Abstract. Atmospheric ammonia (NH3) is a precursor to fine particulate matter and a source of nitrogen (N) deposition that can adversely affect ecosystem health. The main sources of NH3 – agriculture and biomass burning – are undergoing or expected to undergo substantial changes in Africa. Although evidence of increasing NH3 over parts of Africa has been observed, the mechanisms behind these trends are not well understood. Here we use observations of atmospheric NH3 vertical column densities (VCDs) from the Infrared Atmospheric Sounding Interferometer (IASI) along with other satellite observations of the land surface and atmosphere to evaluate how NH3 concentrations have changed over Africa from 2008 through 2017, and what has caused those changes. We find that NH3 VCDs have increased over several regions, including much of West Africa and parts of the Lake Victoria Basin. In West Africa NH3 VCDs are observed to increase during the late dry season, with increases of over 6 % yr−1 in Nigeria during February and March. These positive trends are associated with increasing burned area and CO trends during these months, likely related to agricultural preparation. Increases are also observed in the Lake Victoria Basin, where they are associated with expanding agricultural area. In contrast, South Sudan NH3 VCDs declined by over 2 % yr−1 during the February through May period, with the largest rates of change over the Sudd wetlands. Annual maxima in NH3 VCDs in South Sudan occur during February through May and are associated with drying of temporarily flooded wetland soils, which favor emissions of NH3. The change in mean NH3 VCDs over the Sudd and all of South Sudan during February through May is strongly correlated with variation in wetland extent in the Sudd: in years when more area remained flooded during the dry season, NH3 concentrations were higher (r = 0.69, p = 0.03). Relationships between agriculture and NH3 can be observed when evaluating national-scale statistics: countries with the largest declines in NH3 VCDs concentrations over time tended to have the smallest growth rates in crop productivity and livestock numbers as well as smaller negative changes in burned area than other countries. Fertilizer use in Africa is currently low but growing; implementing practices that can limit NH3 losses from fertilizer as agriculture is intensified may help mitigate impacts on health and ecosystems.
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