The iron isotopic composition and the elemental ratios (Cd/Ti) in the size-fractionated aerosols collected over the East China Sea
Data files
Sep 12, 2024 version files 4.80 KB
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README.md
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Table_2.csv
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Table_Cd_to_Ti_ratio.csv
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Table_S2.csv
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Table_S3.csv
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Abstract
Aerosol deposition is one of the major processes providing bioavailable Fe to the surface ocean. However, the quantification of aerosol Fe flux in the surface ocean is highly challenging operationally. In this study, we measured both Fe isotopic composition and specific elemental ratios in 5 size-fraction aerosols collected over the East China Sea (ECS) to quantify the relative contribution of lithogenic and anthropogenic aerosol Fe. Both the isotopic and elemental ratios indicate that anthropogenic aerosol Fe mainly originates from high temperature combustion activities, with the end member of the δ56Fe to be -4.5 ‰. We found that the Cd/Ti ratio is a much more reliable proxy to quantify the contribution of anthropogenic aerosol Fe in coarse aerosols than δ56Fe in the ECS. Attributed to extremely high deposition velocities and low solubilities for large size aerosols, lithogenic aerosols are still the dominant dissolved aerosol Fe source in the ECS.
README: The iron isotopic composition and the elemental ratios (Cd/Ti) in the size-fractionated aerosols collected over the East China Sea
The files (Table 2, Table S2, Table Cd to Ti ratio, and Table S3) shown in the following link are the key data set for the manuscript submitted to Journal of Geophysical Research: Oceans.
The manuscript entitled, "Contribution of anthropogenic and lithogenic aerosol Fe in the East China Sea" are written by Chih-Chiang Hsieh and Tung-Yuan Ho.
https://doi.org/10.5061/dryad.rv15dv4h3
Description of the data and file structure
The details of the dataset contents and the experimental procedures for obtaining the data are described in the manuscript.
The file of Table 2 is the data of the Fe isotopic compositions of total, ultrapure water leached, and buffer leached of the size-fractionated aerosol samples collected in the East China Sea. The file of Table S2 is the contribution (%) of anthropogenic aerosol Fe of size-fractionated aerosols in ultrapure, buffer, and total pools estimated by Fe isotopic composition.
The file of Table Cd to Ti ratio is the elemental data of Cd to Ti molar ratios in total digested samples of the size-fractionated aerosols collected in the East China Sea. The file of Table S3 stands for the contribution (%) of anthropogenic aerosol Fe of size-fractionated aerosols in bulk fraction estimated by Cd to Ti ratios. The note of n.a. stands for 'data not available'.
Methods
Method
Sampling site and procedures
The aerosol samples were collected in a small islet in the ECS, Pengjia islet (PJ, 1.1 km2, 25.63°N, 122.08°E), which is located at 66 km from the northernmost of Taiwan (Fig. 1). There are only about 30 governmental staff stayed on the volcanic islet for meteorological observation and coastal guarding. Based on aerosol optical depth data, PJ receives aerosols from the north during the northeast monsoon season and from the south during the southwest monsoon season (Fig. 1). This makes it an ideal time series sampling site to investigate the impact of East Asian aerosols on the surface water of the Northwestern Pacific Ocean (NWPO). Size-fractionated aerosol samples were collected by polytetrafluoroethylene filters (TE-230-PTFE, Tisch Environmental Inc., US) installed on the high volume aerosol sampler (TISCH Environmental Inc., US, MODEL-TE-5170) and coupled with a cascade impactor (TISCH Environmental Inc., US, Series 235). The cascade impactor separated aerosols into five size fractions, including stage 1 (>7.3 mm), stage 2 (3.1-7.3 mm), stage 3 (1.6-3.1 mm), stage 4 (1.0-1.6 mm), and stage 5 (0.57-1.0 mm). The stage 5 and stage 1-4 aerosols are referred to as fine and coarse aerosols in this study, respectively. The collection time for one single filter generally lasted for 7-8 days each month from September 2019 to August 2020 to ensure sufficient mass for isotopic composition analysis in different treatments. Detailed sampling information, including sampling dates and volumes, is shown in Table S1. The filters with aerosol samples were freeze-dried and weighed after collection. The aerosol samples were then stored at -20 °C freezer prior to further chemical processing.
