Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Ammonia (NH<sub>3</sub>) is the most abundant alkaline component in the atmosphere. Changes in NH<sub>3</sub> concentrations have important implications for atmospheric chemistry, air quality, and ecosystem integrity. We present a long-term ammonia (NH<sub>3</sub>) assessment in the Western and Central Africa region within the framework of the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) program. We analyze seasonal variations and trends of NH<sub>3</sub> concentrations and total columns densities along an African ecosystem transect spanning dry savannas in Banizoumbou, Niger and Katibougou, Mali, wet savannas in Djougou, Benin and Lamto, C&ocirc;te d&rsquo;Ivoire, and forests in Bomassa, Republic of Congo and Zo&eacute;t&eacute;l&eacute;, Cameroon. We use a 21-year record of observations (1998&ndash;2018) from INDAAF passive samplers and 11-year record of observations (2008&ndash;2018) of atmospheric vertical column densities from the Infrared Atmospheric Sounding Interferometer (IASI) to evaluate NH<sub>3</sub> ground-based concentrations and total column densities, respectively. Annual mean ground-based NH<sub>3</sub> concentrations are around 5.7&ndash;5.8 ppb in dry savannas, 3.5&ndash;4.7 ppb in wet savannas and 3.4&ndash;5.6 ppb in forests. These results suggest that NH<sub>3</sub> emissions from precipitation-induced pulses and volatilization from animal excreta are important emission sources in dry savannas, and biomass burning and agricultural sources are important sources in wet savanna and forest ecosystems. NH<sub>3</sub> total column densities clearly show that the biomass burning source is the most important source in the Lamto wet savanna ecosystem. Annual IASI NH<sub>3</sub> total column densities are 10.1&ndash;11.0x10<sup>15</sup> molec cm<sup>-2</sup> in dry savanna, 16.5&ndash;21.4x10<sup>15</sup> molec cm<sup>-2</sup> in wet savanna and 14.3&ndash;15.1x10<sup>15</sup> molec cm<sup>-2</sup> in forest stations. Non-parametric statistical Mann-Kendall trend tests applied to annual data show that ground-based NH<sub>3</sub> concentrations increase at Bomassa (+2.56 % yr<sup>-1</sup>), but decrease at Zo&eacute;t&eacute;l&eacute; (-2.95 % yr<sup>-1</sup>) over the 21-year period. The 11-year period of IASI NH<sub>3</sub> total column density measurements show yearly increasing trends at Katibougou (+3.98 % yr<sup>-1</sup>) and Djougou (+2.24 % yr<sup>-1</sup>). At Zo&eacute;t&eacute;l&eacute;, we calculated an increasing trend of leaf area index associated to a significant anticorrelation with ground-based NH<sub>3</sub> concentrations. Leaf area index increase could enhance deposition processes and could contribute to the decrease of ground-based NH<sub>3</sub> concentrations.

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