Seasonal and spatial distributions and dry deposition fluxes of atmospheric total and labile iron over the tropical and subtropical North Atlantic Ocean

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Abstract
The tropical and subtropical region of the North Atlantic Ocean is under the influence of mineral dust plumes transported from North Africa, resulting in high atmospheric iron (Fe) deposition. Atmospheric Fe, as a major Fe source to this open oceanic region, may influence the rate of nitrogen fixation since Fe is a critical nutrient cofactor for the nitrogenase enzyme in diazotrophic microorganisms. Field measurements of Fe concentrations and Fe speciation in aerosols provide crucial information to understanding the biological role of this atmospheric Fe flux to the ocean. In this study, 24‐hour aerosol samples were collected during winter (6 January 2001 to 18 February 2001) and summer (27 June 2001 to 15 August 2001) research cruises over this North Atlantic region. Three labile Fe species, that included ferrous iron (Fe(II)) and reducible ferric iron (Fe(III)) species, were measured in aerosol samples using a sequential aqueous extraction method. Microwave digestion of the aerosol samples followed by inductively coupled plasma‐mass spectrometry was used to quantify total elemental concentrations. A spatial gradient of over nearly 3 orders of magnitude in the total Fe concentrations (from 1.6 ng m−3 at 28.6°N to 1688 ng m−3 at 10.9°N) was observed during the winter, while this gradient was not as strong in the summer. The mean total Fe concentrations were approximately a factor of 2 higher during the winter (mean value of 670 ng m−3 between 5°N and 26°N) than in the summer (mean value of 324 ng m−3 between 6°N and 26°N). The highest percentage of labile Fe to total Fe was observed between 26°N and 30°N in the winter with a mean value of 32%, which corresponded to low concentrations of total Fe. At latitude 0°N to 10°N, where the lowest Fe concentrations were observed in the summer, the labile Fe fraction with a mean of 5.0% was similar to the region from 10°N to 20°N where there were much higher mineral aerosol concentrations. Air mass back trajectories showed that mineral dust transport from North Africa is a significant atmospheric Fe source in this Atlantic region. However, the highest labile Fe to total Fe ratios were observed in air masses that had been over the ocean for greater than 7 days and also corresponded to high ratios of non‐seasalt‐sulfate (NSS‐sulfate) to total Fe and oxalate to total Fe. The correlation with NSS‐sulfate and oxalate suggests that labile Fe concentrations may have been influenced by anthropogenic activities from North America or Europe.