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Ammonia (NH3) is the most abundant alkaline component in the atmosphere. Changes in NH3 concentrations have important implications for atmospheric chemistry, air quality, and ecosystem integrity. We present a long-term ammonia (NH3) assessment in the Western and Central Africa region within the framework of the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) program. We analyze seasonal variations and trends of NH3 concentrations and total columns densities along an African ecosystem transect spanning dry savannas in Banizoumbou, Niger and Katibougou, Mali, wet savannas in Djougou, Benin and Lamto, Côte d’Ivoire, and forests in Bomassa, Republic of Congo and Zoétélé, Cameroon. We use a 21-year record of observations (1998–2018) from INDAAF passive samplers and 11-year record of observations (2008–2018) of atmospheric vertical column densities from the Infrared Atmospheric Sounding Interferometer (IASI) to evaluate NH3 ground-based concentrations and total column densities, respectively. Annual mean ground-based NH3 concentrations are around 5.7–5.8 ppb in dry savannas, 3.5–4.7 ppb in wet savannas and 3.4–5.6 ppb in forests. These results suggest that NH3 emissions from precipitation-induced pulses and volatilization from animal excreta are important emission sources in dry savannas, and biomass burning and agricultural sources are important sources in wet savanna and forest ecosystems. NH3 total column densities clearly show that the biomass burning source is the most important source in the Lamto wet savanna ecosystem. Annual IASI NH3 total column densities are 10.1–11.0x1015 molec cm-2 in dry savanna, 16.5–21.4x1015 molec cm-2 in wet savanna and 14.3–15.1x1015 molec cm-2 in forest stations. Non-parametric statistical Mann-Kendall trend tests applied to annual data show that ground-based NH3 concentrations increase at Bomassa (+2.56 % yr-1), but decrease at Zoétélé (-2.95 % yr-1) over the 21-year period. The 11-year period of IASI NH3 total column density measurements show yearly increasing trends at Katibougou (+3.98 % yr-1) and Djougou (+2.24 % yr-1). At Zoétélé, we calculated an increasing trend of leaf area index associated to a significant anticorrelation with ground-based NH3 concentrations. Leaf area index increase could enhance deposition processes and could contribute to the decrease of ground-based NH3 concentrations.

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).

In the framework of the INDAAF (International Network to study Deposition and Atmospheric chemistry in AFrica) programme, part of the ACTRIS European Research Infrastructure for the long-term observation of Aerosols, Cloud, and Trace gases, this paper aims to study trends and seasonal variability of surface atmospheric NO2 and HNO3 concentrations, and OMI (Ozone Monitoring Instrument) NO2 over 6 sites in tropical Africa. Sites are located in west and central Africa to represent the major African biomes: dry savanna (Banizoumbou, Niger and Katibougou, Mali), wet savanna (Djougou, Benin and Lamto, Cote d’Ivoire) and forest (Bomassa, Republic of Congo and Zoetele, Cameroon). Ground-based NO2 and HNO3 concentrations were obtained over the period 1998–2015 using INDAAF passive samplers at a monthly basis, and NO2 Vertical Column Densities (VCDs) from OMI for a 1° grid cell around each sites were obtained from 2005 to 2015. Mean annual NO2 concentrations ranged from 2.3 ± 1.2 to 0.9 ± 0.4 ppb from dry savannas to forests, representing a north south gradient. In dry savannas, we observe two concentration peaks of NO2 appearing at the beginning and the end of the wet season both for ground-based and satellite measurements, whereas at wet savannas and forest sites, NO2 concentrations are highest in the dry season. The seasonality of surface NO2 observations provide further evidence for a large role of microbial soil NOx emissions in dry savannas and of biomass burning NOx emissions in wet savanna and forest sites. Mean annual HNO3 concentrations ranged from 0.4 to 0.5 ppb in dry and wet savannas to 0.2–0.3 ppb in forest. In dry ecosystems, higher HNO3 concentrations are measured in the early wet season, consistent with NO2 results. The analysis of a long-term dataset of surface O3 concentrations indicates that HNO3 production can mainly be explained by the photooxidation of NOx. Mann-Kendall and Seasonal Kendall statistical tests showed that NO2 surface concentrations have a significant decreasing seasonal and annual trends at multiple sites (p-value

