Measurements of nitric oxide and ammonia soil fluxes from a wetsavanna ecosystem site in West Africa during the DACCIWA fieldcampaign

It is important to correctly simulate biogenic fluxes from soil in atmospheric chemistry models at a local and regional scale to study air pollution and climate in an area of the world, West Africa, that has been subject to a strong increase in anthropogenic emissions due to a massive growth in population and urbanization. Anthropogenic pollutants are transported inland and northward from the mega cities located on the coast, where the reaction with biogenic emissions may lead to enhanced ozone production outside urban areas, as well as secondary organic aerosols formation, with detrimental effects on humans, animals, natural vegetation and crops. Here we present field measurements of soil fluxes of nitric oxide (NO) and ammonia (NH 3 ) 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. 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 , maximum soil emissions of NO are recorded over bare soil. NH 3 is dominated by deposition for all land cover types. NH 3 fluxes range between −6.59 and 4.96 ngN m −2  s −1 . NH 3 fluxes averaged over all land cover types are −0.91 ± 1.27 ngN m −2  s −1 and maximum NH 3 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 daily 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 GgN/month and 1.44 GgN/month, respectively. Total NH 3 deposition estimated by upscaling point measurements for the month of July is 0.21 GgN/month.