DOI: 10.5061/dryad.rv15dv4h3
title: The iron isotopic composition and the elemental ratios (Cd/Ti) in the size-fractionated aerosols collected over the East China Sea

description: 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.
description: <strong>Method</strong>
 <strong>Sampling site and
 procedures</strong> The aerosol samples were
 collected in a small islet in the ECS, Pengjia islet (PJ, 1.1
 km<sup>2</sup>, 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 (&gt;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.
 <strong>Quantification of dissolved and total
 concentrations, and isotopic composition</strong>
 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 HNO<sub>3</sub><sub>
 </sub>(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
 <sup>54</sup>Cr correction on <sup>54</sup>Fe and
 <sup>58</sup>Ni correction on <sup>58</sup>Fe. The
 d<sup>56</sup>Fe measurements were conducted using the
 double-spike technique, involving the addition of mixed spike
 (<sup>57</sup>Fe and <sup>58</sup>Fe), with a
 sample-to-spike ratio of 1:2, as described by Dauphas et al. (2017). The
 δ<sup>56</sup>Fe data
 (<sup>56</sup>Fe/<sup>54</sup>Fe) 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
 HNO<sub>3</sub> 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 δ<sup>56</sup>Fe
 value of NIST-3126a is 0.36±0.03 ‰, averaged from 5 different analytical
 periods spanning for half year. The precision and accuracy of
 δ<sup>56</sup>Fe data in this study were validated by using
 reference materials BCR2 and BHVO2 (Table 1, Craddock and Dauphas
 (2010)).
description: # 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](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'.