Summertime surface PMsub1/sub aerosol composition and size by source region at the Lampedusa island in the central Mediterranean Sea

摘要

Measurements of aerosol composition and size distributions were taken duringthe summer of 2013 at the remote island of Lampedusa in the southern centralMediterranean Sea. These measurements were part of the ChArMEx/ADRIMED(Chemistry and Aerosol Mediterranean Experiment/Aerosol Direct RadiativeForcing on the Mediterranean Climate) framework and took place duringSpecial Observation Period 1a (SOP-1a) from 11 June to 5 July 2013. From compact time-of-flight aerosol mass spectrometer (cToF-AMS)measurements in the size range below 1 µm in aerodynamic diameter(PM1), particles were predominately comprised of ammonium and sulfate. Onaverage, ammonium sulfate contributed 63 % to the non-refractory PM1mass, followed by organics (33 %). The organic aerosol was generally veryhighly oxidized (f44 values were typically between 0.25 and 0.26). Thecontribution of ammonium sulfate was generally higher than organic aerosolin comparison to measurements taken in the western Mediterranean but isconsistent with studies undertaken in the eastern basin. Source apportionment of organics using a statistical (positive matrixfactorization) model revealed four factors: a hydrocarbon-like organicaerosol (HOA), a methanesulfonic-acid-related oxygenated organic aerosol(MSA-OOA), a more oxidized oxygenated organic aerosol (MO-OOA) and a lessoxidized oxygenated organic aerosol (LO-OOA). The MO-OOA was thedominant factor for most of the campaign (53 % of the PM1 OA mass). It waswell correlated with SO42-, highly oxidized and generally moredominant during easterly air masses originating from the easternMediterranean and central Europe. The LO-OOA factor had a very similarcomposition to the MO-OOA factor but was more prevalent during westerlywinds, with air masses originating from the Atlantic Ocean, the westernMediterranean and at high altitudes over France and Spain from mistralwinds. The MSA-OOA factor contributed an average 12 % to the PM1 OA andwas more dominant during the mistral winds. The HOA, representing observedprimary organic aerosol, only contributed 8 % of the average PM1 OA duringthe campaign. Even though Lampedusa is one of the most remote sites in the Mediterranean,PM1 concentrations (10 ± 5 µg m−3) were comparable to thoseobserved in coastal cities and sites closer to continental Europe. Cleanerconditions corresponded to higher wind speeds. Nucleation and growth of newaerosol particles was observed during periods of north-westerly winds. From aclimatology analysis from 1999 to 2012, these periods were much moreprevalent during the measurement campaign than during the preceding 13 years.These results support previous findings that highlight the importanceof different large-scale synoptic conditions in determining the regional andlocal aerosol composition and oxidation and also suggest that a non-pollutedsurface atmosphere over the Mediterranean is rare.