Chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the Arabian Sea: Labile-Fe(II) and other trace metals

Atmospheric deposition of iron (Fe) to certain regions of the oceans is an important nutrient source of Fe to the biota, and the ability of the biota to uptake Fe is dependent on the speciation of the Fe. Therefore understand ing the speciation of Fe in the atmosphere is critical to understanding the role of Fe as a nutrient source in surface ocean waters. Labile ferrous ir on (Fe(II)) concentrations as well as total concentrations for Fe and other important trace metals, cations, and anions were determined over the Arabi an Sea for two nonconsecutive months during 1995. Ambient aerosol samples w ere collected during the Indian Ocean intermonsoon and southwest monsoon se asons over the Arabian Sea. Sampling took place aboard the German research vessel Meteor in the months of May (leg M32/3; intermonsoon) and July/Augus t (leg M32/5; southwest monsoon). Both cruise tracks followed the 65th east meridian, traveling for 30 days each (from north to south during leg M32/3 and from south to north during leg M32/5). A high-volume dichotomous virtu al impactor with an aerodynamic cutoff size of 3 mu m was used to collect t he fine and coarse aerosol:Fractions for metal analysis. A low volume colle ctor was used to collect aerosol samples for anion and cation analysis. The analysis for labile-Fe(II) was done immediately after sample collection to minimize any possible Fe redox reactions which might occur during sample s torage. The analytical procedure involved filter extraction in a formate/fo rmic acid buffered solution at pH 4.2 followed by colorimetric quantificati on of soluble Fe(II). Metals, anions, and cations were analyzed after the c ruise. Total atmospheric aqueous-labile-Fe(II) concentrations during the in termonsoon were between 4.75 and <0.4 ng m(-3), of which most (>80%) was pr esent in the fine fraction (<3.0 mu m). During the southwest monsoon, atmos pheric aqueous-labile-Fe(II)) concentrations were consistently below the de tection limit (<0.34 to <0.089 ng m(-3) depending on the volume of air samp led). Air mass back trajectories (5 day, three dimensional) showed that air masses sampled during the southwest monsoon had advected over the open Ind ian Ocean, while air masses sampled during the intermonsoon had advected ov er northeast Africa, the Saudi Arabian peninsula, and southern Asia. These calculations were consistent with the results of the statistical analysis p erformed on the data set which showed that the variance due to crustal spec ies during the intermonsoon samples was greater than the variance due to cr ustal species during the southwest monsoon. The factor scores for the crust al components were also greater when the back trajectories had advected ove r the nearby continental masses. Principal component analysis was also perf ormed with the intermonsoon samples where aqueous labile Fe(II) was above t he detection limit. Aqueous labile Fe(II) did not correlate well with other species indicating possible atmospheric processing of the iron during adve ction.