Stable Isotope Ratios of Combustion Iron Produced by Evaporation in a Steel Plant

Combustion iron (Fe) in aerosols is one of the sources of dissolved Fe in the surface ocean. The iron isotope ratio (delta Fe-56) is an important tool for source apportionment of Fe because combustion Fe emitted by evaporation possibly yields lower delta Fe-56 values than natural materials. However, there are insufficient data of delta Fe-56 for combustion Fe. Hence, delta Fe-5(6) values of Fe emitted from a steel plant were investigated, and the representative delta Fe-56 value of combustion Fe was discussed. The presence of a large number of submicron spherical Fe oxide particles suggested that the particles were emitted by high-temperature evaporation. Fine particles yielded much lower delta Fe-56 (as low as -3.53 parts per thousand) than original materials, indicating that Fe isotope fractionation occurred during evaporation. Based on this study and our previous data, we suggest -3.9 parts per thousand to -4.7 parts per thousand as the representative delta Fe-56 range of combustion Fe. Mass balance calculations using this range suggest that the contribution of combustion Fe is approximately 57-83% of the total soluble Fe in aerosols in the northwest Pacific, implying a large contribution of combustion Fe. The remarkably low delta Fe-56 value of combustion Fe emitted by evaporation enables us to evaluate its contribution to marine aerosols and to understand Fe cycles in the surface ocean.