Chemical composition of aerosols collected over the tropical North Atlantic Ocean

Abstract

Ambient aerosol samples were collected over the tropical northern Atlantic Ocean during the month of April 1996 onboard the R/V Seward Johnson. Dichotomous high-volume collector samples were analyzed for ferrous iron immediately after collection, while trace metals, anions, and cations were determined upon return to the laboratory. Data are analyzed with the aid of enrichment factor, principal component, and weighted multiple linear regression analyses. Average mineral aerosol concentrations amounted to 19.3±16.4 μg m^(−3) whereby the chemical characteristics and air mass back trajectories indicated the dust to be of a typical shale composition and Saharan origin. Calcite accounted for 3.0 and 7.9% of the mineral aerosol during the first and second halves of the cruise, respectively. Total iron concentrations (averaging 0.84±0.61 μg 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 μm diameter) aerosol fraction and correlates with NSS-SO_4^(2−) and oxalate. Approximately 23% of the measured NSS-SO_4^(2−) in both size fractions appears to be biogenically derived, and the rest is of anthropogenic nature. Biogenic SO_4^(2−) /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 characteristics of the aerosol chemistry and air temperature during the cruise. Because of the presence of anthropogenic SO_4^(2−), the non-sea-salt (NSS)- SO_4^(2−)/MSA ratio, 37.4±6.4, is elevated over what would be expected if the NSS - SO_4^(2−) were purely biogenic. Cl^− depletion is seen in all samples and averages 18.3±9.1%. The release of Cl from the aerosol phase appears to occur through acid displacement reactions with primarily HNO_3 in the coarse and H_2SO_4 in the fine fraction.