Sakiko Ishino

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

Michel Legrand

Q Chen

Becky Alexander

J Shao

J Huang

L Jaeglé

Bruno Jourdain

Suzanne Preunkert

L Jaegle

A Yamada

N Yoshida

J Savarino

Regional characteristics of atmospheric sulfate formation in East Antarctica imprinted on 17O-excess signature

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

Introduction

Sulfate is a major component of impurities trapped in Antarctic ice cores and widely used for reconstruction of paleoclimate conditions (e.g., [START_REF] Legrand | Glaciochemistry of polar ice cores: A review[END_REF]. For instance, since the main source of sulfate in Antarctica is oxidation of dimethyl sulfide (DMS) emitted by marine biota [START_REF] Minikin | Sulfurcontaining species (sulfate and methanesulfonate) in coastal Antarctic aerosol and precipitation[END_REF][START_REF] Cosme | Origin of dimethylsulfide, nonsea-salt sulfate, and methanesulfonic acid in eastern Antarctica[END_REF], sulfate in the Antarctic ice cores are often discussed in light of the past bioproductivity of the Southern Ocean [START_REF] Legrand | Climate forcing implications from Vostok ice-core sulphate data[END_REF][START_REF] Wolff | Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles[END_REF][START_REF] Goto-Azuma | Reduced marine phytoplankton sulphur emissions in the Southern Ocean during the past seven glacials[END_REF]. In addition to the use of sulfate content in ice cores, 17 O-excess ( 17 O =  17 O -0.52 ×  18 O) of sulfate ( 17 O(SO 4 2-)) is expected to be a potential tool assessing past oxidant chemistry involving ozone (O 3 ) and hydroxyl radicals (OH), those playing central roles in tropospheric chemistry but not directly preserved in ice cores manuscript submitted to Journal of Geophysical Research: Atmospheres 3 [START_REF] Alexander | Paleo-Perspectives on Potential Future Changes in the Oxidative Capacity of the Atmosphere Due to Climate Change and Anthropogenic Emissions[END_REF][START_REF] Kunasek | Sulfate sources and oxidation chemistry over the past 230 years from sulfur and oxygen isotopes of sulfate in a West Antarctic ice core[END_REF][START_REF] Sofen | WAIS Divide ice core suggests sustained changes in the atmospheric formation pathways of sulfate and nitrate since the 19th century in the extratropical Southern Hemisphere[END_REF][START_REF] Murray | Factors controlling variability in the oxidative capacity of the troposphere since the Last Glacial Maximum[END_REF].

 17 O(SO 4 2-
) is generally assumed to reflect relative importance of different sulfate formation pathways, since the sulfate produced via gas-phase oxidation of SO 2 by OH possesses  17 O = 0 ‰ [START_REF] Dubey | Isotope specific kinetics of hydroxyl radical (OH) with water (H 2 O): Testing models of reactivity and atmospheric fractionation[END_REF][START_REF] Barkan | High precision measurements of 17 O/ 16 O and 18 O/ 16 O ratios in H 2 O[END_REF], whereas those produced via aqueous-phase oxidations of dissolved SO 2 (S(IV) = SO 2 ・H 2 O + HSO 3 -+ SO 3 2-) by O 3 or hydrogen peroxide (H 2 O 2 ) possess  17 O > 0 ‰ [START_REF] Savarino | Analytical procedure to determine both δ 18 O and δ 17 O of H 2 O 2 in natural water and first measurements[END_REF][START_REF] Savarino | Laboratory oxygen isotopic study of sulfur (IV) oxidation: Origin of the mass-independent oxygen isotopic anomaly in atmospheric sulfates and sulfate mineral deposits on Earth[END_REF][START_REF] Vicars | Quantitative constraints on the 17 O-excess ( 17 O) signature of surface ozone: Ambient measurements from 50°N to 50°S using the nitrite-coated filter manuscript submitted to Journal of Geophysical Research: Atmospheres 33 technique[END_REF]. [START_REF] Sofen | WAIS Divide ice core suggests sustained changes in the atmospheric formation pathways of sulfate and nitrate since the 19th century in the extratropical Southern Hemisphere[END_REF] ) in aerosol samples (ca.

1.5 ‰; [START_REF] Hill-Falkenthal | Seasonal variations in 35 S and Δ 17 O of sulfate aerosols on the Antarctic plateau[END_REF][START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF][START_REF] Walters | Assessing the Seasonal Dynamics of Nitrate and Sulfate Aerosols at the South Pole Utilizing Stable Isotopes[END_REF] and those in icecores corresponding to the present-day climate conditions (ca. 3 ‰; [START_REF] Alexander | Climate driven changes in the oxidation pathways of atmospheric sulfur[END_REF][START_REF] Alexander | East Antarctic ice core sulfur isotope measurements over a complete glacial-interglacial cycle[END_REF][START_REF] Kunasek | Sulfate sources and oxidation chemistry over the past 230 years from sulfur and oxygen isotopes of sulfate in a West Antarctic ice core[END_REF][START_REF] Sofen | WAIS Divide ice core suggests sustained changes in the atmospheric formation pathways of sulfate and nitrate since the 19th century in the extratropical Southern Hemisphere[END_REF]. Despite the significance of this ~1.5 ‰ shift in

 17 O(SO 4 2-
) compared to the observed variability in ice cores (1.3-4.8 ‰ for glacial-interglacial time scale), there has been no study pointing it out so far. Thus, the interpretation of ice-core

 17 O(SO 4 2-
) records requires a better understanding of atmospheric sulfate formation in Antarctica.

In Antarctica, where the impact of anthropogenic emissions is still insignificant, there exist unique oxidative conditions associated with natural emissions of reactive trace gases from snow and sea-ice surfaces [START_REF] Grannas | An overview of snow photochemistry: evidence, mechanisms and impacts[END_REF][START_REF] Simpson | Halogens and their role in polar boundary-layer ozone depletion[END_REF]. One characteristic is drastic enhancements of photochemical oxidants, represented by OH and O 3 , over the Antarctic Plateau after polar sunrise to the austral mid-summer [START_REF] Grannas | An overview of snow photochemistry: evidence, mechanisms and impacts[END_REF][START_REF] Crawford | Evidence for photochemical production of ozone at the South Pole surface[END_REF][START_REF] Mauldin | Measurements of OH, H 2 SO 4 and MSA at the South Pole during ISCAT[END_REF], which is mainly triggered by nitrate photolysis within snowpack [START_REF] Frey | Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling[END_REF][START_REF] Erbland | Airsnow transfer of nitrate on the East Antarctic Plateau -Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium in summer[END_REF][START_REF] Noro | Spatial variation of isotopic compositions of snowpack nitrate related to post-depositional processes in eastern Dronning Maud Land, East Antarctica[END_REF] emitting reactive nitrogen species (e.g., NO x ) to the atmosphere (Davis et al., 2008). It has been recently found that the concentration of methanesulfonate (MS -), a second abundant product of DMS oxidation after sulfate, suddenly decreased in the highly oxidative atmosphere in mid-summer at Dome C, inland Antarctica [START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF]. It was hypothesized that this may be due to a chemical destruction of MS -, possibly into sulfate, but the hypothesis needs confirmation and quantification. Since it is also known that MS -is partially lost in snow after its deposition [START_REF] Wagnon | Loss of volatile acid species from upper firn layers at Vostok, Antarctica[END_REF]Delmas et al., 2003;[START_REF] Weller | Postdepositional losses of methane sulfonate, nitrate, and chloride at the European Project for Ice Coring in Antarctica deep-drilling site in Dronning Maud Land, Antarctica[END_REF], it is important to examine the impact of MS -destruction on sulfate formation and  17 O(SO 4 2-) nss values in snow and ice. Another characteristic is the elevated reactive bromine over coastal Antartica during austral spring, as indicated from satellite observations of tropospheirc BrO columns [START_REF] Theys | Global observations of tropospheric BrO columns using GOME-2 satellite data[END_REF]. Hypobromous acids (HOBr), which is produced from the BrO + HO 2 reaction, was proposed to represent up to 50% of total sulfate production in the summertime marine boundary layer over the southern ocean [START_REF] Chen | Isotopic constraints on the role of hypohalous acids in sulfate aerosol formation in the remote marine boundary layer[END_REF]. The contribution of this reaction is thus expected to be also significant in the Antarctic troposphere.

