Aerosol Inorganic Ions in a Semiarid Region on the Southeastern Spanish Mediterranean Coast

Abstract

PM2.5 and PM10 samples were collected in the urban atmosphere of Elche (southeastern Spain) between December 2004 and November 2005. The samples were analyzed for mass and water-soluble inorganic ions (Na⁺, ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$, K⁺, Ca²⁺, Mg²⁺, Cl⁻, ${\text{NO}}^{{\text{ - }}}_{{\text{3}}}$ and ${\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}}$) with the aim of investigating the influence of the climatic and geographic features of a coastal semiarid area on the contribution of these species to PM levels. Secondary inorganic ions (${\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}}$, ${\text{NO}}^{{\text{ - }}}_{{\text{3}}}$, ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$) were the major components in the fine fraction (PM2.5), accounting for 40% of the total mass. The relationship between non-marine ${\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}}$ and ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$ indicated that fine sulfate particles were completely neutralized by ammonium. In the coarse fraction (PM10–2.5), nitrate (as NaNO₃ and Ca(NO₃)₂), together with crustal (CaCO₃) and marine species (NaCl) accounted for almost 50% of the total mass. Fine sulfate and coarse nitrate showed summer maximums. In contrast, the concentrations of fine ${\text{NO}}^{{\text{ - }}}_{{\text{3}}}$ were lowest in the warm period. Ammonium presented both winter and summer maximums. The levels of marine ions, except for coarse Cl⁻, were highest in summer when the dominant wind flow is from the sea. No significant seasonal variations were observed for coarse Ca²⁺ and ${\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}}$. The concentrations of all inorganic ions increased during Saharan dust events, in particular, fine ${\text{NH}}^{{\text{ + }}}_{{\text{4}}}$ and ${\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}}$ and coarse $NO_3^ -$. Coarse calcium was proved not to be a good tracer for this type of episode in our region since the average levels of this cation are elevated and the relative increase in its concentrations during African events was not as high as expected.