Measurement report: Long-term measurements of surface ozone and trends in semi-natural sub-Saharan African ecosystems

Abstract. For nearly 30 years, the International Network to study Deposition and Atmospheric chemistry in AFrica (INDAAF) programme has measured surface ozone from 14 sites in Africa representative of the main African ecosystems: dry savannas (Banizoumbou, Niger; Katibougou and Agoufou, Mali; Bambey and Dahra, Senegal), wet savannas (Lamto, Côte d'Ivoire; Djougou, Benin), forests (Zoétélé, Cameroon; Bomassa, Republic of the Congo) and agricultural–semi-arid savannas (Mbita, Kenya; Louis Trichardt, Amersfoort, Skukuza and Cape Point, South Africa). The data are collected with passive samplers and archived as monthly averages; quality assurance is maintained by INDAAF's calibration and intercomparison protocols with other programmes employing the same systems. This analysis reports on correlations of INDAAF ozone time series (1995–2020) with local meteorological parameters and with ozone precursors, biogenic volatile organic compounds (BVOCs) and nitrogen oxides (NOx), derived from standard global databases. Mean annual averages of surface ozone range from 3.9 ± 1.1 ppb (Bomassa) to 30.8 ± 8.0 ppb (Louis Trichardt), reflecting a general positive gradient from west central Africa to South Africa. At the decade scale, from 2000 to 2020, the Katibougou and Banizoumbou sites (dry savanna) experienced a significant decrease in ozone of around −2.4 and −0.8 ppb per decade, respectively. These decreasing trends are consistent with those observed for nitrogen dioxide (NO2) and BVOCs. An increasing trend is observed in Zoétélé (2001–2020), estimated at +0.7 ppb per decade, and at Skukuza (2000–2015; +3.4 ppb per decade). The increasing trends are consistent with increasing biogenic emissions at Zoétélé and NO2 levels at Skukuza. Very few surface O3 measurements exist in Africa, and the long-term results presented in this study are the most extensive for the ecosystems studied. The importance of maintaining long-term observations like INDAAF cannot be overstated. The data can be used to assess ozone impacts on African crops. For the Tropospheric Ozone Assessment Report Phase II (TOAR II), they provide invaluable constraints for models of chemical and climate processes in the atmosphere.