Satellite mapping of rain-induced nitric oxide emissions from soils

We use space-based observations of NO₂ columns from the Global Ozone Monitoring Experiment (GOME) to map the spatial and seasonal variations of NO_x emissions over Africa during 2000. The GOME observations show not only enhanced tropospheric NO₂ columns from biomass burning during the dry season but also comparable enhancements from soil emissions during the rainy season over the Sahel. These soil emissions occur in strong pulses lasting 1-3 weeks following the onset of rain, and affect 3 million km² of semiarid sub-Saharan savanna. Surface observations of NO₂ from the International Global Atmospheric Chemistry (IGAC)/Deposition of Biochemically Important Trace Species (DEBITS)/Africa (IDAF) network over West Africa provide further evidence for a strong role for microbial soil sources. By combining inverse modeling of GOME NO ₂ columns with space-based observations of fires, we estimate that soils contribute 3.3 ± 1.8 TgN/year, similar to the biomass burning source (3.8 ± 2.1 TgN/year), and thus account for 40% of surface NO_x emissions over Africa. Extrapolating to all the tropics, we estimate a 7.3 TgN/year biogenic soil source, which is a factor of 2 larger compared to model-based inventories but agrees with observation-based inventories. These large soil NO_x emissions are likely to significantly contribute to the ozone enhancement originating from tropical Africa.