Changes in biomass burning, wetland extent, or agriculture drive atmospheric NH 3 trends in select African regions

TLDR: In this article, the Infrared Atmospheric Sounding Interferometer (IASI) was used along with other satellite observations of the land surface and atmosphere to evaluate how NH 3 concentrations have changed over Africa from 2008 through 2018, and what has caused those changes.

Abstract: . Atmospheric ammonia (NH 3 ) is a precursor to fine particulate matter and a source of nitrogen (N) deposition that can adversely affect ecosystem health. The main sources of NH 3 – agriculture and biomass burning – are undergoing are or expected to undergo substantial changes in Africa. Although evidence of increasing NH 3 over parts of Africa has been observed, the mechanisms behind these trends are not well understood. Here we use observations of atmospheric NH 3 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 NH 3 concentrations have changed over Africa from 2008 through 2018, and what has caused those changes. In West Africa NH 3 VCDs are observed to increase during the late dry season, with increases of over 6 % yr −1 in Nigeria during February and March ( p ). 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, NH 3 VCDs declined over the Sudd wetlands in South Sudan by over 1.5 % yr −1 , though not significantly ( p=0.28 ). Annual maxima in NH 3 VCDs in South Sudan occur during February through May and are associated with the drying of temporarily flooded wetland soils, which favor emissions of NH 3 . The change in mean NH 3 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, NH 3 VCDs were lower ( r=0.64 , p ). Relationships between biomass burning and NH 3 may be observed when evaluating national-scale statistics: countries with the highest rates of increasing NH 3 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 NH 3 VCD growth. Fertilizer use in Africa is currently low but growing; implementing practices that can limit NH 3 losses from fertilizer as agriculture is intensified may help mitigate impacts on health and ecosystems.

Figures

Figure 1. Livestock density and annual averages and trends in burned area and 245 atmospheric NH3 concentrations across seven sub-Saharan African ecoregions. Mean 246 annual (a) and trend (b) in atmospheric NH3 VCDs from IASI for the period 2008 through 247 2017. Mean annual (c) and trend (d) in annual atmospheric CO VCDs from IASI for the 248 same period. Mean annual (e) and trend (f) in annual burned area from MODIS for 2008-249 2017. Livestock densities for 2007 from the FAO (f), and key for the outlines of seven 250 major African ecoregions (g). 251

Figure 2. Spatial correlations between mean annual atmospheric NH3 concentrations and 253 livestock density (a), population density (b), and mean cropland area (c), as well as trend in 254 crop area for 2008 through 2016 (d). In correlations, NH3 trend is based on data for 2008 255 through 2017, livestock density data are for the year 2007, population density data are for 256 the year 2017, and cropland area are the mean of 2008 through 2016. 257

Figure 5. Mean monthly flooded extent of the Sudd, surface temperatures over 377 South Sudan, and NH3 VCDs over the Sudd and the entirety of South Sudan for the period 378 2008 through 2017. 379

Full text

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https://doi.org/10.5194/acp-2020-945
Preprint. Discussion started: 25 November 2020
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 Author(s) 2020. CC BY 4.0 License.

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https://doi.org/10.5194/acp-2020-945
Preprint. Discussion started: 25 November 2020
c
 Author(s) 2020. CC BY 4.0 License.