and solid surfaces.

The first successful evaluation study in central Cambridge, UK, shows that aerosol concentrations at the pedestrian level are governed by the local wind conditions and emissions. Nevertheless, aerosol dry deposition on tree leaves and solid surfaces can decrease concentrations locally by over 20%. Similarly, condensation and dissolutional growth were shown to increase particulate mass by over 10%. However, in a city-planning-scenario study that originates from the needs of local urban planners in Helsinki, Finland, we show that improvement of pedestrian level air quality by aerosol dry deposition on vegetation is clearly weaker than its deterioration due to decreased ventilation in a street canyon with street trees.