Speciation of organic aerosols in the Saharan Air Layer and in the
free troposphere westerlies
M. Isabel García1,2, Barend L. van Drooge3, Sergio Rodríguez1, and Andrés Alastuey31Izaña Atmospheric Research Centre, AEMET, Associated Unit to CSIC “Studies on Atmospheric Pollution”, Santa Cruz de Tenerife, 38001, Spain 2Department of Chemistry (T.U. Analytical Chemistry), Faculty of Science, University of La Laguna, La Laguna, 38206, Spain 3Institute of Environmental Assessment and Water Research, CSIC, Barcelona, 08034, Spain
Received: 03 Feb 2017 – Accepted for review: 26 Feb 2017 – Discussion started: 27 Feb 2017
Abstract. 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 flows from North America through 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, ~ 2400 m a.s.l. in Tenerife Island. 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 µg 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 (~ 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 to mass-spectrometry) accounted for a 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 the organic matter accounted for ~ 28 % of bulk mass of the aerosol cocktail. In the westerlies, the determined organic aerosol species accounted for 64 % of the bulk organic matter, being SOA ISO and dicarboxylic acids the most abundant; the highest concentration of organic matter (3.6 µg 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 the enhancement in the formation of secondary organic aerosols due to interaction with anthropogenic NOx emissions.
García, M. I., van Drooge, B. L., Rodríguez, S., and Alastuey, A.: Speciation of organic aerosols in the Saharan Air Layer and in the
free troposphere westerlies, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-108, in review, 2017.