The transport of PM10 over Cape Town during high pollution episodes

Abstract

Listen

Translate article

This study investigates the influence of PM10 from remote sources on PM10 episodes over the city of Cape Town. For the study, we analysed observation data from Cape Town's air quality monitoring stations as well as high-resolution simulation data from the Weather Research and Forecasting model with Chemistry (WRF-Chem). The observation data were used to identify PM10 episodes during the study period (2008–2014) and WRF-Chem was applied to simulate the atmospheric conditions and PM10 transport over southern Africa during each episode. The capability of WRF-Chem to simulate the wind and PM10 concentration over Cape Town was quantified and the paths of air parcels over Cape Town during each episode were tracked. Results of the study show that WRF-Chem gives a realistic simulation of observed wind (speed and direction) over the city during the episodes, but the model struggles to reproduce the observed PM10 concentration. For most episodes, the magnitude of the simulated PM10 is lower than the observed due to lack of local emissions in the simulations. In some cases, the model reproduces the peak in PM10 concentration some days earlier or later than observed. The simulations show that most air parcels over Cape Town during the episodes have travelled over major dust source regions (i.e., the Kalahari or Namib Desert) before reaching the city. Most of episodes are associated with a southward transport of a plume of PM10 from the north-west coast of southern Africa to Cape Town. This PM10 plume is induced by a coastal trough and a continental high pressure system. In some cases, local topography influences the intrusion of the PM10 plume into Cape Town by blocking some of the pollution, thereby minimising the amount of PM10 that reaches the city. Results of the study suggest that the transport of PM10 from the north-west coast of southern Africa may contribute to PM10 episodes in Cape Town.