Chemical characterization and source apportionment of submicron aerosols measured in Senegal during the 2015 SHADOW campaign

Laura-Hélèna Rivellini (1, 2) , Isabelle Chiapello (2) , Emmanuel Tison (1) , Marc Fourmentin (3) , Anais Feron (4) , Aboubacry Diallo , Thierno N'Diaye , Philippe Goloub (2) , Francesco Canonaco (5) , André Stephan Henry Prévôt (6) , Véronique Riffault (7, 1)

1 IMT Lille Douai - Ecole nationale supérieure Mines-Télécom Lille Douai
2 LOA - Laboratoire d’Optique Atmosphérique - UMR 8518
3 LPCA - Laboratoire de Physico-Chimie de l'Atmosphère
4 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
5 LAC - Laboratory of Atmospheric Chemistry [Paul Scherrer Institute]
6 PSI - Paul Scherrer Institute
7 CERI EE - IMT Nord Europe - Centre for Energy and Environment

The present study offers the first chemical characterization of the submicron (PM 1) fraction in western Africa at a high time resolution, thanks to collocated measurements of nonrefractory (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 PM 1 determined by a TEOM-FDMS (tapered element oscillating microbalance-filtered dynamic measurement system) for mass closure. The field campaign was carried out over 3 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 Mbour, Senegal. With an averaged mass concentration of 5.4 µg m −3 , levels of NR PM 1 in Mbour were 3 to 10 times lower than those generally measured in urban and suburban polluted environments. Nonetheless the first half of the observation period was marked by intense but short pollution events (NR PM 1 concentrations higher than 15 µg m −3), sea breeze phenomena and Saharan desert dust outbreaks (PM 10 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 three-quarters of the total PM 1 were explained by NR PM 1 , BC (black carbon) and Fe (a proxy for dust) concentrations, leaving approximately one-quarter for other refractory species. A mean value of 4.6 % for the Fe / PM 1 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 to nonrefractory particulate chloride. Its rose plot and daily pattern pointed to local combustion processes, i.e., 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 location. 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 (approximately three-quarters) but also local (approximately one-quarter) sources due to enhanced photo-chemical processes.

Domaines: Océan, Atmosphère; Milieux et Changements globaux

Laura-Hélèna Rivellini, Isabelle Chiapello, Emmanuel Tison, Marc Fourmentin, Anais Feron, et al.. Chemical characterization and source apportionment of submicron aerosols measured in Senegal during the 2015 SHADOW campaign. Atmospheric Chemistry and Physics, 2017, 17, pp.10291-10314. ⟨10.5194/acp-17-10291-2017⟩. ⟨hal-02567651⟩