Urbanization is an issue that is strongly emerging in southern West Africa (sWA). There is a lack of full understanding on chemical compositions and personal exposure levels to fine particulate matter (hereafter defined as PE PM2.5 ) and its health risks related to various anthropogenic sources in this region. In this study, PE PM2.5 was studied in dry (January) and wet (July) seasons of 2016 for the first time to characterize the contributions of a domestic fire site (DF) to the exposure of women and a waste burning site (WB) to that of students in Abidjan, Cote d'Ivoire, and a motorcycle traffic site (MT) to that of drivers in Cotonou, Benin. The average PE PM2.5 mass concentrations were 331.7±190.7 , 356.9±71.9 and 242.8±67.6 µ g m −3 at DF, WB and MT sites for women, students and drivers, which were 2.4, 10.3 and 6.4 times the ambient PM2.5 concentrations, respectively. Elevated PE PM2.5 levels in the dry season were found at DF ( 358.8±100.5 µ g m −3 ), WB ( 494.3±15.8 µ g m −3 ) and MT ( 335.1±72.1 µ g m −3 ) sites, on average 15 % higher than that at DF and 55 % higher at both WB and MT sites in the wet season. The seasonal variations were attributed to emission sources, meteorological factors and personal activities. In addition, the results show that geological material (35.8 %, 46.0 % and 42.4 %) and organic matter (34.1 %, 23.3 % and 24.9 %) were the major components of PE PM2.5 at DF, WB and MT sites. It is worth noting that the contribution of heavy metals was higher at WB (1.0 %) than at DF (0.7 %) and MT (0.4 %) sites, strongly influenced by waste burning emission. This results in the highest non-cancer risks of heavy metals to students, 5.1 and 4.8 times the values for women and drivers, respectively. By conducting organic speciation, fingerprints were used to access the exposure and identify the source contributions from typical local anthropogenic sources. The women's exposure concentration to particulate polycyclic aromatic hydrocarbons (PAHs) at DF ( 77.4±47.9 ng m −3 ) was 1.6 and 2.1 times, respectively, that of students at WB ( 49.9±30.7 ng m −3 ) and of drivers at MT ( 37.0±7.4 ng m −3 ). This can be associated with the higher contributions from solid fuels' burning and meat grilling activities to women, resulting in a level 5 times in exceedance of the cancer risk safety threshold ( 1 × 10 - 6 ). Phthalate esters (PAEs), commonly used as plasticizers in products, were in high levels in the student exposure PM2.5 samples ( 1380.4±335.2 ng m −3 ), owing to obvious waste burning activities nearby. The drivers' exposures to fossil fuel combustion markers of hopanes in PE PM2.5 at MT ( 50.9±7.9 ng m −3 ) was 3.0–3.3 times those for women at DF ( 17.1±6.4 ng m −3 ) and students at WB ( 15.6±6.1 ng m −3 ). Overall, the current study shows that wood combustion, waste burning, fugitive dust and motor vehicle emissions were the dominant sources of PE PM2.5 and mainly contributed to its toxicities. The exposure to the heavy metals Pb and Mn caused high non-cancer risks to students at WB, while the severe cancer risk of PAHs was found for women at DF via inhalation. The result of this study provides original data, initial perspective of PM2.5 personal exposure and health risk assessment in the developing areas. The information encourages the governments to improve the air quality and living standards of residents in this region.

The physico-chemical characteristics of particulate matter (PM) in African cities remain poorly known due to scarcity of observation networks. Magnetic parameters of PM are robust proxies for the emissions of Fe-bearing particles. This study reports the first magnetic investigation of PM2.5 (PM with aerodynamic size below 2.5 m) in Africa performed on weekly PM2.5 filters collected in Abidjan (Ivory Coast) and Cotonou (Benin) between 2015 and 2017. The magnetic mineralogy is dominated by magnetite-like low coercivity minerals. Mass normalized SIRM are 1.65e−2A□m/kg and 2.28e−2A□m/kg for Abidjan and Cotonou respectively. Hard coercivity material (S-ratio = 0.96 and MDF = 33 mT) is observed during the dry dusty season. Wood burning emits less iron oxides by PM2.5 mass when compared to traffic sources. PM2.5 magnetic granulometry has a narrow range regardless of the site or season. The excellent correlation between the site-averaged element carbon concentrations and SIRM suggests that PM2.5 magnetic parameters are linked to primary particulate emission from combustion sources.

This study was carried out within the framework of the Improving Air Quality in West Africa (IAQWA) project funded by the Make Our Planet Great Again (MOPGA) program. In recent years, West African countries have experienced an economic upturn driven by GDP growth of nearly 3.7% in 2019 (AfDB, 2020). This economic boom is mainly felt in the cities where it promotes the construction of highway infrastructure, real estate development, and industry. All these activities are sources of air pollution. Unfortunately, there is almost no air quality monitoring in these cities partly due to the high cost of monitoring instruments. Low-cost air quality monitoring instruments can help improve the spatial and temporal resolution of measurements at relatively lower cost. However, the installation of these instruments in West African environments characterized by high relative humidity requires their calibration through collocation with reference instruments. The IAQWA project aims to improve our understanding of air pollutants such as fine particulate matter mass (PM2.5), ozone (O3), nitrogen oxides (NOx), sulfur dioxide (SO2), and carbon monoxide (CO) in Abidjan and Accra, two major West African capitals, through the deployment of Real-time Affordable Multi-Pollutant (RAMP) monitors.Since February 2020, five RAMPs have been installed and are operating continuously at various sites in Abidjan and Lamto in Cote d'Ivoire, and four RAMPs have been operating in Accra, Ghana. Some of the RAMPs have been collocated with PM and/or NOx reference instruments. At other sites the RAMPs have been collocated with INDAAF passive samplers and passive aerosol collectors. These collocations have allowed for the development of calibration models for these low-cost sensors. The performance of these calibration models is presented here along with the diurnal and seasonal variations of air pollution at the different sites in Abidjan and Accra. These results will eventually be used to improve our understanding of the drivers of air pollution in these major West African cities, which is essential to choosing sustainable development pathways in the future.

International audience; Particulate pollution in West African cities seriously impacts public human health. Assessing the levels of pollution to which populations are exposed is problematic, as to date very few countries in Africa have an air quality monitoring network in place. However, given the specific anthropogenic sources present in West African countries and the increase in their projected emissions in the coming years if no regulations are put in place, solutions must be found. This study evaluates with a simple existing methodology particulate air pollution in two West African cities (Korhogo and Abidjan in Cote d'Ivoire) which have different population practices and local meteorology. From two measurement campaigns at about ten sites during the dry season and using the inverse distance-weighted interpolation method, maps of spatial variation in PM10 and PM2.5 concentrations are obtained at the scale of each of these cities, which provide a snapshot of the concentration levels to which populations are actually exposed. The results of this study showed that populations in low-and middle-income areas are exposed to higher concentrations than those in high-income areas. The analysis of the relative contribution of fine particles to coarse particles shows that PM2.5 contributes to PM10 at less than 40% in Korhogo but more than 50% in Abidjan. Daily average concentrations of PM2.5 are all above WHO standards, and are 2 to 8 times higher in Korhogo, while in Abidjan they are up to 4 times higher. The daily profiles of the mean hourly concentrations of the sites are similar across all sites in Korhogo, while in Abidjan, that of some sites show some differences. This study, by the methodology used, offers an opportunity for different research teams to assess at lower cost the population's exposure levels to urban particulate pollution.

During monsoon season in southern West Africa (SWA), nocturnal stratiform low-level clouds (LLSC) frequently form over a region extending from Guinean coast to several hundred kilometers inland. The cloud deck persists at least until sunrise next day, affecting surface-energy budget and related processes. However, LLSC lifetime is underestimated by numerical weather prediction and climate models.The DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions-over-West-Africa) field campaign, in June-July 2016, paved the way for studying LLSC over SWA based on high-quality-observational dataset. The first analyzes of this data highlighted that the LLSC diurnal cycle consists of four main stages: the stable, jet, stratus and convective phases. Unlike the first three, the convective phase, which starts after sunrise and ends when LLSC breaks up, has not been well documented yet.This study analyzes the LLSC evolution during stratus and convective phases, specifically addressing the LLSC transition toward other low-cloud types during sunlight hours. It is based on comprehensive dataset acquired during twenty-two precipitation-free LLSC occurrences at Savè (Benin) during the DACCIWA fiel campaign. The cloud-characteristics are deduced from ceilometer and cloud-radar measurements. The associated atmospheric conditions are provided by surface meteorological and energy balance stations, radiosoundings and an Ultra-High-Frequency wind profiler.The LLSC forms (beginning of the stratus phase) decoupled from surface. In thirteen cases, the LLSC remains decoupled until the convective phase (case D). Conversely, in the other nine cases, the cloud gets coupled with surface before sunrise, within the four hours after cloud formation (case C). The coupling is accompanied by cloud base lowering and near-neutral thermal stability in subcloud-layer. Almost all cases C are observed during a period with well-established monsoon-flow over SWA. But, the weak differences of thermodynamical conditions between cases C and D suggest that, contributions of both mesoscale and local processes are crucial for coupling LLSC to the surface before sunrise. In early morning, the macrophysical and thermodynamical characteristics of the LLSC in case C are slightly different from the case D, suggesting that, even during night, the coupling with surface impacts the cloud characteristics.The LLSC evolution during convective phase depends upon the coupling at initial stage. In cases C, the evolution pattern is quite similar, the cloud base rises up under solar heating and shallow cumulus form when the cloud deck breaks up, around 11:30 UTC or later. For some of cases D, the LLSC couples with surface as the convective atmospheric boundary-layer grows and reaches the cloud base. The subsequent evolution and breakup time are then similar to case C. For most of cases D, LLSC remains decoupled from surface, and shallow cumulus form at the convective mixed layer top, under the LLSC deck. In this scenario, the LLSC breakup-time mostly occurs before 11:30 UTC. Thus, the coupling between LLSC and surface is a key factor for its evolution and maintenance after sunrise. Correct simulation of this feature may improve models performance over SWA. The impacts of LLSC on surface-energy budget and verical development of boundary-layer are also quantified.

International audience; 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.

Biomass Burning (BB) aerosol has attracted considerable attention due to its detrimental effects on climate through its radiative properties. In Africa, fire patterns are anticorrelated with the southward-northward movement of the intertropical convergence zone (ITCZ). Each year between June and September, BB occurs in the southern hemisphere of Africa, and aerosols are carried westward by the African Easterly Jet (AEJ) and advected at an altitude of between 2 and 4 km. Observations made during a field campaign of Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) (Knippertz et al., Bull Am Meteorol Soc 96:1451–1460, 2015) during the West African Monsoon (WAM) of June–July 2016 have revealed large quantities of BB aerosols in the Planetary Boundary Layer (PBL) over southern West Africa (SWA).This chapter examines the effects of the long-range transport of BB aerosols on the climate over SWA by means of a modeling study, and proposes several adaptation and mitigation strategies for policy makers regarding this phenomenon. A high-resolution regional climate model, known as the Consortium for Small-scale Modelling – Aerosols and Reactive Traces (COSMO-ART) gases, was used to conduct two set of experiments, with and without BB emissions, to quantify their impacts on the SWA atmosphere. Results revealed a reduction in surface shortwave (SW) radiation of up to about 6.5 W m−2 and an 11% increase of Cloud Droplets Number Concentration (CDNC) over the SWA domain. Also, an increase of 12.45% in Particulate Matter (PM25) surface concentration was observed in Abidjan (9.75 μg m−3), Accra (10.7 μg m−3), Cotonou (10.7 μg m−3), and Lagos (8 μg m−3), while the carbon monoxide (CO) mixing ratio increased by 90 ppb in Abidjan and Accra due to BB. Moreover, BB aerosols were found to contribute to a 70% increase of organic carbon (OC) below 1 km in the PBL, followed by black carbon (BC) with 24.5%. This work highlights the contribution of the long-range transport of BB pollutants to pollution levels in SWA and their effects on the climate. It focuses on a case study of 3 days (5–7 July 2016). However, more research on a longer time period is necessary to inform decision making properly.This study emphasizes the need to implement a long-term air quality monitoring system in SWA as a method of climate change mitigation and adaptation.

We present a long term assessment trend of atmospheric inorganic nitrogen deposition in Sub Saharan Africa (2000–2015) using observational and model data. This work proposes a compilation of International Network to study Deposition and Atmospheric chemistry in Africa wet and dry nitrogen deposition fluxes collected at the wet savanna site of Lamto (Côte d’Ivoire). Total deposition calculation takes in account: (a) gaseous (NO2, NH3, HNO3) dry deposition fluxes estimated by considering nitrogen compound concentrations at the monthly scale and modeling average monthly dry deposition velocities, (b) particulate PM10 (pNO3 −, pNH4 +) dry deposition fluxes calculated using the same inferential method and (c) wet deposition (WD) fluxes including ions concentration measurements (NO3 −, NH4 +) in rainwater combined with rainfall amount. We demonstrate for the first time the monthly and annual decreasing trends for dry nitrogen deposition of N-NO2 (−2.33% month−1 and −2.54% yr−1) and N-NH3 (−2.55% month−1 and −2.89% yr−1), but increasing trends for dry deposition of N-HNO3 (+1.00% month−1) and WD of N-NO3 − (+1.67% month−1 and +2.13% yr−1) and N-NH4 + (+2.33% month−1 and +3.36% yr−1). Dry season N-NO2 deposition flux decline shows agreement with long term trend in NOx emissions by biomass burning. Increasing trends for wet N deposition signals a gradual increase of nitrogen fertilizers use in agricultural practices in the Lamto area. Results also show no significant trend in total N deposition over the 16 year study period explained by the compensation of decreasing and increasing trends for dry and wet N deposition, respectively. However, at the annual scale, the mean total N deposition flux is estimated to 10.3 ± 1.2 kgN ha−1 yr−1 over the 16 year period, indicating an increase of 8% compared to the period 2000–2007.

