Adrien Létoffé

Sandrine Hoppe

Richard M Laine

Nadia Canilho

Andreea Pasc

Didier Rouxel

Rafael Jimenez Rioboo

Sebastien Hupont

Isabelle Royaud

Marc Ponçot

Richard Lainé

Rafael J Jiménez Riobóo

Sébastien Hupont

Resilience improvement of an isotactic polypropylene-g-maleic anhydride by crosslinking using polyether triamine agents

Keywords: isotactic polypropylene grafted maleic anhydride, polyether triamine, crosslinking, microstructure, molecular architecture, Brillouin spectroscopy, mechanical properties

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The development of new multi-materials is an important topic of investigation that contributes to the reduction of ecological footprint of human activities. Furthermore, steel/polymer multilayer composites, used in different industrial applications, allow the generation of lighter structures. However, due to poor compatibility between the polymer and the metallic surface, processing is still being optimized. Since, functionalized polyolefins have adhesive properties that can promote for metal adhesion or nano-filler dispersion, they have been used either in blends with other polymers [1][2][3], or in the elaboration of polymer composites [4][5][6].

Polypropylene grafted with maleic anhydride (iPP-g-MAH), one of those classic functionalized polyolefins, is used in the elaboration of layered metallic/polymer composites. The main drawback of iPP-g-MAH is poor mechanical behavior due to the chain scissions that take place during the MAH grafting reaction [7][8][9][10][11][12][13][14][15][16][17]. In some cases, elastomer nodules are used to improve the mechanical properties of the polymer matrix [18]. Another type of functionalized polypropylene is polypropylene grafted with amine groups (iPP-g-NH2) usually produced from iPP-g-MAH by a grafting reaction of amino groups and the consumption of the MAH. The presence of amino groups change the properties of the polymer matrix. This may be reflected 2 in an increased ability to promote the exfoliation of silicate or graphene fillers [2,4,19]. However, the mechanical properties of iPP-g-NH2 remain low. The physical properties of the genuine iPP-g-MAH are mainly maintained.

In the work reported here, the elaboration of a functionalized polyolefin matrix with significant shock absorption properties and free chemical groups, as MAH or NH2, has been investigated. The material generated is based on isotactic polypropylene grafted with maleic anhydride (iPPg-MAH), which exhibits, as does classic polypropylene (PP), low price, low density and good processing ability. The chemical treatment used is based on a recently developed new crosslinking method for modification of the mechanical properties of different iPP-g-MAH [20].

This method was developed to overcome the detrimental effects of the maleic anhydride grafting reaction on the iPP polymer chains. The iPP-g-MAH, even at low MAH contents, display a brittle mechanical behaviour, with inefficient shock absorption properties. The crosslinking reaction was based on the reaction between maleic anhydride and the amino groups of a polyether diamine molecule [2,[19][20][21][22]. In previous work, two iPP-g-MAH at 1 and 3 wt % of MAH was crosslinked with a polyether diamine at different NH2/MAH molar ratios. The main results were a switch from brittle to ductile mechanical behaviour observed using videocontrolled uniaxial tensile tests for the polymer obtained with an equimolar NH2:MAH molar ratio. In this situation, no MAH or amino groups remain free inside the matrix for further reaction. The aim of this new study was to use a polyether triamine in place of a common linear polyether diamine to always generate a three-dimensional macromolecular network, but permitting at the same time to reach interesting mechanical properties and ready-to-use free chemical functionality for molar ratios others than the equimolar one. The focus was led on molar ratios between amino and maleic anhydride groups from 0.33 to 1.5.

The reaction and the material structure were studied by infrared spectroscopy and determination of the gel proportion. The impacts of the reaction on the semi-crystalline microstructure were studied with the self-successive auto-nucleation technique (SSA) and X-ray scattering. The evolution of elastic constants and optical properties were studied using Brillouin spectroscopy and finally the VidéoTraction™ system was used to determine the true mechanical behaviour of the different materials.

Materials preparation

In this work, the crosslinking agent used was a polyether triamine distributed by Huntsman as the Jeffamine® T3000 [START_REF] Polyetheramines | Technical Specifications Sheet by Hunstman[END_REF] and used as received. The T3000 presents aliphatic polypropylene glycol (PPG) chains end capped by three amine groups and has a molecular weight of 3000 g.mol -1 (Fig. 1). The T3000 polyether triamine is chemically more reactive toward maleic anhydride groups than aromatic amines [2].

The comparative polymer matrix used in this work is an isotactic polypropylene grafted with 1 wt% of maleic anhydride, noted as CA 100 and distributed by Arkema under the commercial name, Orevac® CA 100. More information may be obtained from the Arkema specification sheet [START_REF] Orevac® | Technical Specifications Sheet by Arkema[END_REF] or the previous paper [20]. It should be noted that the grafting process for MAH units involving peroxide molecules induces a significant decrease in the macromolecular mass of iPP. As a result, CA 100 shows a brittle mechanical behaviour.

The chemical reaction involved here, as shown in Fig. 2, is the reaction between MAH groups grafted on the polypropylene macromolecular chains and the T3000 amino groups. As a first step, the reaction between an amino group and a MAH group leads to the formation of an amide and a carboxylic acid. Then, at a much lower rate, cyclization to form an imide group occurs [1,2,19]. Previous analysis using FTIR spectroscopy has demonstrated that part of the amide/carboxylic acid pairs did not form an imide group in the final material [20]. It was also supposed that some MAH groups could react with two amines to form two amide groups [19]. This situation is now considered unlikely since the carboxylic acid vibrational bands have remained at the same intensity in the excess material [20]. It is supposed that at an equimolar ratio, no MAH groups or free amino group are remaining. The elaboration by reactive extrusion and the forming process by injection molding were carried out at the Reactions and Chemical Engineering Laboratory (LRGP) using a Thermofischer reactive extruder with exactly the same parameters as those used in previous work [20].

Experimental methods:

Gel Content determination:

Gel content determination is a classic method for characterization of crosslinked polyolefins. In this work the ASTM D2765-84 standard method was used. The extraction solvent was boiling xylene and the test was carried out inside a copper pouch.

A square pouch made of copper is used to contain the sample, the pouch weight is noted W1. The material sample of approximately 0.3 g is put inside the pouch. The weight of the pouch and the sample is noted W2. The pouch is finally sealed and the weight is noted W3. The sealed pouch is placed in boiling solvent for 8 hours, then dried at reduced pressure oven at 150°C for 30 min. The final weight of the pouch is noted W4. Finally, the solvent extraction is calculated as:

= ( )( ) X 100 (1)
F is the fraction filler in the polymer, insoluble in the xylene. The gel content is estimated at 100 -E. The measurements were performed at least three times to ensure the good reproducibility of results.

Fourier Transform Infrared (FTIR) spectroscopy:

FTIR analyses were performed at room temperature using an FTIR Magna Protégé 460 from Nicolet. The FTIR spectra were recorded from 400 to 4000 cm -1 with a 4 cm -1 resolution using an accumulation of 64 scans in the transmission mode. The samples were compressed at 190°C and 8 bars to form thin films between 100 and 200 µm. The spectra were treated with the Fityk software and a reference band at 998 cm -1 , which come from the methyl groups rocking, was used to normalize the overall spectral intensity.

Wide angle X-rays Scattering (WAXS):

The WAXS analyses were performed with a parabolic multilayer mirror (Osmic) and a cylindrical capillary. The selected wavelength is the copper radiation Kα1 (λ= 0.154 nm) applied with a tension and an intensity of 30 kV and 40 mA. The samples are 1 cm by 1 cm square cut directly from the material sample and polished to remove any skin/core effect.

The 1D WAXS diffractograms presented reflect the average crystal diffraction in the sample plane. The effect of the potential crystalline texture was removed by the rotation of the sample holder. These diffractograms were analysed using the Hermans and Weidinger's method [START_REF] Hermans | [END_REF].

A series of Gaussian and Lorentzian functions were used to fit the amorphous and crystalline phases contribution, respectively [26,[START_REF] Kakudo | X-ray Diffraction by Polymers[END_REF]. The crystallinity ratio was estimated as the ratio of the crystalline peaks areas divided by the total area of the diffractogram.

The 2D WAXS patterns were obtained using a photosensitive plane film scanned using a FUJIFILM scanner BAS 5000. The sample was put at a distance of 75 mm from the film detector.

Differential Scanning Calorimetry analysis (DSC):

Calorimetric data were recorded using a Q200 TA Instruments apparatus. A 10 mg sample of each material were encapsulated with aluminium pans. The analyses were performed under an inert flux of azote atmosphere at a flow rate of 50 ml.min -1 . The samples were treated at a constant rate of 10°C•min -1 (divide in heating, cooling, heating) from 30 to 200°C. The crystallinity ratios of the different materials were estimated from the melting enthalpy (ΔHm) as follows:

= ∆ ∆ (2)
∆ =165 J•g -1 is the enthalpy of a 100% crystalline isotactic polypropylene [START_REF] Wunderlich | Macromolecular physics, Crystal Melting[END_REF].

The main crystalline lamellar thickness population (Lc) of the polymer is calculated from the Gibbs-Thomson equation:

= (1 -∆ ! " ) (3)
where the melting peak temperature Tm is estimated from the materials endotherms, is the theoretical melting temperature of a 100% crystalline isotactic polypropylene given equal to 460 K, σe is the surface free energy of iPP crystalline lamellae at 0.0496 J.m -2 and ρc the crystalline phase density at 0.9361 g.cm -3 [START_REF] Wunderlich | Macromolecular physics, Crystal Melting[END_REF][START_REF] Bassett | [END_REF].

Successive Self-Nucleation and Annealing (SSA):

The Successive Self-Nucleation and Annealing experiments (SSA) is an experimental technique for characterization of the thickness distribution of crystalline lamella of a polymer [30,31]. This technique based on the thermal fractionation of a semi crystalline polymer shows the linearity defects in the macromolecular chains, as co-monomers, stereo-defects or crosslinking [32]. This technique is used to study the decrease of the main lamellar thicknesses of a polymer as a function of its crosslinking degree [20,33]. A SN protocol (self-nucleation) was first performed to identify the ideal self-nucleation temperature Ts of the material. Once Ts was determined, the SSA protocol was carried out.

Uniaxial tensile tests:

The VidéoTraction™ system was used to characterise the true mechanical behaviours of the different materials of this study [18,[34][35][36][37]. The shape of the samples was dumb-bell type specimen with a geometrical defect in the middle of the useful zone. This defect ensures the localization of the plastic deformation (necking) where the video measurements were performed. A specific Representative Volume Element (RVE) was then defined by a seven dot markers pattern applied to the probe surface.

