A number of campaigns have been
carried out to establish the emission factors of pollutants from fuel
combustion in West Africa, as part of work package 2 (“Air Pollution and
Health”) of the DACCIWA ...(Dynamics-Aerosol-Chemistry-Cloud Interactions in
West Africa) FP7 program. Emission sources considered here include wood
(hevea and iroko) and charcoal burning, charcoal making, open trash burning,
and vehicle emissions, including trucks, cars, buses and two-wheeled
vehicles. Emission factors of total particulate matter (TPM), elemental
carbon (EC), primary organic carbon (OC) and volatile organic compounds
(VOCs) have been established. In addition, emission factor measurements were
performed in combustion chambers in order to reproduce field burning
conditions for a tropical hardwood (hevea), and obtain particulate emission
factors by size (PM0.25, PM1, PM2.5 and PM10). Particle
samples were collected on quartz fiber filters and analyzed using gravimetric
method for TPM and thermal methods for EC and OC. The emission factors of 58
VOC species were determined using offline sampling on a sorbent tube.
Emission factor results for two species of tropical hardwood burning of EC,
OC and TPM are 0.98 ± 0.46 g kg−1 of fuel burned (g kg−1),
11.05 ± 4.55 and 41.12 ± 24.62 g kg−1, respectively. For
traffic sources, the highest emission factors among particulate species are
found for the two-wheeled vehicles with two-stroke engines
(2.74 g kg−1 fuel for EC, 65.11 g kg−1 fuel for OC and
496 g kg−1 fuel for TPM). The largest VOC emissions are observed for
two-stroke two-wheeled vehicles, which are up to 3 times higher than
emissions from light-duty and heavy-duty vehicles. Isoprene and monoterpenes,
which are usually associated with biogenic emissions, are present in almost
all anthropogenic sources investigated during this work and could be as
significant as aromatic emissions in wood burning (1 g kg−1 fuel). EC
is primarily emitted in the ultrafine fraction, with 77 % of the total mass
being emitted as particles smaller than 0.25 µm. The particles and
VOC emission factors obtained in this study are generally higher than those
in the literature whose values are discussed in this paper. This study
underlines the important role of in situ measurements in deriving realistic
and representative emission factors.
To tackle the current pressing atmospheric science issues, as well as those in the future, a robust scientific community is necessary in all regions across the globe. Unfortunately, this does not yet ...exist. There are many geographical areas that are still underrepresented in the atmospheric science community, many of which are in the Global South. There are also larger gaps in the understanding of atmospheric composition, processes, and impacts in these regions. In this opinion, we focus on two geographical areas in the Global South to discuss some common challenges and constraints, with a focus on our strengths in atmospheric science research. It is these strengths, we believe, that highlight the critical role of Global South researchers in the future of atmospheric science research.
In this study, we investigate changes in temperature and precipitation extremes over West and Central Africa (hereafter, WAF domain) as a function of global mean temperature with a focus on the ...implications of global warming of 1.5 °C and 2 °C according the Paris Agreement. We applied a scaling approach to capture changes in climate extremes with increase in global mean temperature in several subregions within the WAF domain: Western Sahel, Central Sahel, Eastern Sahel, Guinea Coast and Central Africa including Congo Basin. While there are several uncertainties and large ensemble spread in the projections of temperature and precipitation indices, most models show high-impact changes in climate extremes at subregional scale. At these smaller scales, temperature increases within the WAF domain are projected to be higher than the global mean temperature increase (at 1.5 °C and at 2 °C) and heat waves are expected to be more frequent and of longer duration. The most intense warming is observed over the drier regions of the Sahel, in the central Sahel and particularly in the eastern Sahel, where the precipitation and the soil moisture anomalies have the highest probability of projected increase at a global warming of 1.5 °C. Over the wetter regions of the Guinea Coast and Central Africa, models project a weak change in total precipitation and a decrease of the length of wet spells, while these two regions have the highest increase of heavy rainfall in the WAF domain at a global warming of 1.5 °C. Western Sahel is projected by 80% of the models to experience the strongest drying with a significant increase in the length of dry spells and a decrease in the standardized precipitation evapotranspiration index. This study suggests that the 'dry gets drier, wet gets wetter' paradigm is not valid within the WAF domain.
