Tropospheric NO2 concentrations over West Africa are influenced by climate zone and soil moisture variability

The annual cycles of soil moisture and NO2 have been analysed across the climate zones of West Africa using two satellite data sets (OMI on AURA and ASCAT on MetOp-A). Exploring the sources and sinks for NO2 it is clear that the densely populated urban cities including Lagos and Abuja had the highest mean NO2 concentrations (>1.8 × 10 15 molecules cm -2 ) indicative of the anthropogenic urban emissions. The data analysis shows that rising soil moisture levels may influence the 15 sink of NO2 concentrations after the biomass burning. The results also show significant soil moisture changes in areas of high humidity especially in the east equatorial monsoon climate zone where most of the Niger delta is located (4 %/yr.). A decline in NO2 (0.9 %/yr.) was also observed in this climate zone. Beyond seasonal linear regression models, climate based Granger’s causality tests show that tropospheric NO2 concentrations from soil emissions in the arid steppe (Sahel) and arid desert climate zones of West Africa are significantly affected by soil moisture variability (F > 10, p < 0.01). The arid steppe 20 and arid deserts regions showed no significant changes in soil moisture levels but significant increase in tropospheric NO2 concentrations (> 0.8 %/yr). The results demonstrate the critical sensitivity of the West African emissions of NO2 on soil moisture and climate zone.