Comparison of Measures of PM2.5 and Carbonaceous Aerosol in Air at Cotonou, Benin in 2005 and 2015

. This study focuses on the comparison of carbonaceous aerosol measurements in the air at Cotonou in 2015 compared to 2005. In the framework of two international programs, AMMA (Afri-can Monsoon Multidisciplinary Analysis) and DACCIWA (Dynamics Aerosol-Cloud-Chemistry In-teractions in West Africa) monitoring data for PM2.5 microns were collected at one of the most pol-luted urban site of Cotonou (Dantokpa) in Benin (West Africa) respectively in 2005 and 2015. The results obtained indicate that the carbonaceous aerosol measures, Black carbon (BC), and organic carbon (OC) in 2005 are higher than those obtained in 2015. PM2.5 concentrations related mainly to traffic sources for 2 wheeled vehicles, were 34 g/m 3 in May 2005 and 28 µg/m 3 in May 2015. In May 2005, OC and BC concentrations were from 15 µg/m 3 and 2.3µg/m 3 while in May 2015, they were from 8 µg/m 3 for OC and 1.3 µg/m 3 for BC. In May 2005 and 2015, the total carbon (TC) accounted for 50% and 32% of the PM2.5, respectively. In this study, the OC/EC ratio exceeds 2.0, which confirms the presence of secondary organic aerosols.


Introduction
The pollution in African cities has become an important factor in West Africa and is amplified by climatic conditions (intense photochemistry) (Liousse and Galy-Lacaux, 2010). The follow-up of air quality in these African cities is practically non-existent and with timid moreover regulations of pollutant emissions (Liousse Galy-Lacaux, 2010). Preliminary studies led by researchers of Laboratoire d'Aérologie on atmospheric pollution in several big cities of Africa have confirmed the importance of the problem. We can cite, for example, the experience-test POLCA1 (Pollution of African Capitals) (Yoboué, personal communication) that took place in February-March 2004 in eight African capitals: Abidjan, Dakar, Bamako, Niamey, Ouagadougou, Bangui, Brazzaville, and Yaoundé. Recently, in AMMA programs (Analysis Multiscale of the Monsoon African) and POLCA2 (Pollution of African Capitals), experiment such anthropogenic pollution has been put in evidence in Cotonou (Benin), Bamako (Mali) (Doumbia et al., 2012, Val et al., 2013. For these reasons, the program DACCIWA (Dynamics Aerosol -Chemistry-Cloud Interactions in West Africa) has been developed while creating the Pollution/Health axis.
The idea here is to compare aerosol pollution before and after this change. For such a purpose in this paper, we will compare on the same experimental site called Dantokpa in Cotonou, PM2.  Let is note that May is in the rainy season with temperatures of the order of 24.1 °C to 30.8 °C in 2005 and 26.6 °C to 31.5 °C in 2015.
The same sampling system was used in the 2 periods to measure PM2.5 and carbon aerosol concentrations.

Materials and Methods
The experimental site is a ''so-called'' traffic site at 5 m above a big crossroad with high traffic density (see Figure 1 below). It has been chosen since Cotonou traffic is characterized by a high density of 2 wheels. Experiments interest here occurred in May 2005 and 2015.

Discussion
Our result shows that the PM2.5 Concentrations are 34 µ g/m 3 and 28 µ g/m 3 in May 2005 and 2015, respectively. We can say that the PM2.5 concentration in 2005 is higher than the one found in 2015. In 2007, French Development Agency encouraged the implementation of an innovative program to curb air pollution in Cotonou. This program makes it possible to switch from two-stroke vehicles to four-stroke vehicles, which emit close to 85% fewer greenhouse gases and are much less polluting. More than 10,000 two-stroke vehicles have been replaced by 10,000 four-stroke vehicles (approximately 10% renewal of Cotonou vehicles). The change from two wheels to four wheels participated contributed to the reduction of local pollution and emissions of greenhouse gases of 15,000 tons/year (French Development Agency) and a general impact on the development of urban transport. This confirms the decrease of PM2.5 particles and these constituents observed in 2015. The mass ratio of OC to EC can be used to identify the origins, emission, and transformation characteristics of carbonaceous aerosols (Ouafo et al., 2017). Such a high OC/EC value could indicate the presence of secondary organic carbon at the Dantokpa site.

Conclusions
The results presented in this study permit to do the comparison of carbonaceous aer-