Abstract. Biogenic fluxes from soil at a local and regional scale are crucial to study
air pollution and climate. Here we present field measurements of soil fluxes
of nitric oxide (NO) and ammonia (NH3) observed over four different
land cover types, i.e. bare soil, grassland, maize field, and forest, at an
inland rural site in Benin, West Africa, during the DACCIWA field campaign in
June and July 2016. At the regional scale, urbanization and a massive growth
in population in West Africa have been causing a strong increase in
anthropogenic emissions. Anthropogenic pollutants are transported inland and
northward from the megacities located on the coast, where the reaction with
biogenic emissions may lead to enhanced ozone production outside urban areas,
as well as secondary organic aerosol formation, with detrimental effects on
humans, animals, natural vegetation, and crops. We observe NO fluxes up to
48.05 ngN m−2 s−1. NO fluxes averaged over all land cover types
are 4.79±5.59 ngN m−2 s−1, and maximum soil emissions of NO
are recorded over bare soil. NH3 is dominated by deposition for all
land cover types. NH3 fluxes range between −6.59 and
4.96 ngN m−2 s−1. NH3 fluxes averaged over all land
cover types are -0.91±1.27 ngN m−2 s−1, and maximum
NH3 deposition is measured over bare soil. The observations show high
spatial variability even for the same soil type, same day, and same
meteorological conditions. We compare point daytime average measurements of
NO emissions recorded during the field campaign with those simulated by
GEOS-Chem (Goddard Earth Observing System Chemistry Model) for the same site
and find good agreement. In an attempt to quantify NO emissions at the
regional and national scale, we also provide a tentative estimate of total NO
emissions for the entire country of Benin for the month of July using two
distinct methods: upscaling point measurements and using the GEOS-Chem model.
The two methods give similar results: 1.17±0.6 and
1.44 GgN month−1, respectively. Total
NH3 deposition estimated by upscaling point measurements for the
month of July is 0.21 GgN month−1.