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

Abstract. 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 × 10¹⁵ molecules cm⁻²) indicative of the anthropogenic urban emissions. The data analysis shows that rising soil moisture levels may influence the 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 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.