There are only a few reports of  17 O(SO 4

2-

) observations in the present Antarctic atmosphere, and little is known about the influence of characteristic oxidation processes in ) observations of aerosols at three different sites, Dome C [START_REF] Hill-Falkenthal | Seasonal variations in 35 S and Δ 17 O of sulfate aerosols on the Antarctic plateau[END_REF] and South Pole [START_REF] Walters | Assessing the Seasonal Dynamics of Nitrate and Sulfate Aerosols at the South Pole Utilizing Stable Isotopes[END_REF] on the Antarctic Plateau and coastal Antarctic station Dumont d'Urville (DDU) [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF], show similar seasonality with minima in the austral summer and higher values in the autumn to spring, which likely reflects a seasonal shift from OH-and H 2 O 2 -to O 3 -dominated chemistry. In addition, [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF] and [START_REF] Walters | Assessing the Seasonal Dynamics of Nitrate and Sulfate Aerosols at the South Pole Utilizing Stable Isotopes[END_REF] ) in those seasons. However, the importance of S(IV) + HOBr remains inconclusive, since both results in those two previous works can also be explained by the ) in Antarctica, a comparison of the isotope signatures at inland and coastal sites can be helpful, since the MS -destruction appears most significant on the Antarctic Plateau [START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF] while reactive bromine is more abundant in coastal regions [START_REF] Theys | Global observations of tropospheric BrO columns using GOME-2 satellite data[END_REF]. Here, we conduct an inter-site comparison of year-round

 17 O(SO 4 2-
) values of atmospheric sulfate, using weekly

 17 O(SO 4 2-
) observations newly obtained for the inland site Dome C in this study and those previously obtained for coastal site DDU [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF] in the same year 2011. We also compare the observations with the

 17 O(SO 4 2-
) values estimated using a global chemical-transport model GEOS-Chem, which includes reactive bromine production from sea-salt aerosols that originate from both the open ocean and blowing snow sublimation over sea-ice [START_REF] Huang | Evaluating the impact of blowing-snow sea salt aerosol on springtime BrO and O 3 in the Arctic[END_REF]. The uncertainties were estimated based on the typical uncertainty of the ion chromatography analysis (5%). ))

Materials and Methods

2

 17 O(SO 4 2-
) values were measured with an isotope ratio mass spectrometer (IRMS) (MAT253; Thermo Fisher Scientific, Bremen, Germany), coupled with an in-house measurement system built following original setup by [START_REF] Savarino | Sulfur and oxygen isotope analysis of sulfate at micromole levels using a pyrolysis technique in a continuous flow system[END_REF] and a series of improvements [START_REF] Schauer | Oxygen isotope exchange with quartz during pyrolysis of silver sulfate and silver nitrate[END_REF][START_REF] Geng | Analysis of oxygen-17 excess of nitrate and sulfate at sub-micromole levels using the pyrolysis method[END_REF]. The detailed method is described in [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF].

Briefly, ca. 1 mol of SO ) was corrected for oxygen isotope exchange with quartz ( 17 O = 0 ‰; Matsuhisa et al., 1978) by estimating the magnitude of isotopic exchange based on the set of working standard measurements along with the sample measurement runs as described in [START_REF] Schauer | Oxygen isotope exchange with quartz during pyrolysis of silver sulfate and silver nitrate[END_REF]. The precision (1) of corrected  17 O was ± 0. ) ss = 0 ‰), both total sulfate concentrations and  17 O values were corrected for their ss-SO 4 2-component to obtain their non-sea-salt sulfate (nss-SO 4

2-

) content, using the following Eq. ( 1) and [START_REF] Riley | Introduction to Marine Chemistry[END_REF].

[SO 4 2-] nss = [SO 4 2-] total -𝑘 × [Na + ] (1)
Δ 17 O(SO 4 2-) nss = [SO 4 2-] total [SO 4 2-] nss × Δ 17 O(SO 4 2-) total (2)
where "total" is the sum of ssand nss-SO 4 2-components; and k is the mass ratio of

[SO 4 2-
] ss /[Na + ] in seawater (0.25; [START_REF] Holland | Evolution of the atmosphere and oceans[END_REF]. To take into account sea salt chemical fractionation processes that occurs in the Antarctic region in winter, when temperatures drop below -8 °C in the presence of sea-ice [START_REF] Wagenbach | Sea-salt aerosol in coastal Antarctic regions[END_REF], k value of 0.16 ± 0.05 (Legrand et al., 2017a) ) nss at Dome C were compared to those previously obtained at DDU [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF] ) nss was measured for fine mode [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF]. The results were also compared to datasets of oxidants available year-round at both Dome C and DDU, O 3 mixing ratios (Legrand et al., 2016a) ] total , cf., Eq. ( 1)), and sulfur isotopic composition of non-sea-salt sulfate ( 34 S nss ) [START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF]. [H + ], estimated by using the following equation, was used to consider the contribution of aqueous-phase S(IV) + O 3 pathway, which is highly dependent on pH of liquid water in the atmosphere [START_REF] Seinfeld | Atmospheric chemistry and physics: from air pollution to climate change[END_REF].

[H + ] = ([Cl -] + [Br -] + [NO 3 -] + 2[SO 4 2-] + 2[C 2 O 4 2-]) -([Na + ] + [NH 4 + ] + [K + ] + 2[Mg 2+ ] + 2[Ca 2+ ]) (3)
Also [SO 4

2-] ss /[SO 4 2-] total was used to consider the potential importance of aqueous-phase S(IV)

+ O 3 reaction, since it has been recognized that S(IV) + O 3 proceeds rapidly in alkaline (pH ~ 8) deliquescent solution in fresh sea-salt particles, subsequently shutting off due to acidification of sea salt aerosol by the produced sulfate as well as by uptake of acidic gases such as SO 2 , HNO 3 , and H 2 SO 4 [START_REF] Alexander | Sulfate formation in sea-salt aerosols: Constraints from oxygen isotopes[END_REF].  34 S nss was used to test the impact of sulfur sources on

 17 O(SO 4 2-
) nss , since it reflects the relative contributions of marine biogenic (i.e., DMS-sourced) sulfate and non-marine sulfate (nmb-SO 4 2-

) including volcanic and continental sulfur sources [START_REF] Patris | Isotopic signatures of sulfur in shallow Antarctic ice cores[END_REF][START_REF] Pruett | Assessment of sulfate sources in high-elevation Asian precipitation using stable sulfur isotopes[END_REF][START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF]. Additionally, 210 Pb was used to trace the contribution of long-range transport of continental submicron aerosols (Elsasser et al., 2011;[START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF]. The relationships were examined separately for each season: December-January-February (DJF), March-April-May (MAM), June-July-August (JJA), September-October-November (SON), as well as October 29th to December 23rd (OND), where the last one was defined based on the specifically positive  DC-DDU values as explained later in Section 3.1.

The obtained correlation coefficients (r and p values) are summarized in Table S1.

Model description and limitations

We used v11-02d of the GEOS-Chem global chemical-transport model of coupled aerosol-oxidant chemistry [START_REF] Park | Natural and transboundary pollution influences on sulfate-nitrate-ammonium aerosols in the United States: Implications for policy[END_REF] GEOS-Chem v11-02d includes detailed bromine chemistry as described in [START_REF] Parrella | Tropospheric bromine chemistry: Implications for present and pre-industrial ozone and mercury[END_REF], [START_REF] Schmidt | Modeling the observed tropospheric BrO background: Importance of multiphase chemistry and implications for ozone, OH, andmercury[END_REF], Sherwen et al. (2016a[START_REF] Sherwen | Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS[END_REF], and [START_REF] Chen | Sulfate production by reactive bromine: Implications for the global sulfur and reactive bromine budgets[END_REF]. We also applied the reactive bromine emission scheme from sea-salt aerosols produced by blowing snow sublimation over sea-ice as described in [START_REF] Huang | Using CALIOP to constrain blowing snow emissions of sea salt aerosols over Arctic and Antarctic sea ice[END_REF][START_REF] Huang | Evaluating the impact of blowing-snow sea salt aerosol on springtime BrO and O 3 in the Arctic[END_REF], with surface snow salinity of 0.1 psu and 0.03 psu over the Arctic and the Antarctic, reapectively, for both first-year-ice and multi-year-ice. We assumed an enrichment factor of 9 for Br -/Na + ratio in surface snow on seaice relative to seawater. Sea-salt emission from open ocean is simulated as a function of sea surface temperature and wind speed [START_REF] Jaeglé | Global distribution of sea salt aerosols: new constraints from in situ and remote sensing observations[END_REF], with updates from [START_REF] Huang | Wintertime enhancements of sea salt aerosol in polar regions consistent with a sea ice source from blowing snow[END_REF] for cold ocean waters (SST < 5°C). The model is able to reproduce observed Br y * concentrations at DDU from winter to spring (June to November), within the range of standard deviations of the monthly mean observations [START_REF] Legrand | Year-round records of sea salt, gaseous, and particulate inorganic bromine in the atmospheric boundary layer at coastal (Dumont d'Urville) and central (Concordia) East Antarctic sites[END_REF]; Fig. S1). DMS emission is parameterized as a function of sea surface temperature, wind speed, and DMS concentration in seawater obtained from [START_REF] Lana | An updated climatology of surface dimethlysulfide concentrations and emission fluxes in the global ocean[END_REF]. Note that the model does not include reactive nitrogen emissions from snow nitrate photolysis [START_REF] Grannas | An overview of snow photochemistry: evidence, mechanisms and impacts[END_REF]. This would lead to underestimates in oxidants over large area of the Antarctic continents, as exhibited by the underestimates in surface O 3 concentrations at both Dome C and DDU by a factor of ~1.5 (Fig. 1e). [START_REF] Zatko | The impact of snow nitrate photolysis on boundary layer chemistry and the recycling and redistribution of reactive nitrogen across Antarctica and Greenland in a global chemical transport model[END_REF] previously simulated that the inclusion of their snow NO x emission scheme in GEOS-Chem model will increase surface O 3 concentrations over the Antarctic continent by factors of 1.1-1.8. It is also recently pointed out that the underestimaes in O 3 over the Southern Ocean is improved by the inclusion of a new O 3 deposition scheme associated with chemical reaction of O 3 with iodide on the ocean surface [START_REF] Pound | Influences of oceanic ozone deposition on tropospheric photochemistry[END_REF], which is not implemented in this study.

