Air quality and local climates in cities are strongly influenced by the urban environment both directly through alterations in surface roughness and the surface energy balance and also indirectly via atmospheric boundary layer dynamics as these affect the transport of air pollutants, heat, and moisture.
Areas of reduced roughness (e.g. river) may act as ventilation pathways along which the flow brings fresh air into the city while displacing polluted and often warmer air towards surrounding areas, which can again result in a heating-up of downwind suburbs. In addition to surface roughness variations, the flow responds to urban heat emitted by anthropogenic activities or stored in the canopy. Enhanced buoyancy over built-surfaces can encourage the vertical dispersion and mixing of pollutants and other constituents and hence lead to greater mixed layer heights compared to the rural surroundings. Under certain synoptic conditions (weak wind, low cloud cover), thermal contrasts can induce local circulations at neighbourhood (e.g. park breeze) to regional scales (e.g. urban breeze). Effects of such circulations on near-surface air quality are complex as they can reduce near-surface pollution concentrations through dilution but can also entrain polluted air circulating aloft (e.g. aerosol plumes from long range transport, or accumulated pollution in the residual layer) or advect emissions form rural sources (e.g. agriculture).
Despite its importance for various applications, it remains challenging to describe the complex characteristics of the three-dimensional wind, temperature, aerosol, and moisture fields within the urban atmospheric boundary layer. While numerical simulations and wind tunnel experiments increasingly provide valuable insights into flow effects under idealised conditions, real world monitoring of vertical and horizontal atmospheric profiles within the urban boundary layer are still scarce.