Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel
Bibliographic record
Abstract
Atmospheric ammonia (NH 3 ) 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 NH 3 , 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 NH 3 after the wetting of dry soils; however, the potential regional importance of these emissions remains poorly constrained. Here we use satellite retrievals of atmospheric NH 3 using the Infrared Atmospheric Sounding Interferometer (IASI) regridded at 0.25 ∘ resolution, in combination with satellite-based observations of precipitation, surface soil moisture, and nitrogen dioxide concentrations, to reveal substantial precipitation-induced pulses of NH 3 across the Sahel at the onset of the rainy season in 2008. The highest concentrations of NH 3 occur in pulses during March and April when NH 3 biomass burning emissions estimated for the region are low. For the region of the Sahel spanning 10 to 16 ∘ N and 0 to 30 ∘ E, changes in NH 3 concentrations are weakly but significantly correlated with changes in soil moisture during the period from mid-March through April when the peak NH 3 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). Average emissions for the entire Sahel from a simple box model are estimated to be between 2 and 6 mg NH 3 m −2 d −1 during peaks of the observed pulses, depending on the assumed effective NH 3 lifetime. These early season pulses are consistent with surface observations of monthly concentrations, which show an uptick in NH 3 concentration at the start of the rainy season for sites in the Sahel. The NH 3 concentrations in April are also correlated with increasing tropospheric NO 2 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 emissions of NH 3 -N from the Sahel. We conclude that precipitation early in the rainy season is responsible for substantial NH 3 emissions in the Sahel, likely representing the largest instantaneous fluxes of gas-phase N from the region during the year.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".