Thus, the model has limitations in reproducing oxidants abundances, even though the general seasonality in O 3 with increases in winter and decreases in summer was reproduced (Fig. 1e). We therefore note that our estimates in the relative contributions of sulfate formation processes bear an uncertainty associated with oxidant abundances despite good reproducibilities in

 17 O(SO 4 2-
) nss as shown later in Section 3.2 (Fig. 2). The further precise prediction of  17 O(SO 4 2-) nss will require improvements in oxidants reproducibility in the future.

This version of the model includes gas phase oxidation of SO 2 by OH, in-cloud aqueous phase oxidations of S(IV) by H 2 O 2 , O 3 [START_REF] Park | Natural and transboundary pollution influences on sulfate-nitrate-ammonium aerosols in the United States: Implications for policy[END_REF], and HOBr [START_REF] Chen | Sulfate production by reactive bromine: Implications for the global sulfur and reactive bromine budgets[END_REF], and oxidation of S(IV) by O 3 on sea salt particles [START_REF] Alexander | Sulfate formation in sea-salt aerosols: Constraints from oxygen isotopes[END_REF]. For in-cloud reactions, the cloud fraction and the liquid water content of cloud are obtained from MERRA-2 meteorological fields. For pH-dependent reactions such as in-cloud S(IV) + O 3 and S(IV) + HOBr, the effect of heterogeneity of cloud pH on S(IV) partitioning is accounted as described in [START_REF] Alexander | Isotopic constraints on the formation pathways of sulfate aerosol in the marine boundary layer of the subtropical northeast Atlantic Ocean[END_REF]. The model assumes in-cloud sulfate formations are prohibited at temperature < -15°C as originally designed by [START_REF] Park | Natural and transboundary pollution influences on sulfate-nitrate-ammonium aerosols in the United States: Implications for policy[END_REF]. We confirmed that this assumption limits the annual tropospheric sulfate production via aqueous-phase reactions to 13.8 Gg-S on the Antarctic Plateau (> 68°S) in contrast to 137.0 Gg-S on the Southern Ocean (60-68°S) within longitudes of 0-180°E (Fig. S3). This result is consistent with the limited occurrence frequency of supercooled-liquid-water containing cloud (0-10%) over the Antarctic Plateau compared to over the Southern Ocean (20-60%) [START_REF] Listowski | Antarctic clouds, supercooled liquid water and mixed phase, investigated with DARDAR: geographical and seasonal variations[END_REF]. S(IV) + O 3 in sea-salt is assumed as a function of SO 2 transfer rate constant from the gas to the aerosol phase, because the rate limiting step is not the aqueous-phase reaction rate of S(IV) + O 3 in alkaline solution on fresh sea-salt but is gas-phase diffusion of SO 2 to the aerosol surface [START_REF] Alexander | Sulfate formation in sea-salt aerosols: Constraints from oxygen isotopes[END_REF]. This reaction is calculated only within MBL column, where the newly emitted sea-salt is available, assuming that sea-salt alkalinity is rapidly consumed by this reaction in addition to the uptake of gas form H 2 SO 4 and HNO 3 [START_REF] Alexander | Sulfate formation in sea-salt aerosols: Constraints from oxygen isotopes[END_REF].

We modified the model to tag sulfate produced via each oxidation pathway as different tracers which are transported, as originally described in [START_REF] Alexander | Sulfate formation in sea-salt aerosols: Constraints from oxygen isotopes[END_REF]. As atmospheric SO 2 rapidly attains isotopic equilibrium with H 2 O ( 17 O = 0 ‰) [START_REF] Barkan | High precision measurements of 17 O/ 16 O and 18 O/ 16 O ratios in H 2 O[END_REF][START_REF] Holt | Oxygen-18 study of the aqueous-phase oxidation of sulfur dioxide[END_REF],  17 O value of sulfate produced via each formation pathway is determined by  17 O value of corresponding oxidant and their transferring factors [START_REF] Savarino | Laboratory oxygen isotopic study of sulfur (IV) oxidation: Origin of the mass-independent oxygen isotopic anomaly in atmospheric sulfates and sulfate mineral deposits on Earth[END_REF]. Since OH also efficiently exchanges its oxygen isotopes with water vapor [START_REF] Dubey | Isotope specific kinetics of hydroxyl radical (OH) with water (H 2 O): Testing models of reactivity and atmospheric fractionation[END_REF],

 17 O(OH)
is also 0 ‰ under regions where water vapor is abundant (e.g., throughout most of the troposphere) [START_REF] Lyons | Transfer of mass-independent fractionation in ozone to other oxygencontaining radicals in the atmosphere[END_REF][START_REF] Morin | Signature of Arctic surface ozone depletion events in the isotope anomaly (Δ 17 O) of atmospheric nitrate[END_REF]. Therefore, gas-phase SO 2 + OH produces sulfate with  17 O(SO 4

2-) is assumed to be 0 ‰. Note that it has been previously suggested that [START_REF] Savarino | Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign[END_REF] due to the limited availability of water vapor in inland Antarctica. We also conducted the calculation with  17 O(OH) = 3 ‰ as a maximum case, which is expected to produce sulfate with  17 O(SO 4

 17 O(OH) can be 1-3 ‰ at Dome C (
2-) = 0.75 ‰ with assuming oxygen atom transferring factor of 0.25. We note that there remains possibility that  17 O(OH) is higher than 3 ‰ [START_REF] Savarino | Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign[END_REF], that might cause underestimates in  17 O(SO 4

2-) nss . The [START_REF] Savarino | Analytical procedure to determine both δ 18 O and δ 17 O of H 2 O 2 in natural water and first measurements[END_REF] with a transferring factor of 0.5 [START_REF] Savarino | Laboratory oxygen isotopic study of sulfur (IV) oxidation: Origin of the mass-independent oxygen isotopic anomaly in atmospheric sulfates and sulfate mineral deposits on Earth[END_REF]. Note that the  17 O(H 2 O 2 ) is derived from only one set of observations at La Jolla, California, and thus needs further verification in various environment in the future. For  17 O(O 3 ), among the whole sets of observations to the present, the two early studies using cryogenic technique had shown large variabilities (24.7 ± 11.4 ‰ and 26.5 ± 5.0 ‰; [START_REF] Krankowsky | Measurement of heavy isotope enrichment in tropospheric ozone[END_REF]Johnston and Theimens, 1997). Such variabilities were much greater than those expected from the experimentally determined pressure and temperature dependency of  17 O(O 3 ), e.g., a decrease of only ~2 ‰ for an pressure increase from 500 to 760

 17 O(SO 4 2-) value of sulfate produced via aqueous-phase S(IV) + H 2 O 2 is assumed to be 0.8 ± 0.2 ‰ based on  17 O(H 2 O 2 ) of 1.6 ± 0.3 ‰ (
Torr [START_REF] Morton | Laboratory studies of heavy ozone[END_REF][START_REF] Thiemens | Pressure dependency for heavy isotope enhancement in ozone formation[END_REF]) and a increase of only ~5 ‰ for an temperature increase from 260 to 320 K [START_REF] Morton | Laboratory studies of heavy ozone[END_REF][START_REF] Janssen | Temperature dependence of ozone rate coefficients and isotopologue fractionation in 16 O-18 O oxygen mixtures[END_REF]. Based on these experimental data, it has been pointed out that these observations would have random errors associated with sampling artifacts [START_REF] Mauersberger | Oxygen Isotope Processes and Transfer Reactions[END_REF]. Therefore, we exclude the data of these two studies from the consideration. Given the consistency of the  17 O(O 3 ) observations using nitrite-coated method among various locations and seasons including at Dome C and DDU [START_REF] Vicars | Quantitative constraints on the 17 O-excess ( 17 O) signature of surface ozone: Ambient measurements from 50°N to 50°S using the nitrite-coated filter manuscript submitted to Journal of Geophysical Research: Atmospheres 33 technique[END_REF][START_REF] Savarino | Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign[END_REF][START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF], we decided to use the average value of the  17 O(O 3 ) observations, which comes to 25.6 ± 1.3 ‰.