Ambient air pollution is a leading cause of premature mortality across the world, with an estimated 258,000 deaths in Africa (UNICEF/GBD 2017). These estimated impacts have large uncertainties as many major cities in Africa do not have any ground-based air quality monitoring. The lack of data is due in part to the high cost of traditional monitoring equipment and the lack of trained personnel. As part of the “Make Air Quality Great Again” project under the “Make Our Planet Great Again” framework (MOPGA), we propose filling this data gap with low-cost sensors carefully calibrated against reference monitors.Fifteen real-time affordable multi-pollutant (RAMP) monitors have been deployed in Abidjan, Côte d'Ivoire; Accra, Ghana; Kigali, Rwanda; Nairobi, Kenya; Niamey, Niger; and Zamdela, South Africa (near Johannesburg). The RAMPs use Plantower optical nephelometers to measure fine particulate matter mass (PM2.5) and four Alphasense electrochemical sensors to detect pollutant gases including nitrogen dioxide (NO2) and ozone (O3).Using a calibration developed in Créteil, France, the deployments thus far reveal morning and evening spikes in combustion-related air pollution. The median hourly NO2 in Accra and Nairobi for September-October 2019 was about 11 ppb; a similar value was observed across November-December 2019 in Zamdela. However, a previous long-term deployment of the RAMPs in Rwanda showed that, for robust data quality, low-cost sensors must be collocated with traditional reference monitors to develop localized calibration models. Hence, we acquired regulatory-grade PM2.5, NO2, and O3 monitors for Abidjan and Accra. We also collocated RAMPs with existing reference monitors in Zamdela, Kigali, Abidjan, and Lamto (a rural site in Côte d'Ivoire). In this talk, we will present results on spatio-temporal variability of collocation-based sensor calibrations across these different cities, source identification, and challenges and plans for future expansion.

Biomass Burning (BB) aerosol has attracted considerable attention due to its detrimental effects on climate through its radiative properties. In Africa, fire patterns are anticorrelated with the southward-northward movement of the intertropical convergence zone (ITCZ). Each year between June and September, BB occurs in the southern hemisphere of Africa, and aerosols are carried westward by the African Easterly Jet (AEJ) and advected at an altitude of between 2 and 4 km. Observations made during a field campaign of Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) (Knippertz et al., Bull Am Meteorol Soc 96:1451–1460, 2015) during the West African Monsoon (WAM) of June–July 2016 have revealed large quantities of BB aerosols in the Planetary Boundary Layer (PBL) over southern West Africa (SWA).This chapter examines the effects of the long-range transport of BB aerosols on the climate over SWA by means of a modeling study, and proposes several adaptation and mitigation strategies for policy makers regarding this phenomenon. A high-resolution regional climate model, known as the Consortium for Small-scale Modelling – Aerosols and Reactive Traces (COSMO-ART) gases, was used to conduct two set of experiments, with and without BB emissions, to quantify their impacts on the SWA atmosphere. Results revealed a reduction in surface shortwave (SW) radiation of up to about 6.5 W m−2 and an 11% increase of Cloud Droplets Number Concentration (CDNC) over the SWA domain. Also, an increase of 12.45% in Particulate Matter (PM25) surface concentration was observed in Abidjan (9.75 μg m−3), Accra (10.7 μg m−3), Cotonou (10.7 μg m−3), and Lagos (8 μg m−3), while the carbon monoxide (CO) mixing ratio increased by 90 ppb in Abidjan and Accra due to BB. Moreover, BB aerosols were found to contribute to a 70% increase of organic carbon (OC) below 1 km in the PBL, followed by black carbon (BC) with 24.5%. This work highlights the contribution of the long-range transport of BB pollutants to pollution levels in SWA and their effects on the climate. It focuses on a case study of 3 days (5–7 July 2016). However, more research on a longer time period is necessary to inform decision making properly.This study emphasizes the need to implement a long-term air quality monitoring system in SWA as a method of climate change mitigation and adaptation.

From January 2005 to December 2009, a total of 457 and 444 samples were collected respectively rains in humid savannas of Djougou and Lamto. Using Henry's law, we determined the content in the air of the major organic monoacid (HCOOH and CH3COOH) from their concentration in rainwater. At Lamto, annual partial pressure of formic acid (FA) and acetic acid (AA) on the five year study is very little variable. It covers a range of 0.1 ppbv and 0.2 ppb to 0.4 to 0.7 ppbv respectively for formic acid (FA) and acetic acid (AA). While in Djougou, it is very variable. It varies from 0.01 to 0.19 ppbv to formic acid (FA) and 0.04 to 0.54 ppbv to acetic acid (FA). For both stations, it is a two times higher in the dry season than the wet season factor. This difference is related to the enrichment of a monoacid organic acid content in the air by supplying various sources of these acids. However, the correlation analysis that enables the production of formic acid (FA) and acetic acid (AA) Djougou like Lamto is not related to the marine source although the latter is close to the Gulf of Guinea.

