To characterize atmospheric particulate matter equal or less than 2.5 µm in diameter (PM2.5) over the Tropical Atlantic Ocean, aerosol sampling was carried out at Cape San Juan, Puerto Rico during August and September, 2006. Aerosol particles were analyzed by ion chromatography for water soluble inorganic and organic ions (Na+, NH4+, Mg2+, Ca2+, K+, Cl-, SO42-, NH4+, F-, methanesulfonate (MSA), and oxalate), by induced coupled plasma mass spectrometry (ICPMS) for trace elements (Al, Cr, Cd, Co, Cu, Fe, Mn, Ni, Pb, Sb, Sc, V, and Zn), and by scanning electron microscopy for individual particle composition and morphology. The results show that dominant cations were: sodium (631 ng m-3) and ammonium (164 ng m-3), accounting for 64% and 14% of the total cation concentration, respectively. Main inorganic anions were: chloride (576 ng m-3, 54%) and sulfate (596 ng m-3, 38%), and the key organic anion was oxalate (18 ng m-3, 0.1%). Non-sea salt sulfate (316 ng m-3) dominated in days with volcanic influences, accounting for 68% - 91% of the total sulfate; these fine sulfate particles may serve as cloud condensation nuclei (CCN) and influence the radiation balance. The crustal enrichment factor calculations identified 62% of the selected trace elements (Al, Cr, Co, Cu, Fe, Mn, Ni, Sc, and V) with crustal origin. Al (144 ng m-3) and Fe (76 ng m-3) were the major elements, accounting for 63% and 34% of the total trace elements concentrations, respectively. Single particle analysis demonstrated that 40% of the total aerosol particles examined was Cl- rich particles in the form of sodium chloride from seawaters and 34% of the total particles were Si- rich particles, mainly in the form of aluminosilicates from soil dust. About 42% of the total aerosol particles examined by SEM were under 0.8 µm in diameter in size, suggesting their potential roles in serving as CNN and light scatters. Based on the combination of air-mass back trajectories, cluster analysis and principal component analysis, the major sources of these PM2.5 particles include marine, volcanic emissions from the Soufriere Hills in Montserrat, Saharan dust and biomass burning from West Africa, in addition to possible anthropogenic influences from northern continental sources.