Abstract. Atmospheric ammonia (NH3) 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 NH3, 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 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 nitrogen dioxide
concentrations, to reveal 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 NH3 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 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). Average emissions
for the entire Sahel from a simple box model are estimated to be between 2
and 6 mg NH3 m−2 d−1 during peaks of the observed
pulses, depending on the assumed effective NH3 lifetime. These early
season pulses are consistent with surface observations of monthly
concentrations, which show an uptick in
NH3 concentration 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 emissions of
NH3-N 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 gas-phase N
from the region during the year.
Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel
