Surface ozone over southern Africa; synthesis of monitoring results during the cross border air pollution impact assessment project

Abstract

Measurements of surface ozone in the southern African region are limited to a few active and passive monitoring sites. Over the region, the mean surface ozone concentrations exhibit strong seasonal and diurnal variations. The seasonal maximum generally occurs in the spring months from August to November and the minimum occurs in December and January. With the exception of Cape Point, a strong diurnal variation is observed at all sites. Ozone concentrations increase from a minimum near sunrise to a maximum in the afternoon, then decrease again to the early morning minimum. The highest ozone concentrations occur over Botswana and the Mpumalanga highveld. In both regions the springtime maximum is between 40 and 60 ppb, but reached more than 90 ppb as a mean in October 2000. In these two regions the monthly minimum is between 20 and 30 ppb. The mean daytime ozone concentrations in Botswana and on the highveld reach 40 ppb as early as 10:00 and remain above this level for up to 10 h. At the background stations at Cape Point, in Namibia and areas adjacent to the highveld the maximum concentrations are between 20 and 30 ppb with minimums between 10 and 20 ppb. In the Cross border Air Pollution Impact Assessment project, CAPIA, a threshold value of 40 ppb is used to assess the potential risk of damage to maize by ozone. Measured data show that this threshold is exceeded over Botswana and on the South African highveld.