Quantification of dissolved and total concentrations, and isotopic composition
All sample pretreated procedures were carried out in an ISO Class 4 positive pressure cleanroom by operators wearing powder-free polyvinyl chloride (PVC) gloves. We followed the suggested protocols of GEOTRACES Cookbook to clean sample vials (Cutter et al., 2017). Regarding the dissolved fraction, we conducted two different leaching protocols, including instantaneous ultrapure water leached (ultrapure water) and acetate buffer leached (buffer) treatments. The ultrapure water Fe was obtained by passing ultrapure water through 0.2 mm filters instantaneously (Buck et al., 2006; Morton et al., 2013). For buffer leached Fe (buffer Fe), an ammonia acetate buffer solution at pH 4.7 was used to mimic aerosol metals dissolution processes by rainwater (Baker and Jickells, 2006; Sarthou et al., 2003) or short period complexation processes by seawater ligands (Perron et al., 2020). For total digestion (referred as ‘total’ hereafter), samples were heated for 4 h at 120°C (heater temperature) in 2 ml of a freshly prepared mixed solution containing 4M HF, 4M HCl, and 4M HNO3 (Eggimann and Betzer, 1976). Filter blanks for each treatment were obtained by subjecting new filters to the same leaching procedures as the samples. In over 90% of the cases, the sample concentrations were at least two orders of magnitude higher than the concentrations of the blanks, and in all cases, the samples were at least one order of magnitude higher than the blank value. The detailed information about the pretreatment procedures, analytical method, blank test, and the validation of precision and accuracy were described in our previous studies (Hsieh et al., 2022; Hsieh et al., 2023).
Iron isotopic compositions were determined by using multi-collector Inductively coupled Plasma Mass Spectrometer (MC-ICP-MS, Neptune Plus, Thermo Fisher Scientific) equipped with a sample inlet system APEX-IR (no gas added), normal Ni sampler cone, and X-type skimmer cone from Elemental Scientific. Samples were measured in high-resolution mode with 54Cr correction on 54Fe and 58Ni correction on 58Fe. The d56Fe measurements were conducted using the double-spike technique, involving the addition of mixed spike (57Fe and 58Fe), with a sample-to-spike ratio of 1:2, as described by Dauphas et al. (2017). The δ56Fe data (56Fe/54Fe) ratios are reported in per mil notation (‰) relative to the IRMM-014 Fe isotope reference material (Institute for Reference Materials and Measurements) and described below:
To minimize the Cr and Ni interferences, we purified all samples by using an anion exchange resin (AG1-X8, Bio-Rad, 100-200 mesh). The samples were loaded in 1.4 mL 7 N HCl solution onto perfluoroalkoxy alkanes (PFA, Savillex) microcolumns filled with the resin. We then applied ultrapure 0.4 ml 7N HCl thrice to remove Ni and Cr. The Fe was retained on the resin and later eluted out with 0.7 N HCl (Fig. S1). The eluted off purified Fe samples were dried in open cap vials then were redissolved subsequently in 1 ml of 0.5 M HNO3 at 120 °C for 2 hours with closed caps, and then were ready for isotopic analysis. We measured IRMM-014 right after 4 sample analysis and measured NIST-3126a right after 8 sample analysis to validate analytical accuracy. Our long-term δ56Fe value of NIST-3126a is 0.36±0.03 ‰, averaged from 5 different analytical periods spanning for half year. The precision and accuracy of δ56Fe data in this study were validated by using reference materials BCR2 and BHVO2 (Table 1, Craddock and Dauphas (2010)).