The axial true strain noted ε33 was estimated by a polynomial interpolation of the markers displacement using Lagrange Transform and following Hencky's strain theory. In the case of uniaxial tensile testing, the two transversal strains, ε11 and ε22, were estimated equal (the strain field is supposed transversally isotropic at the middle of the neck) [38][39][40][41][42]. All the mechanical analyses were performed at 20°C with a constant true strain rate of 5.10 -3 s -1 . This rate was chosen because it ensures the perfect control of the constant true strain rate of the testing machine, and suppress the risk of self-heating from the deformation [41,42].

The volume strain εv in the RVE is calculated from the trace of the true strain tensor:

εv = ε11 + ε22 + ε33 (4)
When the volume strain is positive, it is called volume damage since the "reversible" elastic part can be neglected.

The axial true stress (Cauchy stress) is determined in the RVE as:

( )) = (*/, ) * . / (5)
with S0 is the initial cross-section at the RVE of the sample.

Brillouin Spectroscopy:

Brillouin Scattering spectroscopy (BS), like Raman spectroscopy, is a light scattering technique, providing direct information on the acoustic phonons of the material i.e. on collective oscillations of atoms and on the propagation of acoustic waves in the material, and therefore on the mechanical properties of the material. It avoids direct contact of the sample, which may be crucial for many experiments. It is remarkably well adapted to study soft materials [START_REF] Krüger | Brillouin Spectroscopy and its Application to Polymers[END_REF][START_REF] Rouxel | Brillouin spectroscopy of polymer nanocomposites[END_REF], since the opto-mechanical coupling between photons and acoustic phonons is then much more important than for hard materials [START_REF] Krüger | Brillouin Spectroscopy and its Application to Polymers[END_REF], except in the case of induced phonons [45].

Therefore, BS provides noninvasive measurement of acoustic and elastic properties of oligomers [46,[START_REF] Krüger | Mechanical Interphases in Epoxies as seen by Nondestructive High-Performance Brillouin Microscopy[END_REF], polymers [START_REF] Nguyen | [END_REF] and polymer nanocomposites [49][50][51][52][53]. In the scattered light, the interaction between the photons of the incident beam and the acoustic phonons in material, which occur at hypersonic frequencies (10 MHz -100 GHz), leads to specific frequency shifts (f) of the scattered laser light. Two conservation laws are followed by the photons and the phonons involved in the scattering process: conservation of energy, involving the frequencies of the incident and scattered light and of the acoustic wave (5), and conservation of momentum, which involves the light wave vectors and the acoustic wave vector q (6):

As in Raman spectroscopy, Stokes/anti-Stokes scattering occurs, corresponding to the creation or annihilation of a phonon, hence the ± signs. Assuming a negligible acoustic attenuation, the relation between the sound velocity v, the acoustic phonon frequency f and its wave vector q can be calculated using equations ( 5) and ( 6): (8) where λo is the laser wavelength, and θi the angle between the incident and scattered wave vectors and . From which is obtained the elastic modulus, related to the sound velocity and the mass density ρ of the medium:

0(1 2) = 3 * 4 (1 2) (9)
The Brillouin spectroscopy were performed in a 90A geometry to obtain the transmission mode (f90A). The samples were obtained by microtomy cuts to produce a 20 µm thin element put between two glass slides [START_REF] Rouxel | Brillouin spectroscopy of polymer nanocomposites[END_REF]. The reflection of the laser on the second blade leads to simultaneously backscattering mode acquisitions (f180) [START_REF] Rouxel | Brillouin spectroscopy of polymer nanocomposites[END_REF], and then enable to obtain the phononic frequencies f90A and f180 in the same spectrum.

In this configuration, the equation ( 8) can be simplified as: √ * > @ A (12) From which we can deduce in particular the optical index of the material noted n [START_REF] Rouxel | Brillouin spectroscopy of polymer nanocomposites[END_REF], besides the elastic constant. The reproducibility of our tests was done by checking different samples.

s i ω ω Ω = ± h h h s i k k q = ± h h h       = = 2 sin 2 2 ν 0 i n f q f θ λ π i k → s k → 7
The longitudinal phonon mode which gives rise to the elastic constant c11 was observed, defined by the first elasticity tensor constant in the Hooke's law. This elastic constant is calculated with the equation ( 9) from:

0 = 3 * 4 6 7
(13)

Scanning Electron Microscopy:

Scanning electron microscopy (SEM), like in our previous work, is used to analyse the fracture surface of the mechanical analysis samples. The SEM micrographies enlighten the differences between the mechanical behaviours, and the switch from a brittle to a ductile material. Post mortem micrographies were recorded with an environmental Quanta FEG 650 electron microscope from the FEI Company. The parameters selected were a pressure of 100 Pa of water vapour and a 4 kV acceleration voltage.

Results and discussion:

Study of the crosslinking reaction:

The gel content evolution, as a function of the NH2:MAH molar ratio, is shown in Fig. 3. The gel content shows an increase with NH2:MAH ratio from the unmodified material to the 1:1 molar ratio material. The CA 100 unmodified shows 0% of gel content. This null value is logical due to the low molecular weight, consequence of the MAH grafting reaction and was already seen before [7][8][9][10][11][12][13][14][15][16][17]20]. The increase of the gel content observed is then attributed to the crosslinking reaction of the polymer chains. The crosslinking reaction, with the use of polyether triamine, induces a three dimensional molecular network. The gel content stabilizes for the 1:1 and the 1.5:1 molar ratio. This evolution was expected, and is similar to the material obtained using polyether diamine [20]. The difference with polyether diamine is, in this situation of amino groups in excess (1.5:1), part of the polyether triamine molecule reacts with two MAH groups and maintained the 3D network.

This effect of amine excess was observed in previous work with high NH2:MAH molar ratios [7][8][9][10][11][12][13][14][15][16][17], but also in the case of polyether diamine [20]. The FTIR analyses were performed to study the chemical evolution of the materials. The results between 1900 and 1500 cm -1 are presented in Fig. 4. For the unmodified CA 100 spectra, three vibrational bands are observed, at 1860, 1780 and 1715 cm -1 . The first two bands are assigned to the symmetric and asymmetric νC=O vibrations of the maleic anhydride group, and the third one to the νC=O vibrations, associated with carboxylic groups. The carboxylic acids must be formed by hydrolysis of the maleic anhydride groups under wet storage conditions. Those three bands intensities decrease with the NH2:MAH molar ratios, and, as expected, disappear at the 1:1 ratio.

Four other bands that appeared for the 0.33:1 material and reach a maximum at the 1:1 prove the presence of imide and amide groups. First, two bands at respectively 1770 and 1700 cm -1 can be assigned to the symmetric and asymmetric νC=O vibrations of the imide group, and two others at 1735 and 1650 cm -1 can be assigned to associated carboxylic acid/amide bonds. Those bands confirmed the reaction between the MAH groups and the amino groups of the T3000.

The presence of amide/carboxylic acid bonds at 1735 and 1650 cm -1 indicated an incomplete imidization, as seen in previous report [20]. Those four bands stay at the same level of intensity for the 1.5:1 ratio, the excess of amino group did not stop the reaction, but a new band at 1550 cm -1 is observed. This band is assigned to primary amino groups that did not react. Those bands evolution is consistent with the chemical reaction that takes place during the extrusion process [1,2,[19][20][21][22].

This evolution, with a complete use of amino groups for the materials between 0.33:1 and 1:1, and the disappearance of MAH groups for the 1:1 molar ratio confirmed the complete extent of the reaction for this specific ratio. The amino bands found for the 1.5:1 material, which characterized the excess of crosslinking agent, is also in agreement with this. As seen as gel content evolution shows in Fig. 3. The gel content determination and the FTIR results confirmed a chemical and structural modification of the CA 100 by the T3000, close to the evolution observed before [20]. The reaction, proved by the FTIR results, create a three-dimensional molecular network. These structures evolution should influence the macromolecules mobility and/or the semi-crystalline microstructure of the materials, and may finally modify their mechanical behaviours.

Microstructural evolution

The impact of a crosslinking reaction on the semi-crystalline structure because of the threedimensional network can change a lot depending on the reaction. In the case of the MAH/NH2 grafting reaction used is this work that takes place in melt state, previous studies showed a diminution of the lamellar thickness with the NH2:MAH ratio for the iPP-g-MAH at 1 wt% of MAH content [20]. To investigate these effects for these new materials crosslinked via the T3000 one-dimensional WAXS and DSC experiments were performed. Fig. 5 a) shows the 1D WAXS results. Those diffractograms highlight the presence of the monoclinic α crystalline phase of the iPP in majority [53,54] but also a small amount of the β crystalline phase. The β phase was neglected since it represents less than 1% of the crystalline structure. No significant shift of the diffraction peak positions and no modification of the crystallinity ratio was induced with the increase of the NH2:MAH molar ratio.

The DSC thermograms of the different materials are presented on Fig. 5 b). For all samples, large endotherms around 160°C could be assigned to the melting of α crystalline phase of iPP.

The maximum peaks associated with the main melting temperature Tm and the crystallinity ratios are given for the first and second endotherms in Table 1. The crystallinity ratios between the first and the second heating ramp remain close for all the materials with a maximum gap of 2%; this value is inferior to the experimental error. The same observation can be done between the different materials with the maximum gap of 3%. The values of the crystallinity ratios determined from WAXS analyses and deduced from the endotherm areas of the DSC first heating ramp are significantly different but put in evidence the same tendency towards the chemical reaction advancement. The two experimental techniques give results that cannot be compared in absolute values for a specific material, but it is relevant to compare their trends. In the two cases only small variations of crystallinity ratio appeared with the rise of the NH2:MAH molar ratio. The crosslinking reaction does not induce significant modification of the crystallinity ratio as compared to the unmodified CA 100.