The  17 O(SO 4 2-) for S(IV) + O 3 , both in cloud and in sea salt, is assumed to be 6.4 ± 0.3 ‰, by a transferring factor of 0.25 [START_REF] Savarino | Laboratory oxygen isotopic study of sulfur (IV) oxidation: Origin of the mass-independent oxygen isotopic anomaly in atmospheric sulfates and sulfate mineral deposits on Earth[END_REF]. Since aqueous-phase S(IV) oxidation by HOBr gives an oxygen atom from liquid water to produce sulfate [START_REF] Fogelman | Non-Metal Redox Kinetics: Hypochlorite and Hypochlorous Acid Reactions with Sulfite[END_REF][START_REF] Troy | Non-Metal Redox Kinetics: Hypobromite and Hypobromous Acid Reactions with Iodide and with Sulfite and the Hydrolysis of Bromosulfate[END_REF][START_REF] Liu | Kinetics and mechanisms of aqueous ozone reactions with bromide, sulfite, hydrogen sulfite, iodide, and nitrite ions[END_REF] ] nss is enhanced during austral summer and reduced during winter, a seasonality that is driven by marine biogenic emission of DMS [START_REF] Preunkert | Interannual variability of dimethylsulfide in air and seawater and its atmospheric oxidation byproducts (methanesulfonate and sulfate) at Dumont d'Urville, coastal Antarctica (1999-2003)[END_REF][START_REF] Preunkert | Seasonality of sulfur species (dimethyl sulfide, sulfate, and methanesulfonate) in Antarctica: Inland versus coastal regions[END_REF][START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF]. This seasonal cycle in [SO 4

2-

] nss is partially intensified by atmospheric dynamics due to enhanced efficiency of meridional long-range transport and the weakened inversion layer on the Antarctic Plateau during summer, as previously suggested by the similar seasonal cycle with 210 Pb [START_REF] Elsässer | Continuous 25-yr aerosol records at coastal Antarctica: Part 2: Variability of the radionuclides 7Be, 10Be 210Pb[END_REF][START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF].  17 O(SO 4 2-) nss show lower values in austral summer and higher values in winter, with monthly-mean values ranging from 1.1 ± 0.1 ‰ in February to 2.5 ± 0.2 ‰ in August, with a mass-weighted annual average of 1.7 ± 0.1 ‰ (Table 1). These trends and values are generally consistent with previous observations at Dome C in 2010 [START_REF] Hill-Falkenthal | Seasonal variations in 35 S and Δ 17 O of sulfate aerosols on the Antarctic plateau[END_REF] as well as at DDU in 2011 [START_REF] Ishino | Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica[END_REF]. ] nss at DDU and Dome C, as DMS oxidation is thought to be the main source of sulfate in these locations (e.g., [START_REF] Minikin | Sulfurcontaining species (sulfate and methanesulfonate) in coastal Antarctic aerosol and precipitation[END_REF][START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF]. We note that the overestimate of [SO 4

2-

] nss could lead to an underestimate of F S(IV)+O3 and thus ) nss in the model. Although similar seasonal trends for sulfate formation pathways are found for DDU (Fig. 2f), F SO2+OH is significantly higher at Dome C (34% in summer) than DDU (16% in summer) (Table 1). This is because the model prohibits aqueous-phase sulfate production at temperatures lower than -15°C and therefore most sulfate formation along with the transport of precursors towards inland Antarctica occurs through gas-phase SO 2 + OH pathway.

 17 O(SO 4 2
As a result of these estimates in sulfate formation pathways, the model roughly reproduces the observed seasonality and magnitude of  17 O(SO 4 2-) nss (Fig. 2c,d). However, the model largely underestimates observed  17 O(SO 4 2-) nss from August to December at Dome C by 0.5 to 1.1 ‰ (Fig. 2c), partially overlapping the period when the significantly positive  DC-DDU values were observed in October to December. This underestimate implies that the model might lack sulfate formation processes that causes  17 O(SO 4 2-) nss of higher than 2.0 ± 0.3 ‰ at the inland site (Table 1) and thus the positive  DC-DDU in this time period. Meanwhile for DDU, the model also underestimates  17 O(SO 4 2-) nss from September to October by 0.6 to 0.8 ‰, but rather slightly overestimates in January by 0.5 ‰ (Fig. 2d). This result is further discussed in the following sections within the focus of the observed inter-site differences.

Discussion

The  DC-DDU values different from 0 ‰ during the austral spring to summer (October-December) and during the austral autumn (March-May) (Fig. 1d) suggest that some fractions of sulfate existing at these inland and coastal sites experienced different oxidation processes. The possible processes may include long-range transport of sulfate produced above the other continents or in the stratosphere (i.e., nmb-SO 4 2-

), in addition to DMS-sourced sulfate produced within the troposphere above the Antarctic continents and the Southern Ocean. In the former case, it is expected that  34 S nss values in the same aerosol samples would specifically decrease during the corresponding periods, since nmb-SO 4 2-has lower  34 S nss values than DMS-sourced sulfate [START_REF] Patris | Isotopic signatures of sulfur in shallow Antarctic ice cores[END_REF][START_REF] Pruett | Assessment of sulfate sources in high-elevation Asian precipitation using stable sulfur isotopes[END_REF][START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF]. Indeed, [START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF] found an unexpected decrease of  34 S nss at Dome C in November, suggesting a significant input manuscript submitted to Journal of Geophysical Research: Atmospheres 14 of nmb-SO 4 2-, which is likely attributed to long-range transport of continental submicron aerosols based on a significant correlation with 210 Pb tracer. This input of nmb-SO 4 2-in November overlaps the period of the positive  DC-DDU values. However, there were not significant correlations observed for the  DC-DDU compared to  34 S nss (p = 0.39) and 210 Pb (p = 0.15) at Dome C (Fig. 3a, 3b, and Table S1). Therefore, this nmb-SO 4 2-is not likely the main factor causing the positive  DC-DDU , while it may dilute or perturb the high  17 O(SO 4 2-) nss signature in that period. Since  34 S nss values were homogeneous between Dome C and DDU for the rest period of the year [START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF], the negative  DC-DDU values during the autumn would neither be associated with the contribution of nmb-SO 4 2-. Additionally, whereas the deposition of polar stratospheric clouds (PSCs) is thought to be a potential source of tropospheric sulfate in Antarctica, it is likely to occur during mid-winter (July-August) [START_REF] Morin | Signature of Arctic surface ozone depletion events in the isotope anomaly (Δ 17 O) of atmospheric nitrate[END_REF], not coinciding with the positive and negative  DC-DDU values. Furthermore, the relative abundance of 35 S, a radioactive tracer often used as a indicator of stratospheric sulfate, relative to total sulfate is maximized in June [START_REF] Hill-Falkenthal | Seasonal variations in 35 S and Δ 17 O of sulfate aerosols on the Antarctic plateau[END_REF]. Therefore, the intrusion of stratospheric sulfate is not the likely reason for the  DC-DDU values discussed here. Thus, below we discuss possible influences of regional characteristic chemistry taking place at the scale of the Antarctic continent on  17 O(SO 4 2-) nss during these periods. ]/[SO 4 2-] nss at DDU is less significant than Dome C, while MS -destruction may also occur at DDU where is frequently exposed to the highly oxidative atmosphere from the interior Antarctica during summer due to katabatic wind (Legrand et al., 2016a). The co-occurrence of MSdestruction at Dome C and positive  DC-DDU (Fig. 1d and1f) is striking, suggesting the possibility that MS -destruction produces sulfate with significantly high  17 O(SO 4 2-) nss at Dome C and leads to the high  DC-DDU . This possibility is supported by the negative co-variation between the  DC-DDU and [MS -]/[SO 4

2-] nss at Dome C for October to December with p value of 0.01 (Fig. 3c and Table S1), where  DC-DDU tends to be higher as [MS -]/[SO 4

2-] nss becomes lower.

