Interactive comment on “ Year-round record of bulk and size-segregated aerosol composition in central Antarctica ( Concordia site ) Part 2 : Biogenic sulfur ( sulfate and methanesulfonate ) aerosol ”

This manuscript discusses the sources and processes affecting sulfate and MSA concentrations, and the MSA to sulfate concentration ratio, in the continental Antarctica. This is a very important topic, since the MSA to sulfate ratio has been widely used in estimating biogenic vs anthropogenic sulfur source sources as well as in interpreting ice core data. The measurement data used in this paper appears to be of good quality and the analysis itself is, in most part, scientifically sound. I have a few minor issues to be considered before accepting this paper for publication.


Introduction
The coupling between climate and atmospheric aerosol involves complex processes that are not yet fully elucidated.In the southern hemisphere, aside from the primarily emitted sea-salt particles, the oxidation of dimethyl sulfide (DMS) emitted by phytoplankton is an important source of secondary aerosol (Gondwe et al., 2003).In the atmosphere, DMS is oxidized into small sulfate and methanesulfonate aerosols that interact with solar radiations reaching Earth's surface by scattering of solar energy and by acting as condensation nuclei for cloud droplets, thereby affecting the cloud albedo (Shaw, 1983;Charlson et al., 1987).
Polar ice cores provide a unique archive of climate and past atmospheric aerosol (composition and load) that may help to address some relevant key questions (Legrand and Mayewski, 1997).In contrast to sulfate, methanesulfonate (MS − , also denoted MSA) is exclusively formed by photooxidation of DMS.Pioneering studies dedicated to its records extracted from Antarctic ice cores proposed its use to investigate changes of the marine biota in response to past climatic fluctuations (Legrand and Feniet-Saigne, 1991;Legrand et al., 1991) or sea-ice extent (Welch et al., 1993;Curran et al., 2003).However, it rapidly appears that the interpretation of MSA ice core profiles in terms of past oceanic DMS emissions is far less straightforward than initially thought.First, highly complex mechanisms control the DMS marine emissions.For instance, it is now recognized that the concentration and the oceanic emission of DMS is controlled not only by the phytoplankton biomass or activity alone but also by numerous ecological and biogeochemical processes that are not well understood (Simo' and Dachs, 2002).Second, the atmospheric behavior of DMS, which is characterized by a variable MSA oxidation yields (Gondwe et al., 2004), renders more difficult than expected the use of the ratio of MSA to non-sea-salt sulfate (R MSA ) to separate the contribution of marine biogenic emissions from other sulfate sources like volcanic activity, terrestrial sources, and possibly the stratospheric sulfate reservoir.It is now well recognized that R MSA is highest in the polar region and lowest within the tropics due to a more efficient MSA production from the OH oxidation of DMS at low temperatures (Bates et al., 1992;Gondwe et al., 2004).However, at very high latitudes the atmospheric behavior of DMS may be even more complex than elsewhere due to the presence of halogenated radicals (Read et al., 2008) and a possible role of heterogeneous chemistry on the behavior of dimethyl sulfoxide (DMSO) (Davis et al., 1998).Whereas a good relationship between the MSA level in air and fresh snow has been observed (Jaffrezo et al., 1994;Wolff et al., 1998), the existence of post-depositional losses and migration of MSA signals within annual firn layers have been recognized by several studies (Wagnon et al., 1999;Pasteur and Mulvaney, 2000;Delmas et al., 2003;Weller et al., 2004).Concerning the loss of MSA from the Antarctic snowpack towards the atmosphere, the subsequent presence of MSA in the gas phase is, however, still unclear (Weller et al., 2004;Piel et al., 2006;Mauldin et al., 2004).This loss of MSA was proposed to explain the previous observations of a decreasing trend of R MSA in snow deposited at the coast compared to inland Antarctica (Legrand, 1997).Finally, the calculations of the non-sea-salt sulfate present in Antarctica are more difficult than at any other place in the world due to a depletion of sulfate relative to sodium caused by precipitation of mirabilite (Na 2 SO 4 •10 H 2 O) during freezing of seawater in winter (Wagenbach et al., 1998).