This evolution is quite similar to the one observed with the same iPP-g-MAH modified with polyether diamine [20]. Only a slight decrease of the main melting temperature can be observed between the two heating ramps. There is certainly an influence of the injection on the microstructure. This difference is maximal for the unmodified material and the 0.33:1 material and decreases for higher NH2:MAH molar ratio. A double shoulder shaped endotherms is observed for materials under the 0.5:1 molar ratio. For the unmodified CA 100 material, the main crystalline population appears to be the one at the highest melting temperature for the first endotherms, but switch to the lowest one for the second. For the 0.33:1 material, the main crystalline population remains the same between the two endotherms. Also, a slight rise of the melting temperature is observed between the unmodified CA 100 and the 0.33:1 material, from 163 to 165°C for the first heating ramp, and from 161 to 164°C for the second heating ramp. The evolution of the two crystalline population rise was unexpected compared to previous results, and indicates a small nucleation effect at low level of crosslinking. This effect seems to counteract at higher molar ratio. The first heating endotherms show a decrease of the main melting temperature from 165 to 163°C from the 0.33:1 to the 0.5:1 molar ratio, and finally decreases to 161°C for the equimolar ratio. For the 1.5:1 NH2:MAH molar ratio, this effect disappears, probably due to the presence of unreacted T3000 molecules, that tend to facilitate the chain mobility. The diminution of the melting temperatures implies, with the Gibbs-Thomson relation (Equation 2), a fall of the crystalline lamellae thicknesses, from 13 nm for the 0.33:1 material to 11 nm for the 1:1 material. It is assumed that the crosslinked part of the material is located in the amorphous phase and does not alter the crystalline phase characteristics such as monoclinic cell parameters, crystalline density and crystalites surface energy. The crosslinked network created by the reaction shows an impact on the crystallisation of the material which is small compared to other crosslinking methods [21,22,[56][57][58]. To properly highlight the impact of the reaction and the molar ratio on the melting temperature ranges for the modified samples SN/SSA studies were performed. To start the SSA protocol, an SN protocol was performed on the material with the higher melting temperature (0.33:1). The Ts temperature ware determined at 170°C for the 0.33:1 molar ratio. between the unmodified CA 100 and the 0.33:1 material, whereas the second thickest ones (Lc = 15.5 nm) present a rise from 22 to 26% of the crystallinity ratio. This rise explains the evolution of the melting temperature between the two materials, and the evolution of the double shoulder shaped endotherms. The reaction, at low levels of crosslinking, impacts positively the crystallisation of the medium chain elements of the polypropylene and implied the rise of the melting temperature observed by DSC analysis. For the highest molar ratios from the 0.5:1 material, a fall of the thickest crystalline population associated with the longest chain elements is observed, from 62% for the unmodified CA 100 and the 0.33:1 material to 55% for the 0.5:1 material and 41% for the 1:1 molar ratio. This decrease is in favour of the second thickest population associated to the medium chain elements, which represents 41 % of the 1:1 material crystallinity. The crosslinking reaction, with the creation of chemical network, affects the crystallisation of the longest chain elements. This evolution explains the decrease of the melting temperature observed by DSC. The microstructure evolution observed with the increase of the NH2/MAH molar ratio is similar to the previous results obtained with similar method [20], or in case of other crosslinked polymers in the literature, but at a smaller scale [21,22,[56][57][58].

The small rise of melting temperature for the 0.33:1 material, in correlation with the SSA protocol results, indicates a small nucleation effect of T3000 that was not previously observed [20]. This effect looks counterbalanced at higher molar ratio by the three-dimensional network formation, with a slight decrease of the melting temperatures ranges, as pointed out by SSA results.

Mechanical and optical properties evolution

To analyse the evolution of the mechanical behaviour of the different materials with the crosslinked reaction, Brillouin spectroscopy and VidéoTraction™ tests in uniaxial tension deformation path were performed. Fig. 7 presents a typical Brillouin response in case of CA 100 1:1. As expected, the two frequencies f90A and f180 of the acoustic phonon were obtained. Fig. 8. present the evolution of the elastic modulus (c11) and refractive index (n) for the different materials, considering a constant material density to within 0.905. The elastic modulus shows a rise from the unmodified materials to the 1:1 molar ratio, with 26% augmentation from 9.9 to 12.5 GPa, and finally falls to 11.5 GPa for the 1.5:1 molar ratio. The evolution is consistent with previous results, and was expected in particular from the plasticizing effect of the polyether triamine for molar ratio higher than the 1:1. This evolution can be easily explained by the creation of the crosslinked network inside the materials by the T3000. As seen by FTIR analysis and gel content, this network depends on the molar ratio, and shows no important evolution above the 1:1 molar ratio. This network does not present an important impact on the crystalline phase, as discussed earlier in the microstructure part, and remains located inside the amorphous phase. This network implied a stiffening of the amorphous phase and the rise of elastic properties. For the 1.5:1 molar ratio, the unreacted amine groups from the T3000 act like a plasticizer agent.

Furthermore, one expects a close correlation between mechanical and optical properties and in the particular case of these materials, there is a symmetrical look of both curves (it is clear from eq.12 that the frequency f90A, which shows the greatest variation, controls the evolution of the optical index): the higher the elastic constant, the lower the optical index. Interesting is to notice that this effect is not universal and for instance in Phosphate glasses, among others, the higher the Young's modulus the higher the refractive index [START_REF] Muñoz | crosslinking enables maleated isotactic polypropylene to get higher mechanical resilience while keeping its functional property with available free MAH or NH2 grafts, confirmed by the FTIR analysis[END_REF].

To highlight the impact of the reaction on the true mechanical behaviour of the materials, VidéoTraction™ tests in uniaxial tension deformation path were performed on the different material at a constant true strain rate of 5.10 -3 s -1 . Fig. 9 shows the true mechanical behaviours of the different materials, with the evolution of the true stress and the volume strain as a function of the true strain. As seen before, the iPP-g-MAH unmodified (CA 100) shows a brittle mechanical behaviour [20]. The 0.33:1 material, the one with the lowest crosslinking level, also presents a brittle behaviour. In this situation, the three-dimensional network seems not enough to affect positively the mechanical properties of the material. The first main change appears for the 0.5:1 material, with a true strain at break that rises from εb CA100 = 0.04 to εb 0.5:1 = 0.12.

With the use of a polyether triamine, a switch from brittle to ductile mechanical behaviour starts from the 0.5:1 molar ratio. This evolution appears clearly for the 0.66:1 material, with a strain at break of εb 0.66:1 = 0.45, but no modification of the level of true stress at break (σb CA100 = 26 MPa and σb 0.66:1 = 27 MPa). The rise of true strain at break continues for the equimolar material, with also a modification of the true stress at break twice higher than the unmodified material (εb 1:1 = 1.22 and σb 1:1 = 49 MPa). This evolution is easily explained by the three dimensional network highlighted by the rise of the gel content. The 1.5:1 material that presents an excess of amino group shown by the FTIR, presents also a ductile behaviour with a diminution of the true stress level at the beginning of the plastic deformation compared with the 1:1 material. It also presents a more important amplitude of its stress hardening (εb 1.5:1 = 1.3 and σb 1.5:1 = 63 MPa) that could be explained by a higher level of molecular chain orientation towards the stretching direction. This final evolution between the 1:1 and 1.5:1 material was unexpected, due to previous results obtained with polyether diamine [20]. In the previous work the excess of amino group presents a plasticizing effect with a diminution of true stress level. This evolution of mechanical properties is in perfect correlation with the previous result like the gel content determination and Brillouin spectroscopy. It is important to note that due to the brittle mechanical behavior of the unmodified material (CA100), and the low ductility profile of the materials with the lowest molar ratios, the calculation of the Young moduli, by the slope of the elastic part of the VidéoTraction™ system curves, is difficult and exhibits a high level of uncertainty. Moreover, due to the slight differences observed at low frequencies, Dynamical Mechanical Thermal Analysis are so much relevant. These two reasons justify the use of Brillouin spectroscopy. The change of volume strain with the molar ratio indicates that a complete reaction (1:1 molar ratio) induces a fall of volume strain at the same level of true strain. The excess of amino groups also decreases it, with a volume strain at break twice lower than the 1:1 material (εvb 1:1=0.35 and εvb 1.5:1=0.18). This fall is in good agreement with the highest level of molecular chain orientation supposed from the true mechanical behaviour and the most important amplitude of its stress hardening observed for the 1.5:1. For this material, the FTIR analysis confirmed the complete extent of the reaction, with the total disappearance of the MAH bands and the presence of unreacted amino groups. However, the close values of gel content between the 1:1 and 1. ) presents the heterogeneous surface of the 0.5:1 material. It put in evidence the creation of a fibrillar microstructure distributed amongst some brittle zones that confirms the transition of mechanical behaviour around this molar ratio. This evolution is completed for the 1:1 molar ratio as seen in the Fig 10 c), with a complete fibrillar microstructure that is conform to the complete switch of mechanical behaviour observed by the tensile test results. Finally, the structure is maintained for the 1.5:1 material, the excess of crosslinking agent does not modify this observation. To take in consideration the difference of volume strain level between the materials and to compare the real mechanical behavior of all the materials at high values of true strain, the intrinsic true mechanical behaviors were determined as developed in a previous paper [18]. The shape of cavities was estimated to be tube-like shaped. This way, the intrinsic true strain in the axial direction and the intrinsic true stress are given by:

B )) C = B )) (14)
( )) C = D E = ( )) D D E = ( )) . / F (15)
As a result, Fig. 11 presents the intrinsic true mechanical behaviours of the different materials.

Thanks to this correction of the volume strain the mechanical resistance increases as a function of the amine/MAH molar ratio, with a maximum for the 1:1 material. It is interesting to look at the evolution between the 1:1 and 1.5:1 material. Both material present close intrinsic true mechanical behaviours, but at equal true strain the 1:1 material presents the higher level of true stress. The plasticizing effect of the unreacted amino groups is here again put in evidence. Finally, to compare the effects of two crosslinking agents that differs in terms of geometry, the polyether diamine and the polyether triamine, the intrinsic true mechanical behaviours and the volume strain evolutions for both agents at the 1:1 and 1:5 molar ratios are given in Fig. 12.

The two crosslinking agents impact the mechanical behaviour with the switch from brittle to ductile, but not with the same final properties. Indeed, the materials modified with the THF 100 (linear polyether diamine [20]) show higher intrinsic true stress level and higher volume strain level, but the T3000 materials present higher intrinsic true strain at break, and a lower impact of the excess of amino groups on the intrinsic properties. In addition, the T3000 materials show a high impact of the excess of amine on volume strain level. In conclusion, with the same reaction but with different molecular structure, different mechanical properties are reached.

