Photochemical smog in greater Cape Town

Loewenheim, L

Photochemical smog in greater Cape Town

Master Thesis

1988

Publisher

University of Cape Town

Department

Department of Environmental and Geographical Science

Faculty

Faculty of Science

Abstract

Photochemical smog is the name given to a complex sequence of chemical reactions that occurs in the presence of sunlight. These reactions comprise a mix of organic and inorganic compounds, including a number of toxic secondary pollutants such as ozone (O3) and peroxyacetyl nitrate. These substances are commonly referred to as oxidants and are the result of numerous reactions of primary pollutants or precursors (including nitrogen oxides (NOx) and non- methane hydrocarbons (NHHC)) emitted from vehicle exhausts and to some extent industry, O3 is the major constituent of the photochemical oxidants, and its concentration is often used to determine the severity of photochemical smog. Limited research on photochemical smog in Cape Town has been undertaken, and this study has concentrated on providing a more detailed understanding of photochemical precursor and oxidant levels in the urban atmosphere of Greater Cape Town. This was approached by the investigation and assessment of the spatial and temporal behaviour of photochemical pollutants, making use of automatic monitor data collected from 1984 to 1986, and supplemented by data collected during a spatial survey in April and Hay of 1987. Precursor levels were found to be strongly influenced by the seasonal cycle of the weather and were highest in winter when stable atmospheric conditions prevailed, particularly during morning rush hours, O3 behaviour was complex and lacked any definite relationship to season or to selected meteorological variables, although the limited data indicated high levels during the early spring months. Peak levels were generally experienced on fair weather days during the early afternoon hours at the time of maximum ultraviolet radiation, O3 levels did not exceed the USEPA 1-hour standard of 0.12ppm during 1985 and 1986. The spatial distribution of precursor and oxidant concentrations showed the NOx levels to be spatially dependent, following the major arterial roads. NHHC levels were spatially less well defined than NOx, and O3 levels were spatially relatively uniform, exhibiting depletion due to scavenging by nitric oxide (NO) in areas close to main traffic routes. High NOx levels were experienced predominantly in the city centre, while the suburbs tended to experience the higher O3 levels. Cape Town was not considered to have a photochemical smog problem of the same magnitude as Los Angeles or Sydney, ( due to a number of factors which contributed to the complex situation (such as high NO levels, relatively low NMHC levels and strong winds in summer). However in the Northern Suburbs, the absence of high NO levels together with additional NMHC emissions from nearby industry led to the recognition of this area as one of potential photochemical smog formation.