Organic, elemental and water-soluble organic carbon in size segregated aerosols, in the marine boundary layer of the Eastern Mediterranean

To assess the origin and transformation of carbonaceous material in the marine boundary layer of the Eastern Mediterranean, a total of 111 size segregated aerosol samples have been collected using a 12-stage Small-Deposit-area low-volume-Impactor (SDI) covering an almost 3 year period. The samples have been analyzed for organic (OC), elemental (EC), water-soluble organic carbon (WSOC) and ionic components. Maxima of OC, EC and WSOC mass size distributions were found in the accumulation mode (0.449 μm) with occasionally a minor, secondary peak in the coarse mode (2.68 μm). OC and WSOC concentrations peak during summertime due to photochemistry, while EC during autumn, and spring. In general, almost 2/3 of OC and EC concentrations are found in the PM1 fraction of the aerosol with OC being mostly secondary and therefore highly oxidized and water-soluble to a great extent (∼70%). Using the EC-tracer method, it was found that 83 ± 11% of the PM10 organic carbon is secondary, with the percentage reaching ∼70% for the PM1 fraction, a value in very good accordance to WSOC/OC ratio. Ammonium sulfate accounts for 75.5 ± 21.7% and 9.3 ± 1.9% of the aerosol mass in the fine and coarse fraction respectively, exhibiting maximum concentrations also in the accumulation mode. It was estimated that, on average, sea salt and mineral dust account for 33% and 45% of the coarse inorganic mass fraction, respectively.

► Carbonaceous aerosol constitutes a significant part of submicron mass fraction.
► Size distribution of OC, EC, WSOC and ions was performed during a 3 year period.
► OC, EC and WSOC exhibit bimodal distribution with maximum in the accumulation mode.
► 83.5 ± 11% of the PM10 OC is secondary.
► 70% of the fine mode OC is water-soluble with high summer and low winter values.