This modeling study is conducted to examine the potential impact of the reforestation (greenbelt) location (either in Sahel or in Guinean region) on West African summer climate system. To this end, ...three simulations using the regional climate model RegCM4 driven by ERA-Interim reanalysis were performed at 50 km horizontal resolution over a West African domain for the period 2000–2011. The first experiment, namely the control (CTRL), uses the standard vegetation cover, while the two others incorporate throughout the model integration, a zonal reforestation band of evergreen broadleaf over different locations: (i) over a 13° N–17° N band latitudes in a Sahel-Sahara region (experiment hereafter referred to as GB15N) and (ii) between 8.5° N–11.5° N in the Guinea Coast region (experiment hereafter referred to as GB10N). A comparison of the CTRL experiment with observation reveals a faithful reproduction of the mean boreal and summer seasonal precipitation pattern, though substantial dry/wet biases remain, especially in the Atlantic Ocean. In addition, the seasonal cycle over sub-regions matches satisfactory the observed pattern. The GB15N reforestation leads to a precipitation increase in the range of 2–4 mm/day over the forested areas, whereas in the GB10N reforestation, precipitation increase is weaker and not necessarily located in the forested areas. Temperature cooling is observed over the reforested area and may be explained by a decrease of ground heat flux related to a reduction of the surface albedo.
Road traffic emission inventories based on bottom-up methodology, are calculated for each road segment from fuel consumption and traffic volume data obtained during field measurements in Yopougon. ...High emissions of black carbon (BC) from vehicles are observed at major road intersections, in areas surrounding industrial zones and on highways. Highest emission values from road traffic are observed for carbon monoxide (CO) (14.8 t/d) and nitrogen oxides (NOx) (7.9 t/d), usually considered as the major traffic pollution tracers. Furthermore, peak values of CO emissions due to personal cars (PCs) are mainly linked to the old age of the vehicle fleet with high emission factors. The highest emitting type of vehicle for BC on the highway is PC (70.2%), followed by inter-communal taxis (TAs) (13.1%), heavy vehicles (HVs) (9.8%), minibuses (GBs) (6.4%) and intra-communal taxis (WRs) (0.4%). While for organic carbon (OC) emissions on the main roads, PCs represent 46.7%, followed by 20.3% for WRs, 14.9% for TAs, 11.4% for GB and 6.7% for HVs. This work provides new key information on local pollutant emissions and may be useful to guide mitigation strategies such as modernizing the vehicle fleet and reorganizing public transportation, to reduce emissions and improve public health.
This study assessed the sensitivity of the West African climate to varying vegetation fractions. The assessment of a such relationship is critical in understanding the interactions between land ...surface and atmosphere. Two sets of convection‐permitting simulations from the UK Met Office Unified Model at 12 km horizontal resolution covering the monsoon period May–September (MJJAS) were used, one with fixed vegetation fraction (MF‐V) and the other with time‐varying vegetation fraction (MV‐V). Vegetation fractions are based on MODIS retrievals between May and September. We focused on three climatic zones over West Africa: Guinea Coast, Sudanian Sahel, and the Sahel while investigating heat fluxes, temperature, and evapotranspiration. Results reveal that latent heat fluxes are the most strongly affected by vegetation fraction over the Sahelian and Sudanian regions while sensible heat fluxes are more impacted over the Guinea Coast and Sudanian Sahel. Also, in MV‐V simulation there is an increase in evapotranspiration mainly over the Sahel and some specific areas in Guinea Coast from June to September. Moreover, it is noticed that high near‐surface temperature is associated with a weak vegetation fraction, especially during May and June. Finally, varying vegetation seems to improve the simulation of surface energy fluxes and in turn impact on climate parameters. This suggests that climate modelers should prioritize the use of varying vegetation options to improve the representation of the West African climate system.
Comparison between the use of varying vegetation fraction and fix vegetation fraction in regional climate models over West Africa: Case study of the Unified Model with focus on three different climatic zones.
Traffic source emission inventories for the rapidly growing West African urban cities are necessary for better characterization of local vehicle emissions released into the atmosphere of these ...cities. This study is based on local field measurements in Yopougon (Abidjan, Côte d’Ivoire) in 2016; a site representative of anthropogenic activities in West African cities. The measurements provided data on vehicle type and age, traveling time, fuel type, and estimated amount of fuel consumption. The data revealed high traffic flow of personal cars on highways, boulevards, and backstreets, whereas high flows of intra-communal sedan taxis were observed on main and secondary roads. In addition, the highest daily fuel consumption value of 56 L·day−1 was recorded for heavy vehicles, while the lowest value of 15 L·day−1 was recorded for personal cars using gasoline. This study is important for the improvement of uncertainties related to the different databases used to estimate emissions either in national or international reports. This work provides useful information for future studies on urban air quality, climate, and health impact assessments in African cities. It may also be useful for policy makers to support implementation of emission reduction policies in West African cities.