Conclusion:

The in situ crosslinking reaction performed by twin-screw extrusion on the iPP-g-MAH by the Jeffamine™ polyether triamine T3000 was confirmed by the infrared spectroscopy and gel content determination, and the 3D network appears more complex compared with the one obtained by using linear diamine [20]. A slight nucleating effect is observed for the low molar ratio (0.33:1) but is counterbalanced for higher molar ratios with a small decrease of the crystalline lamellae thickness. This modification is shown by the fall of the main melting temperature and the crystallisation of smallest iPP macromolecular elements. The crosslinked network is maximal for the equimolar ratio. It induces an increase of stiffness of the matrix, as seen with the Brillouin spectroscopy results and more importantly, a switch of mechanical behavior from brittle to ductile. This switch starts to appear with the 0.33:1 molar ratio and the best mechanical properties are obtained for the 1:1. The 1.5:1 material shows that the excess of triamine induces a slight decrease of the elastic modulus and the volume strain, but presents finally a very close true mechanical behavior in comparison to the one obtained for the 1:1 material.

Compared with the same initial material crosslinked with polyether diamine [20], the polyether triamine materials present a highest true strain level at break, and lower level of volume strain and true intrinsic stress, as revealed by VidéoTraction™. An interesting result is that the impact of the polyether triamine excess, on the 1.5:1 molar ratio material, is less important compared with the polyether diamine at the same molar ratio. Statistically, in this situation, two of the three amine groups of each T3000 molecules react with different MAH groups from the CA 100 and maintained the 3D network, with remaining free NH2 groups. The other interesting material is the 0.66:1 molar ratio, which exhibits an important rise of the mechanical properties, with remaining MAH groups for further reaction.

Such materials show interesting level of ductility and volume strain (essential to ensure good shock absorption properties) and remaining MAH or NH2 groups grafted to the matrix. As described before, different techniques are used to obtain functionalized polyolefin with free MAH or amine groups, but such materials present low mechanical properties compared to those new materials.

Fig. 1 .

 1 Fig. 1. Molecular structure of the Jeffamine® T3000.

Fig. 2 .

 2 Fig. 2. Chemical reaction scheme illustrating the crosslinking steps between T3000 and iPP-g-MAH through the formation of either amide or imide units, with different final structures

Fig. 3 .

 3 Fig. 3. Gel content measurements evolution as a function of the NH2:MAH molar ratio.

Fig. 4 .

 4 Fig. 4. Evolution of the FTIR spectra as a function of the MAH: NH2 molar ratio.

Fig. 5 .

 5 Fig. 5. a) 1D WAXS diffractograms and b) DSC thermograms evolution as a function of the NH2:MAH molar ratio.

Fig. 6 .

 6 Fig. 6. a) SSA last thermograms for the different materials as a function of the NH2:MAH molar ratio, and b) distribution of the different lamellae populations between the different thickness according to their melting temperature.

Fig. 6

 6 Fig. 6 shows the final heating ramp of the SSA protocol for the materials from the unmodified CA 100 to the 1.5:1 material (Fig 6 a)), and the repartition of the crystallinity between the different crystallite size population (Fig 6 b)). Six different crystalline lamellae populations appear for each material. No change appeared for the first thickest crystallites (Lc = 19.5 nm)between the unmodified CA 100 and the 0.33:1 material, whereas the second thickest ones (Lc = 15.5 nm) present a rise from 22 to 26% of the crystallinity ratio. This rise explains the evolution of the melting temperature between the two materials, and the evolution of the double shoulder shaped endotherms. The reaction, at low levels of crosslinking, impacts positively the crystallisation of the medium chain elements of the polypropylene and implied the rise of the melting temperature observed by DSC analysis. For the highest molar ratios from the 0.5:1 material, a fall of the thickest crystalline population associated with the longest chain elements is observed, from 62% for the unmodified CA 100 and the 0.33:1 material to 55% for the 0.5:1 material and 41% for the 1:1 molar ratio. This decrease is in favour of the second thickest population associated to the medium chain elements, which represents 41 % of the 1:1 material crystallinity. The crosslinking reaction, with the creation of chemical network, affects the crystallisation of the longest chain elements. This evolution explains the decrease of the melting temperature observed by DSC. The microstructure evolution observed with the increase of the NH2/MAH molar ratio is similar to the previous results obtained with similar method[20], or in case of other crosslinked polymers in the literature, but at a smaller scale[21,22,[56][57][58].

Fig. 7 .

 7 Fig. 7. Typical Brillouin response of the the 1:1 NH2:MAH molar ratio crosslinked polypropylene.

Fig. 8 .

 8 Fig. 8. C11 elastic constant and refractive index n evolution as a function of the NH2:MAH molar ratio.

Fig. 9 .

 9 Fig. 9. Evolution of the a) true mechanical behaviour and b) volume strain in uniaxial tension for the different crosslinked materials.

  5:1 materials indicate the same level of crosslinking. In this configuration each polyether triamine, statistically reacts with two different MAH groups and has one free branch with an unreacted amino group. Here, due to the important size of each free branches (1000 g.mol -1 ) the three dimensional network is denser compared with the 1:1 material that induces the difference of the true mechanical behaviours. SEM micrographies of the fractured surface of the different stretched materials are given in Fig 10. In Fig 10 a), the unmodified CA 100 micrography indicates a brittle behaviour with no plastic deformation. Fig 11 b

Fig. 10 .

 10 Fig. 10. Evolution of the fracture surfaces of a) unmodified CA 100, b) 0.5:1, c) 1:1, and d) 1.5:1 materials by SEM micrographies.

Fig. 11 .

 11 Fig. 11. True intrinsic mechanical behaviours of the different materials as function of the NH2:MAH molar ratio.

Fig. 12 .

 12 Fig. 12. Comparison of the a) true intrinsic mechanical behaviour and b) volume strain evolution for the crosslinked materials with the two polyether amines in the cases of 1:1 and 1.5:1 molar ratios.

Table 1

 1 Evolution of the crystallinity ratio and melting temperatures as function of the NH2:MAH molar ratio.

	Materials unmodified	0.33:1	0.5:1	0.66:1	1:1	1.5:1
	Xc (± 3%) (1 st heating)	45	46	47	47	44	44
	Tm (± 1°C)(1 st heating)	165/160	165/161	163	162	161	162
	Xc (± 3%)(2 nd heating)	47	45	47	46	44	44
	Tm (± 1°C)(2 nd heating)	161	164/163	161	160	161	161
	Xc (± 3%)(1D WAXS)	62	59	60	62	61	61

Acknowledgements

The authors thank Huntsman, Eastman and Arkema for providing chemicals and polymers and Provisys Engineering Company for facilities with the VidéoTraction™ system. AL and MP thank the French Ministry for the PhD grant and the LabEX DAMAS for the financial support.


S Hattori
email: hattori.s.ab@m.titech.ac.jp

Y Iizuka

B Alexander

S Ishino

K Fujita

S Zhai

T Sherwen

N Oshima

R Uemura

A Yamada

N Suzuki

S Matoba

A Tsuruta

J Savarino

N Yoshida

Isotopic Evidence for Acidity-driven Enhancement of Sulfate Formation after SO2 emission control

After the 1980s, atmospheric sulfate has remained relatively high despite dramatic reductions in sulfur dioxide (SO2) emissions. However, a lack of observational evidence has hindered the identification of causal feedback mechanisms. Here we report an increase in the oxygen isotopic composition of sulfate (D 17 OSO 4 2 ) in a Greenland ice core, implying an enhanced role of aciditydependent in-cloud oxidation by ozone (up to 18%) in sulfate production since the 1960s. A global chemical transport model reproduces the magnitude of the increase in observed D 17 OSO 4 2 with a 10 15% enhancement in the conversion efficiency from SO2 to sulfate in Eastern North America and Western Europe. With an expected continued decrease in atmospheric acidity, this feedback will continue in the future and partially hinder air quality improvements.

One Sentence Summary: Observed 17 O excess of sulfate in an ice core record suggests an acidity-driven enhancement of atmospheric sulfate formation in the recent past.

Main Text:

Atmospheric sulfate (SO4 2-) has a significant but uncertain impact on the global radiation budget and cloud lifetimes (1). sulfate also accounts for a major component of fine particulate matter mass in urban regions, affecting visibility (2) and public health (3). Since the Industrial Revolution, increased emissions of sulfur dioxide (SO2) have resulted in an increase in sulfate load. The period from the 1950s to the 1970s had increased SO2 emissions leading to highpollution; however, switching to cleaner technology and fuels decreased these emissions after the 1980s across North America (NA) and Western Europe (WE) (4). These decreases successfully lowered sulfate concentrations, which avoided hundreds of thousands of deaths and illnesses from exposure to particulate matter smaller than 2.5 µm (PM2.5) in the USA alone (3). However, atmospheric sulfate declined less rapidly than SO2 emissions, especially in wintertime (4,5). This unexpected phenomenon suggests the existence of feedback processes which render SO2 emission reductions less efficient than expected for mitigation of sulfate aerosol.

In the atmosphere, the majority (60-80%) of sulfate formation occurs through oxidation of SO2 in the aqueous-phase (i.e., in clouds), with gas-phase SO2 oxidation by hydroxyl radicals (•OH) accounting for most of the remainder (20-40%) (6). Upon dissolution in the aqueous phase, SO2 dissociates into S(IV) species (mainly HSO3 -and SO3 2-) that react with hydrogen peroxide (H2O2), ozone (O3), and molecular oxygen (O2) to form sulfate (7). S(IV) oxidation is thought to be dominated by H2O2 oxidation, which is largely insensitive to cloud acidity, whereas the more minor S(IV) + O3 pathway exhibits strong pH dependence (7). Among these chemical processes, several studies have proposed enhancement of S(IV) + H2O2 (8,9), S(IV) + O3 (10), or SO2 + •OH (9,11) oxidation pathways as mechanisms causing the weakened response of sulfate abundances to decreases in SO2 emissions. These conclusions are based on a comparison between observed and modeled sulfate concentrations, and model estimates of atmospheric sulfate production mechanisms. However, observational evidence so far has not pointed to a specific mechanism. These uncertainties limit confidence in the model forecast and hindcasts of management of current and future tropospheric sulfate aerosol and its environmental impacts.

One tool for providing insight into sulfate formation mechanisms is offered by the massindependent oxygen isotopic composition (D 17 O) (12) of sulfate. D 17 OSO 2 equals 0‰ because of rapid oxygen exchange between SO2 and H2O in the atmosphere (13), and thus the D 17 OSO 4 2- value reflects the oxidation pathway of SO2 to SO4 2-. Gas-phase SO2 oxidation by •OH, where D 17 O•OH = 0‰, produces SO4 2-with D 17 OSO 4 2-= 0‰ (14). Aqueous-phase S(IV) oxidation by H2O2 and O3 leads to D 17 OSO 4 2 values of 0.8 and 6.5‰, respectively, since D 17 OH 2 O 2 = 1.6‰ (15) and D 17 OO 3 = 26‰ (16) (see Supplementary Text). Other production pathways, such as S(IV) + O2 catalyzed by trace metal ions (TMI) and in-cloud HOBr lead to D 17 OSO 4 2-at or near 0‰. Primary SO4 2-from natural and anthropogenic sources also has D 17 OSO 4 2-= 0‰ (17).

