Diel variability of soluble Fe(II) and soluble total Fe in North African dust in the trade winds at Barbados

There is increased interest in the deposition of wind-transported mineral dust to the oceans because of the possible role that iron might play as a limiting nutrient in primary productivity. It has been suggested that photochemical processes taking place on mineral particles suspended in the atmosphere could reduce large fractions of the Fe(III) to Fe(II) and thereby increase the soluble fraction of iron; when the particle is deposited in the ocean, the soluble iron would be more readily available to chemical and biological processes. In this study we collected mineral aerosol (carried by the trade winds from North African sources) al Barbados over a diel cycle; we immediately measured Fe(II) and total soluble Fe in filter extracts using as the extractant an acidic saline solution comparable to that expected to be present on mineral aerosols in the marine atmospheric boundary layer. The concentration of mineral dust was quite variable during the experiment, ranging from 1.99 to 18.4 mu g/m(3) (mean, 6.59 mu g/m(3)); Fe constituted on average 3.4% of the mineral dust mass. On average, 6.2% of the Fe in the dust was extracted in our process. The concentration of total dissolved Fe varied within a relatively narrow range, closely tracking the variations in dust concentration with no apparent day/night pattern. In contrast, there was a clear day/night pattern in the concentration of soluble Fe(II), with day values (mean 3.7 ng m(-3)) about twice night values (1.5 ng m m(-3)); expressed as a fraction of total dissolved Fe, Fe(II) ranged from 5.1% to 55% in the entire (day and night) data set (mean, 25%). Thus soluble Fe(II) constitutes only a small fraction (mean 1.6%) of the total Fe in the mineral aerosol. Furthermore, the Fe(II) concentration varied independently of the concentration of soluble Fe. The well-defined diel cycle that we observed demonstrates that photochemical processing of Fe does take place; the relatively small dissolved Fe fractions and the absence of a relationship between Fe(II) and total dissolved Fe suggest, however, that the photochemical processes in mineral aerosols do not have a substantial effect on the total Fe solubility.