Ionic and elemental composition of PM_2.5 aerosols over the Caribbean Sea in the Tropical Atlantic

To characterize atmospheric particulate matter equal or less than 2.5 μm in diameter (PM_2.5) over the Tropical Atlantic Ocean, aerosol sampling was carried out in Puerto Rico during August and September, 2006. Aerosols were analyzed by ion chromatography for water-soluble inorganic and organic ions (including Na⁺, NH₄ ⁺, Mg²⁺, Ca²⁺, K⁺, Cl⁻, SO₄ ²⁻, NH₄ ⁺, F⁻, methanesulfonate (MSA), and oxalate), by inductive coupled plasma mass spectrometry (ICPMS) for trace elements (Al, Fe, Zn, Mn, Cu, Ni, V, Pb, Cr, Sb, Co, Sc, Cd), and by scanning electron microscopy for individual aerosol particle composition and morphology. The results show that the dominant cations in aerosols were Na⁺, (mean: 631 ng m⁻³), accounting for 63.8 % of the total cation and NH₄ ⁺ (mean: 164 ng m⁻³), accounting for 13.8 % of the total cation measured in this study. The main inorganic anions were Cl⁻ (576 ng m⁻³, 54.1 %) and SO₄ ²⁻ (596 ng m⁻³, 38.0 %). The main organic anion was oxalate (18 ng m⁻³). Crustal enrichment factor calculations identified 62 % of the trace elements measured (Cu, Ni, V, Co, Al, Mn, Fe, Sc, and Cr) with crustal origin. Single particle analysis demonstrated that 40 % of the aerosol particles examined were Cl⁻ rich particles as sodium chloride from seawater and 34 % of the total particles were Si-rich particles, mainly in the form of aluminosilicates from dust material. Based on the combination of air-mass trajectories, cluster analysis and principal component analysis, the major sources of these PM_2.5 particles include marine, Saharan dust and biomass burning from West Africa; however, volcanic emissions from the Soufriere Hills in Montserrat had significant impact on aerosol composition in this region at the time of sample collection.