Abstract:
Air quality degradation is a major issue in the large
conurbations on the shore of the Gulf of Guinea. We present
for the first time PM2:5 time series collected in Cotonou,
Benin, and Abidjan, Côte d’Ivoire, from February 2015 to
March 2017. Measurements were performed in the vicinity
of major combustion aerosol sources: Cotonou/traffic
(CT), Abidjan/traffic (AT), Abidjan/landfill (AL) and Abidjan/
domestic fires (ADF). We report the weekly PM2:5 mass
and carbonaceous content as elemental (EC) and organic
(OC) carbon concentrations. We also measure the aerosol
optical depth (AOD) and the Ångström exponent in both
cities. The average PM2:5 mass concentrations were 32 32,
32 24 and 28 19 μgm3 at traffic sites CT and AT and
landfill site AL, respectively. The domestic fire site shows
a concentration of 145 69 μgm3 due to the contribution
of smoking and roasting activities. The highest OC and EC
concentrations were also measured at ADF at 71 29 and
15 9 μgm3, respectively, while the other sites present OC
concentration between 8 and 12 μgm3 and EC concentrations
between 2 and 7 μgm3. The OC= EC ratio is 4.3 at
CT and 2.0 at AT. This difference highlights the influence of
two-wheel vehicles using gasoline in Cotonou compared to
that of four-wheel vehicles using diesel fuel in Abidjan. AOD
was rather similar in both cities, with a mean value of 0.58 in
Cotonou and of 0.68 in Abidjan. The seasonal cycle is dominated
by the large increase in surface mass concentration
and AOD during the long dry season (December–February) as expected due to mineral dust advection and biomass burning
activities. The lowest concentrations are observed during
the short dry season (August–September) due to an increase
in surface wind speed leading to a better ventilation.
On the other hand, the high PM2:5 =AOD ratio in the short
wet season (October–November) indicates the stagnation of
local pollution.