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 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 to what extent mixing related to cloud venting contributes to the downward transport of the biomass burning plume. Based on a monthly climatology, model simulations compare satisfactory with wind fields from reanalysis data, cloud observations, and satellite retrieved CO mixing ratio. For a case study on 02 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. Looking into the CO dispersion, used as an indicator for the biomass burning plume, we identified 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 boundary layer. This cloud venting is modulated by the underlying sea surface temperatures. Idealized tracer experiments suggest that about 20 % of the CO mass from the 2–4 km layer are mixed below 1km within two days over the Gulf of Guinea. There is even stronger vertical mixing when the biomass burning plume reaches land due to daytime heating. 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 cloud venting effect over the Gulf of Guinea and include aspects of aerosol deposition.

Several field campaigns were deployed in the framework of the project Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) to measure a broad range of atmospheric constituents. Here, we present the analysis of an unprecedented and comprehensive dataset integrating up to fifty-six VOC from ambient sites and emission sources. VOCs were collected on sorbent tubes in the coastal city of Abidjan, Côte d'Ivoire, in winter and summer 2016 and analysed by gas chromatography coupled with flame ionization and mass spectrometer detectors (GC-FID and GC-MS) at the laboratory. The comparison between VOC emission source profiles and ambient profiles suggests the substantial impact of two-wheelers and domestic fires on the composition of Abidjan's atmosphere. However, despite the high VOC concentrations near-source, moderate ambient levels were observed (by a factor of 10 to 4000 lower) and similar to the ones observed in northern mid-latitude urban areas. Beyond photochemistry, reported high wind speeds suggest that meteorology is an essential factor that regulates air pollution in Abidjan. Emission ratios (ΔVOC / ΔCO) were established based on real-world measurements achieved on a selected number of representative combustion sources. Maximal molar mass contributions were observed from two-wheeler (TW) emissions, overpassing other regional sources by two orders of magnitude. This source also largely governs the VOC atmospheric impacts in terms of OH reactivity, secondary aerosol formation (SOAP) and ozone production (POCP). While the contribution of aromatics dominates the atmospheric impact, our measurements reveal the systematic presence of anthropogenic terpenoids in all residential combustion sectors. Finally, emission factors were used to retrieve and quantify VOC emissions from the main anthropogenic source sectors at national level. Our detailed VOC emissions estimation suggests that the road transport sector is the dominant source in Cote d'Ivoire by emitting around 1200 Gg yr−1 of gas-phase VOCs. These new estimations are 100 and 160 times larger than previous global inventory estimations like MACCity or Edgar (v4.3.2). Additionally, the residential sector is also largely underestimated by a factor of 13 to 43. For the only Cote d'Ivoire, these new estimates for VOCs are three to six times higher than the whole Europe. Given the significant underestimation of VOC emissions from transport and residential sectors for Côte d'Ivoire country there is an urgent need for such an effort in the whole West Africa region for building more realistic and region-specific emission inventories. This is not only true for VOCs but all atmospheric pollutants. The dearth of waste burning, fuelwood burning and charcoal representation in regional inventories need also to be addressed, particularly in low-income areas where these types of activities are essential sources of VOC emissions.

This work provides a complete chemical characterization of rains collected in the tropical rural site of Mbita (Kenya) on the shores of Lake Victoria (annual rainfall 1259.3 mm). We present a wet nitrogen deposition budget including inorganic and organic dissolved nitrogen in relation with atmospheric sources of gases and particles, precipitation rate and air mass transport. A unique 2 yr monitoring data set (2017–2019), providing 183 rain samples was collected and analyzed according to international standards (WMO/GAW). Considering that precipitation represents the largest contributor of water to the Lake Victoria (80%), this study gives new insights in the seasonality of nutrients wet deposition (WD) inputs in the unique natural resource represented by Lake Victoria and its catchment. Four main contributions to the chemical composition of precipitation, were identified: (a) a 28% terrigenous contribution related to crustal and biomass sources, (b) a 14% marine contribution related to Indian ocean air masses intrusion, (c) a 15% organic contribution due to volatile organic carbon emissions from biomass burning and vegetation and (d) a predominant nitrogenous contribution of 39% due to livestock and fertilizers, biomass burning and neighboring agricultural fires. Ammonium and nitrate volume weighed mean concentrations are 36.75 and 8.88 μeq l−1, respectively. Rain in Mbita is alkaline (pH = 5.8) highlighting neutralization by heterogeneous chemistry. Total nitrogen WD is 8.54 kgN ha−1 yr−1, 58 760 tN yr−1 for the entire lake, with 26% attributed to dissolved organic nitrogen. A total atmospheric deposition of 15 kgN ha−1 yr−1 is estimated taking into account dry deposition estimate from literature, showing that the Lake Victoria ecosystem is exposed to eutrophication. An extensive and regular monitoring of wet and dry nitrogen deposition is highly recommended both in-shore and off-shore to help improving the efficiency of nitrogen use in agricultural areas and reduce nitrogen losses around Lake Victoria.