This hypothesis requires that MS -possesses significantly high  17 O signature or MS - destruction occurs via its oxidation by O 3 to produce sulfate. It is known that MS -formation in the marine boundary layer involves O 3 as well as BrO, which is produced via Br + O 3 [START_REF] Zhang | Crossed Molecular Beam Study of the Reaction Br + O 3 †[END_REF], as important oxidants [START_REF] Von Glasow | Model study of multiphase DMS oxidation with a focus on halogens[END_REF][START_REF] Hoffmann | An advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry[END_REF]. These oxidants would imprint a high  17 O value on MS -. To date, however, there are no observations or estimates of  17 O(MS -).The mechanism and the subsequent products of MS -destruction in inland Antarctica remain unclear. While MS -oxidation by OH, SO 4 -, Cl, and Cl 2 -have been proposed so far (Zhu et al., 2003a[START_REF] Zhu | Temperature-dependent kinetics studies of aqueous phase reactions of SO 4 radicals with dimethylsulfoxide, dimethylsulfone, and[END_REF]Zhu, 2004), there is no evidence for a reaction with O 3 . A previous box model study simulating multi-phase sulfur chemistry [START_REF] Hoffmann | An advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry[END_REF] indicated that aqueous-phase oxidation of MS -by OH to produce sulfate (Zhu et al., 2003a) is the dominant pathway under typical pristine marine boundary layer (MBL) conditions [START_REF] Bräuer | Mechanism development and modelling of tropospheric multiphase halogen chemistry: The CAPRAM Halogen Module 2.0 (HM2)[END_REF]. Furthermore, it is also shown by a flow tube chamber experiment that MS -can be oxidized on deliquesced aerosols to form sulfate, which may lead to shorter lifetime of MS -than in condenced aqueous-phase in MBL [START_REF] Mungall | Heterogeneous Oxidation of Particulate Methanesulfonic Acid by the Hydroxyl Radical: Kinetics and Atmospheric Implications[END_REF]. [START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF] mentioned that, although the chance of aerosol experiencing aqueous-phase chemistry is far lower than in the marine boundary layer, far more acidic conditions on the Antarctic Plateau compared to the marine boundary layer would favor the production of OH via the reaction of O 3 with O 2 - [START_REF] Ervens | CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application[END_REF]. Thus, here we assume MS -oxidation by OH in aqueous-phase or on aerosols as the mechanism for MS -destruction, and for the first time estimate  17 O transferred from DMS oxidation to MS -and then to sulfate. CH 3 S(O)CH 3 ), methyl sulfinic acid (MSIA; CH 3 S(O)OH), and then to MS -, as well as production of SO 2 from each species, whose importance in the marine boundary layer are recognized [START_REF] Barnes | Dimethyl Sulfide and Dimethyl Sulfoxide and Their Oxidation in the Atmosphere[END_REF][START_REF] Von Glasow | Model study of multiphase DMS oxidation with a focus on halogens[END_REF][START_REF] Hoffmann | An advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry[END_REF][START_REF] Chen | DMS oxidation and sulfur aerosol formation in the marine troposphere: a focus on reactive halogen and multiphase chemistry[END_REF]. Note that gas-phase and aqueous-phase reactions using the same oxidants are considered as one pathway for simplification, since they will result in the same  17 O(SO 4 2-). We also summarize the formula for calculating the  17 O value of each sulfur species X (DMSO, MSIA, MS -, and SO 4 2-) produced by each reaction j ( 17 O(X) j ) in Table 2, which is determined based on mechanisms of respective oxidation pathways as follows.

At the first step, DMS oxidation into DMSO includes four different oxidation pathways reacting with OH, BrO, O 3 , and Cl. These reactions generally occur through adduct of the oxidant to the sulfur atom of DMS [START_REF] Barnes | Dimethyl Sulfide and Dimethyl Sulfoxide and Their Oxidation in the Atmosphere[END_REF][START_REF] Ingham | Kinetics and Products of the Reactions BrO + DMS and Br + DMS at 298 K[END_REF][START_REF] Gershenzon | Simultaneous uptake of DMS and ozone on water[END_REF], transferring oxygen atoms of oxidants to the produced DMSO. [START_REF] Zhang | Crossed Molecular Beam Study of the Reaction Br + O 3 †[END_REF]. DMS + O 3 and DMS + BrO are thus expected to produce DMSO with  17 O of 39 ‰. Since DMS + Cl pathway is expected to form CH 3 S(Cl)CH 3 , which is followed by subsequent oxidation by O 2 [START_REF] Barnes | Dimethyl Sulfide and Dimethyl Sulfoxide and Their Oxidation in the Atmosphere[END_REF],  17 O DMS+Cl is assumed to be equal to  17 O(O 2 ) (= -0.3 ‰; [START_REF] Barkan | High precision measurements of 17 O/ 16 O and 18 O/ 16 O ratios in H 2 O[END_REF]. During DMSO oxidation by OH into MSIA, one of the two oxygen atoms of produced MSIA is from DMSO while another comes from OH [START_REF] Bardouki | Kinetics and mechanism of the oxidation of dimethylsulfoxide (DMSO) and methanesulfinate (MSI-) by OH radicals in aqueous medium[END_REF] [START_REF] Bardouki | Kinetics and mechanism of the oxidation of dimethylsulfoxide (DMSO) and methanesulfinate (MSI-) by OH radicals in aqueous medium[END_REF]. In MSIA + O 3 , it is experimentally indicated that O 3 -terminal transfers to MS - [START_REF] Flyunt | OHradical-induced oxidation of methanesulfinic acid. The reactions of the methanesulfonyl radical in the absence and presence of dioxygen[END_REF]. In both reactions, two of three O-atoms of MSare preserved from MSIA. Therefore,  17 O transferring to MS -via each reaction is determined as sum of 2/3  17 O(MSIA) and 1/3  17 O(oxidant). Finally, assuming MS -+ OH provides one O-atom from OH to produce sulfate,  17 O(MS -) MS+OH is determined as the sum of 3/4  17 O(MS -) and 1/4  17 O(OH) (Table 2).

With the above assumptions on  17 O transferring processes,  17 O value of species X is determined by the isotopic mass balance as the following equation:

∆ 17 O(𝑋) = ∑ ∆ 17 O(𝑋) 𝑗 𝑓 𝑗 , (6)
𝑓 𝑗 = 𝑃(𝑋) 𝑗 ∑ 𝑃(𝑋) 𝑗 ⁄ , (7)
where f j is relative contribution of reaction j for production of X (P(X)). To obtain  17 O(MS -), we here used P(X) j and thus f j estimated by the previous simulation using GEOS-Chem by Chen et al. ( 2018), which incorporated whole sulfur chemistry shown in Figure 4. We used the mean P(X) j within the troposphere in 60-90°S during October to December. For production of DMSO, f DMS+OH , f DMS+BrO , f DMS+O3 , and f DMS+Cl are estimated to be 36%, 49%, 8%, and 7%, respectively.

By applying these estimated f j with  17 O(X) j defined in Table 2 and Eq. ( 6 ]/[SO 4

2-] nss relationship (Fig. 3) suggests that  17 O(SO 4 ) MS+OH × 0.14 ± 0.07 = 1.7 ± 0.9 ‰. These consistencies imply that the positive  DC-DDU observed in the austral spring-summer is mainly caused by MS -destruction. We therefore conclude that MS -destruction and subsequent sulfate production along with transport over the Antarctic Plateau is the most likely process responsible for the positive  DC-DDU in the austral spring-summer at inland Antarctica.

The model underestimate of  17 O(SO 4 2-) nss for Dome C during August to December by 0.5-1.1 ‰ (Fig. 2c) are also within the possible range of the expected shift in  17 O(SO 4 2-) nss by MS -destruction, i.e., 1.6‰ at maximum (Fig. 3). Note that the MS -destruction becomes significant from November (Fig. 1f), only for the latter period of this underestimate of This finding that confirms the suspected occurrence of an efficient atmospheric oxidation of MS -into sulfate over the Antarctic plateau has an important implication for the interpretation of ice-core  17 O(SO 4 2-) nss records. Previous studies revealed that, in Antarctica, MS -in snow is largely lost after deposition [START_REF] Wagnon | Loss of volatile acid species from upper firn layers at Vostok, Antarctica[END_REF][START_REF] Delmas | Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn[END_REF][START_REF] Weller | Postdepositional losses of methane sulfonate, nitrate, and chloride at the European Project for Ice Coring in Antarctica deep-drilling site in Dronning Maud Land, Antarctica[END_REF].

