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 Izana Observatory, similar to 2400 m 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 0.02-4.0 mu g m(-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 (similar to 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 alpha-pinene and isoprene, determined by gas chromatography coupled with mass spectrometry) accounted for 15% of the bulk organic matter (determined by the thermooptical 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, Izana was within the westerlies stream and organic matter accounted for similar to 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 (3.6 mu g m(-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 NOx emissions.