The 10-year observations of the atmospheric molar fractions of CO2, CH4 and CO in West Africa were analyzed using a high precision measurement of the Lamto (LTO) station (6 degrees 31N and 5 degrees 02W) in Cote d'Ivoire. At daily scale, high concentrations appear at night with significant peaks around 7 a.m. local time and minimum concentrations in the afternoon for CO2 and CH4. The CO concentrations show two peaks around 8h and 20h corresponding to the maximum in road traffic of a northern motorway located 14km from the station. The long-term increase rates of CH4 (similar to 7ppb year(-1)) and CO2 (similar to 2.24ppm year(-1)) at Lamto are very close to global trends. The variations of the concentrations of the three gases show strong seasonality with a peak in January for all gases and minima in September for CO2 and CH4, and in June for CO. The CO variation suggests a significant impact of fires on the CO, CO2 and CH4 anomalies in the Lamto region during the dry season (December to February). CO and CH4 show strong correlations (at synoptic-scale and monthly based) in January (r=0.84), February (r=0.90), April (r=0.74), November (r=0.79) and December (r=0.72) reflecting similar sources of emission for both gases. The trajectories of polluted air masses at LTO, also indicate continental sources of emission associated with Harmattan winds.

CO₂, CH₄, and CO are the most critical atmospheric gases in terms of their impact on the radiative system, air quality, and health. This work provides information on the direction of source areas and potential sources of emissions and shows many aspects of these gases by a statistical analysis using bivariate polar diagrams and local weather conditions (e.g., temperature, wind speed, and wind direction), recorded at the Lamto station (LTO, 6°31′ N and 5°2′ W) in Côte d’Ivoire over the 2014–2018 period. The results show that the main regions contributing to the high concentrations of CH₄ (>1925 ppb) and CO₂ (>420 ppm) in the great dry season (GDS), great wet season (GWS), short dry season (SDS), and short wet season (SWS) are the north and north-west sectors of Lamto. In these directions, CH₄ and CO₂ concentrations are associated with wind speeds less than 6 m.s⁻¹, due to the influences of local sources as emissions resulting from the degradation of organic matter submerged during the impoundment of the Taabo dam, and/or human activities linked to the practice of intensive agriculture. In addition, the high concentrations of CO (>350 ppb) are observed in GDS in the north, north-west, north-east, and east sectors for wind speeds less than or equal to 9 m.s⁻¹, due to the influences of both local and distant sources. The correlation coefficients between CH₄ and CO and between CH₄ and CO₂ are positive and significant in all sectors. However, those calculated between CO₂ and CO have showed both low and high values in all seasons.

The contribution in terms of long-range transport of CO₂, CH₄, and CO concentrations to measurements at Lamto (5°02′ W–6°13′ N) was analyzed for the 2014–2017 period using the FLEXPART model that calculates the retro-plumes of air masses arriving at the station. The identification of the source-receptor relationships was also studied with a clustering technique applied on those retro-plumes. This clustering technique enabled us to distinguish four categories of air mass transports arriving at Lamto site described as follows: oceanic and maritime origin (≈37% of the retro-plumes), continental origin (≈21%), and two hybrid clusters (≈42%). The results show that continental emission sources contribute significantly to the increases in concentrations of CO₂, CH₄, and CO and explain ≈40% of their variance. These emission sources are predominantly from north and north-east directions of the measurement point, and where densely populated and economically developed areas are located. In addition, the transport of air masses from these directions lead to the accumulation of CO₂, CH₄, and CO. Furthermore, the ratios ΔCO/ΔCH₄ and ΔCO/ΔCO₂ observed in the groups associated with Harmattan flows clearly show an influence of combustion processes on the continent. Thus, the grouping based on FLEXPART footprints shows an advantage compared to the use of simple trajectories for analyzing source–receptor relationships.

This study analyzes the potential response of the seasonal cycle of heatwaves (HWDI) and dry (CDD) and wet (CWD) spell indices over West Africa for the near- (2031–2060) and the far-future periods (2071–2100) under RCP4.5 and RCP8.5 scenarios using Coordinated Regional Climate Downscaling Experiment (CORDEX) simulations. Despite the fact that some relative biases (an underestimation of 30% for CDD, an overestimation of about 60% for CWD, and an overestimation of about 50% for HWDI) exist, during the historical period (1976–2005) in general, the CORDEX simulations and their ensemble mean outperform the seasonal variability in the above-mentioned indices over three defined subregions of West Africa (i.e., the Gulf of Guinea and Western and Eastern Sahel). They show high correlation coefficients (0.9 in general) and less RMSE. They project an increase (about 10 and 20 days) in heatwave days for both the near- and far-future periods over the whole West African region under both RCP scenarios. In addition, projections indicate that the Sahel regions will experience a decrease (about 5 days) in wet spell days from March to November, while in the Gulf of Guinea, a decrease (about 3 days) is projected throughout the year, except in the CCCLM simulation, which indicates an increase (about 5 days) during the retreat phase of the monsoon (October to December). Our results also highlight an increase (about 80%) in dry spells over the Sahel regions that are more pronounced during the March–November period, while over the Gulf of Guinea, an increase (about 40%) is projected over the entire year. Moreover, the months of increasing dry spells and decreasing wet spells coincide, suggesting that countries in these regions could be simultaneously exposed to dry seasons associated with a high risk of drought and heatwaves under future climate conditions.

This study analyzes the potential response of the seasonal cycle of heatwave (HWDI), dry (CDD) and wet (CWD) spells indices over West Africa for the near (2031-2060) and the far (2071-2100) future periods, under RCP4.5 and RCP8.5 scenarios using CORDEX simulations. Although some relative biases during the historical period (1976-2005), the CORDEX simulations and their ensemble mean outperform the seasonal variability of the above indices over three defined sub-regions of West Africa (i.e., Guinea gulf, west and east Sahel). They have shown significant correlation coefficients and less RMSE. They project an increase in heatwave days for both near and far future periods over whole west Africa region under both RCP scenarios. In addition, the Sahel regions will face to a decrease in wet spells days from March to November, whereas, the Gulf of Guinea will face to a decrease during all the year, except CCCLM simulation which indicates an increase during the retreat phase of the monsoon (October to December). The results also have shown an increase in dry spells over Sahel regions, more pronounced during March-November period, whereas, over Guinea gulf, the increase is observed over the entire year. On the other hand, the months of increasing dry spells and decreasing wet spells coincide, suggesting that countries in these regions could be exposed simultaneously to dry season associated with a high risk of drought and heatwave under future climate conditions.