Consequently, D 17 OSO 4 2-is solely determined by the proportions of different sulfate formation pathways that yield non-zero values of D 17 OSO 4 2-(18): D 17 OSO 4 2-= 6.5 fO 3 + 0.8 fH 2 O 2 + ( 0.1) fTMI + 0 fzero (‰), (1) where the " # (x = O3, H2O2, TMI, and zero) terms indicate the respective fractions of SO2 oxidized by O3, H2O2, and TMI catalyzed O2, and fO 3 + fH 2 O 2 + fTMI + fzero = 1. To date, this approach using D 17 OSO 4 2-has enabled observation-based quantification of atmospheric sulfate formation in different regions and time periods, including glacial-interglacial cycles (19), stratospheric volcanic eruptions (20), preindustrial biomass burning (21), and transition after the Industrial Revolution (22). Particularly, comparison of observed and modeled D 17 OSO 4 2-has enabled the recognition and quantification of sulfate formation mechanisms often ignored in models, including S(IV) + O2 oxidation catalyzed by TMI (22,23), sulfate formation by ozone oxidation on sea salt aerosol, S(IV) oxidation by hypohalous acids (24,25), and heterogeneous reactions in extreme haze events (18,26). However, there is no record of D 17 OSO 4 2-that can provide information on changes in sulfate formation pathways in response to the reduction in air pollution following the implementation of governmental reduction policy such as US Clean Air Act of 1970.

Here, we present the first observations of changes in Northern Hemisphere D 17 OSO 4 2- between 1958 to 2015, based on a continuous and high-resolution ice core record from a highelevation dome site in southeast Greenland called SE-Dome. The record from the past 60 years covers the high-pollution decades of the 1950s-1970s as well as the substantial SO2 emissionsreduction period from the 1980s to the present (1). The SE-Dome ice core preserves atmospheric aerosols that originate mainly from NA and WE, with no notable change in the air mass origin over the period (27). We reconstructed D 17 OSO 4 2-in three to six years resolution with an accuracy of dating better than two months, which is given by precise age-depth scaling with the oxygenisotope matching method (28).

The ice-core D 17 Onss-SO 4 2-ranges from 1.0‰ to 1.7‰ and shows a substantial increase over the period of record (Fig. 1A), with a ~0.4‰ difference (p < 0.05) between the D 17 Onss-SO 4 This increase in D 17 Onss-SO 4 2 clearly indicates that the sulfate formation pathways responsible for sulfate preserved in the SE-Dome have changed from the 1960s to the present. Given that the D 17 OSO 4 2 signatures for the sulfate formation pathways are all lower than 0.8‰ except for the S(IV) + O3 pathway (D 17 O = 6.5‰), the D 17 Onss-SO 4 2 increase can reasonably be interpreted as the reflection of an increase in the relative importance of the S(IV) + O3 pathway. On the basis of simple mass balance calculation using Eq. ( 1), the estimated maximum and minimum fO 3 are increased from 3.5 16.5% to 15.7 27.7%, respectively, when the D 17 Onss-SO 4 2-is increased from 1.0‰ to 1.7‰ (see Supplement). The increase of the relative importance of the S(IV) + O3 pathway can be explained by (i) enhancement of the S(IV) + O3 pathway itself and (ii) decrease of other oxidation pathways that possess lower D 17 OSO 4 2-values. First, we consider the possible reasons that could cause an enhancement of the S(IV) + O3 pathway. Given that high pH conditions promote the S(IV) + O3 pathway (7), acidity changes may be an important factor accounting for the increase in D 17 Onss-SO 4 2 . Indeed, the best correlation between D 17 Onss-SO 4 2 and other ice-core measurements is found for the neutralization ratio, NH4 + /(2 nss-SO4 2 + NO3 ) (Fig. 1D), where $ = 0.80 (% < 0.01; Table S1). Additionally, the H + flux (Fig. 1E), the ice acidity indicator, also shows a strong correlation with D 17 Onss-SO 4 2 ($ = 0.71, % < 0.01; Table S1). The increase in the neutralization ratio and the decrease in the acidity result from the simultaneous decrease in nss-SO4 2 flux (Fig. 1B) and increase in NH4 + flux (Fig. 1C). This is consistent with previous studies that observed an increase in the pH of precipitation in NA and WE after the 1970s (29) and in a Greenland ice core (30), mainly because of the mitigation of SO2 emission and the simultaneous increase in NH3 emission from agricultural and industrial sectors (31). Changes in O3 concentrations over the period may also contribute to changes in sulfate formation pathways. Tropospheric O3 concentrations in the free troposphere (~3 km above sea level, a.s.l.) were ~30 ppb in the 1970s and increased to ~50 ppb by 2000, but there is no significant subsequent increase (32). Thus, an increase in tropospheric O3 might partially contribute to the increase of the S(IV) + O3 pathway before 2000, but is not consistent with the substantial increase of D 17 Onss-SO 4 2 for the post year 2000 period (Fig. 1A).

Secondly, we discuss the possibility of inhibition of other oxidation pathways which possess low D 17 Onss-SO 4 2 , because the decrease of other sulfate formation pathways also could increase the relative importance of the S(IV) + O3 pathway. Influence from changes in the contribution from SO2 + OH due to changes in tropospheric •OH concentrations is thought to be minimal, given that no significant decrease in •OH is observed between 1980 and 2010 (33). Greenland ice core shows a post-1950 increase of H2O2 (34), but this H2O2 increase is not consistent with the observed D 17 Onss-SO 4 2 increase, given the D 17 OSO 4 2-values of only 0.8‰ from S(IV) + H2O2

reaction. An observed 0.6‰ decrease in D 17 Onss-SO 4 2 in a Greenland ice core from 1900 to 1980 was partially attributed to an increase in the TMI catalyzed S(IV) + O2 oxidation pathway resulting from increases in anthropogenic metal emission after the Industrial Revolution (22). A decrease in anthropogenic metal emissions since 1980 would tend to decrease the importance of the TMI catalyzed S(IV) + O2 oxidation pathway, resulting in an increase in D 17 Onss-SO 4 2 as is observed.

To examine the relative importance changes in sulfate formation mechanisms on observed icecore D 17 Onss-SO 4 2 , we simulate global tropospheric chemistry using the GEOS-Chem chemical transport model (version 12.5.0, DOI:10.5281/zenodo.3403111) with an updated "offline" taggedsulfate aerosol simulation (22,23,[START_REF] Alexander | Isotopic constraints on sulfate aerosol formation pathways in the marine boundary layer of the subtropical northeast Atlantic Ocean[END_REF]. We performed model simulations using meteorology and anthropogenic emissions for the years 1960,1973,1986,1999, and 2013 (see Materials and Methods). This model calculates bulk cloud pH based on the local concentrations of inorganic species and CO2 (g) and considers the impact of heterogeneity in cloud water pH within a cloud [START_REF] Alexander | Isotopic constraints on sulfate aerosol formation pathways in the marine boundary layer of the subtropical northeast Atlantic Ocean[END_REF]. Since the main source region of sulfate aerosol to the Arctic region is transported from Eastern North America (ENA) and WE, we compare the ice-core observations with modeled tropospheric D 17 Onss-SO 4 2-in ENA (-90--60°E, 30-60°N, n = 56) and WE (15 to 40°E, 40-70°N, n = 72) (Fig. S2). We also compute modeled D 17 Onss-SO 4 2-in the region -90 to 30°E and 35 to 90°N based on computed 10-day back trajectories (n = 300, Trajectory (TRAJ) region hereafter) (27) (Fig. S2).

The GEOS-Chem model yields an increase of annual-mean, mass-weighted average of tropospheric D 17 OSO 4 2 from 1973 (0.79 to 1.31‰) to 2013 (1.45 to 1.75‰) for all three regions for TRAJ, NA, WE, an increase that is quantitatively consistent with the observed increase of ice-core D 17 OSO 4 2 (Fig 1A) over this same time period. The observed D 17 OSO 4 2 falls within modeled values in all years, consistent with a mixture of sulfate originating in ENA and WE. The increase in modeled D 17 OSO 4 2 is due to an increase in fO 3 between 1973 and 2013 from 2.0 to 8.2% and 7% to 21% for ENA, and WE regions, respectively (Fig. S3), consistent with the observation-based estimate of an increase in fO 3 from [3. 5 16.5% (1960 1970s) to 15.7 27.7% (present)]. GEOS-Chem also reproduces the observed D 17 Onss-SO 4 2 trend, and other trends for modeled sulfate (Fig. 1B), NH4 + (Fig. 1C), neutralization ratio (Fig. 1D), and bulk cloud pH (Fig. 1E) between 1973 and 2013 (see Supplementary Text).

The TMI catalyzed S(IV) + O2 pathway decreases by 10% in the model between 1973 to 2013, yielding a 0.1‰ increase in D 17 OSO 4 2 (Fig. S3). Although this goes in the right direction, it is not large enough to explain the observed 0.7‰ increase. Modeled O3 concentrations also increase over this same time period, but the increase from 1960 to 1986 is higher than from 1986 -2013 (Fig. S4). Modeled bulk cloud pH increased by 0.5 pH units between 1973 to 2013 (Fig. 1E) and neutralization ratio also shows substantial increases between 1973 to 2013 (Fig. 1D), with good agreement with modeled changes in D 17 OSO 4 2 for ENA and WE ($ > 0.75, % < 0.01; Table S2). In addition, no increase in the modeled D 17 OSO 4 2 by is found when cloud pH is set constant to pH = 4.5 in the model (Fig. S6). Thus, modeled increase in D 17 Onss-SO 4 2 is driven by an increase in the cloud water pH over the same time period.