 17 O(SO 4 2
While the mechanism of this MS -loss in snow is under debated, there are two proposed ideas: 2-) nss in Antarctic ice corresponding to the present-day warm climate period (Holocene) average 2.8 ± 0.4 ‰ [START_REF] Alexander | Climate driven changes in the oxidation pathways of atmospheric sulfur[END_REF][START_REF] Alexander | East Antarctic ice core sulfur isotope measurements over a complete glacial-interglacial cycle[END_REF][START_REF] Kunasek | Sulfate sources and oxidation chemistry over the past 230 years from sulfur and oxygen isotopes of sulfate in a West Antarctic ice core[END_REF][START_REF] Sofen | WAIS Divide ice core suggests sustained changes in the atmospheric formation pathways of sulfate and nitrate since the 19th century in the extratropical Southern Hemisphere[END_REF], which is significantly higher than the annual mass-weighted average of  17 O(SO 4 2-) nss of 1.7 ± 0.1 ‰ in aerosols at Dome C (Table 1). Even the maximum monthly mean  17 O(SO 4 2-) nss of 2.5 ± 0.1 ‰ in July cannot reach the ice-core  17 O(SO 4 2-) nss of 2.8 ± 0.4 ‰, indicating the additional sulfate production with the higher  17 O(SO 4 2-) nss in snow after deposition. The degree of postdepositional loss of MS -tends to be higher at sites with lower snow accumulation rates [START_REF] Delmas | Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn[END_REF], and reaches 80-90% at Vostok where the present-day snow accumulation rate is 2.2 g cm -2 yr -1 [START_REF] Wagnon | Loss of volatile acid species from upper firn layers at Vostok, Antarctica[END_REF]. Assuming 90% of MS -in snow is converted into sulfate at Dome C where the accumulation rate (2.7 g cm -2 yr -1 ) is similar to the one in Vostok, combined with the annual mean [MS -]/[SO 4

2-] nss in aerosols of 0.11 [START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF] However, although the model also shows the larger decrease in [Br y * ] during autumn at DDU (3.5 ± 1.4 ng m -3 for MAM compared to 12.8 ± 2.7 ng m -3 for JJA) than Dome C (5.1 ± 1.4 ng m -3 for MAM compared to 2.2 ± 0.5 ng m -3 for JJA) (Fig. S1), the change in the modeled F S(IV)+HOBr were smaller at DDU (23% for MAM compared to 33% for JJA) than Dome C (18% for MAM compared to 33% for JJA) (Table 1). Therefore, the larger decrease in Rather, given that BrO in 60 to 90°S is underestimated year-round (Fig. S2), it could be a reason for overestimate of  17 O(SO 4 2-) nss such that seen for DDU in January (Fig. 2d). We note that, prior to aerosol acidification [START_REF] Alexander | Sulfate formation in sea-salt aerosols: Constraints from oxygen isotopes[END_REF]. It is thus expected that, while sea-salt loading in autumn (0.9 ± 0.4 g m -3 for MAM) is lower than in summer (1.1 ± 0.4 g m -3 for DJF) by only a factor of 1.2, the [SO 4

2-] nss level in autumn (57 ± 46 ng m -3 ) is a factor of 4 lower than that in summer (215 ± 79 ng m -3 ), leading to slower sea-salt acidification, possibly allowing S(IV) + O 3 to proceed. This possibility seems to be supported by the negative covariation between the  DC-DDU and [SO 4

2-] ss /[SO 4 2-] total at DDU, an assumed index of the degree of sea-salt acidification, for the data during March to May with r = -0.96 and p < 0.01 (Fig. 5 and Table S1), where the  DC-DDU tends to be lower as [SO 4 2-] total as an index of sea-salt loading relative to titrating acids without using k value, the correlation coefficient compared to  DC-DDU becomes less significant (r = -0.74 and p = 0.06; Table S1). Hence, it is currently difficult to conclude the In both cases, to obtain the negative  DC-DDU values, sulfate with high  17 O(SO 4 2-) nss of 1.9 ± 0.2 ‰ during autumn at DDU needs to be reduced and sulfate with lower  17 O(SO 4 2-) nss needs to be increased along with the transport of sulfate and its precursors towards inland Antarctica. It is reasonable that, since liquid water content becomes zero at -40°C [START_REF] Jeffery | Homogeneous nucleation of supercooled water: Results from a new equation of state[END_REF][START_REF] Pruppacher | A New Look at Homogeneous Ice Nucleation in Supercooled Water Drops[END_REF] we looked into the processes that would be more important around coastal regions than inland such as the decrease in F S(IV)+HOBr and the increase in F S(IV)+O3 in sea-salt particles at DDU.

However, both of them are not decisive. Given the larger abundances of reactive bromine at coastal regions of West Antarctica than those of East Antarctica [START_REF] Theys | Global observations of tropospheric BrO columns using GOME-2 satellite data[END_REF][START_REF] Saiz-Lopez | Boundary layer halogens in coastal Antarctica[END_REF][START_REF] Grilli | First investigations of IO, BrO, and NO 2 summer atmospheric levels at a coastal East Antarctic site using mode-locked cavity enhanced absorption spectroscopy[END_REF], further observations of  17 O(SO 4 2-) nss at different coastal sites around Antarctica would help to constrain the importance of those processes.

DMS

  suggested the possibility of an increased contribution of S(IV) + HOBr during austral spring at DDU and summer at South Pole, respectively, based on the relatively low  17 O(SO 4 2-

  .1 Aerosol sampling and measurement of soluble species Aerosol samples were collected at Dome C (75°10′S, 123°30′E; 3233 m above sea level), located on the East Antarctic Plateau 1100 km from the nearest coast. The aerosol sampling is performed continuously at Dome C from the year 2010 as a part of Sulfate and Nitrate Evolution at Dome C (SUNITEDC) program (e.g.,[START_REF] Erbland | Airsnow transfer of nitrate on the East Antarctic Plateau -Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium in summer[END_REF]. Bulk aerosol was collected on glass fiber filters using high-volume air sampler (HVAS; General Metal Works GL 2000H Hi Vol TSP; Tisch Environmental, Cleves, OH, USA) at the flow rate of 1.7 m 3 min -1 with time resolution of 1-2 week. The HVAS was placed at ~1 km distant from the main building of research activity at Dome C. A field blank was checked once per month by mounting filters onto the filter holder and running for 1 min. After each collection run, filters were removed from the HVAS and wrapped in aluminium foil, which were sealed in plastic bags, stored at -20 °C, and shipped to Institut des Géosciences de l'Environnement (Grenoble, France) for chemical analyses. Samples collected during January to December 2011 were used in this study.The procedures for extraction and quantification of soluble species concentrations in aerosols are detailed in[START_REF] Ishino | Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle[END_REF]. Some anions (were measured at Tokyo Tech using ion chromatograph ICS2100, DIONEX with a guard column (Dionex IonPac AG19) and a separation column (Dionex IonPac AS19) for anions, and 881 Compact IC Pro, Metrohm with a guard column (Metrosep C4 S-Guard/4.0) and a separation column (Metrosep C 4-150/4.0) for cations. Considering high blank loading on filters or the lack of ion standard materials at Tokyo Tech., the concentration of other ions (MS -, Cl -, Br -, oxalate (C 2 O 4 2-), and Na + ) were obtained from other aerosol samples as described inLegrand et al. (2017a[START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF]. The measured ion concentrations were corrected for blank values, and reported as atmospheric concentration in standard temperature and pressure (T = 273.15 K, p = 101325 Pa) based on meteorological data of Dome C provided by IPEV/PNRA (www.climantartide.it).

Figure

  Figure 1d shows  DC-DDU , the differences in weekly  17 O(SO 4 2-) nss values between the two sites in the year 2011. Throughout most of the year,  DC-DDU is 0 ‰ within the range of the estimated uncertainty. However, there are two specific time periods exhibiting  DC-DDU values different from 0 ‰. One is from October to December, a transition from the austral spring to summer, when a group of positive  DC-DDU values ranging 0.4 ± 0.3 ‰ to 1.4 ± 0.3 ‰ are observed. The second is found from March to May, the austral autumn, when negative  DC-DDU values ranging -1.4 ± 0.6 ‰ to -0.5 ± 0.3 ‰ are observed. These  DC-DDU values different from zero suggest that sulfate at Dome C and DDU experienced different oxidation processes during their transport from source regions. The possible processes corresponding to these  DC-DDU values are discussed in Section 4.

Figure 1 .