To explain these phenomena, atmospheric records of both DMS and sulfur aerosol are needed, particularly in the vicinity of sites where Antarctic ice cores are extracted.While detailed long-term records of sulfur-derived aerosol species (sometimes completed by DMS and DMSO measurements) are available for the coastal sites of Neumayer (NM) and Dumont d'Urville (DDU) (Wagenbach, 1996;Minikin et al., 1998;Jourdain and Legrand, 2001), only very scattered atmospheric observations of both MSA and sulfate have been obtained so far at central Antarctic positions.Except the study conducted at Concordia by Preunkert et al. (2008) covering a complete annual cycle of MSA, non-sea-salt sulfate, and DMS, most inland records were restricted to austral summer with only few data obtained during polar night (Arimoto et al., 2001(Arimoto et al., , 2004(Arimoto et al., , 2008;;Udisti et al., 2004;Piel et al., 2006).
We here report on multiple year-round (2006-2015) records of bulk aerosol composition of sulfur-derived aerosol (MSA and sulfate) at the Concordia site located on the high East Antarctic plateau.The record of bulk aerosol is complemented by a study of the size-segregated aerosol composition conducted by running a 12-stage impactor over 3 years (2009)(2010)(2011).As discussed in the companion paper (Legrand et al., 2017), these impactor data are essential to evaluate the degree of sulfate depletion relative to sodium of sea-salt aerosol in winter and consequently to accurately calculate here the nssSO 4 level and the R MSA ratio.Here we present and discuss the temporal variability of the composition of sulfur-derived aerosol (MSA, non-sea-salt sulfate, and R MSA ) and its dependence to the aerosol size in relation to seasonal change of marine biogenic DMS emissions and the contribution of non-biogenic sources of sulfate.

Sites, samplings and methods
Bulk aerosol sampling was initiated in 2006 at the inland site of Concordia (75 • 06 S, 123 • 20 E; 3233 m a.s.l.) located near Dome C (DC; 1100 km away from the nearest coast of East Antarctica).Working conditions are detailed in Legrand et al. (2017).Given the weekly sampling time, a large air volume was sampled (∼ 8000 m 3 ), permitting the blank values to remain well below 1 ng m −3 (0.17 ± 0.15 ng m −3 for sodium, 0.4 ± 0.3 ng m −3 for sulfate, and 0 for MSA).The wind was occasionally blowing from the generator building of the Concordia station, disturbing measurements of atmospheric species like ozone (Legrand et al., 2016).Ozone measurements were also occasionally disturbed under very low wind speed conditions (< 2 m s −1 ).The effect of such sporadic contamination of the station activities on the sulfate levels was here examined in the light of weekly denuder tube sampling of acidic gases done at the site, as detailed by Legrand et al. (2017) for HCl and HNO 3 .Indeed, the denuder tube sampling of acidic gases conducted at Concordia also documents SO 2 by measuring sulfate on the extracts.After subtraction of a mean blank of sulfate of 1.5 ng m −3 (i.e., 0.3 pptv of SO 2 ), the average mixing of SO 2 collected from January 2013 to April 2016 (170 samples) is 0.7 ± 0.6 pptv.It is therefore unlikely that the station activities had emitted enough SO 2 to disturb the sulfate levels.As reported in Fig. 1, bulk aerosol chemical measurements were backed up by measurements of the 210 Pb activities (310 samples) using γ -spectrometric quantification, as detailed by Wagenbach et al. (1988) and Elsässer et al. (2011).Due to the short half-life (53 days) of 7 Be, its γ -spectrometric quantification was mainly done on September-January samples, just after their retrograde in Europe at the end of the austral summer season.In this way, 7 Be measurements were obtained on 48 HV filters collected in summer 2006-2007, 2008-2009, 2009-2010, 2010-2011, and the end of 2015, permitting the documentation of the most important change of the 7 Be / 210 Pb activity ratio, as previously reported for the coastal Antarctic site of Neumayer (see Sect. 3.3.1).In addition to the non-destructive γ spectrometry of 210 Pb and 7 Be, 10 Be was chemically extracted from aliquot of filters collected in 2008 to allow quantification by accelerator mass spectrometry (AMS) at Dresden (DREAMS; Rugel et al., 2016).It is important to emphasize here that, whereas 10 Be is rather routinely measured in ice cores, its measurement in air samples is rather rare.The basic steps of chemical treatment are (1) the leaching of the filter with 10 mL very diluted HCl (ultrasonic bath for 5 min, resting overnight) in the presence of ∼ 300 µg of 9 Be carrier (Scharlau, 2 % HCl, 9 Be concentration of 980.4 ± 4.9 µg g −1 ); (2) filtration through PVDF filter (pore size of 0.45 µm); (3) precipitation of beryllium hydroxide by ammonia solution (25 %); (4) rinsing three times with dilute ammonia solution (pH 8-9); (5) drying and ignition to BeO at 900 • C; and (6) mixing with Nb powder (1 : 6 by weight) and pressing into Cu cathodes.Every ninth sample was accompanied by a processing blank, which was treated identically as the filter samples. 10Be data and subsequent 10 Be / 9 Be AMS measurements result in ratios of 2 × 10 −13 to 1 × 10 −11 with total uncertainties from 2.0 to 4.4 % (mean uncertainty 2.4 %).