We use the model to further investigate how changes in sulfate formation pathways alter the conversion efficiency (η) from SO2 to SO4 2 using the following metric:

ηx = P(SO4 2 )x / S(SO2), (2)
where P(SO4 2 )x indicates the tropospheric sulfate production rate from the oxidation of SO2 by oxidant x (i.e. OH, H2O2, O3, O2 (TMI), and hypohalous acids (H)), and S(SO2) represents total tropospheric source SO2 calculated from SO2 emission, photochemical SO2 production and net transport (import export). Although there is regional variability (Fig. S7), the common feature in all regions is that η increases from year 1973 to year 2013. The increase in η is caused mainly by increases in ηO 3 and ηH 2 O 2 , particularly in ENA, partially compensated by decreases in ηTMI and ηOH (Fig. 2AB). The modeled abrupt increase of η after 1999 is mainly caused increases in wintertime (Fig. S8B), consistent with a weakened response of reduction of US SO2 emission due to a weakening H2O2 limitation on the S(IV) + H2O2 pathway (8) and cloud pH-induced promotion of S(IV) + O3 pathway at low SO2 (10). The WE region has relatively high coal combustion and thus higher metal emissions, yielding a higher contribution from the TMI catalyzed SO2 + O2 oxidation in wintertime (Fig. S8C). Increases of ηO 3 and ηH 2 O 2 for WE between 1973 and 2013 are 10% and 8%, respectively, but these increases were compensated by a decrease of ηTMI by 5% (Fig. 2B). Notably, the main contribution for total η for WE was switched from the TMI catalyzed S(IV) + O2 oxidation in 1973 to the S(IV) + O3 oxidation at 2013 (Fig. 2B). An increase in ηO 3 in WE thus plays a greater role for the increase of η compared to ENA, because of relatively high cloud pH conditions (Fig. 1E) and more limited oxidant concentrations at higher latitude. The relatively small decreases in ηOH and increase in ηH in both regions are likely driven by an increase in cloud water pH which enhances SO2 solubility in clouds (7). As consequence, the increase in η over the ENA and WE regions kept sulfate reduction slower than the reduction of source SO2 during the past 60 years (Fig. 2AB), by increasing solubility of SO2 and promoting the acidity-dependent S(IV) + O3 pathway.

In this study, we present ice core D 17 OSO 4 2 record providing the first observational evidence that atmospheric acidity has driven changes in sulfate formation pathways and enhanced sulfate production rates after SO2 emissions control in NA and WE. The increases in sulfate production efficiency (η) has so far been partially compensated by the reduction of other pathways, particularly in WE (Fig. 2A), but this offset may not be significant in the future. The importance of S(IV) + O3 pathway, on the other hand, will continue to increase in the future due to the increase of cloud water pH by expected future growth of anthropogenic NH3 emission [START_REF] Paulot | Ammonia emissions in the United States, European Union, and China derived by high resolution inversion of ammonium wet deposition data: Interpretation with a new agricultural emissions inventory (MASAGE_NH3)[END_REF] and possible future usage of NH3 as hydrogen storage/transport medium in hydrogen energy technologies [START_REF] Lamb | Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and purification[END_REF]. SO2 emissions have increased in other parts of the world (e.g. China and India) over the last decades, but they then have been decreasing more recently and are expected to continue to decrease in coming decades [START_REF] Li | India Is Overtaking China as the World's Largest Emitter of Anthropogenic Sulfur Dioxide[END_REF][START_REF] Si | Assessment of satellite-estimated near-surface sulfate and nitrate concentrations and their precursor emissions over China from 2006 to 2014[END_REF]. Thus, the acidity-driven enhancement of atmospheric sulfate formation causing non-linear sulfate responses to SO2 is expected to occur for the wide area in the world as long as NH3 emissions are also not simultaneously controlled. The future feedback driven by atmospheric acidity for sulfate formation should be predicted to design effective mitigation policies for air quality and climate change, and observations of D 17 OSO 4 2 provide the ability to validate and give confidence in the model forecasts.

Materials and Methods

Samples

This study is based on the 90.45 m depth ice core obtained at a southeastern Greenland dome site (67.18°N, 36.37°W, 3170 m above sea level, SE-Dome hereafter) (1). The age-depth scale was determined by the oxygen-isotope matching method with an estimated age error of two months (2). The ice core samples were stored in the cold room (-50 °C) of The Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan.

The ion fluxes of the species used in Fig. 1 in the main text are from a previous study (3). To match the resolution of ion fluxes and D 17 OSO 4 2-, we reanalyzed the data with our resolutions as summarized in Table S1. In addition to ion fluxes, we calculated the neutralization ratio (N) and H + flux (F(H + )) determined by ion balance: The molar ratio of [SO4 2-]seawater / [Na + ]seawater in seawater is 0.06 (4).

N = NH4 + / (2 nss-SO4 2 + NO3 ) (S1) F(H + ) = F(Cl ) + 2F(SO4 2-) + F(NO3 -) F(Na + ) F(K + ) -2F(Mg 2+ ) 2F(Ca 2+ ), ( S2

Ice core cut

The ice sample was cut by a band saw in the cold room (-20 °C) and decontaminated by cutting the outside ice with a ceramic knife to ~70% of the sample's original weight in a class 10,000 clean booth. The sample (approximately 1~2 kg) was then melted in a pre-cleaned bottle and shipped frozen (~ 20 °C) to the Tokyo Institute of Technology, Yokohama, Japan. The sample was then stored in a freezer kept at 30 °C until the time of the experiments described below. The samples and corresponding years covered are summarized in Table S1.

Oxygen isotope analysis of sulfate

The measurement system for D 17 OSO 4 2 follows Savarino et al. (5), with modifications described in our previous study (6). SO4 2-was separated from other ions using ion chromatography and converted to H2SO4. 1 µmol of H2SO4 was then chemically converted to Na2SO4, and subsequently added 1 mL of 30% H2O2 solution and dried up. Next, the Na2SO4 was converted to silver sulfate (Ag2SO4) using an ion exchange resin (6). This Ag2SO4 powder GEOS-Chem model description and simulations. GEOS-Chem is a global 3-D model of atmospheric composition (www.geos-chem.org) originally developed by Bey et al. (11). In this study, we use GEOS-Chem (version 12.5.0, DOI: 10.5281/zenodo.3403111) driven by assimilated meteorological fields from MERRA-2 reanalysis data product from NASA Global Modeling and Assimilation Office's GEOS-5 Data Assimilation System. We simulate aerosol-oxidant tropospheric chemistry containing detailed HO x -NO x -VOC-ozone-BrO x chemistry (11,12,13). The model was run at 4 ×5 ̊ horizontal resolution and 47 vertical levels up to 0.01hPa. The model was spun up for 1 year before each of the five years simulated. In the model, sulfate is produced from gas-phase oxidation of SO2 (g) by OH, aqueous-phase oxidation of S(IV) by H2O2, O3, HOBr, metal-catalyzed O2, and heterogeneous oxidation on sea-salt aerosols by O3 (14,15).

The parameterization of the metal-catalyzed S(IV) oxidation is described in Alexander et al (16). We consider Fe and Mn which catalyze S(IV) oxidation in the oxidation states of Fe(III) and Mn(II). Dust-derived Fe ([Fe]dust) is scaled to the modeled dust concentration as 3.5% of total dust mass, and dust-derived Mn is a factor of 50 times lower than [Fe]dust. Anthropogenic Fe ([Fe]anthro) is scaled as 1/30 of primary sulfate and anthropogenic Mn ([Mn]anthro) is 10 times lower than that of [Fe]anthro. In the model, 50% of Mn is dissolved in cloud water as Mn(II) oxidation state, and 10% of [Fe]dust and 1% of [Fe]anthro are dissolved in cloud water as Fe(III) oxidation states. This parameterization might underestimate the anthropogenic Fe and Mn, especially for the US region, which is discussed in the Supplementary Text below.

For pH-dependent S(IV) partitioning, bulk cloud water pH is calculated as described in Alexander et al. (14). We use the Yuen et al. parameterization to account for the effect of heterogeneity of cloud water pH on S(IV) partitioning and subsequent aqueous phase sulfate formation (14). Sulfate formed from each oxidation pathway was treated as a different "tracer" in the model to calculate D 17 O as described elsewhere (17,18).

Given that meteorological fields from MERRA-2 are not available before 1979, we use meteorological fields and non-anthropogenic emissions such as biogenic VOCs (42), soil NOx (43), lightning (44), and stratospheric sources (44) from the year 1986, and set anthropogenic emissions, biomass burning emissions, and CH4 concentrations to specific years. For anthropogenic emissions we use the Community Emissions Data System (CEDS) inventory (http://www.globalchange.umd.edu/ceds/) (19). Emission species for CEDS include aerosol (black carbon (BC), organic carbon (OC)), aerosol precursors, and reactive compounds (SO2, NOx, NH3, CO, non-methane volatile organic carbon (NMVOC)). We use the biomass burning emissions from the CMIP6 (BB4CMIP) inventory for each individual year (20). Emission species for BB4CMIP include the following species: BC, CH4, CO, NH3, NMVOC, NOx, OC, SO2, and HCl. We prescribe latitudinal CH4 concentrations for historical simulations. For years after 1979, CH4 concentrations are based on NOAA GMD flask (observationswww.esrl.noaa.gov/gmd/ccgg/trends_ch4/), and for pre-1979 the CMIP6 monthly mean surface CH4 is used (21).

The simulations were performed for years 1960, 1973, 1986, 1999, and 2013, which enables us to isolate the impact of anthropogenic emissions on historical changes in sulfate formation pathways and D 17 OSO 4 2 based on Eq. 1 in the manuscript. In addition to the five model years with calculated cloud pH, we test the same model but assume cloud water pH is constant (pH = 4.5) for 1973 and 2013, in order to examine the importance of changes in bulk cloud pH for modeled D 17 OSO 4 2 over the period.

In addition to the calculation of the transported sulfate in the model, we also extracted online diagnostics to calculate sulfate production efficiency (η) using Eq. ( 2) in the main text. S(SO2) was calculated by summing the emission of SO2 from anthropogenic and volcanic activity, photochemical production of SO2 from DMS oxidation in the atmosphere, and the net import/export budget of SO2 by transportation. The production of SO4 2-(P(SO4 2-)) was also calculated online, as the sum from all aqueous, heterogeneous, and gas-phase production pathways.

