Trends and seasonal variability in ammonia across major biomes in western and central Africa inferred from long-term series of ground-based and satellite measurements

Ammonia (NH 3 ) is the most abundant alkaline component in the atmosphere. Changes in NH 3 concentrations have important implications for atmospheric chemistry, air quality, and ecosystem integrity. We present a long-term ammonia (NH 3 ) assessment in the western and central African regions within the framework of the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) programme. We analyse seasonal variations and trends in NH 3 concentrations and total column densities along an African ecosystem transect spanning dry savannas in Banizoumbou, Niger, and Katibougou, Mali; wet savannas in Djougou, Benin, and Lamto, Cote d'Ivoire; and forests in Bomassa, Republic of the Congo, and Zoetele, Cameroon. We use a 21-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 NH 3 ground-based concentrations and total column densities, respectively. Climatic data (air temperature, rainfall amount, and leaf area index), as well as ammonia emission data of biomass combustion from the fourth version of the Global Fire Emissions Database (GFED4) and anthropogenic sources from the Community Emissions Data System (CEDS), were compared with total NH 3 concentrations and total columns over the same periods. Annual mean ground-based NH 3 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 NH 3 total column densities are 10.0-10.7 x 10 15 molec. cm - 2 in dry savanna, 16.0-20.9 x 10 15 molec. cm - 2 in wet savanna, and 12.4-13.8 x 10 15 molec. cm - 2 in forest stations. Non-parametric statistical Mann-Kendall trend tests applied to annual data show that ground-based NH 3 concentrations increase at Bomassa ( + 2.56 % yr - 1 ) but decrease at Zoetele ( - 2.95 % yr - 1 ) over the 21-year period. The 11-year period of IASI NH 3 total column density measurements show yearly increasing trends at Katibougou ( + 3.46 % yr - 1 ), Djougou ( + 2.24 % yr - 1 ), and Zoetele ( + 3.42 % yr - 1 ). From the outcome of our investigation, we conclude that air temperature, leaf area index, and rainfall combined with biomass burning, agricultural, and residential activities are the key drivers of atmospheric NH 3 in the INDAAF stations. 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.