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

Hickman, Jonathan E.; Dammers, Enrico; Galy-Lacaux, Corinne; van der Werf, Guido R.

Atmospheric ammonia (inline-formulaNH3) is a precursor to fine particulate matter formation and contributes to nitrogen (N) deposition, with potential implications for the health of humans and ecosystems. Agricultural soils and animal excreta are the primary source of atmospheric inline-formulaNH3, but natural soils can also be an important emitter. 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 inline-formulaNH3 after the wetting of dry soils; however, the potential regional importance of these emissions remains poorly constrained. Here we use satellite retrievals of atmospheric inline-formulaNH3 using the Infrared Atmospheric Sounding Interferometer (IASI) regridded at 0.25inline-formula resolution, in combination with satellite-based observations of precipitation, surface soil moisture, and nitrogen dioxide concentrations, to reveal substantial precipitation-induced pulses of inline-formulaNH3 across the Sahel at the onset of the rainy season in 2008. The highest concentrations of inline-formulaNH3 occur in pulses during March and April when inline-formulaNH3 biomass burning emissions estimated for the region are low. For the region of the Sahel spanning 10 to 16inline-formula N and 0 to 30inline-formula E, changes in inline-formulaNH3 concentrations are weakly but significantly correlated with changes in soil moisture during the period from mid-March through April when the peak inline-formulaNH3 concentrations occur (inline-formular=0.28, inline-formulap=0.02). The correlation is also present when evaluated on an individual pixel basis during April (inline-formular=0.16, inline-formulap<0.001). Average emissions for the entire Sahel from a simple box model are estimated to be between 2 and 6 mg inline-formulaNH3 minline-formula−2 dinline-formula−1 during peaks of the observed pulses, depending on the assumed effective inline-formulaNH3 lifetime. These early season pulses are consistent with surface observations of monthly concentrations, which show an uptick in inline-formulaNH3 concentration at the start of the rainy season for sites in the Sahel. The inline-formulaNH3 concentrations in April are also correlated with increasing tropospheric inline-formulaNO2 concentrations observed by the Ozone Monitoring Instrument (inline-formular=0.78, inline-formulap<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 emissions of inline-formulaNH3-N from the Sahel. We conclude that precipitation early in the rainy season is responsible for substantial inline-formulaNH3 emissions in the Sahel, likely representing the largest instantaneous fluxes of gas-phase N from the region during the year.

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Hickman, Jonathan E. / Dammers, Enrico / Galy-Lacaux, Corinne / et al: Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel. 2018. Copernicus Publications.

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