Monitoring of Atmospheric Pollutant Concentrations in the City of Dakar, Senegal

The lack of data on air quality monitoring and neglected and overlooked pollutant emissions in the transportation and industrial sectors are motives for the government of Senegal to set up, in 2009, an air quality management center, the CGQA (Centre de Gestion de la Qualité de l’Air). Air quality monitoring at CGQA deals with mainly six pollutants: carbon monoxide (CO), nitrogen oxides (NO x ) (with nitrogen dioxide (NO 2 ) and nitrogen monoxide (NO)), sulfur dioxide (SO 2 ), ground-level ozone (O 3 ), Benzene-Toluene-Xylenes (BTX), and particulate matters (PM) with diameters less than 10 µm (PM 10 ) and 2.5 µm (PM 2.5 ). The concentration levels of gaseous pollutants (CO, O 3 , NO 2 and SO 2 ) in the city generally remain below the limit value set by the WHO (World Health Organization). However, particulate matters (PM 10 and PM 2.5 ) are the most important pollutants observed in Dakar, they far exceed the annual thresholds set by the WHO and the national standard (NS 05-062). This situation results in an Air Quality Index (AQI) around bad and very bad during the dry season (November to May) and good to moderate during the rainy season (June to October). The concentrations of PM 10 and PM 2.5 vary respectively from 120 to 180 µg∙m −3 and from 25 to 48 µg∙m −3 . The average concentrations of pollutants therefore vary from one area to another and depending on the location of the air quality monitoring station (near industrial sites, traffic, etc.).


Introduction
Atmospheric air pollution is a complex phenomenon with variety of origins and sources. In urban areas, it is caused mainly by mobile combustion in transportation vehicles which burn large quantities of fossil fuels. The city of Dakar, is home to more than four (4) million inhabitants, nearly a quarter of the population of Senegal. It has one of the highest urbanization rates in Africa, and is therefore particularly faces growing transport demand. The demographic growth is due to several factors: natural births, migration from other cities and rural areas as well, and the concentration of infrastructure and improved services and economic attractiveness. Dakar holds 80% of the country's economic and industrial activities. Road transport is the main mode of transport in Senegal [1], and faces many challenges: poor maintenance, lack of infrastructure, mismanagement, etc. Dakar's car fleet represents 73% of the national vehicle fleet, and the consequence is intense road traffic and congestions. According to studies carried out by Tchanche (AIMS, COAE, Earth), road vehicles use oil products (gasoline and diesel oil) and are distributed as follows: gasoline (38%) and diesel (59%) [2]. Transport sector represents 30% of total energy consumed in the country and is second after the residential sector [3]. Transport sector is dominated by road transport which consumes 95% of total energy consumed and distributed between diesel (80%) and gasoline (20%) [4]. In addition, the industrial sector depends more than 70% on fossil fuels [3]. The consequence of the utilization of these fossil fuels is, among other things, an obvious deterioration in air quality.
The first study on air pollution in Dakar was carried out by The World Bank in the framework of its initiative on air quality in sub-Sahara Africa in 1998.
That study was based on simulation of the traffic in the city of Dakar. Since then, a study financed by the Nordic Development Fund (FND) has made it possible to obtain more precise data on the basis of a sampling and analysis program during two assessment campaigns carried out by the Norwegian Institute of Air Research (NILU) [5]. This study allowed Senegal to have standard values for NO 2 , SO 2 , O 3 , VOC, PM 10 and PM 2.5 [6]. There is some research work on air pollution in Dakar that can be cited [7]- [14] but more is still needed.
In 2009, with the support of The World Bank, the Senegalese government has set up an air quality management center, the CGQA (Centre de Gestion de la Qualite de l'Air) with as mission the monitoring of the air quality in the city of Dakar. Several measurement stations, six in total have been installed so far: Cathedrale, Yoff and a new one is to be installed in the upcoming year in Guediawaye. Pollutants monitored are particle matters (PM): PM 10 , PM 2.5 , and atmospheric gases: O 3 , CO, SO 2 , NO 2 and BTX. An air quality index (AQI) is calculated on daily basis to establish the concentration of pollutants and has become an important means to characterize the pollution and helps to send warnings in case of critical air quality degradation. This study is collaboration between the CGQA and the Alioune Diop University in which a focus is given to the air quality monitoring. The air quality monitoring system is presented along with the regulation framework on air pollution and recorded IQA is discussed.