Urbanisation is a strongly emerging issue in Southern West African (SWA) region. There is a general lack of understanding about the personal exposure to fine particulate matter (PM2.5), its chemical components and health risks related to the various anthropogenic sources in this region. In the current study, personal exposure to PM2.5 (PE PM2.5) sampling was for the first time carried out in dry season (January) and wet season (July) of 2016 to characterize PE PM2.5 from Domestic Fires (DF) for women and Waste Burning (WB) for students in Abidjan, Côte d'Ivoire and Motorcycle Traffic (MT) for drivers in Cotonou, Benin. The average PE PM2.5 mass concentrations were 331.7 ± 190.7, 356.9 ± 71.9 and 242.8 ± 67.6 μg m−3 at DF, WB and MT for women, students and drivers, which were 2.4, 10.3 and 6.4 times of the ambient PM2.5 concentrations, respectively. Mean concentrations of PE PM2.5 at DF (358.8 ± 100.5 μg m−3), WB (494.3 ± 15.8 μg m−3) and MT (335.1 ± 72.1 μg m−3) were much elevated in dry season, 15 % higher than that at DF and 55 % higher at both WB and MT. The changes in PE PM2.5 can be attributed to the source emissions, meteorological factors and personal activities. The results also show that geological material (35.8 %, 46.0 % and 42.4 %) and organic matter (34.1 %, 23.3 % and 24.9 %) were always the major components in PE PM2.5 at DF, WB and MT sites. It is worth noting that the contribution to PE PM2.5 from heavy metals was higher at WB (1.0 %) than at DF (0.7 %) and MT (0.4 %), which was influenced by the waste burning emission strongly, leading to the highest heavy metal non-cancer risks for students (5.1 and 4.8 times of women and drivers' non-cancer risks). In organic species of PE PM2.5, some fingerprints can be used to quantify the exposure concentrations and trace the source contributions from local typical anthropogenic sources to different samples. Women exposure concentration to polycyclic aromatic hydrocarbons (PAHs) in PM2.5 at DF (77.4 ± 47.9 ng m−3) was 1.6 times that for students at WB (49.9 ± 30.7 ng m−3) and 2.1 times for drivers at MT (37.0 ± 7.4 ng m−3), which is related to the solid fuels burning and grilling meat activities, resulting in 5 times higher of cancer risk safety threshold (1 × 10−6) to women. Phthalate esters (PAEs), commonly used as plasticizers in many products, were observed to be extremely high in student exposure PM2.5 samples (1380.4 ± 335.2 ng m−3) at WB site, owing to the waste burning emission obviously. Drivers exposure to fossil fuel emission (especially traffic) markers-hopanes in PE PM2.5 at MT (50.9 ± 7.9 ng m−3) was 2.0–2.3 times higher than women at DF (17.1 ± 6.4 g m−3) and students at WB (15.6 ± 6.1 ng m−3), correlating with the elevated exposure to traffic emissions for drivers. Overall, the study shows that wood combustion, waste burning, fugitive dust and motor vehicle emissions dominated PE PM2.5 mass and contributed to its toxicities mainly. Heavy metals and organic chemicals in PE PM2.5 in SWA brought about Pb and Mn non-cancer health risks for students at WB site and serious PAHs cancer risks for women at DF site via inhalation pathway. This study provides basic data and initial perspective of PM2.5 personal exposure and health risk assessment in underdeveloped area to encourage the government to improve the air quality and living standard of residents in this region.

Traffic source emissions inventories for the rapidly growing West African urban cities are necessary for better local characterization of vehicle emissions released into these cities atmosphere. This study based on local field campaign in a representative site of anthropogenic activities over West African cities such as Yopougon (Abidjan, Côte d'Ivoire) during 2016, provided useful information on vehicle type and age, traveling time, fuel type and amount for fuel consumption estimation. Also, high traffic flow of personal car were recorded on highway, boulevard and backstreet whereas high flows of intra-communal sedan taxi are recorded on main and secondary roads. In addition, the highest daily fuel consumption value of 56 L.day-1 was recorded in heavy vehicle while the lowest value of 15 L.day-1 is recorded for personal car using gasoline. This study will be useful for the improvement of uncertainties related to the different databases used to estimate inventories emissions either national or international reports. This work provides useful information for future studies on urban air quality, climate and health impacts assessment in African cities. It may also be useful for policy makers to support implementation of emission reduction policy in West African cities.

International audience; Unprecedented ground-based and aircraft measurements in June-July 2016 in southern West Africa characterize atmospheric composition and dynamics, low-level cloud properties, the diurnal cycle, and air pollution impacts on health.The EU-funded project DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) investigates the relationship between weather, climate, and air pollution in southern West Africa, an area with rapid population growth, urbanisation, and increase in anthropogenic aerosol emissions. The air over this region contains a unique mixture of natural and anthropogenic gases, liquid droplets and particles, emitted in an environment, in which multi-layer clouds frequently form. These exert a large influence on the local weather and climate, mainly due to their impact on radiation, the surface energy balance, and thus the diurnal cycle of the atmospheric boundary layer.In June and July 2016, DACCIWA organized a major international field campaign in Ivory Coast, Ghana, Togo, Benin, and Nigeria. Three supersites in Kumasi, Savè, and Ile-Ife conducted permanent measurements and 15 Intensive observation periods. Three European aircraft together flew 50 research flights between 27 June and 16 July 2016 for a total of 155 hours. DACCIWA scientists launched weather balloons several times a day across the region (772 in total), measured urban emissions, and evaluated health data. The main objective was to build robust statistics of atmospheric composition, dynamics, and low-level cloud properties in various chemical landscapes to investigate their mutual interactions.This article presents an overview of the DACCIWA field campaign activities as well as some first research highlights. The rich data obtained during the campaign will be made available to the scientific community and help to advance scientific understanding, modeling, and monitoring the atmosphere over southern West Africa.

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, Cote 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/Waste Burning (AWB) 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 proceed to the measurements of the Aerosol Optical Depth (AOD) and the Angström exponent in both cities. The average PM2.5 mass concentrations were 32 ± 32 μg/m3, 32 ± 24 μg/m3, 28 ± 19 μg/m3 at traffic sites CT and AT and waste burning site AWD, respectively. The domestic fire site shows concentration of 145 ± 69 μg/m3 due the contribution of smoking and grilling activities. The highest OC and EC concentrations were also measured at ADF at 71 ± 29 μg/m3 and 15 ± 9 μg/m3, respectively. While the other sites present OC concentration between 8 and 12 μg/m3 and EC concentrations between 2 and 7 μg/m3. The OC / EC ratio is 4.3 at CT and 2.0 at AT. This difference highlighs the influence of 2-wheels vehicles using 2-stoke mix in Cotonou compared to Abidjan. AOD were rather similar in both cities with a mean value of 0.58 in Cotonou and 0.68 in Abidjan. The seasonal cycle is dominated by the large increase in surface mass concentration and AOD during the main dry season (Dec.–Feb.) as expected due to mineral dust advection and biomass burning activities. The lowest concentrations are observed during the minor dry season (Aug.–Sept.) 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 minor wet season (Oct.–Nov.) indicates the stagnation of local pollution.

