Size distribution of water-soluble ions and carbon content in the Southwest Mediterranean Basin: results from the ChArMEx project

Abstract

In order to understand the origin and behaviour of aerosols in the west of the Mediterranean basin, a comprehensive investigation of water-soluble inorganic ions (WSII) with size segregation and Organic carbon (OC) and Element carbon (EC) in ultra fine fraction has been carried out in a coastal Algerian measurement station, namely BouIsmaïl, during seven months covering July 2013 to January 2014. A cascade impactor for six particle sizes with cut-off diameters of 0.49, 0.95, 1.5, 3, 7.2, and 10 µm was used for the whole campaign. The WSII were analysed by ion chromatography, and their distribution mode and potential sources were evaluated. The weekly mass concentration varied between 8.7 and 87 µg m⁻³ (with an average of 39.3 µg m⁻³). The three major ions were Cl⁻, Na⁺, and SO₄²⁻, contributing 65% of the total water-soluble inorganic ions (TWSII). Na⁺ and Cl⁻ manifested comparable an unimodal distributions dominated in coarse mode, and the Cl⁻/Na⁺ molar ratio was close to that attributed to marine sources. Whilst SO₄²⁻ and methane sulfonic acid (MSA) presented an inverted bimodal distribution mainly concentrated in the fine mode and contributed over 60%, ammonium (NH₄⁺) showed strong and significant Pearson correlations with sulfate in the fine mode (r = 0.88, p-value < 0.01), indicating that NH₄⁺ was an important neutralising agent of SO₄²⁻ leading to the production of (NH₄)₂SO₄ and NH₄HSO₄. The size distributions of the rest of the ions Cl⁻, NO₃⁻, Na⁺, K⁺, Mg²⁺, and Ca²⁺ were unimodal mainly in the coarse mode, while oxalate and NH₄⁺ were unimodal in the fine mode. The average concentrations of OC and EC during the investigated campaign were 4.5 and 1.3 μg m⁻³, respectively. Finally, according to the aerosol chemical composition and backwards trajectory analysis, BouIsmaïl air was affected by long-range air mass transported from the northwest and the west, and local emissions have an important impact on ions and carbonaceous particles in the aerosol of the investigated site.