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We focused this research on the composition of the organic aerosolstransported in the two main airflows of the subtropical North Atlantic freetroposphere: (i) the Saharan Air Layer – the warm, dry and dusty airstreamthat expands from North Africa to the Americas at subtropical and tropicallatitudes – and (ii) the westerlies, which flow from North America overthe North Atlantic at mid- and subtropical latitudes. We determined theinorganic compounds (secondary inorganic species and elemental composition),elemental carbon and the organic fraction (bulk organic carbon and organicspeciation) present in the aerosol collected at Izaña Observatory, ∼  2400 m a.s.l. on the island of Tenerife. The concentrations of allinorganic and almost all organic compounds were higher in the Saharan AirLayer than in the westerlies, with bulk organic matter concentrations withinthe range 0.02–4.0 µg m−3. In the Saharan Air Layer, thetotal aerosol population was by far dominated by dust (93 % of bulk mass),which was mixed with secondary inorganic pollutants ( <  5 %) and organicmatter ( ∼  1.5 %). The chemical speciation of the organic aerosols(levoglucosan, dicarboxylic acids, saccharides, n-alkanes, hopanes,polycyclic aromatic hydrocarbons and those formed after oxidation ofα-pinene and isoprene, determined by gas chromatography coupled withmass spectrometry) accounted for 15 % of the bulk organic matter(determined by the thermo-optical transmission technique); the most abundantorganic compounds were saccharides (associated with surface soils), secondaryorganic aerosols linked to oxidation of biogenic isoprene (SOA ISO) anddicarboxylic acids (linked to several primary sources and SOA). When theSaharan Air Layer shifted southward, Izaña was within the westerlies streamand organic matter accounted for  ∼  28 % of the bulk mass of aerosols.In the westerlies, the organic aerosol species determined accounted for64 % of the bulk organic matter, with SOA ISO and dicarboxylic acids beingthe most abundant; the highest concentration of organic matter(3.6 µg m−3) and of some organic species (e.g. levoglucosanand some dicarboxylic acids) were associated with biomass burning linked to afire in North America. In the Saharan Air Layer, the correlation foundbetween SOA ISO and nitrate suggests a large-scale impact of enhancement ofthe formation rate of secondary organic aerosols due to interaction withanthropogenic NOx emissions

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