Supplementary Text

Correlation between D 17 Onss-SO 4 2

and proxies of atmospheric composition

In order to determine the cause of the observed D 17 Onss-SO 4 2-trend, we compared D 17 Onss-SO 4

2-with other proxies of atmospheric composition preserved in the same ice core. The Mt. Pinatubo eruption in 1991 strongly influenced the sulfate flux (3), and probably influenced the D 17 Onss-SO 4 2-and other proxies preserved in the SE-Dome ice core. Thus, the two samples covering the periods from 1989 to 1991 (Sample 10, Table S1) and from 1992 to 1994 (Sample 11, Table S1) were excluded for the correlations discussed in the main text. After the exclusion, we found a weak correlation (J = 0.56, K < 0.05) between D 17 Onss-SO 4 2 and SO4 2 flux (Table S1). The correlation of D 17 Onss-SO 4 2 with SO4 2 flux is weaker than that with NH4 + flux (J = 0.62, K < 0.05, Table S1). The best correlation with D 17 Onss-SO 4 2 is found for the neutralization ratio, NH4 + /(2 nss-SO4 2 + NO3 ) (Fig. 1D), with a correlation coefficient of J = 0.80 (K < 0.01; Table S1). This suggests that acidity changes may be an important factor controlling the trends in D 17 Onss-SO 4 2 . Conversely, there is no correlation between D 17 Onss-SO 4 2 and Mg 2+ (J = 0.36, K = 0.22; Table S1), indicating the D 17 Onss-SO 4 2 variation is not the result of changing terrestrial dust fluxes (22).

GEOS-Chem model results and data extraction

In 2013 are calculated over a wide region, the D 17 OSO 4 2 at SE-Dome region did not show an increase, instead of decreasing by 0.1‰ from 1973 to 2013 (Fig. S1). Given that the increase in D 17 OSO 4 2 were clearly observed in the SE-Dome ice core (Fig. 1A in the main manuscript), we consider that several uncertainties in the model associated with aerosol transport, aerosol wet deposition, annual precipitation, and snow accumulation in complex mountain terrains in coarse resolution of the model (4° × 5°) caused this difference between model and observation. We therefore extract the modeled tropospheric D 17 Onss-SO 4 2 from three selected regions of TRAJ, ENA, and WE (Fig. S2), to obtain regional characteristics of D 17 OSO 4 2 . The changes in D 17 OSO 4 2 and other proxies show good agreement between the SE-Dome observation and the model from 1974 to the present (Fig. 1).

In Fig. S3, the relative proportion of each sulfate formation pathways in the model is shown. In the main manuscript, the calculated increase of fO 3 between 1973 and 2013 in the model was 2.0 to 8.2% and 7% to 21% for ENA and WE regions, respectively, but this figure shows the results including all formation pathways for all model years.

Correlation between modeled D 17 OSO 4 2 and the proxies in the model In Fig. S4, the modeled tropospheric oxidant concentrations are summarized. The trend of the oxidant concentrations is generally consistent with the broad expected trends, which is discussed in the main manuscript. Briefly, as noted earlier in a review for global trends (23), tropospheric O3 concentrations in the model increase after 1960s, but the increase found in the models are smaller than observations. The correlation between O3 concentrations to the modeled D 17 OSO 4 2 is not significant (Table S2), likely due to the smaller increase of O3 after the end of 20 th century. This disagreement supports that the proportional contribution of the S(IV) + O3 pathway is mainly driven by cloud pH, and not by O3 changes. The model calculates and increase in H2O2 from 1973 to 2013, and this is slightly correlated with D 17 OSO 4 2 (J = 0.63) over the model period, but the increased proportion of the S(IV) + H2O2 pathway does not explain the increase of D 17 OSO 4 2 because of the D 17 OSO 4 2 from the S(IV) + H2O2 pathway possesses a value (0.8‰) to low to explain the observed increase from 1973 (D 17 OSO 4 2 = 0.79 to 1.31‰) to 2013 (D 17 OSO 4 2 = 1.45 to 1.75‰) for three selected regions of TRAJ, NA, and WE.

In Fig. 1B-E, the modeled sulfate and NH4 + concentrations, neutralization ratio (calculated from (NH4 + + NH3) / [2 nss-SO4 2 + (NO3 + HNO3)] based on the modeled concentrations of these species), and modeled bulk cloud pH are shown. For the neutralization ratio, the absolute value is different among observation and model (Fig. 1D), possibly owing to the uncertainty originating from transport and wet and dry deposition. Note that the modeled bulk cloud pH represents an average in the grid boxes of each region, and actual pH for sulfate formation, especially for the S(IV) + O3 pathway, is not identical to the average value. We note that the important finding of this study is that acidity indicators (neutralization ratio, H + flux, and bulk cloud pH) for the observations and the model show good agreement with modeled changes in D 17 OSO 4 2 (Table S1 andS2).

The uncertainly in modeled D 17 OSO 4 2- The end member for D 17 OSO 4 2-of the S (IV) + O3 oxidation (= 6.5‰) value used in this study is on the basis of 1/4 slope between D 17 OSO 4 2-and D 17 O of O3(bulk) from a laboratory experiment (24). This assumption hypothesizes that 1/4 of D 17 O for SO4 2-originates from O3, and that all 3 oxygen atoms of O3 are equally likely to transfer to the product SO4 2-. However, we note that if O atom transfer occurs only from one of the two terminal oxygen atoms of O3 (O3(term) = ~39.2‰), which is expected from theoretical calculations (25), D 17 OSO 4 2-from S(IV) + O3 would be 9.8‰. When assuming D 17 OSO 4 2-= 9.8 ‰ for the S(IV) + O3 oxidation, the modeled D 17 OSO 4 2-is increased for all three regions by 0.3-0.5‰ relative to the model when using D 17 OSO 4 2-= 6.5‰ for the S (IV) + O3 oxidation (Fig. S5). If this assumption is true, only the variation of D 17 OSO 4 2-for ENA is similar to the observation in the SE-Dome ice core (Fig. S5), and the model is biased high in D 17 OSO 4 2-for TRAJ and WE regions. This may indicate that the observed D 17 Onss-SO 4 2-record reflects in the changes in sulfate formation pathway mainly for ENA. We note that the case assuming D 17 OSO 4 2-= 6.5‰ in the model showed better agreement with the observations. This determination of end member of D 17 OSO 4 2-of the S (IV) + O3 oxidation from more comprehensive laboratory experiments is needed in future studies. The assumption of D 17 OSO 4 2-of the S (IV) + O3 oxidation pathway does not change the trends in modeled D 17 OSO 4 2-, nor our conclusions.

Secondly, the model may underestimate pFe concentrations, especially for ENA region. In the US, pFe is emitted from coal combustion proportional to primary SO 2 4 -emissions with an emission ratio of 0.15 kg kg -1 (26), which is ~5 times higher than our estimate in the model. These uncertainties for metal emissions might result in a model underestimate the proportion of TMI catalyzed S(IV) + O2 pathway and overestimate of D 17 OSO 4 2 in the model. In particular, the proportional contribution of TMI catalyzed S(IV) + O2 pathway in ENA region only possess 2~3% for total sulfate, but it is smaller than the earlier estimate of Shah et al. (27) which modeled ~10% contribution of this pathway for the atmosphere below in 2015. In that case, D 17 OSO 4 2 in the model would be lower than current result shown in Fig. 1A for ENA. This underestimate and uncertainty may cause the inconsistent result between 1960 and 1973, which shows no significant difference of D 17 Onss-SO 4 2 for observation but a significant decrease in the model.

Thirdly, the model-calculated cloud water pH and its heterogeneity has a large influence on the importance of the S (IV) + O3 pathway, and also represents a source of uncertainty in the model. In this study, we use the model's default setting of calculated bulk cloud water pH (14) and its heterogeneity according to Yuen's parameterization (28). We note that the increase of D 17 OSO 4 2 due to the Yuen et al. parameterization contributes ~10% of total P(SO4 2 ) at maximum of for North Atlantic Ocean, and ~5% increase of η for TRAJ regions. Since atmospheric acidity plays a central role for aqueous chemistry in general, cloud and aerosol pH have implications for many other chemical reactions, including nitrate, ammonium, halogen, metals, and organic compounds reactions that link emission to impacts on air quality, human health, ecosystem, and climate change (29). Prior to the 1990s, cloud pH was too low in many regions for S(IV) + O3 reaction to be significant, leading earlier studies to often neglect the S(IV) + O3 reaction completely (30,31). Recent studies have proposed an increase in cloud water pH promoting the S(IV) + O3 pathway to explain the sublinear response of SO4 2 concentrations to the decrease in SO2 emission (32,33), but even the latest study (34) prescribes cloud pH to investigate past sulfate production and its radiative effect. Although this uncertainty should be considered in future studies, we note that, when we set a constant value for cloud water pH in the model, the increase of D 17 OSO 4 2-does not occur (Fig. S6). This result strongly supports our conclusion that the increase in D 17 OSO 4 2-reflects the enhanced role of O3 for atmospheric sulfate formation pathway induced by an increase in the cloud water pH after SO2 emission control.

As discussed above, although there are several factors leading to uncertainty in the calculation of the absolute value of D 17 OSO 4 2-in the model, the increase of modeled D 17 OSO 4 2- after 1973 is driven by increases in cloud water pH. Thus, our main conclusion in the manuscript is not changed.

Sulfate production rates and efficiency in the GEOS-Chem model Fig. S7 shows the spatial distribution of η calculated in GEOS-Chem. The modeled increase of η from 1973 to 2013 occurs in both the NA and WE regions, and is mainly caused by an increase in ηO 3 and ηH 2 O 2 . The decrease in ηTMI and ηOH partially compensated for the total increase of η as discussed in the manuscript. The decrease in ηTMI was caused by decrease in metal emission, especially for WE, and the decrease of ηOH was caused by increase of SO2 solubility to the aqueous-phase by increase cloud water pH as mentioned in the main manuscript.

Fig. S8 shows the seasonal pattern of η. The increase in η occurred in both summer (JJA) and winter (DJF) for WE (Fig. S8C andS8F). These increases in η for WE are partially compensated by decreases in ηTMI in wintertime and ηTMI in summertime (Fig. S8C andS8F). For ENA, the increase in η is mainly for wintertime and was caused by increases in ηO 3 and ηH 2 O 2 (Fig. S8B). Smaller increases in summertime η is mainly caused by an increase in ηO 3 (Fig. S8E). More detailed analysis of these feedback mechanisms causing non-linear sulfate responses to SO2 for other regions of the world is desirable, and future model predictions under different emission control strategies may provide valuable information for designing effective pollution control and climate change prediction in the future.

2 average

 2 of 1960-1970 [(1.14±0.06)‰, n = 4)] and that of 2005-2015 [(1.51±0.19)‰, n = 3)].

Fig. 1 .Fig. 2 .