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 µg m−3 at traffic sites CT and AT and landfill
site AL, respectively. The domestic fire site shows a concentration of
145 ± 69 µg m−3 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 µg m−3,
respectively, while the other sites present OC concentration between 8 and
12 µg m−3 and EC concentrations between 2 and
7 µg m−3. 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.
There are very few African regional inventories providing
biofuel and fossil fuel emissions. Within the framework of the DACCIWA
project, we have developed an African regional anthropogenic emission
...inventory including the main African polluting sources (wood and charcoal
burning, charcoal making, trucks, cars, buses and two-wheeled vehicles, open
waste burning, and flaring). To this end, a database on fuel consumption and
emission factors specific to Africa was established using the most recent
measurements. New spatial proxies (road network, power plant geographical
coordinates) were used to convert national emissions into gridded
inventories at a 0.1∘ × 0.1∘ spatial resolution. This
inventory includes carbonaceous particles (black and organic carbon) and
gaseous species (CO, NOx, SO2 and NMVOCs) for the period 1990–2015
with a yearly temporal resolution. We show that all pollutant emissions are
globally increasing in Africa during the period 1990–2015 with a growth rate of 95 %, 86 %, 113 %, 112 %, 97 % and 130 % for BC, OC,
NOx, CO, SO2 and NMVOCs, respectively. We also show that Western Africa is the highest emitting region of BC, OC, CO and NMVOCs, followed by
Eastern Africa, largely due to domestic fire and traffic activities, while
Southern Africa and Northern Africa are the highest emitting regions of
SO2 and NOx due to industrial and power plant sources. Emissions
from this inventory are compared to other regional and global inventories,
and the emissions uncertainties are quantified by a Monte Carlo simulation.
Finally, this inventory highlights key pollutant emission sectors in which
mitigation scenarios should focus on. The DACCIWA inventory (https://doi.org/10.25326/56, Keita et al., 2020) including the annual
gridded emission inventory for Africa for the period 1990–2015 is
distributed by the Emissions of atmospheric Compounds and Compilation of
Ancillary Data (ECCAD) system (https://eccad.aeris-data.fr/, last access: 19 July 2021). For review
purposes, ECCAD has set up an anonymous repository where subsets of the
DACCIWA data can be accessed directly through https://www7.obs-mip.fr/eccad/essd-surf-emis-dacciwa/ (last access: 19 July 2021).
Between June and September large amounts of biomass burning
aerosol are released into the atmosphere from agricultural fires in central
and southern Africa. Recent studies have suggested that this ...plume is
carried westward over the Atlantic Ocean at altitudes between 2 and 4 km and
then northward with the monsoon flow at low levels to increase the
atmospheric aerosol load over coastal cities in southern West Africa (SWA),
thereby exacerbating air pollution problems. However, the processes by which
these fire emissions are transported into the planetary boundary layer (PBL)
are still unclear. One potential factor is the large-scale subsidence
related to the southern branch of the monsoon Hadley cell over the tropical
Atlantic. Here we use convection-permitting model simulations with COSMO-ART
to investigate for the first time the contribution of downward mixing
induced by clouds, a process we refer to as downward cloud venting in
contrast to the more common process of upward transport from a polluted PBL.
Based on a monthly climatology, model simulations compare satisfactory with
wind fields from reanalysis data, cloud observations, and satellite-retrieved carbon monoxide (CO) mixing ratio. For a case study on 2 July
2016, modelled clouds and rainfall show overall good agreement with Spinning
Enhanced Visible and InfraRed Imager (SEVIRI) cloud products and Global
Precipitation Measurement Integrated Multi-satellitE Retrievals (GPM-IMERG)
rainfall estimates. However, there is a tendency for the model to produce
too much clouds and rainfall over the Gulf of Guinea. Using the CO
dispersion as an indicator for the biomass burning plume, we identify
individual mixing events south of the coast of Côte d'Ivoire due to
midlevel convective clouds injecting parts of the biomass burning plume into
the PBL. Idealized tracer experiments suggest that around 15 % of the CO
mass from the 2–4 km layer is mixed below 1 km within 2 d over the
Gulf of Guinea and that the magnitude of the cloud venting is modulated by
the underlying sea surface temperatures. There is even stronger vertical
mixing when the biomass burning plume reaches land due to daytime heating
and a deeper PBL. In that case, the long-range-transported biomass burning
plume is mixed with local anthropogenic emissions. Future work should
provide more robust statistics on the downward cloud venting effect over the
Gulf of Guinea and include aspects of aerosol deposition.
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