Chemical characterization and source apportionment of submicron aerosols measured in Senegal during the 2015 SHADOW campaign
Laura-Hélèna Rivellini1,2, Isabelle Chiapello2, Emmanuel Tison1, Marc Fourmentin3, Anaïs Féron4, Aboubacry Diallo5, Thierno N'Diaye5, Philippe Goloub2, Francesco Canonaco6, André S. H. Prévôt6, and Véronique Riffault11IMT Lille Douai, Univ. Lille, SAGE – Département Sciences de l'Atmosphère et Génie de l'Environnement, 59000 Lille, France 2Laboratoire d'Optique Atmosphérique, Université de Lille – CNRS, Villeneuve d'Ascq, 59655, France 3Laboratoire de Physico-Chimie de l'Atmosphère, Université du Littoral Côte d'Opale, Dunkerque, 59140, France 4Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS – Université Paris Est Créteil – Université Paris Diderot, Créteil, 94010, France 5Institut de Recherche pour le Développement, M'Bour, Senegal 6Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
Received: 05 Jan 2017 – Accepted for review: 18 Feb 2017 – Discussion started: 27 Feb 2017
Abstract. The present study offers the first chemical characterization of the submicron (PM1) fraction in West Africa at a high time resolution, thanks to collocated measurements of non-refractory (NR) species with an Aerosol Chemical Speciation Monitor (ACSM), black carbon and iron concentrations derived from absorption coefficient measurements with a 7-wavelength aethalometer, and total PM1 determined by a TEOM-FDMS for mass closure. The field campaign was carried out during four months (March to June 2015) as part of the SHADOW (SaHAran Dust Over West Africa) project at a coastal site located in the outskirts of the city of M'Bour, Senegal. With an average mass concentration of 5.4 µg m−3, levels of NR-PM1 in M'Bour were three to ten times lower than cities like Paris or Beijing. Nonetheless the first half of the observation period was marked by intense but short pollution events (concentrations higher than 15 µg m−3), sea breeze phenomena and Saharan desert dust outbreaks (PM10 up to 900 µg m−3). During the second half of the campaign, the sampling site was mainly under the influence of marine air masses. The air masses on days under continental and sea breeze influences were dominated by organics (36–40 %), whereas sulfate particles were predominant (40 %) for days under oceanic influence. Overall, measurements showed that about 3/4 of the total PM1 were explained by NR-PM1, BC and Fe (a proxy for dust) concentrations, leaving ~ 1/4 for other refractory species. A mean value of 4.6 % for the Fe / PM1 ratio was obtained. Source apportionment of the organic fraction, using Positive Matrix Factorization (PMF) highlighted the impact of local combustion sources, such as traffic and residential activities, which contribute on average to 52 % of the total organic fraction. A new organic aerosol (OA) source, representing on average 3 % of the total OA fraction, showed similar variation as non-refractory particulate chloride. Its rose plot and daily pattern pointed out to local combustion processes, that is to say two open waste burning areas located about 6 and 11 km away from the receptor site and to a lesser extent a traditional fish smoking place. The remaining fraction was identified as oxygenated organic aerosols (OOA), a factor that prevailed regardless of the day type (45 %) and was representative of regional but also local sources due to enhanced photochemical processes.
Rivellini, L.-H., Chiapello, I., Tison, E., Fourmentin, M., Féron, A., Diallo, A., N'Diaye, T., Goloub, P., Canonaco, F., Prévôt, A. S. H., and Riffault, V.: Chemical characterization and source apportionment of submicron aerosols measured in Senegal during the 2015 SHADOW campaign, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1127, in review, 2017.