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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-550
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Jul 2018

Research article | 09 Jul 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel

Jonathan E. Hickman1,a, Enrico Dammers2, Corinne Galy-Lacaux3, and Guido R. van der Werf1 Jonathan E. Hickman et al.
  • 1Earth and Climate Cluster, Vrije Universiteit, Amsterdam, 1081 HV, the Netherlands
  • 2Environment and Climate Change Canada, Toronto, Ontario, M3H 5T4, Canada
  • 3Laboratoire d'Aérologie UPS-CNRS UMR 5560, Toulouse, 31400, France
  • anow at: NASA Goddard Institute for Space Studies, New York, NY, 10025, USA

Abstract. Atmospheric ammonia (NH3) is a precursor to fine particulate matter formation and contributes to nitrogen deposition, with potential implications for the health of humans and ecosystems. Agricultural soils and animal excreta are the primary source of atmospheric NH3, but natural soils can also be an important emittor. In regions with distinct dry and wet seasons such as the Sahel, the start of the rainy season triggers a pulse of biogeochemical activity in surface soils known as the Birch effect, which is often accompanied by emissions of microbially-produced gases such as carbon dioxide and nitric oxide. Field and lab studies have sometimes, but not always, observed pulses of NH3 after the wetting of dry soils; however, the potential regional importance of these emissions remains poorly constrained. Here we use satellite retrievals of atmospheric NH3 using the Infrared Atmospheric Sounding Interferometer (IASI) regridded at 0.25° resolution, in combination with satellite-based observations of precipitation, surface soil moisture, and nitric dioxide concentrations, to present evidence of substantial precipitation-induced pulses of NH3 across the Sahel at the onset of the rainy season in 2008. The highest concentrations of NH3 occur in pulses during March and April, when biomass burning emissions estimated for the region by the Global Fire Emissions Database database are low. For the region of the Sahel spanning 10° to 16°N and 0° to 30°E, changes in NH3 concentrations are weakly but significantly correlated with changes in soil moisture during the period from mid-March through April, when the peak NH3 concentrations occur (r=0.28, p=0.02). The correlation is also present when evaluated on an individual pixel-basis during April (r=0.16, p<0.001). Using a simple box model, average emissions for the entire Sahel are between 2 and 6mgNH3m−2day−1 during peaks of the observed pulses, depending on the assumed effective lifetime. These early season pulses are consistent with surface observations of monthly deposition, which show an uptick in NH3 deposition at the start of the rainy season for sites in the Sahel. The NH3 concentrations in April are also correlated with increasing tropospheric NO2 concentrations observed by the Ozone Monitoring Instrument (r=0.78, p<0.0001), which have previously been attributed to the Birch effect. Box model results suggest that pulses occurring over a 35-day period in March and April are responsible for roughly one fifth of annual NH3 emissions from the Sahel. We conclude that precipitation early in the rainy season is responsible for substantial NH3 emissions in the Sahel, likely representing the largest instantaneous fluxes of nitrogen gas from the region during the year.

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Jonathan E. Hickman et al.
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Ammonia gas emitted from soils and combustion, which contributes to air pollution. In regions with distinct dry and rainy seasons, the first rainfall events each year trigger biogeochemical activity in soils. We used satellite observations of the atmosphere over the African Sahel savanna ecosystem to show that increases in soil moisture at the onset of the rainy season are responsible for large pulsed emissions of ammonia, equal to roughly a fifth of annual ammonia emissions from the region.
Ammonia gas emitted from soils and combustion, which contributes to air pollution. In regions...
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