Iron speciation in different Saharan dust advections and effect of the procedural blank on the results from X-ray Absorption Spectroscopy and selective leaching experiments

: In this work, we applied X-ray Absorption Spectroscopy (XAS) and selective leaching experiments for investigating iron speciation in different dust advections collected on different unwashed quartz ﬁber ﬁlters. XAS analysis evidenced a predominance of Fe(III) in 6-fold coordination for Saharan dust and a trend towards Fe(II) and 4-fold coordination in the order: Saharan dust, mixed 4 Saharan, and non-Saharan aerosol samples. The role of the sampling substrate was evaluated 5 explicitly, including in the analysis a set of blank ﬁlters. We were able to pinpoint the possible contribution to the overall XAS spectrum of the residual Fe on quartz as the concentration decrease towards the blank value. In particular, the ﬁlter substrate showed a negligible effect on the structural 8 trend mentioned above. Furthermore, selective leaching experiments evidenced a predominance 9 of the residual fraction on Fe speciation and indicated the lowest Fe concentrations for which the 10 blank contribution is < 20% are 1 µ g for the ﬁrst three steps of the procedure (releasing the acid-labile, 11 reducible and oxidizable phases, respectively) and 10 µ g for the last step (dissolving the insoluble 12 residuals). 13

[13] applied a combined XANES and wet chemical techniques approach for studying the effect of reported XAS data on Nuclepore [26] Teflon ([13,21,27] and cellulose ester filters [25]. Moreover, 56 acid-cleaning procedures are usually implemented on filters to be used for XAS analysis in order to 57 minimize the impact of the substrate's trace metals contamination on the analysis [21]. However, filter 58 acid cleaning procedures are expensive and time-consuming and cannot be routinely applied to all 59 the filters used for automated aerosol sampling in air quality monitoring sites. In order to investigate 60 iron speciation in episodic events such as Saharan dust advections on routinely collected aerosol 61 samples, the filter impurities contribution to the XAS spectrum needs to be assessed. The aim of this 62 work is to perform XAS analyses and selective leaching experiments on samples collected during 63 different long-range dust advections on different unwashed quartz fiber filter substrates. To this aim, 64 iron speciation in two severe Saharan dust advections, two mixed Saharan dust advections and two Table 1. Samples description: sampling date, filter code, aerosol mass concentration (PM 10 and PM 2.5 in µg m −3 ) and iron concentrations determined by ICP-OES for HVS (high volume sampler) PM 10 and SWAM PM 2.5 samples expressed in µg m −3 and converted to atoms cm −2 . Errors on the last digit are indicated in parentheses. the contribution of Saharan dust to mix-SH_Feb resulted higher than in mix-SH_Apr.

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The first non-Saharan advection occurred on October 31 st 2014 (non-SH_Oct). The sampled air 93 mass came from Eastern Europe and the Balkans where it passed over a very polluted industrialized 94 area in proximity of ground level ( Figure S2e). Anthropogenic sources would explain the high PM air 95 mass concentration (18.9 µg m −3 ) and the extremely high PM 2.5 /PM 10 ratio (0.9) of the sample.

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The second non-Saharan advection occurred on January 7 th 2015 (non-SH_Jan) and came from 97 Northern Europe. In this case, the air masses spent most of the time in the high troposphere at a great 98 distance from continental Europe ( Figure S2f). The long-range advection combined with a regional 99 contribution by anthropogenic aerosols could justify the high PM 2.5 mass concentrations (18.9 µg m −3 )   as hematite, rather than in more disordered compounds like ferrihydrite.

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The pre-edge features were fitted in all the samples, and the results are reported in Table 2. The centroid 127 position ranges between 7115.04 eV in Saharan dust samples (HVS PM 10 for SH_Dec-3011) to 7113.93  Figure 2. A gradual shift is observed, from high-energies/low-intensities for the 131 pure Saharan dust samples (a-d) towards lower-energies/higher-intensities for non-Saharan samples.

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Moreover, the coordination geometry shows a decreasing trend from 6-coordinated in pure Saharan  The EXAFS spectrum was reproduced with a two-shell ferrihydrite model, and the fit parameters 139 are shown in Table 3. The coordination number (N) was strongly correlated with the Debye-Waller 140 factors, and was thus obtained by a different first shell fit, linking N and r(Fe-O) using the results of coordination number (5.5 to 4.9) are observed for PM 2.5 samples of SH_Dec to non-SH_Jan. Therefore, 147 the overall trend follows the order: SH_Dec>mix-SH_Feb>mix-SH_Apr>non-SH_Oct>non-SH_Jan.

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It is worth noting that both SWAM PM 2.5 and HVS PM 10 samples from SH_Dec show the same spectral 149 features and fit parameters. We can, therefore, exclude any influence of the sampling system (sampler 150 and substrate) for this event, which is, however, the most intense amongst the presented ones.

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The second shell Fe-Fe distance is about 2.95Å in the majority of the samples. The slight variations 152 observed between the samples can be ascribed to structural defects and partial substitution of Fe(II) 153 and Fe(III) on the same site, while the much shorter distance in the blank filter is in line with a different 154 coordination geometry.

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In order to get an insight on blank filter contribution to the XAS spectrum, a SWAM quartz fiber 156 filter was analysed. Despite the spectrum of a blank HVS filter is not available, we assume the iron 157 local structure to be similar in both cases because the filter material and manufacturer are the same.

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The XANES spectrum of the blank filter is shown in Figure 1b Table S1), and a 4.4 coordination.

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Considering that XANES data are not quantitative, they are in good agreement with EXAFS data. In Leaching test results are reported in Table 4. Fe concentrations in non-SH_Oct and non-SH_Jan 166 samples were too low to obtain reliable Fe fractionation data after blank subtraction, and they are 167 hence excluded from further analyses. and lower residual (step IV) with respect to the coarse fraction.

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The blank contribution in the sequential extraction solutions is evaluated for ten atmospheric 178 aerosol samples (listed in Supplementary Material, Table S2) and reported as a function of the total Fe 179 amount ( Figure 4). The influence of the procedural blank on the first three steps of the procedure 180 is quite similar and increases as the total Fe amount decreases. The fourth step is characterized by 181 higher blank values than the previous ones, thus affecting the analytical results more severely. We 182 estimate that the reagents account for about 30% of the total blank values, whereas the remaining 70% 183 is due to the filter contribution. Finally, Figure 4 shows that the lowest Fe content for which the blank 184 contribution is <20%, which can be regarded as a good target, resulted to be 1 µg for the steps I-II-III 185 and 10 µg for step IV.  In this study, we considered the most significant dust advections recorded at the rural regional intrusion. Filter samples were kept frozen until analysis, as this preservation method was proven to 286 minimize the loss of Fe(II) due to oxidating processes [14,39]. Filters were then defrost and cut into 287 pieces of suitable size for the different analyses described in the following paragraphs.

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The obtained spectra have been then analysed by means of appropriate codes (FEFF8, Athena, Artemis;

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[43]) to obtain information from both the XANES and the EXAFS regions, separately. The pre-edge 330 features of the XANES part of the spectrum were fitted keeping the number of components at a minimum in all cases in order to determine the centroid position and the intensity of the peaks using