 12 Fig. 1. D 17 Onss-SO 4 2 and chemical fluxes at the SE-Dome and GEOS-Chem model results during the last 60 years. Open black circles represent ice core record and closed black circle represents data from snow pit, and red symbols represent model results for given years for TRAJ, ENA, and WE regions. The observed chemical fluxes and neutralization ratio were obtained from Iizuka et al. (27). The thin lines represent observed data for each year, and the open circles with thick lines represent the weighted average flux data corresponding to D 17 OSO 4 2-sample resolution (see Supplementary Text). (A) D 17 OSO 4 2-record. Open black circles represent ice core record and closed circle represents data for shallow snow with 1 sigma uncertainty (± 0.1‰ or 0.2‰) shown as error bar. Red sybols represent annual-mean, mass-weighted average of tropospheric for given years. (B) nss-SO4 2-flux and modeled annual-mean SO4 2-concentration normalized to 1973 (C) NH4 + flux and modeled annual-mean concentrations of NH3 + NH4 + normalized to1973, (D) neutralization ratio: NH4 + /(2 nss-SO4 2 + NO3 ) for observation, and (NH4 + + NH3) / [2 nss-SO4 2 + (NO3 + HNO3)] calculated from modeled, annual-mean tropospheric concentrations, and (E) H + flux and modeled tropospheric annual-mean, cloud liquid water weighted, bulk cloud-pH.

  )where F(X) (nmol m 3 yr -1 ) represents the flux of X as determined by Iizuka et al.(3), and nss refers to non-sea salt component.Since sea salt sulfate aerosols (ss-SO4 2-) are of little importance to atmospheric sulfur oxidation processes (i.e., D 17 Oss-SO 4 2 = 0 ), D 17 O values were corrected for their sea salt sulfate (ss-SO4 2 ) component by using Eq. S3 to obtain their corresponding nss-SO4 2 content, using Eq. S3 below.

  Fig. S1, the modeled annual-mean mass-weighted average of tropospheric D 17 OSO 4 2 for 1960, 1973, 1986, 1999, and 2013 are shown. Although clear increases in D 17 OSO 4 2 from 1973 to

Fig. S1 .

 S1 Fig. S1. Modeled annual-mean, mass-weighted average of tropospheric D 17 OSO 4 2-for 1960 (A), 1973 (B), 1986 (C), 1999 (D), and 2013 (E), and differences in D 17 OSO 4 2-relative to 1973 (F to K).

Fig. S2 .

 S2 Fig. S2. Map for data extracted regions from the model, Trajectory region (TRAJ) (A), Eastern North America (ENA) (B), and Western Europe (WE) (A). (A) Color indicate probability distribution of an air mass arriving at the SE-Dome site from a 10-days 3-D backward-trajectory analysis for two elevations (1,000 and 1,500 m agl) (3).

Fig. S3 .

 S3 Fig. S3. Proportional contributions of each sulfate (SO4 2-) formation pathway and modeled weighted average of D 17 OSO 4 2-for TRAJ, ENA, and WE over the period.

Fig. S4 .Fig. S5 .Fig. S6 .

 S4S5S6 Fig. S4. Modeled annual-mean tropospheric concentrations of oxidants (ozone, H2O2, OH, pFe) from 1960 to 2013. The color symbols are the same as Fig. 1.

Fig. S7 .

 S7 Fig. S7. The calculated conversion efficiency of SO2 to sulfate (η) for the model of 1973 and 2013. (A) total η for 1973; (B) total η for 2013; (C) Difference in total η between 1973 and 2013. (D-G) η in each sulfate formation pathway for 2013: SO + OH (D), S(IV) + H2O2 (E), S(IV) + O3 (F), S(IV) + O2 (TMI) (G). (H-K) Difference in each sulfate formation pathway between 1973 and 2013: SO + OH (H), S(IV) + H2O2 (I), S(IV) + O3 (J), S(IV) + O2 (TMI) (K).

Fig. S8 .

 S8 Fig. S8. Seasonal difference of modeled η, S(SO2), and P(SO4 2-) between wintertime (DJF) and summertime (JJA) at TRAJ (A, D), ENA (B, E), and WE (C, F) regions.

  1,* , Yoshinori Iizuka 2 , Becky Alexander 3 , Sakiko Ishino 1 † , Koji Fujita 4 , Shuting Zhai 3 , Tomás Sherwen 5 , Naga Oshima 6 , Ryu Uemura 4 , Akinori Yamada 7 , Nozomi Suzuki 1 , Sumito Matoba 2 , Asuka Tsuruta 1 , Joel Savarino 7 , Naohiro Yoshida 1,8

	Materials and Methods
	Supplementary Text
	Figs. S1 to 8
	Tables S1 to S2
	Reference (1-34)

Correspondence to: hattori.s.ab@m.titech.ac.jp This PDF file includes:

Table S1 .

 S1 Data used in Fig.1and correlations between D 17 OSO 4 2 and fluxes atmospheric components over the last 60 years.

	Sample ID	Start year	End year	Periods of year	SO4 2	D 17 O nss-SO4 2	SO4 2	Anion species nss-SO4 2 Cl	NO3	Na +	Cation species NH4 + Mg 2+	Ca 2+	Neutralization ratio	H +
						‰						mmol m 2 yr 1
	1	1959		3	1.03	1.06	0.64	0.62	0.39	1.08	0.31	0.50	0.02	0.07	0.216	1.74
	2	1962		3	1.14	1.16	0.67	0.66	0.32	1.10	0.22	0.63	0.02	0.07	0.260	1.73
	3	1965		4	1.14	1.17	0.60	0.59	0.27	0.79	0.18	0.32	0.02	0.07	0.164	1.58
	4	1969		3	1.12	1.15	0.57	0.55	0.33	0.80	0.24	0.41	0.02	0.06	0.216	1.46
	5	1972		3	1.15	1.20	0.60	0.57	0.47	0.86	0.40	0.43	0.03	0.11	0.211	1.42
	6	1975		3	0.95	0.98	0.87	0.85	0.43	1.21	0.31	0.72	0.03	0.12	0.247	2.05
	7	1978		4	1.25	1.30	0.57	0.56	0.39	1.04	0.27	0.58	0.02	0.08	0.271	1.52
	8	1982		3	1.39	1.43	0.81	0.79	0.54	1.21	0.37	0.70	0.03	0.10	0.252	2.04
	9	1985		4	1.30	1.35	0.50	0.48	0.43	1.30	0.26	0.51	0.04	0.10	0.226	1.68
	10	1989		3	1.13	1.21	0.70	0.67	0.75	1.50	0.54	0.74	0.05	0.08	0.262	2.11
	11	1992		3	1.51	1.57	0.83	0.82	0.48	1.67	0.30	1.13	0.03	0.09	0.340	2.16
	12	1995		5	1.27	1.34	0.34	0.32	0.40	1.19	0.27	0.58	0.02	0.08	0.320	1.21
	13	2000		5	1.21	1.30	0.37	0.35	0.57	1.40	0.40	0.72	0.05	0.22	0.342	1.08
	14	2005		6	1.44	1.55	0.35	0.33	0.56	1.10	0.36	0.78	0.04	0.14	0.443	0.86
	15	2011		4	1.38	1.67	0.46	0.42	0.81	1.01	0.68	0.97	0.03	0.24	0.524	0.54
	Correlati on with D 17 O	Entire period Without (1989-1994)		r p r p	0.32 0.238 0.56 0.047	0.34 0.210 0.58 0.037	0.56 0.031 0.76 0.003	0.28 0.303 0.16 0.610	0.47 0.080 0.65 0.016	0.66 0.007 0.62 0.023	0.21 0.452 0.36 0.222	0.50 0.056 0.36 0.035	0.79 0.0004 0.80 0.0003	0.45 0.089 0.71 0.007

Table 2 .

 2 Correlations between D 17 OSO 4 2 and oxidants concentrations and acidity indicators calculated from the model.

			Neutralization ratio	Bulk cloud pH	O3	H2O2	OH	pFe
	TRAJ	r p	0.57 0.2525	0.90 0.0039	-0.18 0.7523 0.9405 0.04	0.88 0.0085	-0.87 0.0102
	ENA	r p	0.97 0.00003	0.98 0.00001	0.35 0.529	0.63 0.182	0.00 0.994	-0.93 0.001
	WE	r p	0.76 0.07	0.98 0.00001	-0.02 0.9765 0.6174 0.28	0.91 0.0031	-0.94 0.0005

Acknowledgments: We gratefully acknowledge the drilling and initial analysis teams of SE-Dome. We thank Pete Akers for editing a draft of this manuscript. Funding: This work was supported by MEXT/JSPS KAKENHI grant numbers JP16H05884, JP18H05292, JP17H06105 JP18H03363. S. H. appreciates support for this project from JSPS and CNRS under the JSPS-CNRS Joint Research Program. We are grateful to the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center for providing the MERRA-2 metereological

was transported in a custom-made quartz cup, which was dropped into the 1000 °C furnace of a high-temperature conversion elemental analyzer (TC/EA; Thermo Fisher Scientific, Bremen, Germany) and thermally decomposed into O2 and SO2. Gas products from this sample pyrolysis were carried by ultrahigh-purity He (>99.99995 % purity; Japan Air Gases Co., Tokyo, Japan), which was first purified using a molecular sieve (5Å) held at 196 °C (7). The O2 and SO2 gas products were carried through a clean-up trap (trap 1) held at -196 °C to trap SO2 and trace SO3, while O2 was transported to another 1/16 inch outer diameter tubing trap with a molecular sieve (5 Å) (trap 2) held at 196 °C to trap O2 separately from the other gas products. The O2 was purified using a gas chromatograph, with a CP-Molsieve (5 Å) column (0.32 mm inner diameter, 30 m length, 10 µm film; Agilent Technologies Inc., Santa Clara, CA, USA) held at 40 °C, before being introduced to the IRMS system to measure m/z = 32, 33, and 34. As discussed by Schauer et al. (8), this method results in oxygen isotope exchange between the O2 products and the quartz cups having D 17 O of 0‰ ( 9 where "total" is the quantity measured by ion chromatography, corresponding to the sum of ssand nss-SO4 2-components. The data for D 17 Ototal-SO 4 2 and D 17 Onss-SO 4 2 are summarized in table S1.

Calculation of the maximum and minimum contribution of the S (IV) + O3 oxidation

The maximum fO 3 was calculated assuming that SO4 2-is produced only by the S (IV) + O3 oxidation (D 17 OSO 4 2-= 6.5 ) and the TMI catalyzed S (IV) + O2 oxidation (D 17 OSO 4 2-= -0.1‰). In a similar manner, the minimum fO 3 was calculated assuming that nss-SO4