Spatial distribution of gaseous pollutants (NO2, SO2, NH3, HNO3 and O3) in Abidjan, Cote d'Ivoire
Authors: Bahino Julien , Yoboué Veronique , Galy-Lacaux Corinne , Adon Marcellin , Akpo Aristide , Keita Sekou , Liousse Cathy , Gardrat Eric , Chiron Christelle , Ossohou Money , Gnamien Sylvain , Djossou Julien 
Published: August 2017

Abstract. 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