Chemical characterization and sources of background aerosols in the eastern Mediterranean

Abstract

Measurements of the composition of the gas and particulate phases were conducted from May 9 to June 4 of 2016 at the Finokalia (Greece) remote coastal site in the eastern Mediterranean, continuing the effort to track long term changes of the atmospheric chemical composition in the area. Finokalia is influenced by air masses arriving in the site from regional sources in Europe, Africa, and other locations. In this study, a series of instruments and analysis techniques were used for the first time at the Finokalia site to characterize the composition of the organic aerosol (OA). The PM1 was composed of ammonium sulfate/bisulfate (60%), followed by organics (35%) and black carbon (BC) (4%). The OA was highly oxidized with an average oxygen-to-carbon (O:C) ratio of 0.81. Source apportionment of high-resolution organic aerosol mass spectra via positive matrix factorization (PMF) identified four factors: less oxidized oxygenated organic aerosol (LO-OOA), more oxidized oxygenated organic aerosol (MO-OOA), marine-related oxygenated organic aerosol (marine-OOA) and hydrocarbon-like organic aerosol (HOA). The LO-OOA (O:C = 0.92) was the dominant factor for most of the campaign (51% of the PM1 OA), while the MO-OOA (O:C = 1.1) was responsible for 11% of the OA. The marine-OOA factor was also highly oxidized (O:C = 1.03), contributing on average 24% to the total OA and was found to be well correlated with sulfate (R2 = 0.83). The anthropogenic HOA factor (O:C = 0.36) was related to activities in the island of Crete. Its mass spectrum was quite similar (R2 = 0.86–0.95) to those of other HOA factors in the literature and its average concentration was 0.2 μg m−3 (14% of OA). High molecular weight compounds were found, suggesting the presence of oligomers in agreement with the presence of a highly oxidized quenching agent. Aromatic VOCs, like benzene, toluene, and xylenes were on average 0.17, 0.66 and 0.36 ppb, respectively with toluene concentrations indicating the presence of a local source despite the remote nature of the site. New particle formation (NPF) was observed during 26% of the days. The air masses during NPF events passed over the island of Crete and the Balkans and were characterized by a low condensation sink (5.9 ± 2.2 10−3 s−1). Nucleation mode particle concentrations during NPF days had an additional evening peak between 17:00 and 22:00, which was accompanied by an HOA peak implying the influence of a source of sub-25 nm particles in the extended area. These findings show that even a relatively remote site like Finokalia is occasionally affected by sources located tens of kilometers away, something that should be taken into account in the interpretation of past and future measurements in this location.