Abstract
Abstract. 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.
Highlights
Ammonia (NH3), a reactive nitrogen (N) trace gas, plays a number of important roles in the atmosphere, with implications for human health, climate, and ecosystems
Parts of these regions experience substantial biomass burning (Fig. 1e), high livestock densities (Fig. 1g), and/or high cropland cover (Fig. 1h), all of which can contribute to NH3 emissions
In addition to local emissions, biomass burning emissions and their reactive products are transported to the coast of West Africa during both the Northern Hemisphere rainy season, when they are transported from central and Southern Africa, and during the dry season, when they are transported from biomass burning regions to the east (Sauvage et al, 2007)
Summary
Ammonia (NH3), a reactive nitrogen (N) trace gas, plays a number of important roles in the atmosphere, with implications for human health, climate, and ecosystems. West Africa has been identified as an important NH3 source region (Van Damme et al, 2018), where a trend of increasing NH3 concentrations in recent decades has been attributed at least in part to increased fertilizer use (Van Damme et al, 2021; Warner et al, 2017). The studies by Warner et al (2017) and Van Damme et al (2021) were global in nature, and as such could not include detailed explorations of the drivers of trends such as consideration of emission seasonality or the geographic distribution of emission drivers Consideration of these factors is important across large parts of Africa where both biomass burning and soils are potentially important sources of NH3 (van der A et al, 2008). We use an 11-year satellite record to evaluate trends in atmospheric NH3 concentrations over Africa from 2008 through 2018, including a detailed examination of three regions where changes are pronounced: West Africa, the Lake Victoria basin region, and South Sudan
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