 1 Figure 1. (a) Map of stations cited in this study. (b-g) Observed seasonal variations of (b) nonsea-salt sulfate concentrations, (c)  17 O(SO 4 2-) nss values (from this study and Ishino et al., 2017),

Figure 2 .

 2 Figure 2. Comparison of observed and modeled values of (a, b) non-sea-salt sulfate concentration and (c, d)  17 O(SO 4 2-) nss , and (e, f) calculated relative fraction of sulfate produced via different formation pathways (F i , see Section 2.4), in the model. Left and right columns show results for Dome C and DDU, respectively.

4. 1

 1 Positive  DC-DDU values in austral spring to summer Positive  DC-DDU in spring-summer coincides with a [O 3 ] increase from 25 ppb to 34 ppb at Dome C (Fig. 1e) and a significant drop of [MS -]/[SO 4 2-] nss ratios at Dome C (Fig. 1f) from 0.13 ± 0.04 (October) to 0.05 ± 0.02 (January). [MS -]/[SO 4 2-] nss generally shows a bimodal seasonal cycle with slight increase from winter (July-August; 0.06 ± 0.01) to spring (October; 0.13 ± 0.04), followed by a significant drop in summer (January; 0.05 ± 0.02) and then increases to maximum values in autumn (March; 0.25 ± 0.09) at Dome C (Legrand et al., 2017b). Legrand et al. (2017b) found that the decline of [MS -]/[SO 4 2-] nss during summer coincides with periods of high photochemical activity as indicated by high O 3 levels, suggesting the occurrence of chemical destruction of MS -. Legrand et al. (2017b) also showed that the decrease in [MS -

Figure 3 .Figure 4 .

 34 Figure 3. Variations of  DC-DDU against (a)  34 S nss , (b) 210 Pb, and (c) [MS -]/[SO 4 2-] nss , at Dome C. Data for summer (DJF), autumn (MAM), winter (JJA), and spring (SON) are plotted with crosses, circles, triangles, and squares, respectively. Data for October 29th to December 23rd (OND) are plotted with green diamonds, with the linear least-squares fit shown by green dashed lines.

Table 2 .

 2 Figure 4 summarizes the hypothesized sulfate formation processes via MS -oxidation possibly transferring high  17 O to sulfate, in addition to the recognized processes explained in Section 2.4. The reaction scheme includes DMS oxidation into dimethyl sulfoxide (DMSO;

  ), the  17 O(DMSO) is estimated to be 20.8-23.0 ‰. Since MSIA production occurs via DMSO + OH only (f DMSO+OH = 100%),  17 O(MSIA) is equivalent to  17 O(MSIA) DMSO+OH = 10.4-11.5 ‰. For production of MS -, f MSIA+OH and f MSIA+O3 are estimated to be 31% and 69%, respectively, consequently leading to  17 O(MS -) of 15.2-16.9 ‰. Finally,  17 O(SO 4 2-) derived via MS -destruction, 17 O MS+OH , is estimated to be 11.4-12.7 ‰. If the oxidation of MS -into sulfate fully corresponds to the difference in [MS -]/[SO 4 2-] nss of 0.09 between Dome C (0.05 ± 0.02) and DDU (0.14 ± 0.07) during October to December,  17 O(SO 4 2-) nss can increase by 1.1 ‰ (=  17 O(SO 4 2-) MS+OH × 0.09), which is close to the observed  DC-DDU of ca. 0.7 ‰. Additionally, the intercept of the slope of  DC-DDU versus [MS -

  -) nss in the model. Additionally, the model also underestimates  17 Oto October by 0.6-0.8 ‰, while does not during November to December. The absense of  17 O(SO 4 2-) nss underestimate for DDU during November-December is expected because MS -destruction in the mid-summer is less significant at DDU than Dome C[START_REF] Legrand | Yearround record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) -Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol[END_REF]. On the other hand, the underestimates of  17 O(SO 4 2-) nss in early spring at both sites indicate the other missing processes such that impacts both sites. One idea that might cause the underestimates of  17 O(SO 4 2-) nss is the underestimates of F S(IV)+O3 due to the excessive sulfur loading (Fig.2a and 2b). Since S(IV) + O 3 reaction prefers higher pH condition, the excessive acidification of cloud water and sea-salt aerosols induced by the overestimates of [SO 42-] nss may result in surpression of S(IV) + O 3 . This idea is uncertain because the overestimate of [SO 4 2-] nss is not specific for early spring but seen for year-round. Additionally, there remains possibility that  17 O(OH) is higher than 3 ‰[START_REF] Savarino | Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign[END_REF], that might partially correspond to the underestimates in  17 O(SO 4 2-) nss . Future modeling work incorporating DMSO, MSIA, MS -, and sulfate possessing different  17 O signatures is necessary to examine if this underestimate in  17 O(SO 4 2-) nss at Dome C corresponds to the lack of MS -destruction in the model.

  physical migration of MS -within firn layers and possibly by MS -oxidation by OH in quasibrine layer of snow grain. If being viable, the latter MS -oxidation should increase  17 O(SO 4 2-) nss manuscript submitted to Journal of Geophysical Research: Atmospheres 20 values in snow after deposition of MS -and sulfate. Indeed, the previous measurements of  17 O(SO 4

  mechanisms as well as to constrain the sulfur chemistry in atmospheric chemical transport models.4.2 Negative  DC-DDU values in austral autumnThe negative  DC-DDU values ranging -1.4 ± 0.6 ‰ to -0.5 ± 0.3 ‰ were observed in austral autumn (March to May) (Fig.1d), suggesting that sulfate with higher  17 O(SO 4 2-) nss is produced at or transported to DDU, compared to Dome C. The largest negative  DC-DDU value of -1.4 ± 0.6 ‰ was observed in April. Meanwhile, the model in the present study slightly underestimates the observed  17 O(SO 4 2-) nss at DDU in April (modeled: 1.6-2.1 ‰, observed: 2.4 ± 0.5 ‰), but reproduces the observed  17 O(SO 4 2-) nss at Dome C (modeled: 1.7-2.1 ‰, observed: 1.7 ± 0.1 ‰) within the range of standard deviation of observations (Fig. 2). Below we consider processes that possibly lead to high  17 O(SO 4 2-) nss at DDU in this period and that also can explain the underestimate of  17 O(SO 4 2-) nss at DDU in the model. The negative  DC-DDU values in March to May coincide with the period when [Br y * ] is minimal at both sites (Fig. 1g). This [Br y * ] minimum in autumn is thought to be a result of the decrease in Br y emission from sea-salt provided from both open ocean and sea-ice related process (Legrand et al., 2016b). Since Br y * includes HOBr that reacts with S(IV) to produce sulfate with  17 O(SO 4 2-) = 0 ‰ as mentioned in Section 2.4, the decrease in [Br y * ] may cause the reduction of F S(IV)+HOBr and the increase in  17 O(SO 4 2-) nss . Since Br y * emission sources are close to coastal regions, the decrease of [Br y * ] during autumn compared to the other period is larger at DDU (4.7 ± 1.7 ng m -3 for MAM compared to 8.9 ± 1.8 ng m -3 for JJA) than Dome C (1.0 ± 0.4 ng m -3 for MAM compared to 1.9 ± 0.8 ng m -3 for JJA). Given the larger decrease of [Br y * ] at manuscript submitted to Journal of Geophysical Research: Atmospheres 21 DDU than Dome C during autumn, it seems that F S(IV)+HOBr might decrease and  17 O(SO 4 2-) nss might increase at larger degree at DDU than Dome C, resulting in the negative  DC-DDU values.

