Southern African Ozone Trends (1990-2007): Influences of Climate Variability and Anthropogenic Sources

Abstract:

Studies of tropospheric ozone trends over the southern African Highveld in the 1990-2007 period present a paradox. We used monthly averaged surface ozone data from 5 South African monitoring stations east of Johannesburg in a linear regression model to show that the cycles associated with the El Niño/La Niña make a considerable contribution to interannual ozone variability through perturbations in cloud cover, temperature and precipitation that interact with photochemistry (see Figure). During El Niño periods, typically sunnier and drier, summertime ozone is enhanced, whereas wetter, cloudier conditions of a La Niña are associated with lower ozone. Interestingly, the 5 stations show very little evidence of a statistically significant trend from 1990 through 2007. Over the same time period, the regression model shows that free tropospheric ozone, from 5-11 km, taken from monthly averaged SHADOZ (Southern Hemisphere ADditional OZonesondes) and MOZAIC (Measurement of Ozone and Water Vapour on Airbus in-service Aircraft) profiles, increased significantly (+20-25%/decade) in late autumn and early winter (May-July). There is also a positive ozone trend near the tropopause in summer (Nov.-Dec.) but none during the oft-studied months of biomass fires (Sept.-Oct.). It is difficult to interpret the seemingly contradictory trends in terms of emissions of ozone precursors that are not well characterized over the Highveld and larger southern African region. However, we ran a series of back-trajectories at 500 and 300 hPa to coincide with the profile sampling times in May-August 1990-2007. Regional contributions are implicated by recirculation in the Johannesburg region. Trajectories also point to long-range transport from the greater African continent, south Atlantic and South America, all known regions of high ozone and in the case of South America, growing pollution from emerging mega-cities.