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

Abstract

Combustion iron (Fe) in aerosols is one of the sources of dissolved Fe in the surface ocean. The iron isotope ratio (δ56Fe) is an important tool for source apportionment of Fe because combustion Fe emitted by evaporation possibly yields lower δ56Fe values than natural materials. However, there are insufficient data of δ56Fe for combustion Fe. Hence, δ56Fe values of Fe emitted from a steel plant were investigated, and the representative δ56Fe 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 δ56Fe (as low as −3.53‰) than original materials, indicating that Fe isotope fractionation occurred during evaporation. Based on this study and our previous data, we suggest −3.9‰ to −4.7‰ as the representative δ56Fe 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 δ56Fe 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.