Winter air pollution episodes over Cape Town

Episodes of air pollution over Cape Town are identified using multi-year time series of SO₂ and NO_x concentrations. The associated meteorological conditions are studied both from synoptic and meso-scale perspectives using detailed observations, statistical analyses and numerical model simulations. Atmospheric conditions are most conducive to poor dispersion in the winter months, April to August. Episodes are initiated by the eastward passage of an intense, synoptic anticyclone over the Cape Town area. The 850 hPa geopotential height typically rises to 1600 gpm a day before the episode. Northeasterly berg winds are common and act to dry out the boundary layer. A nocturnal radiation inversion forms with a mean strength of 11 °C and extends from the surface to 953 hPa (500m). Gradient and thermal winds tend to cancel out providing for low net transport rates for near-surface emissions. Acoustic sounder profiles for two episodes illustrate a reduction of winds and turbulence within the boundary layer. The episode surface circulation is simulated using a two-layer model. Hills which lie upstream of Cape Town obstruct the northeasterly flow during the morning.