Contribution of combustion Fe in marine aerosols over the northwestern Pacific estimated by Fe stable isotope ratios

Abstract

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.

Iron (Fe) speciation in size-fractionated aerosol particles in the Pacific Ocean: The role of organic complexation of Fe with humic-like substances in controlling Fe solubility

“…The enrichments of labile Fe and Al in fine fractions have been reported by previous studies (Baker and Jickells, 2006; Fe in fine fractions is the presence of pyrogenic Fe in fine fractions (Kurisu et al, 2021). Oil combustions, including ship emissions, are one of the dominant sources of pyrogenic Fe in fine aerosol particles because several studies have been reported…”

“…However, fractional Fe solubility (Fesol% = (labile Fe/total Fe) × 100) in mineral dust in source regions is usually less than 1.0% because Fe in mineral dust is typically present as insoluble species (e.g., Fe in aluminosilicates and Fe (hydr)oxides). In contrast, a wide range of Fesol% in marine aerosol particles (0.1-90%) has been reported by previous observational studies (Buck et al, 2006;2013, Baker andJickells, 2006;Bakers et al, 2016Bakers et al, , 2021Chance et al, 2015;Kurisu et al, 2021). One of the reasons for high Fesol% in the marine aerosol particles is pyrogenic Fe with high Fesol% (up to 80%, Schroth et al, 2009;Takahashi et al, 2013;Kurisu et al, 2016;Conway et al, 2019).…”

“…HULIS is approximately 5% of total Fe in TSP, which was below the detection limit of XAFS spectroscopy. Previous studies have also suggested the presence of Fe(III)-sulfate as L-Fe species by spot analysis using microscopic XAFS, but Fe(III)-sulfate were not detected by macroscopic XAFS because of lower abundance of the species in TSP (Oakes et al, 2012;Kurisu et al, 2021). Therefore, size-fractionated aerosol sampling is needed to identify L-Fe species in the marine aerosol particles.…”

Iron (Fe) speciation in size-fractionated aerosol particles in the Pacific Ocean: The role of organic complexation of Fe with humic-like substances in controlling Fe solubility

“…In contrast, a wide range of Fe sol % in marine aerosol particles (0.1 %-90 %) has been reported in previous observational studies (Buck et al, 2006(Buck et al, , 2010(Buck et al, , 2013Baker and Jickells, 2006;Bakers et al, 2016Bakers et al, , 2021Chance et al, 2015;Shelley et al, 2018;Kurisu et al, 2021). One of the reasons for the high Fe sol % in marine aerosol particles is pyrogenic Fe with high Fe sol % (up to 80 %, Schroth et al, 2009;Takahashi et al, 2013;Kurisu et al, 2016Kurisu et al, , 2019Kurisu et al, , 2021Conway et al, 2019). It seems that the variation in Fe sol % in marine aerosol particles can be explained by a binary mixing system of mineral dust and anthropogenic aerosols if the Fe sol % of these components at the time of emission is known.…”

“…However, fractional Fe solubility (Fe sol % = (labile Fe / total Fe) × 100) in mineral dust in source regions is usually below 1.0 % because Fe in mineral dust is typically present as insoluble species, e.g., Fe in aluminosilicates and Fe (hydr)oxides. In contrast, a wide range of Fe sol % in marine aerosol particles (0.1 %-90 %) has been reported in previous observational studies (Buck et al, 2006(Buck et al, , 2010(Buck et al, , 2013Baker and Jickells, 2006;Bakers et al, 2016Bakers et al, , 2021Chance et al, 2015;Shelley et al, 2018;Kurisu et al, 2021). One of the reasons for the high Fe sol % in marine aerosol particles is pyrogenic Fe with high Fe sol % (up to 80 %, Schroth et al, 2009;Takahashi et al, 2013;Kurisu et al, 2016Kurisu et al, , 2019Kurisu et al, , 2021Conway et al, 2019).…”