Variability and trends of NO₂ and CO over West Africa from space coupled to climate, fire and soil moisture

Emissions of ozone precursors from biomass burning, soil microbial processes and gas flaring activities over West Africa play major roles in local, regional and global air quality. This thesis investigates the sources, sinks, annual cycles and decadal trends of atmospheric NO2 and CO over West Africa between December 2004 and November 2014 using freely available remotely sensed data and software. Datasets used were soil moisture from ASCAT (MetOp-A/B), CO from IASI (MetOp-A/B), NO2 from OMI, CO from TES (AURA), FRP from SEVIRI (Meteosat) and burned area generated from MODIS (Terra/Aqua) and other sensors. Major contributors to pollution variability considered were climate, biomass burning, gas flaring, soil moisture and atmospheric transport (in the case of CO seasonality). The Savitzky-Golay smoothing algorithm was applied to eliminate false peaks from each time series to improve the identification of seasonality. The trend analysis showed significant decline in NO2 over areas where biomass burning was the main driver of NO2 variability and a significant increase in areas where soil moisture and urban emissions were main drivers. Overall, CO declined inland but trends between December 2008 and November 2014 showed a strong land-ocean difference in linear trends such that inland, CO declined significantly but increased insignificantly offshore. Pixel-wise seasonal relationships between both ozone precursors, biomass burning and soil moisture were assessed using linear regression models. Granger causal tests were carried out to determine the Granger causality of soil moisture and biomass burning on the annual cycles of NO2 and CO. This showed strong soil moisture Granger causality on atmospheric CO in equatorial winter grasslands and NO2 in arid steppe/desert grasslands/shrublands. Burned area showed a strong causality on CO in equatorial winter grasslands. Finally, the dispersion models indicate that beyond biomass burning, gas flaring may contribute to CO transported from the SHA to West Africa in the SHA biomass burning season.