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 and charcoal burning, charcoal making, open waste burning, and vehicles including trucks, cars, buses and two-wheeled vehicles. Emission factors of total particulate matter, black carbon, primary organic carbon and non-methane volatile organic compounds (NMVOC) have been established. In addition, emission factor measurements were performed in combustion chambers in order to reproduce field burning conditions for tropical hardwood, and obtain particulate emission factors by size (PM0.25, PM1, PM2.5 and PM10). Aerosol samples were collected on quartz filters and analysed using gravimetric and thermal methods. The emission factors of 50 NMVOC species were determined using systematic off-line sampling. Emission factors from wood burning for black carbon, organic carbon and total particulate matter were 0.8 ± 0.4 g/kg of dry matter (dm), 9.29 ± 3.82 g/kg dm and 34.54 ± 20.6 g/kg dm, respectively. From traffic sources, the highest emission factors for all particulate species were emitted from two wheeled vehicles with two-stroke engines (2.74 g/kg fuel for black carbon, 65.11 g/kg fuel for organic carbon and 496 g/kg fuel for total particulate matter). The emissions of NMVOCs were lower than those of particles for all sources aside from traffic. The largest NMVOC emissions were observed for two-stroke two-wheeled vehicles, which were up to three times higher than emissions from light-duty and heavy-duty vehicles. Isoprene and monoterpenes, which are usually associated with biogenic emissions, were present in almost all anthropogenic source categories and could be as significant as aromatic emissions in wood burning (1 g/kg dm). Black carbon was primarily emitted in the ultrafine fraction, with 77 % of the total mass being emitted as particles smaller than 0.25 µm. This study observed higher particle and NMVOC emission factors than those in the current literature. This study underlines the important role of in-situ measurements in deriving realistic and representative emission factors.

This work is part of the DACCIWA FP7 program (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) in the framework of the work package 2 « Air Pollution and Health ». This study aims to characterize urban air pollution levels through the measurement of NO2, SO2, NH3, HNO3 and O3 in Abidjan, the economic capital of Cote d’Ivoire. Gases measurements are performed using INDAAF (International Network to study Deposition and Atmospheric chemistry in AFrica) passive samplers exposed in duplicate for two weeks periods. We performed an intensive measurement campaign in Abidjan from December 15th, 2015 to February 16th, 2016 during the dry season. Twenty-one sites were selected in the district of Abidjan to be representative of various anthropogenic and natural sources of air pollution in the city. We collected 672 samples of gas during this period. Results from the intensive campaign show that gas concentrations are strongly linked to pollution sources nearby and show a high spatial variability on the different sites of Abidjan. However, three gases present relative higher levels of concentrations at all the sites: NH3, NO2 and O3. NH3 average concentrations vary between 9.1 ± 1.7 ppb at a suburban site and 102.1 ± 9.1 ppb at a domestic fires site. NO2 mean concentration vary from 2.7 ± 0.1 ppb at a suburban site to 25.0 ± 1.7 ppb at an industrial site. We measured the two highest O3 concentration on the two coastal sites located in the southeast of the city with average concentration of 19.1 ± 1.7 ppb and 18.8 ± 3.0 ppb respectively for Gonzagueville and the Felix Houphouet-Boigny international airport. The SO2 average concentration never exceeds 7.2 ± 1.2 ppb at all the sites with 71.5 % of the sampling sites presenting concentrations ranged between 0.4 ppb and 1.9 ppb. The HNO3 average concentration is comprised between 0.2 ppb and 1.4 ppb. All these results were combined with meteorological parameters to provide the first mapping of gaseous pollutants at the scale of the district of Abidjan using the geostatistical analysis (ArcGIS software). Spatial distribution results emphasize the importance of the domestic fires source and the significant impact of the traffic emissions at the scale of the city. In addition, we propose in this work a first overview of gaseous SO2 and NO2 concentrations at the scale of several African cities from literature compared to our measurements. The daily SO2 standard of WHO is exceeded in most of the cities reported in the overview where concentrations range from 0.2 µg m−3 – 3662 µg m−3. Annual NO2 concentrations ranged from 2 µg m−3 – 175 µg m−3 are lower than the WHO threshold. As a conclusion, this study constitutes an original database to characterize urban air pollution and a first attempt toward a spatialization of the pollution levels at the scale of the metropolis of Abidjan. This work should draw the attention of the African public authorities to the necessity of air quality monitoring network in order to (1) to define national standards and to better control the pollutants emissions and (2) to investigate the impact on the health of the growing population of developing African countries

This work is part of the IDAF program (IGAC-DEBITS-AFRICA) and is based on the long-term monitoring of gas concentrations (1998–2007) established at seven remote sites representative of major African ecosystems. Dry deposition fluxes were estimated by the inferential method using on the one hand surface measurements of gas concentrations (NO 2 , HNO 3 , NH 3 , SO 2 and O 3 ) and on the other hand modeled exchange rates. Dry deposition velocities ( V d ) were calculated using the big-leaf model of Zhang et al. (2003b). The bidirectional approach is used for NH 3 surface–atmosphere exchange (Zhang et al., 2010). Surface and meteorological conditions specific to IDAF sites have been used in the models of deposition. The seasonal and annual mean variations of gaseous dry deposition fluxes (NO 2 , HNO 3 , NH 3 , O 3 and SO 2 ) are analyzed. Along the latitudinal transect of ecosystems, the annual mean dry deposition fluxes of nitrogen compounds range from −0.4 to −0.8 kg N ha −1 yr −1 for NO 2 , from −0.7 to −1.0 kg N ha −1 yr −1 for HNO 3 and from −0.7 to −8.3 kg N ha −1 yr −1 for NH 3 over the study period (1998–2007). The total nitrogen dry deposition flux (NO 2 +HNO 3 +NH 3 ) is more important in forests (−10 kg N ha −1 yr −1 ) than in wet and dry savannas (−1.6 to −3.9 kg N ha −1 yr −1 ). The annual mean dry deposition fluxes of ozone range between −11 and −19 kg ha −1 yr −1 in dry and wet savannas, and −11 and −13 kg ha −1 yr −1 in forests. Lowest O 3 dry deposition fluxes in forests are correlated to low measured O 3 concentrations, lower by a factor of 2–3, compared to other ecosystems. Along the ecosystem transect, the annual mean of SO 2 dry deposition fluxes presents low values and a small variability (−0.5 to −1 kg S ha −1 yr −1 ). No specific trend in the interannual variability of these gaseous dry deposition fluxes is observed over the study period.

