Measurement report: Long-term measurements of ozone concentrations in semi-natural African ecosystems

Abstract. In the framework of the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) program, we present the seasonal variability of atmospheric ozone concentrations at the regional scale. The contributions of local atmospheric chemistry and meteorological parameters to ozone photochemistry are investigated, as are long-term trends in ozone concentrations. Fourteen measurement sites were identified for this study, 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 Congo) and semi-agricultural/arid savanna (Mbita, Kenya, Louis Trichardt, Amersfoort, Skukuza and Cape Point, South Africa). Over the period 1995–2020, monthly ozone concentrations were measured at these sites using passive samplers. Monthly averages of surface ozone range from 4.7±1.4 ppb (Bomassa) to 31.0±10.5 ppb (Louis Trichardt). Ozone levels in the wet season (in dry savanna) are higher and comparable to concentrations in the dry season (in wet savanna and forest). In East Africa, ozone levels show no marked seasonality, with higher levels in Southern Africa. In the Sahel, under the influence of temperature, ozone formation is closely linked to biogenic Volatile Organic Carbon (VOC) emissions. It is also sensitive to nitrogen monoxide (NO) emissions in the presence of high precipitation and humidity. Biogenic VOC (BVOC) emissions, anthropogenic NOx, temperature and radiation are the dominant contributors to O₃ formation in wet savannas and forests. At the southern African sites, the most important parameters influencing surface ozone concentrations are humidity, temperature, VOC emissions (anthropogenic and biogenic) and NOx. At the annual scale, Katibougou and Banizoumbou sites (dry savanna) experienced a significant decrease in ozone concentrations respectively around -0.24 ppb yr^-1 (-2.3 % yr^-1) and -0.15 ppb yr^-1 (-1.9 % yr^-1). Seasonal Kendall statistical tests revealed decreasing trends of -0.07 ppb yr^-1 in Banizoumbou and -0.24 ppb yr^-1 in Katibougou. These decreasing trends are consistent with those observed for nitrogen dioxide (NO₂) and biogenic VOCs. A significant increasing trend is observed in Zoétélé, with the Sen slope estimated at 0.1 ppb yr^-1 and at Skukuza (Sen slope = 0.3 ppb yr^-1). The increasing trends are consistent with the increase in biogenic emissions at Zoétélé and NO₂ levels at Skukuza. Very few surface O₃ measurements exist in Africa, and long-term results presented in this study are the most extensive for the studied ecosystems.