Characterisation of Cape Town brown haze

[摘要] The Cape Town brown haze is a brown-coloured smog that is present over the CapeTown atmosphere during the winter months due to the accumulation of gaseous andparticulate pollutants. The main aim of this research was to evaluate the impact ofatmospheric pollutants to visibility impairment by the brown haze through visibilitymodelling of major pollution sources around the City of Cape Town. The screeningmodel, VISCREEN, the Plume Visibility model, PLUVUE II and the CALPUFFModelling System were employed to model the visual impact of emissions from themajor sources. Two point sources, Caltex Oil Refinery and Consol Glass, and threearea sources, Cape Town Central Business District (CBD), Cape Town InternationalAirport and the townships of Khayelitsha and Mitchell’s Plain were identified as themajor sources. An initial screening analysis indicated that emissions from the twoindustrial sources would be visible and would result in a yellow-brown discolourationof the atmosphere. Detailed modelling using PLUVUE II identified the area sourcesof Cape Town CBD and the townships to be the significant contributors to visibilityimpairment over Cape Town. Plume perceptibility is primarily dependant uponparticulate emissions while NOx emissions influence the colouration of theatmosphere. CALPUFF was employed to assess the distribution of NOx, SO2 andPM10 concentrations over the area and the associated visibility impairment on a nonhaze(13 August 2003) and haze day (22 August 2003). Pollutant concentrations wereconsiderably reduced on the non-haze day compared to the haze day. The Cape TownCBD was an important source of all the major pollutants with the townshipscontributing significantly to the aerosol loading over Cape Town. Pollutantconcentrations are particularly elevated during the late evening and early morningperiods, particularly between 7 am and 8 am. Visibility impairment is greatest on thehaze day, particularly over the central Cape Town region and the townships. Thegreatest reduction in visibility is experienced between midnight and 9 am whichcorresponds with the periods of elevated atmospheric pollutant concentrations.

[发布机构] University of the Witwatersrand