In this paper we present the long term monitoring of ambient gaseous concentrations within the framework of the IDAF (IGAC-DEBITS-AFRICA) program. This study proposes for the first time an analysis of long-term inorganic gas concentrations (1998 to 2007) of SO2, NO2, HNO3, NH3 and O3, determined using passive samplers at seven remote sites in West and Central Africa. Sites are representative of several African ecosystems and are located along a transect from dry savannas-wet savannas-forests with sites at Banizoumbou (Niger), Katibougou and Agoufou (Mali), Djougou (Benin), Lamto (Cote d'Ivoire), Zoetele (Cameroon) and Bomassa (Congo). The strict control of measurement techniques as well as the validation and inter-comparison studies conducted with the IDAF passive samplers assure the quality and accuracy of the measurements. For each type of African ecosystem, the long term data series have been studied to document the levels of surface gaseous concentrations. The seasonal and interannual variability have also been analyzed as a function of emission source variations. We compared the measured West and Central African gas concentrations to results obtained in other parts of the world. Results show that the annual mean concentrations of NO2, NH3, HNO3 measured in dry savannas are higher than those measured in wet savannas and forests that have quite similar concentrations. Annual mean NO2 concentrations vary from 0.9±0.2 in forests to 2.4±0.4 ppb in the dry savannas, NH3 from 3.9±1.4 to 7.4±0.8 ppb and HNO3 from 0.2±0.1 to 0.5±0.2 ppb. Annual mean O3 and SO2 concentrations are lower for all ecosystems and range from 4.0±0.4 to 14.0±2.8 and from 0.3±0.1 to 1.0±0.2 ppb, respectively. A focus on the processes involved in gas emissions from dry savannas is presented in this work, providing explanations for the high concentrations of all gases measured at the three dry savannas sites. At these sites, seasonal concentrations of all gases are higher in the wet season. Conversely, concentrations are higher in the dry season in the wet savannas. In forested regions, we measure no significant difference between wet and dry seasons. This unique database of long term gases concentrations monitoring is available at: http://medias.obs-mip.fr/idaf/.

This work is part of the IDAF (IGAC/DEBITS/AFrica) programme which started in 1995 with the establishment of 10 measurement sites representative of major African ecosystems. The objectives of the programme are to study wet and dry deposition fluxes, to identify the relative contribution of natural and anthropogenic sources and factors regulating these fluxes. This study presents an estimation of the atmospheric nitrogen (N) deposition budget in Africa based on a long term monitoring measurements database including gaseous, precipitation and aerosols chemical composition. Annual nitrogen fluxes including wet and dry processes are estimated to be around 6 kg N ha−1 year−1, 6.5 kg N ha−1 year−1 and 13 kg N ha−1 year−1 respectively over dry savanna, humid savanna and over the forest.

African biomass burning emission inventories for gaseous and particulate species have been constructed at a resolution of 1 km by 1km with daily coverage for the 2000¿2007 period. These inventories are higher than the GFED2 inventories, which are currently widely in use. Evaluation specifically focusing on combustion aerosol has been carried out with the ORISAM-TM4 global chemistry transport model which includes a detailed aerosol module. This paper compares modeled results with measurements of surface BC concentrations and scattering coefficients from the AMMA Enhanced Observations period, aerosol optical depths and single scattering albedo from AERONET sunphotometers, LIDAR vertical distributions of extinction coefficients as well as satellite data. Aerosol seasonal and interannual evolutions over the 2004¿2007 period observed at regional scale and more specifically at the Djougou (Benin) and Banizoumbou (Niger) AMMA/IDAF sites are well reproduced by our global model, indicating that our biomass burning emission inventory appears reasonable., JRC.H.5-Land Resources Management

We use space-based observations of NO2 columns from the Global Ozone Monitoring Experiment (GOME) to map the spatial and seasonal variations of NOx emissions over Africa during 2000. The GOME observations show not only enhanced tropospheric NO2 columns from biomass burning during the dry season but also comparable enhancements from soil emissions during the rainy season over the Sahel. These soil emissions occur in strong pulses lasting 1-3 weeks following the onset of rain, and affect 3 million sq km of semiarid sub-Saharan savanna. Surface observations of NO2 from the International Global Atmospheric Chemistry (IGAC)/Deposition of Biochemically Important Trace Species (DEBITS)/Africa (IDAF) network over West Africa provide further evidence for a strong role for microbial soil sources. By combining inverse modeling of GOME NO2 columns with space-based observations of fires, we estimate that soils contribute 3.3+/-1.8 TgN/year, similar to the biomass burning source (3.8+/-2.1 TgN/year), and thus account for 40% of surface NO(x) emissions over Africa. Extrapolating to all the tropics, we estimate a 7.3 TgN/year biogenic soil source, which is a factor of 2 larger compared to model-based inventories but agrees with observation-based inventories. These large soil NO(x) emissions are likely to significantly contribute to the ozone enhancement originating from tropical Africa.