Coupling and feedback between iron and sulphur in air-sea exchange

Abstract

Iron in surface seawater has been demonstrated to be the limiting nutrient factor for primary productivity in certain oceanic regions. In 1992 we reported inNature at first Fe(II) existence in marine aerosols over the remote Pacific and Atlantic and proposed the hypothesis of coupling and feedback between iron and sulfur in air-sea exchange. We recently detected a considerable amount of Fe(II) in the aerosols collected during dust storm in Beijing. The concentration of Fe(II) in the dust storm was as high as 1.8–4.3 g · m−3 that accounted for 1.4% –2.6% of the total Fe mass concentration in aerosols, while Fe(II) in the aerosols collected from non-dust storm days was 0.7% of the total Fe. Fe and S concentrations in the dust storm aerosol samples represent a very positive correlationship. The size distribution of Fe, Fe(II), and S in either dust storm particles or the normal aerosols showed the same peak at 1–3 μm. The oxidation of S(IV) under natural light and UV in addition of Fe(III) was 6.5 and 14 times faster respectively compared with that of non-addition. The photoreduction of Fe(□) to Fe(II), through the formation of an OH radical, associated with the oxidation of S(IV) to S(VI) could account for ∼3%–20% of the formation of non-sea salt sulfate aerosols at Midway, central Pacific. These results further support the Fe-S coupling hypothesis. From consideration of both the marine biological production of dimethylsulphide (DMS) and the subsequent oxidation of reduced forms of sulfur in the atmosphere, the iron and sulphur cycles in both the atmosphere and the ocean may be closely coupled.