Speciation of organic aerosols in the Saharan Air Layer and in the free troposphere westerlies

We focused this research on the composition of the organic aerosols transported in the two main airflows of the subtropical North Atlantic free troposphere: (i) the Saharan Air Layer - the warm, dry and dusty airstream that expands from North Africa to the Americas at subtropical and tropical latitudes - and (ii) the westerlies, which flow from North America over the North Atlantic at mid- and subtropical latitudes. We determined the inorganic compounds (secondary inorganic species and elemental composition), elemental carbon and the organic fraction (bulk organic carbon and organic speciation) present in the aerosol collected at Izaña Observatory, ∼2400m.a.s.l. on the island of Tenerife. The concentrations of all inorganic and almost all organic compounds were higher in the Saharan Air Layer than in the westerlies, with bulk organic matter concentrations within the range 0.02-4.0μ-3. In the Saharan Air Layer, the total aerosol population was by far dominated by dust (93% of bulk mass), which was mixed with secondary inorganic pollutants ( < 5%) and organic matter (∼1.5%). The chemical speciation of the organic aerosols (levoglucosan, dicarboxylic acids, saccharides, n-alkanes, hopanes, polycyclic aromatic hydrocarbons and those formed after oxidation of α-pinene and isoprene, determined by gas chromatography coupled with mass spectrometry) accounted for 15% of the bulk organic matter (determined by the thermo-optical transmission technique); the most abundant organic compounds were saccharides (associated with surface soils), secondary organic aerosols linked to oxidation of biogenic isoprene (SOA ISO) and dicarboxylic acids (linked to several primary sources and SOA). When the Saharan Air Layer shifted southward, Izaña was within the westerlies stream and organic matter accounted for 28% of the bulk mass of aerosols. In the westerlies, the organic aerosol species determined accounted for 64% of the bulk organic matter, with SOA ISO and dicarboxylic acids being the most abundant; the highest concentration of organic matter (3.6μ-3) and of some organic species (e.g. levoglucosan and some dicarboxylic acids) were associated with biomass burning linked to a fire in North America. In the Saharan Air Layer, the correlation found between SOA ISO and nitrate suggests a large-scale impact of enhancement of the formation rate of secondary organic aerosols due to interaction with anthropogenic NO emissions. © 2017 Author(s).This study was performed within the context of the projects AEROATLAN (CGL2015-66299-P; MINECO/FEDER) and TEAPARTICLE (CGL2011-29621), supported by the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund (ERDF). The authors acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT back-trajectories used in this publication. The excellent work performed by the staff of the Atmospheric Research Centre (Concepción Bayo, Cándida Hernández, Fernando de Ory, Virgilio Carreño, Rubén del Campo and SIELTEC Canarias) and of the Institute of Environmental Assessment and Water Research (Roser Chaler, Dori Fanjul, and Bibiano Hortelano) is appreciated. M. Isabel García acknowledges the grant of the Canarian Agency for Research, Innovation and Information Society (ACIISI), co-funded by the European Social Funds. Measurements at Izaña Observatory are performed within the context Global Atmospheric Watch networks with the financial support of the State Meteorological Agency of Spain (AEMET).Peer reviewe