  [Br y * ] at DDU than Dome C does not lead to a larger decrease in F S(IV)+HOBr at DDU in the model. Furthermore, despite the good reproducibility of [Br y * ] at DDU during MAM by the model (observed: 4.7 ± 1.7 ng m -3 , modeled: 3.5 ± 1.4 ng m -3 ), the model tends to underestimate tropospheric BrO vertical column density in 60 to 90°S during March to April by a factor of ~4 (Fig. S2), implying that the model could also underestimate HOBr abundance. The underestimate of HOBr abundance would lead to underestimate of F S(IV)+HOBr and overestimate of  17 O(SO 4 2-) nss , which is opposed to the obtained result of underestimate in  17 O(SO 4 2-) nss during autumn at DDU.

  based on the kinetics of HSO 3 -+ HOCl investigated by a flow tube experiment, Liu and Abbatt (2020) recently determined reaction rate constant of HSO 3 -+ HOBr (k HOBr+HSO3-) which was estimated to be two orders of magnitude lower than the values used in atmospheric models including GEOS-Chem. However, Chen et al. (2017) had performed a sensitivity test with the two orders of magnitude lower k HOBr+HSO3-and showed that the contribution of S(IV) + HOBr reaction to the global sulfate formation does not change because this reaction is limited by gas diffusion of HOBr into cloud droplets. Thus, it seems that the current understanding in S(IV)+HOBr pathway and related reactive bromine chemistry cannot explain the underestimate of  17 O(SO 4 2-) nss at DDU during the autumn in the model. Another possible explanation is the increase in  17 O(SO 4 2-) nss at DDU associated with the aqueous-phase S(IV) + O 3 pathway in sea-salt aerosol. As mentioned in Section 2.3, fresh sea-salts typically contain alkaline solution (pH ~ 8) where the S(IV) + O 3 pathway is efficient

  role of sea-salt controlling  17 O(SO 4 2-) nss and  DC-DDU values based on the available data. In the meantime, the sea-salt aerosol in the model may be acidified excessively because of the overestimate of [SO 4 2-] nss (Fig. 2a and 2b), while the model fairly reproduces the abundances of sea-salt aerosols at DDU by implementing sea-salts production from blowing snow sublimation (Huang and Jaeglé, 2017; Fig. S1). The excessive acidification of sea-salt in the model would lead to the underestimate of F S(IV)+O3 and then  17 O(SO 4 2-) nss . Therefore, the small underestimate of  17 O(SO 4 2-) nss at DDU during autumn might be partly compensated by correcting [

Figure 5 .

 5 Figure 5. Relationship between [SO 4 2-] ss /[SO 4 2-] total at DDU and  DC-DDU . Symbols for different seasons are same as Fig. 3. The dashed line shows the linear least-squares fit for autumn (MAM).

  Figure1.

-

  Figure3.

  Figure5.

  observed a 1.1 ‰ increase of  17 O(SO 4 2-) within the early 19th century in West Antarctic Ice Sheet Divide ice cores, probably suggesting an increase of O 3 -oxidation relative to OH-and H 2 O 2 -oxidation in sulfate formation in the mid-to high-southern latitude region. However, they estimated by their box model that a 1.1 ‰ increase of  17 O(SO 4 2-) requires a 260% increase of relative abundance of O 3 /OH, which they concluded was highly implausible given a 26% increase of O 3 /OH from a chemistry transport model estimate for the Southern Hemisphere extra-tropics. Based on such results, they pointed out deficiencies in the understanding of sulfate formation other than the recognized SO 2 oxidation by OH, H 2 O 2 , and O 3 . Furthermore, there is increasing evidence for a significant difference between the  17 O(SO 4

	2-

  in the same year 2011. The inter-site difference was evaluated at weekly

	resolution by subtracting the  17 O(SO 4	2-) nss of each DDU sample from that of each Dome C
	sample collected in the closest time period, hereafter denoted  DC-DDU . Note that, at DDU,
	aerosol samples were collected separately for coarse (>1 μm) and fine (<1 μm) mode particles
	and the  17 O(SO 4	2-

  -) nss in the model as we discuss in Section 4.

	The model also reproduces the seasonality of  17 O(SO 4	2-) nss in the observations with
	austral summer minima and austral winter maxima, ranging from 0.9 ‰ (0.8-1.3 ‰, November
	and January) to 2.4 ‰ (2.2-2.6 ‰, June) and from 1.0 ‰ (0.9-1.2 ‰, November) to 2.4 ‰
	formation pathway that leads to  17 O(SO 4	2-) nss > 1 ‰, the modeled  17 O(SO 4	2-) nss values (> 2 ‰
	in winter) mainly reflect the change in the relative importance of this pathway. The sulfate
	formed via S(IV) + HOBr (F S(IV)+HOBr ), which has  17 O(SO 4	2-) = 0 ‰, also increases to 33%
	during winter, limiting the increase of  17 O(SO 4	2-

(2.3-2.6 ‰, June-July) at Dome C and DDU, respectively. The modeled seasonality results from changes in the relative fractions of sulfate formed via different processes, F i in Eq. (

5

) (Figure

2e

, f; Table

1

). During austral summer at Dome C, the relative fractions of sulfate formed by OH (F SO2+OH ) and H 2 O 2 (F S(IV)+H2O2 ) increase to 34% and 49%, respectively (Table

1

), because solar radiation induces production of these oxidants. In contrast, the F S(IV)+O3 increases to 31% during winter, when production of OH and H 2 O 2 is diminished. Since S(IV) + O 3 is the only sulfate-

Table 1 .

 1 Model calculation of relative fraction of sulfate produced via different formation pathways and  17 O(SO 4 2-) nss values in each season in comparison to the observed  17 O(SO 4

	2-) nss

a Mass-weighted average. b Values shown in parenthesis are possible ranges in case taking variabilities in  17 O of oxidants into account (see Section 2.4).

  Therefore, the  17 O transferred to DMSO via DMS + OH, DMS + BrO, and DMS + O 3 reflect  17 O values of each oxidant.  17 O of DMSO produced via DMS + OH is assumed to be equal to  17 O of OH, i.e., 0 ‰. Since 17 O excess is located at the two terminal O atoms of O 3 (O

3 -terminal)

[START_REF] Bhattacharya | Determination of intramolecular isotope distribution of ozone by oxidation reaction with silver metal[END_REF][START_REF] Janssen | A diode laser spectrometer for symmetry selective detection of ozone isotopomers[END_REF]

,  17 O(O 3 ) term is assumed to be 38.4 ± 2.0 ‰ (= 3/2 ×  17 O(O 3 ) bulk ). In addition, BrO receives O 3 -terminal via Br + O 3

  ] nss in snow at various sites with different snow accumulation rates over Antarctica will be required as a future step. Additionally, observations of  17 O(MS -) in aerosols, snow, and ice will provide useful information to prove the proposed

				,  17 O(SO 4	2-) nss
	can increase by 1.1-1.3 ‰ (=  17 O(SO 4	2-) MS+OH × 0.11 × 0.9) at maximum. This estimated post-
	depositional shift in  17 O(SO 4	2-) nss is in agreement with the difference between atmosphere and
	snow  17 O(SO 4	2-) nss , indicating the significance of the post-depositional oxidation of MS -to
	sulfate as a controlling factor of ice-core  17 O(SO 4	2-) nss . Thus, we argue that, to use ice-core
	 17 O(SO 4	2-) nss for assessing past atmospheric oxidant chemistry, it is necessary to correct the
	ice-core  17 O(SO 4	2-) nss for this process. For this purpose, the investigation of the relationship
	between  17 O(SO 4	2-) nss and [MS -]/[SO 4	2-

Acknowledgments

We thank Shuting Zhai for providing support to S.I. and S.H for handling the GEOS-Chem model. We acknowledge financial supports and field supplies for winter and summer campaigns at Dome C and DDU by the Institut Polaire Français Paul Emile Victor (IPEV) from program 1011 (SUNITEDC) and program 1177 (CAPOXI 35-75). We also thank aerosol data provided via the French Environmental Observation Service CESOA, which was supported by IPEV and the National Institute of Sciences of the Universe (INSU-CNRS). We acknowledge JSPS KAKENHI (JP17J08978 (S.I.), JP19J00682 (S.I.), JP16H05884 (S.H.), JP18H05050 (S.H.), JP20H04305 (S.H.), JP20H04969 (S.H.), and JP17H06105 (N.Y. and S.H.)) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We acknowledge Labex OSUG@2020 (Investissements d'avenir -ANR10 LABX56), the French Agence Nationale de la Recherche (ANR) FOFAMIFS project (grant ANR-14-CE33-0009-01) and EAIIST project (grant ANR-16-CE01-0011-01) (J.S.), NSF AGS award 1343077 and 1702266 (B.A.), and NASA award 80NSSC19K1273 (L.J.). The BNP-Paribas foundation is also acknowledged for its financial support under its climate initiative (J.S.). AppSILAS, a scientific program of LEFE-CHAT from the Institut National des Sciences de l'Univers/CNRS, has also provided partial funding for this study (J.S.). S.H. acknowledges JSPS and CNRS under the JSPS-CNRS Joint Research Program. J.S. acknowledges the CNRS/INSU (PRC program 207394) and the PH-SAKURA program of the French Embassy in Japan (project 31897PM) for financial support. Meteo France is acknowledged for providing meteorological data at DDU and IPEV/PNRA for providing routine meteorological observation at station Concordia (www.climantartide.it). manuscript submitted to Journal of Geophysical Research: Atmospheres

Data Availability Statement

Data presented in this article are available at http://dx.doi.org/10.17632/xfr8ffn9xv.1.

Conflict of Interest

The authors declare no conflicts of interest.