Biogenic nitrogen oxide emissions from soils: impact on NO_x and ozone over west Africa during AMMA (African Monsoon Multidisciplinary Analysis): observational study

D. J. Stewart (1) , C. M. Taylor (2) , C. E. Reeves (1) , J. B. Mcquaid (3)

1 School of Environmental Sciences [Norwich]
2 CEH - Centre for Ecology and Hydrology
3 SEE - School of Earth and Environment [Leeds]

Chemical and meteorological parameters measured on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 Atmospheric Research Aircraft during the African Monsoon Multidisciplinary Analysis (AMMA) campaign are presented to show the impact of NO_x emissions from recently wetted soils in West Africa. NO emissions from soils have been previously observed in many geographical areas with different types of soil/vegetation cover during small scale studies and have been inferred at large scales from satellite measurements of NO_x. This study is the first dedicated to showing the emissions of NO_x at an intermediate scale between local surface sites and continental satellite measurements. The measurements reveal pronounced mesoscale variations in NO_x concentrations closely linked to spatial patterns of antecedent rainfall. Fluxes required to maintain the NO_x concentrations observed by the BAe-146 in a number of cases studies and for a range of assumed OH concentrations (1×10⁶ to 1×10⁷ molecules cm^?3) are calculated to be in the range 8.4 to 36.1 ng N m^?2 s^?1. These values are comparable to the range of fluxes from 0.5 to 28 ng N m^?2 s^?1 reported from small scale field studies in a variety of non-nutrient rich tropical and sub-tropical locations reported in the review of Davidson and Kingerlee (1997). The fluxes calculated in the present study have been scaled up to cover the area of the Sahel bounded by 10 to 20 N and 10 E to 20 W giving an estimated emission of 0.03 to 0.30 Tg N from this area for July and August 2006. The observed chemical data also suggest that the NO_x emitted from soils is taking part in ozone formation as ozone concentrations exhibit similar fine scale structure to the NO_x, with enhancements over the wet soils. Such variability can not be explained on the basis of transport from other areas.

Delon et al. (2008) is a companion paper to this one which models the impact of soil NO_x emissions on the NO_x and ozone concentration over West Africa during AMMA. It employs an artificial neural network to define the emissions of NO_x from soils, integrated into a coupled chemistry-dynamics model. The results are compared to the observed data presented in this paper. Here we compare fluxes deduced from the observed data with the model-derived values from Delon et al. (2008).

Domaines: Océan, Atmosphère

D. J. Stewart, C. M. Taylor, C. E. Reeves, J. B. Mcquaid. Biogenic nitrogen oxide emissions from soils: impact on NO_x and ozone over west Africa during AMMA (African Monsoon Multidisciplinary Analysis): observational study. Atmospheric Chemistry and Physics, 2008, 8 (8), pp.2285-2297. ⟨hal-00296532⟩