16 Jan 2023
 | 16 Jan 2023
Status: this preprint is currently under review for the journal ACP.

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

Money Ossohou, Jonathan E. Hickman, Lieven Clarisse, Pierre-François Coheur, Martin Van Damme, Marcellin Adon, Véronique Yoboué, Eric Gardrat, Maria Dias Alvès, and Corinne Galy-Lacaux

Abstract. Ammonia (NH3) is the most abundant alkaline component in the atmosphere. Changes in NH3 concentrations have important implications for atmospheric chemistry, air quality, and ecosystem integrity. We present a long-term ammonia (NH3) 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 NH3 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, Côte d’Ivoire, and forests in Bomassa, Republic of Congo and Zoétélé, Cameroon. We use a 21-year record of observations (1998–2018) from INDAAF passive samplers and 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. 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. These results suggest that NH3 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. NH3 total column densities clearly show that the biomass burning source is the most important source in the Lamto wet savanna ecosystem. Annual IASI NH3 total column densities are 10.1–11.0x1015 molec cm-2 in dry savanna, 16.5–21.4x1015 molec cm-2 in wet savanna and 14.3–15.1x1015 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 (+3.98 % yr-1) and Djougou (+2.24 % yr-1). At Zoétélé, we calculated an increasing trend of leaf area index associated to a significant anticorrelation with ground-based NH3 concentrations. Leaf area index increase could enhance deposition processes and could contribute to the decrease of ground-based NH3 concentrations.

Money Ossohou et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-793', Anonymous Referee #1, 03 Mar 2023
  • RC2: 'Comment on acp-2022-793', Anonymous Referee #2, 13 Apr 2023

Money Ossohou et al.

Money Ossohou et al.


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Short summary
The updated analyses of ground-based concentrations and satellite total vertical columns of atmospheric ammonia help to better understand 21st century ammonia dynamics in Sub Saharan Africa. We conclude that the main atmospheric ammonia sources are alkaline Sahelian soils and agro-pastoralism emissions along the dry savanna ecosystem. Ammonia variability in the wet savanna and forest ecosystems emphasized the importance of two main sources, i.e., biomass burning and agricultural waste burning.