Chemical composition of size-resolved atmospheric aerosols in the eastern Mediterranean during summer and winter

The chemical composition of aerosols has been determined in 30 size-resolved samples collected using a Berner low-pressure impactor during two campaigns conducted at a coastal site in the Eastern Mediterranean in July 2000 and in January 2001. Sulfate (SO42−) and ammonium (NH4+) have been identified as the main ionic components of the sub-micronic aerosol fraction, with SO42− accounting for up to 38% of the total fine mass and up to 65% of the total ionic mass during both seasons. On the other hand, nitrate (NO3−), chloride (Cl−), sodium (Na+) and calcium (Ca2+) were identified as the main components of the super-micron mode. The ionic organic compounds (including carboxylic, dicarboxylic and ketoacids) were distributed both between sub-micron and super-micron mode, indicating origin from both gas-to-particle conversion and heterogeneous reactions on pre-existing particles. The total water-soluble ionic organic fraction although accounting for only up to 1–2% of both coarse (>1 μm) and fine (<1 μm) mass fractions, accounts for up to 15% of the organic carbon (OC) mass. NH4+ was found to be significantly correlated to non-sea-salt sulfate (nss-SO42−), with NH4+/nss-SO42− molar ratio ranging from 1.3 to 2, the lower ratio associated with transport from the W sector. Chloride depletion was observed mainly during summer and was significantly correlated with NO3− concentrations, with a molar ratio of 0.80, indicating the reaction of nitric acid with NaCl as the main source of NO3− in the area. Total ionic mass both in the fine and coarse fraction accounted for up to 58% of the total aerosol mass during both seasons. An attempt to perform a mass closure analysis indicates that nss-SO42− and organic carbon are the main components of the fine fraction with relative contributions of 38% and 16%, respectively. In the coarse fraction, the ionic part accounts for 58%, mineral dust for 32% and the remaining non-identified part of 12–30% could be partly attributed to water.