Changes in biomass burning, wetland extent, or agriculture drive atmospheric NH₃ trends in select African regions

Jonathan E. Hickman , Niels Andela , Enrico Dammers , Lieven Clarisse , Pierre-François Coheur , Martin van Damme , Courtney A. Di Vittorio , Money Ossohou , Corinne Galy-Lacaux (1) , Kostas Tsigaridis , Susanne E. Bauer

1 LAERO - Laboratoire d'aérologie

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 are 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 2018, and what has caused those changes. 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 (p<0.01). 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 region, where they are associated with expanding agricultural area. In contrast, NH₃ VCDs declined over the Sudd wetlands in South Sudan by over 1.5 % yr⁻¹, though not significantly (p=0.28). Annual maxima in NH₃ VCDs in South Sudan occur during February through May and are associated with the drying of temporarily flooded wetland soils, which favor emissions of NH₃. The change in mean NH₃ VCDs over the Sudd is strongly correlated with variation in wetland extent in the Sudd: in years when more area remained flooded during the dry season, NH₃ VCDs were lower (r=0.64, p<0.05). Relationships between biomass burning and NH₃ may be observed when evaluating national-scale statistics: countries with the highest rates of increasing NH₃ VCDs also had high rates of growth in CO VCDs; burned area displayed a similar pattern, though not significantly. Livestock numbers were also higher in countries with intermediate or high rates of NH₃ VCD growth. 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.

Domaines: Planète et Univers [physics]

Jonathan E. Hickman, Niels Andela, Enrico Dammers, Lieven Clarisse, Pierre-François Coheur, et al.. Changes in biomass burning, wetland extent, or agriculture drive atmospheric NH₃ trends in select African regions. Atmospheric Chemistry and Physics, 2021, 21, pp.16277-16291. ⟨10.5194/acp-21-16277-2021⟩. ⟨insu-03671631⟩