Analysis of surface ozone measurements at Cape Point, SANAE and City of Cape Town.

Abstract

This study presents an investigation of surface ozone (O₃) at SANAE (2002 to 2009), Cape Point (1997 to 2009), Molteno (1997 to 2007) and Goodwood (2000 to 2006). The Cape Point data was statistically separated to background and non-background (anthropogenic) contributions. The main aim of the study was to investigate diurnal cycles, seasonal cycles, the weekend effect, and any long term trends in surface O₃, as well as assess meteorological controls on surface O₃ at these stations.

The observed O₃ concentrations were higher during the day than at night at all stations, with urban stations (Molteno and Goodwood) showing more pronounced peak-to-peak variations relative to the marine stations (SANAE and Cape Point). The ‘weekend effect’ was evident at Molteno and Goodwood with higher O₃ and lower NOₓ concentrations on 'weekends relative to weekdays. The weekend effect on O₃ was more pronounced in winter at Goodwood and Molteno. Maximum monthly average concentrations were observed in spring at Goodwood and Molteno, with minimum in winter. The SANAE and Cape Point background O₃ maxima were observed in winter (June to August) with minima in summer (January to February). The Cape Point non-background O₃ maximum was observed in September with a minimum in summer (January). The seasonal cycles at marine sites appear to be driven by O₃ photolysis.

Wind speeds played a critical role in O₃ concentrations, particularly in the continental environment. At Goodwood, high NOₓ levels are associated with low wind speeds. At SANAE, no significant O₃ differences were observed between low wind speeds and high wind speeds while at Cape Point, differences were more evident during the winter months. A decreasing O₃ trend in monthly averages was observed from 1997 to 2006 at Molteno while Goodwood demonstrated an increase of O₃ monthly averages with decreasing NOₓ monthly averages from 2000 to 2006.