Article révisé par les pairs
Résumé : Abstract. Ammonia (NH3) is the most abundant alkalinecomponent in the atmosphere. Changes in NH3 concentrations haveimportant implications for atmospheric chemistry, air quality, and ecosystemintegrity. We present a long-term ammonia (NH3) assessment in thewestern and central African regions within the framework of the InternationalNetwork to study Deposition and Atmospheric chemistry in Africa (INDAAF)programme. We analyse seasonal variations and trends in NH3concentrations and total column densities along an African ecosystemtransect spanning dry savannas in Banizoumbou, Niger, and Katibougou, Mali;wet savannas in Djougou, Benin, and Lamto, Côte d'Ivoire; and forests in Bomassa, Republic of the Congo, and Zoétélé, Cameroon. We use a21-year record of observations (1998–2018) from INDAAF passive samplers and an 11-year record of observations (2008–2018) of atmospheric vertical column densities from the Infrared Atmospheric Sounding Interferometer (IASI) to evaluate NH3 ground-based concentrations and total column densities,respectively. Climatic data (air temperature, rainfall amount, and leaf areaindex), as well as ammonia emission data of biomass combustion from thefourth version of the Global Fire Emissions Database (GFED4) andanthropogenic sources from the Community Emissions Data System (CEDS), werecompared with total NH3 concentrations and total columns over the sameperiods. Annual mean ground-based NH3 concentrations are around 5.7–5.8 ppb in dry savannas, 3.5–4.7 ppb in wet savannas, and 3.4–5.6 ppb in forests. Annual IASI NH3 total column densities are 10.0–10.7 × 1015 molec. cm−2 in dry savanna, 16.0–20.9 × 1015 molec. cm−2 in wetsavanna, and 12.4–13.8 × 1015 molec. cm−2 in forest stations. Non-parametric statistical Mann–Kendall trend tests applied to annual data show that ground-based NH3 concentrations increase at Bomassa (+2.56 % yr−1) but decrease at Zoétélé (−2.95 % yr−1) over the 21-year period. The 11-year period of IASI NH3 total column density measurements show yearly increasing trends at Katibougou (+3.46 % yr−1), Djougou (+2.24 % yr−1), and Zoétélé (+3.42 % yr−1). From the outcome of ourinvestigation, we conclude that air temperature, leaf area index, andrainfall combined with biomass burning, agricultural, and residentialactivities are the key drivers of atmospheric NH3 in the INDAAFstations. The results also show that the drivers of trends are (1) agriculture in the dry savanna of Katibougou; (2) air temperature and agriculture in the wet savanna of Djougou and Lamto; and (3) leaf area index, air temperature, residential, and agriculture in the forest of Bomassa.

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