Chemical composition of aerosols collected over the tropical North Atlantic Ocean

Ambient aerosol samples were collected over the tropical northern Atlantic Ocean during the month of April 1996 onboard the R/V Seward Johnson. Dichot omous high-volume collector samples were analyzed for ferrous iron immediat ely after collection, while trace metals, anions, and cations were determin ed upon return to the laboratory. Data are analyzed with the aid of enrichm ent factor, principal component, and weighted multiple linear regression an alyses. Average mineral aerosol concentrations amounted to 19.3 +/- 16.4 mu g m(-3) whereby the chemical characteristics and air mass back trajectorie s indicated the dust to be of a typical shale composition and Saharan origi n. Calcite accounted for 3.0 and 7.9% of the mineral aerosol during the fir st and second halves of the cruise, respectively. Total iron concentrations (averaging 0.83 +/- 0.61 mu g m(-3)) are crustally derived, of which 0.51 +/- 0.56% is readily released as Fe(II). Eighty-six percent of this Fe(II) is present in the fine (<3 mu m diameter) aerosol fraction and correlates w ith NSS-SO42- and oxalate. Approximately 23% of the measured NSS-SO42- in b oth size fractions appears to be biogenically derived, and the rest is of a nthropogenic nature. Biogenic SO42-/methanesulfonic acid (MSA) ratios could not be easily extracted by employing a multiple linear regression analysis analogous to that of Johansen et al. [1999], possibly due to the varying c haracteristics of the aerosol chemistry and air temperature during the crui se. Because of the presence of anthropogenic SO42-, the non-sea-salt (NSS)- SO42-/MSA ratio, 37.4 +/- 16.3, is elevated over what would be expected if the NSS-SO42- were purely biogenic, Cl- depletion is seen in all samples an d averages 18.3 +/- 9.1%. The release of Cl from the aerosol phase appears to occur through acid displacement reactions with primarily HNO3 in the coa rse and H2SO4 in the fine fraction.