Summertime surface PM<sub>1</sub> aerosol composition and size by source region at the Lampedusa island in the central Mediterranean Sea

Abstract

<p><strong>Abstract.</strong> Measurements of aerosol composition and size distributions were taken during the summer of 2013 at the remote island of Lampedusa in the southern central Mediterranean Sea. These measurements were part of the ChArMEx/ADRIMED (Chemistry and Aerosol Mediterranean Experiment/Aerosol Direct Radiative Forcing on the Mediterranean Climate) framework and took place during the Special Observation Period 1a (SOP-1a) from 11 June until 5 July 2013. From compact time-of-flight aerosol mass spectrometer (cToF-AMS) measurements in the size range below 1 μm in aerodynamic diameter (PM<sub>1</sub>), particles were predominately comprised of ammonium and sulphate. On average, ammonium sulphate contributed 63 % to the non-refractory PM<sub>1</sub> mass, followed by organics (33 %). The organic aerosol was generally very highly oxidised (f<sub>44</sub> values were typically between 0.25 and 0.26). The contribution of ammonium sulphate was generally higher than organic aerosol in comparison to measurements taken in the western Mediterranean but is consistent with studies undertaken in the eastern basin.</p> <p>Source apportionment of organics using a statistical (positive matrix factorisation) model revealed four factors; a hydrocarbon-like organic aerosol (HOA), a methanesulfonic acid related oxygenated organic aerosol (MSA-OOA), a more oxidised oxygenated organic aerosol (MO-OOA) and a less oxidised oxygenated organic aerosol we label (LO-OOA). The MO-OOA was the dominant factor for most of the campaign (53 % of the PM<sub>1</sub> OA mass). It was well correlated with SO<sub>4</sub><sup>2−</sup>, highly oxidised, and generally more dominant during easterly air masses originating from the eastern Mediterranean and central Europe. The LO-OOA factor had a very similar composition to the MO-OOA factor, but was more prevalent during westerly winds with air masses originating from the Atlantic Ocean, the western Mediterranean, and in high altitudes over France and Spain from mistral winds. The MSA-OOA factor contributed an average 12 % to the PM<sub>1</sub> OA and was more dominant during the mistral winds. The HOA, representing observed primary organic aerosol only contributed 8 % of the average PM<sub>1</sub> OA during the campaign.</p> <p>Even though Lampedusa is one of the most remote sites in the Mediterranean, PM<sub>1</sub> concentrations (10 ± 5 μg m<sup>−3</sup>) were comparable to those observed in coastal cities and sites closer to continental Europe. Cleaner conditions corresponded to higher wind speeds. Nucleation and growth of new aerosol particles was observed during periods of northwesterly winds. From a climatology analysis from 1999 until 2012, these periods were much more prevalent during the measurement campaign than during the preceding 13 years. These results support previous findings that highlight the importance of different large-scale synoptic conditions in determining the regional and local aerosol composition and oxidation and also suggest that a non-polluted surface atmosphere over the Mediterranean is rare.</p>