Air Quality Management Center (CGQA)
Dakar has a network of 6 fixed stations, 5 in the inner-city and one in one of its suburbs (Guediawaye) (Figure 1). In addition, the CGQA has a mobile station called a "laboratory truck" which is part of the air quality monitoring system. In

Senegalese Regulations on Air Pollution
Law

Air Quality Index (AQI)
Pollutant concentration AQI 100 Limit value = ×  A global air quality index is calculated daily to assess the level of air pollution in Dakar. It corresponds to the average of the AQI of the six pollutants (NO 2 , SO 2 , O 3 , CO, PM 10 and PM 2.5 ) of the six stations. The limit values used to calculate the indices are those of the Senegalese standard and/or the World Health Organization (WHO).
The air quality index (AQI) is scaled between 0 and 500. The higher the index, the more dangerous it represents to public health. When the pollution level is less than or equal to 50, the quality is said to be good and does not present any health risks. On the other hand, an index greater than or equal to 100 poses health risks and when the 200 mark is crossed, a health alert is launched. In such cases each individual can experience serious health effects. A general alert, accompanied by emergency measures, is required when the index goes above 300. Figure 2 represents the change in the daily air quality index (AQI) from 2010 to 2018. It shows the seasonality of pollution episodes with peaks observed between November and April. The air quality remains in the interval bad to very bad. This period corresponds to the dry season with a dominance of the continental trade winds (Harmattan). During the rainy season from June to October the air quality is generally in the interval good to moderate. This is linked to the phenomenon of leaching of pollutants. Figure 3 and Figure 4 represent the average monthly levels of PM 10 and PM 2.5 concentrations from 2010 to 2018, respectively. There is seasonality in the concentration of pollutants. These concentrations are higher during the dry season from November to May. The weakest values are recorded during the rainy season from June to October.       Furthermore, there is a correlation between pollution episodes and the increase in particle concentrations (Figures 2-4). So particles seem to be the main cause of pollution episodes in Dakar. A study on the degradation of air quality in the first trimester of 2015 showed a correspondence between the increase in the concentrations of PM 10 and PM 2.5 , the peaks of which occurred at the same periods [17]. These particles come mainly from desert dust (PM 10 ) and the transport sector (PM 2.5 ).

Results and Discussions
In the dry period, between November and April, most cities located in the Sahelian zone are confronted, in addition to anthropogenic pollution, to dry winds from the North East (Harmattan). During this period, several towns in the region experience very high levels of dust particles [18]. Dust is lifted into the atmosphere by storm events that can move large amounts of particles into the upper troposphere, where they are then carried over long distances by tropospheric winds [19]. The analysis of these desert aerosols shows a very large variability of the particles in terms of size, most of which have a diameter between 1 and 10 μm. This dust has a huge impact on atmospheric visibility, affects the intensity of solar radiation and is a source of several diseases such as meningitis [20]. A study carried out particularly in Fann-Hospital in the Dakar region showed that the air samples taken during dust episodes contained living organisms (bacteria, fungi, viruses), including opportunistic human pathogens (Penicillium, Aspergillus) that can cause respiratory diseases [21].  [22]. In fact, resuspension dust contributes 13% -23%, the source linked to traffic 32% -42%, the marine source 15% -21%, the source associated with the production of cement and the construction sector 13% -18%, the source of fuel combustion linked to port activities 5% -13% and wear of vehicle brakes 5% -13% [22]. The large part of road traffic is linked to the fact that the vehicle fleet is constantly increasing.
In addition, road transport emits significant amounts of PM 2.5 . In fact, the vehicle fleet, made up mainly of imported used vehicles, is often dilapidated and outdated. The obsolescence of vehicles leads to overconsumption of fuels (gaso-Open Journal of Air Pollution line, diesel) which is accompanied by a high concentration of pollutants and deterioration of air quality in cities [23]. PM 2.5 , mainly emitted by diesel-powered vehicles, is the most dangerous because they cross the pulmonary barrier and enter the bloodstream.

Conclusion
Despite the significant progress recorded by the government of Senegal since 2003 with the implementation of the NS 05-062 standard and recently with the creation of a monitoring system (in 2010), the air people breathe is not always of good quality in the city of Dakar. In fact, the city is confronted throughout the year with particulate pollution with worrying levels far exceeding the limit values. In addition to particulate pollution, some types of pollution such as nitrogen dioxide and tropospheric ozone are also of great concern. These pollutants are dangerous for health, sometimes even at low doses. The mechanisms of their formation and their behaviour are complex. They come from road transport, industry, biomass fires and surface dumps. This work allows realizing the importance of monitoring air quality. It also helps the authorities to put in place strategies to drastically reduce air pollution. Dakar residents, informed day by day about the quality of the air they breathe, expect a lot from the action of the public authorities, but they are also one of the keys to improving the situation.
This necessarily involves a renewal of the vehicle fleet, that is the gradual replacement of old transport vehicles (Car-rapides, ndiaga-ndiaye, clandos) by modern buses and taxis (vehicles with less polluting technologies). The government of Senegal is committed to reducing the negative externalities of transport through the development of public transport systems such as the regional express train (TER) and the rapid transit bus project (BRT). In the industrial sector, the public authorities should strengthen regulatory requirements by imposing heavy taxes on polluting activities to encourage manufacturers to use fewer polluting technologies.