Source–Receptor Relationships and Cluster Analysis of CO2, CH4, and CO Concentrations in West Africa: The Case of Lamto in Côte d’Ivoire

Abstract

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.

CO2, CH4, and CO Emission Sources and Their Characteristics in the Lamto Ecological Reserve (Côte d’Ivoire)

“…In this study, four wind sectors (i = 90 degrees) were chosen and the threshold values were fixed at the 75th percentile over the entire study period in the four seasons of the Lamto rainfall regime. Here, the threshold percentile choice takes into account the distribution of concentrations as a function of wind speed and direction, and the trend of CO 2 , CH 4 and CO species obtained at Lamto area by Tiemoko et al [12] over the 2014-2018 period. The CPF function is defined by:…”

“…This instrument model, based on cavity ring-down spectroscopy, has linear and stable responses, and it is recognized for its high accuracy in measuring the molar fractions of CO 2 (ppm), CH 4 (ppb) and CO (ppb). At Lamto, CRDS shows accuracies below 0.1 ppm, 0.5 ppb, and 16 ppb for CO 2 , CH 4 , and CO measurements, respectively (see Tiemoko et al [4,12]).…”

“…The air analyzed is taken continuously at the top of a 50 m tower. The measuring system, data processing, and calibration strategy are explained by Tiemoko et al [4,12].…”

“…These winds blow from north to south carrying desert dust [35,36,41] and polluted air masses due to Sahelian biomass fires [41,42]. Tiemoko et al [12] showed that these harmattan winds in the GDS season significantly contributed to the increased CH 4 concentration and explained ~64% of its variance. In addition, the heat maps of CH 4 concentrations (Figure 4b) show that the significant concentration values during all seasons (>1970 ppb in GDS, >1925 ppb in GWS, >1850 ppb in SDS, and >1950 ppb in SWS) are recorded in the north-west direction with temperature values between 20 • C and 26 • C. However, at the diurnal scale, a lack of clear and significant correlation between temperature and CH 4 concentrations is found.…”

“…A similar study [43] has pointed out that the lower the wind speeds (~5 m.s −1 ), the higher the concentrations recorded are from local sources. Furthermore, in terms of the contribution of long-range transport, Tiemoko et al [12] pointed out that continental air flows explain 40% of the variance in CO 2 concentration. These different results show that the local and regional influences on air quality in the Lamto region are predominantly from the north, north-east, and north-west sectors, suggesting the main directions of local and regional emission sources.…”