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 (δ 56 Fe) is an important tool for source apportionment of Fe because combustion Fe emitted by evaporation possibly yields lower δ 56 Fe values than natural materials. However, there are insufficient data of δ 56 Fe for combustion Fe. Hence, δ 56 Fe values of Fe emitted from a steel plant were investigated, and the representative δ 56 Fe 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 hightemperature evaporation. Fine particles yielded much lower δ 56 Fe (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 δ 56 Fe 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 δ 56 Fe 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.

Testing Iron Stable Isotope Ratios as a Signature of Biomass Burning

“…Our previous studies analyzed Fe isotope ratios of size-fractionated aerosol particles near anthropogenic emission sources, which were collected in a tunnel, from an incinerator, and near a steel plant. We found that δ 56 Fe values of Fe emitted during the combustion processes ranged from −1‰ to −4‰ [17][18][19], which were much lower than those of terrestrial igneous rocks (0.00 ± 0.10‰, [20]) and other natural materials [21]. The remarkably low δ 56 Fe values were possibly caused by isotope fractionation during evaporation under high-temperature condition.…”

“…Even if all K at stage 6 of the 1 km site is of reed origin, the evaporated Fe concentration is less than 0.32 ng/m 3 , which is approximately 1% of total Fe at stage 6 (30 ng/m 3 ). Assuming that (i) the δ 56 Fe value of evaporated Fe fraction is as low as −4.7‰ based on our previous studies [17][18][19], and (ii) the δ 56 Fe value of the other Fe sources is 0.0‰, the δ 56 Fe value of the aerosol sample becomes −0.047‰, which cannot explain the observed δ 56 Fe value (−0.36 ± 0.08‰). Therefore, evaporation of Fe during the burning event was not a possible Fe source for the observed δ 56 Fe value.…”

“…All equipment was preliminarily washed with acid. Aerosol samples were digested with mixed acids (HNO 3 , HCl, and HF, Tamapure AA-100, Tama Chemicals, Kawasaki, Japan), details of which are described in Kurisu et al [19]. Concentrations of these elements in washed PTFE and unwashed PF050 were subtracted from their concentrations in each sample.…”

“…Spot analysis of aerosol samples was also conducted by µ-XRF-XAFS at the BL-4A of PF. Methods were the same as those reported by Kurisu et al [19].…”

“…Reed and residual ash were not possible sources for the low δ 56 Fe values because their δ 56 Fe values were close to 0‰. The evaporation of Fe during the burning event could be a reason for the low δ 56 Fe values, if Fe isotope fractionation occurred during evaporation [17][18][19]. We calculated the possible amount of Fe evaporated by the burning event on the basis of the concentrations of Fe and K in aerosols and reed.…”