Seven size‐fractionated aerosol samples were collected from Hiroshima, Japan, and were analyzed in terms of chemical composition, soluble fraction of iron (Fe), Fe species, and Fe isotope ratios. The results suggested that Fe in fine particles contained a larger fraction of anthropogenic aerosols than coarse particles did. Iron in the fine particles was more soluble in simulated seawater (up to 25%) than that in the coarse particles and was in the form of Fe (hydr)oxide species, such as ferrihydrite or hematite. The Fe isotope ratios (δ56Fe) of the coarse particles (+0.04‰ to +0.30‰) were close to the crustal mean value (0.0‰). By contrast, the δ56Fe values of fine particles were much lower and ranged from −2.01‰ to −0.56‰. δ56Fe values of the soluble Fe fraction in the fine particles were remarkably low (−3.91 to −1.87‰), suggesting that anthropogenic aerosols yield soluble Fe with low δ56Fe values. Such low values could be explained by kinetic isotope fractionation during evaporation of Fe at high temperatures, coupled with the refractory characteristics of Fe. Marine aerosols from the Northwest Pacific were also analyzed. The δ56Fe values in the fine particles were also lower than those in the coarse particles. These results may be important to quantitatively estimate the contribution of anthropogenic Fe deposited on the surface ocean on the basis of the Fe isotopes.

Iron isotope ratios (δ 56/54 Fe) of various anthropogenic materials produced by combustion processes were analyzed. It was revealed that particles emitted through combustion processes, such as fine aerosols collected from a tunnel or fly ash (extracted by 1.0 M HCl solution), yielded much lower isotope ratios than other materials on the earth's surface environment. These results suggest that the low δ 56/54 Fe was caused by human activities such as combustion processes. Keywords: Iron isotope ratio | Anthropogenic aerosol | Combustion processIron (Fe) has been regarded as a highly important micronutrient to the growth of phytoplankton in open ocean, or more specifically in high-nutrient low-chlorophyll (HNLC) regions.1 The marine primary productivity stimulated by the supply of Fe is closely related to the sequestration of CO 2 from the atmosphere to the surface ocean.1,2 After a pioneering work by Martin and his coworkers, 3,4 many studies have been conducted regarding the source of Fe to HNLC regions. In particular, Fe supply via atmosphere as aerosols has been considered as a predominant process. Three sources of Fe are mainly suggested for the aerosols such as (i) mineral dust from arid areas, 2 (ii) volcanic origins, 5 and (iii) anthropogenic emissions, 6,7 but the relative contributions of the three components are not clear at present.In this study, we focused on Fe in anthropogenic aerosols emitted by human activities. As pointed out above, it is necessary to quantify its contribution for the precise estimation of soluble Fe to HNLC regions and its effect on climate change both in the past and in future. Fe values in non-dust aerosols is not studied so far. Hence, we need a systematic survey of δ 56/54 Fe values in various materials that can be incorporated into non-dust aerosols in environment. In this study, we measured δ 56/54Fe values in anthropogenic materials mainly collected in Hiroshima, Japan. We here found the variation of δ 56/54 Fe values in the materials, which suggests that low δ 56/54 Fe values can reflect the influence of human activities. Metal elements with sufficient volatility can be incorporated into aerosols, 10 possibly through combustion processes mainly related to human activities. If the combustion is responsible for the fractionation of Fe isotope, possible sources can be (i) exhaust gas from automobiles, 11,12 (ii) municipal solid waste incinerator, 11,13 and (iii) industrial activities such as steel manufacturing. 11,12 In this study, we focused on (i) aerosols collected in a tunnel to examine isotope fractionation for Fe emitted from automobiles 14 and (ii) residual bottom ash and fly ash in an incinerator. For sample (i), 9 size-fractionated aerosol samples were collected using an Andersentype air sampler (AN-200, Sibata, Tokyo) at the side of a roadway (National Route 185) in Yasumiyama Tunnel (YT; 34.2450°N, 132.5853°E; entire length: 1706 m) in Hiroshima, Japan. Sizefractionated sampling was adopted here to distinguish natural and anthropogenic particles. 15 A...

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Abstract. The source apportionment of aerosol iron (Fe), including natural and combustion Fe, is an important issue because aerosol Fe can enhance oceanic primary production in the surface ocean. Based on our previous finding that combustion Fe emitted by evaporation processes has Fe isotope ratios (δ56Fe) that are approximately 4 ‰ lower than those of natural Fe, this study aimed to distinguish aerosol Fe sources over the northwestern Pacific using two size-fractionated marine aerosols. The δ56Fe values of fine and coarse particles from the eastern or northern Pacific were found to be similar to each other, ranging from 0.0 ‰ to 0.4 ‰. Most of them were close to the crustal average, suggesting the dominance of natural Fe. On the other hand, particles from the direction of East Asia demonstrated lower δ56Fe values in fine particles (−0.5 ‰ to −2.2 ‰) than in coarse particles (on average −0.02 ± 0.12 ‰). The correlations between the δ56Fe values and the enrichment factors of lead and vanadium suggested that the low δ56Fe values obtained were due to the presence of combustion Fe. The δ56Fe values of the soluble component of fine particles in this region were lower than the total, indicating the preferential dissolution of combustion Fe. In addition, we found a negative correlation between the δ56Fe value and the fractional Fe solubility in air masses from the direction of East Asia. These results suggest that the presence of combustion Fe is an important factor in controlling the fractional Fe solubility in air masses from the direction of East Asia, whereas other factors are more important in the other areas. By assuming typical δ56Fe values for combustion and natural Fe, the contribution of combustion Fe to the total (acid-digested) Fe in aerosols was estimated to reach up to 50 % of fine and 21 % of bulk (coarse + fine) particles in air masses from the direction of East Asia, whereas its contribution was small in the other areas. The contribution of combustion Fe to the soluble Fe component estimated for one sample was approximately twice as large as the total, indicating the importance of combustion Fe as a soluble Fe source despite lower emissions than the natural. These isotope-based estimates were compared with those estimated using an atmospheric chemical transport model (IMPACT), in which the fractions of combustion Fe in fine particles, especially in air masses from the direction of East Asia, were consistent with each other. In contrast, the model estimated a relatively large contribution from combustion Fe in coarse particles, probably because of the different characteristics of combustion Fe that are included in the model calculation and the isotope-based estimation. This highlights the importance of observational data on δ56Fe for size-fractionated aerosols to scale the combustion Fe emission by the model. The average deposition fluxes of soluble Fe to the surface ocean were 1.4 and 2.9 nmol m−2 d−1 from combustion and natural aerosols, respectively, in air masses from the direction of East Asia, which suggests that combustion Fe could be an important Fe source to the surface seawater among other Fe sources. Distinguishing Fe sources using the δ56Fe values of marine aerosols and seawater is anticipated to lead to a more quantitative understanding of the Fe cycle in the atmosphere and surface ocean.