Analysis of the temporal variability of CO<sub>2</sub>, CH<sub>4</sub> and CO concentrations at Lamto, West Africa
Toure Dro Tiemoko, Michel Ramonet, Fidèle Yoroba, Kobenan Benjamin Kouassi, Kouakou Kouadio, Victor Kazan, C. Kaiser, François Truong, C. Vuillemin, Marc Delmotte, B. Wastine, Phillipe Ciais
Abstract
The 10-year observations of the atmospheric molar fractions of CO<sub>2</sub>, CH<sub>4</sub> and CO in West Africa were analyzed using a high precision measurement of the Lamto (LTO) station (6°31 N and 5°02 W) in Côte 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 CO<sub>2</sub> and CH<sub>4</sub>. The CO concentrations show two peaks around 8 h and 20 h corresponding to the maximum in road traffic of a northern motorway located 14 km from the station. The long-term increase rates of CH<sub>4</sub> (∼7 ppb year<sup>−1</sup>) and CO<sub>2</sub> (∼2.24 ppm year<sup>−1</sup>) 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 CO<sub>2</sub> and CH<sub>4</sub>, and in June for CO. The CO variation suggests a significant impact of fires on the CO, CO<sub>2</sub> and CH<sub>4</sub> anomalies in the Lamto region during the dry season (December to February). CO and CH<sub>4</sub> show strong correlations (at synoptic-scale and monthly based) in January (<em>r =</em> 0.84), February (<em>r =</em> 0.90), April (<em>r =</em> 0.74), November (<em>r =</em> 0.79) and December (<em>r =</em> 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.