3:45 PM - 4:00 PM
[ACG46-02] Estimation of the Fe isotope ratio of combustion Fe originated from East Asia using aerosols collected in Fukue Island
Keywords:Aerosol, Combustion Fe, Fe stable isotope ratio, East Asia
Atmospheric aerosol is one of the main sources of iron (Fe) to the surface ocean where primary production is limited by the deficiency of dissolved Fe (Martin and Fitzwater, 1988; Jickells et al., 2005). While natural Fe in aerosols, such as mineral dust, is considered to be an important source of Fe to the surface ocean due to its large amount of emission, combustion Fe emitted by anthropogenic activities can be another important source of Fe due to its high solubility to seawater (Sedwick et al., 2009; Ito et al., 2021). However, there are few examples of observational estimates of the contribution of each source of Fe to the surface ocean.
The stable isotope ratio of Fe (δ56Fe) is a useful tracer to distinguish Fe sources. Our previous studies revealed that δ56Fe of combustion Fe (-4.3±0.4‰) is much lower than natural Fe (0.0‰), which can be applied to the estimation of the contribution of different sources of Fe in marine aerosols (Kurisu et al., 2019; 2021). However, there are still few reports of δ56Fe of combustion Fe. In this study, we focused on seasonal changes in characteristics of δ56Fe in aerosols from East Asia and aimed to discuss differences in typical δ56Fe values depending on emission sources and regions.
The aerosol samples were collected every five days from November 2019 to April 2020 on Fukue Island in Nagasaki Prefecture, where aerosols often originate from East Asia. The aerosol particles were collected on filters and were separated into two fractions at 2.5 μm. Trace metal concentrations were measured by ICP mass spectrometry after decomposition of the filters using mixed acid. Iron stable isotope analysis was carried out using a multi-collector ICP mass spectrometry after Fe separation with an anion exchange resin. We applied the double spike method for the isotope analysis.
Based on backward trajectory analyses, the air masses originated from the northwest or north. The atmospheric mass concentrations of aerosols increased in March and April, and the atmospheric concentrations of Fe and titanium (Ti), prevalent in crustal materials, similarly increased, indicating the contribution of mineral dust in East Asia. On the other hand, there was no significant change in the atmospheric concentrations of lead and nickel, common in combustion aerosols, whereas the enrichment factor (EF = (M/Ti) sample/(M/Ti) crust, M: target element) increased from November to February, indicating that the influence of combustion aerosols was relatively large in this season. The average δ56Fe of coarse particles was 0.07±0.17‰ (2SD), similar to the crustal average for all the samples, while the δ56Fe of fine particles showed a gradual increase in the range of 1.77‰ to 0.10‰ from November to April (on average -0.67±0.88‰, 2SD). The δ56Fe of fine particles negatively correlated with the EF of lead and the inverse of the Fe concentration, indicating that combustion Fe collected in this study had an almost constant δ56Fe regardless of the emission source and season and that the measured δ56Fe values were determined by the mixing of the two components (i.e., natural and combustion Fe). The δ56Fe of the end-member (combustion Fe) was estimated to be approximately 3.5‰, which was higher than that estimated from aerosols collected in Japan (-4.3‰). This is possibly due to differences in the main emission sources or temperatures during the emission of combustion aerosols depending on regions, suggesting that this difference should be taken into account to estimate the contribution of combustion Fe.
The stable isotope ratio of Fe (δ56Fe) is a useful tracer to distinguish Fe sources. Our previous studies revealed that δ56Fe of combustion Fe (-4.3±0.4‰) is much lower than natural Fe (0.0‰), which can be applied to the estimation of the contribution of different sources of Fe in marine aerosols (Kurisu et al., 2019; 2021). However, there are still few reports of δ56Fe of combustion Fe. In this study, we focused on seasonal changes in characteristics of δ56Fe in aerosols from East Asia and aimed to discuss differences in typical δ56Fe values depending on emission sources and regions.
The aerosol samples were collected every five days from November 2019 to April 2020 on Fukue Island in Nagasaki Prefecture, where aerosols often originate from East Asia. The aerosol particles were collected on filters and were separated into two fractions at 2.5 μm. Trace metal concentrations were measured by ICP mass spectrometry after decomposition of the filters using mixed acid. Iron stable isotope analysis was carried out using a multi-collector ICP mass spectrometry after Fe separation with an anion exchange resin. We applied the double spike method for the isotope analysis.
Based on backward trajectory analyses, the air masses originated from the northwest or north. The atmospheric mass concentrations of aerosols increased in March and April, and the atmospheric concentrations of Fe and titanium (Ti), prevalent in crustal materials, similarly increased, indicating the contribution of mineral dust in East Asia. On the other hand, there was no significant change in the atmospheric concentrations of lead and nickel, common in combustion aerosols, whereas the enrichment factor (EF = (M/Ti) sample/(M/Ti) crust, M: target element) increased from November to February, indicating that the influence of combustion aerosols was relatively large in this season. The average δ56Fe of coarse particles was 0.07±0.17‰ (2SD), similar to the crustal average for all the samples, while the δ56Fe of fine particles showed a gradual increase in the range of 1.77‰ to 0.10‰ from November to April (on average -0.67±0.88‰, 2SD). The δ56Fe of fine particles negatively correlated with the EF of lead and the inverse of the Fe concentration, indicating that combustion Fe collected in this study had an almost constant δ56Fe regardless of the emission source and season and that the measured δ56Fe values were determined by the mixing of the two components (i.e., natural and combustion Fe). The δ56Fe of the end-member (combustion Fe) was estimated to be approximately 3.5‰, which was higher than that estimated from aerosols collected in Japan (-4.3‰). This is possibly due to differences in the main emission sources or temperatures during the emission of combustion aerosols depending on regions, suggesting that this difference should be taken into account to estimate the contribution of combustion Fe.