Characterising mixing and pollution transport in the urban boundary layer

Air quality models (AQMs) are a critical tool in the management of urban air pollution. They can be used for short-term air quality (AQ) forecasts, and in making planning and policy decisions aimed at abating poor AQ. Vertical turbulent mixing and horizontal advection of pollution are crucial processes controlling human exposure to pollution, since they are responsible for transporting pollution away from areas of high concentration. A simple two-box model is used to investigate the relative importance of urban boundary layer (UBL) pollution transport processes on AQ throughout the day. The lower and upper boxes represent the pollution concentration in the urban canopy and mixed layer respectively. The investigation utilises UBL meteorology and AQ measurements made during an experimental field campaign in London. The results demonstrate that the vertical exchange timescale at canopy top is a key parameter influencing concentration in the canopy. However, canopy height and wind speed in the canopy are likely less important. These findings can be used to inform parametrisation of urban boundary layer pollution transport processes in AQMs. To analyse the vertical mixing of pollution in the convective boundary layer, where the dominant turbulence ranges from mostly parametrised to mostly resolved in numerical weather prediction (NWP), the Met Office Unified Model (UM) is run at horizontal grid lengths ranging from 1.5 km to 55 m over London. A reduced analytical model is developed and used to determine vertical mixing timescales associated with surface released tracers in the UM. It is found that when vertical mixing is mostly resolved, distinctly different vertical mixing of tracers occurs on O(10 min) timescales. This results in a significant influence on surface level tracer concentrations at the city scale compared to when vertical mixing is mostly parametrised. Turbulence in urban canopies is investigated using simulations of flow through a wide range of urban canopy geometries. The general characteristics of momentum mixing length profiles are established and a first-order mixing length closure is formulated that is appropriate for parametrisation of urban canopy turbulence in AQMs. Unlike in vegetation canopies the inflection in the time- and horizontally space-averaged velocity profile does not control turbulent mixing in urban canopies. Overall, this thesis shows that good representation of pollution vertical mixing, both at the scale of the urban canopy and the entire UBL, is crucial for accurate urban AQ prediction