The concentrations of nssSO 4 corresponding to HV samples were calculated as follows: (1) Examination of the size-segregated composition of aerosol present at Concordia indicates significant sulfate depletion relative to sodium with respect to the seawater composition from May to September (i.e., a k SO 4 /Na value of 0.16 ± 0.09 instead of 0.25 in seawater) (Legrand et al., 2017), resulting from the presence of sea-salt aerosol at the site emitted from both open ocean and sea ice.This value of 0.16 was used in Eq. ( 1) to calculate the nssSO 4 concentrations.From November to April, an absence of sulfate depletion relative to sodium in sea-salt aerosol is assumed and a k SO 4 /Na value of 0.25 (i.e., the seawater reference value) was applied in Eq. ( 1).
The uncertainties in calculating the nssSO 4 level are related to the accuracy of determinations of SO 4 and Na as well as the uncertainties of the calculated value of k SO 4 /Na : where k SO 4 /Na is equal to 0.25 in summer (November-April) and 0.16 ± 0.09 in winter (May-October), SO 2 4 = (0.05 SO 4 ) 2 + σ 2 blank , and Na 2 = (0.05 Na) 2 + σ 2 blank .As discussed above, the HV blanks lead to a σ blank of 0.15 ng m −3 for Na and 0.3 ng m −3 for SO 4 .
Uncertainties in calculating the MSA to nssSO 4 (R MSA ) ratio were estimated as follows: where MSA = 0.05 MSA.On a total of 446 HV filters, in nine cases we calculate R MSA values that are out of order, corresponding to low nssSO 4 values (< 10 ng m −3 ).These data (see the caption of Fig. 1) were not considered when calculating the monthly R MSA means reported in Fig. 2. In addition, the size-segregated aerosol composition was investigated by doing 105 samplings between March 2006 and January 2012 by using a small deposit area impactor, equipped with a 20 µm cutoff diameter inlet (Legrand et al., 2017).Applying a sampling interval of 2 weeks, eight runs per year were done in 2006 and 2007 and a more continuous sampling (25 runs) from 2009 to 2012.The blank values of the deposit remain well below 1 ng m −3 (0.17 ± 0.12 ng m −3 for sodium, 0.08 ± 0.06 ng m −3 for sulfate, and 0 for MSA).All data were blank corrected.The nssSO 4 and R MSA values were calculated by applying in Eq. (1) a k SO 4 /Na value of 0.25 in summer (November-April).For winter sampling (from May to October), the calculations were done for each impactor run by using the individual k SO 4 /Na value derived from the corresponding impactor run, as detailed in Legrand et al. (2017).Briefly, the k SO 4 /Na values are derived by examining the levels of sulfate and sodium present on the stages where most of sea-salt aerosol was collected (0.5-2.0 µm diameter) and having corrected sulfate from its small (but significant) biogenic sulfate contribution, as estimated from MSA levels.
Over the 2009 to 2011 years, aerosol was sampled on both bulk filter and impactor.A good agreement between the two data sets is found for sulfate as well as MSA (not shown).For sulfate, the relationship between the sum of concentrations observed on the impactor ([SO 4 ] impactor ) and the concentration observed on the bulk filter ([SO 4 ] bulk ) is [SO 4 ] impactor = 0.91 (± 0.08)× [SO 4 ] bulk with R 2 = 0.75.For MSA the rela-tionship is [MSA] impactor = 0.75 (± 0.06) × [MSA] bulk with R 2 = 0.8.The slight difference between the two data sets is likely due to differences (up to a few days) in the sampling time intervals.

Results and discussions
The aerosol record at Concordia now covers a decade.The long-term sulfate and MSA trends were examined by calculating the regression line slopes through annual and monthly mean values.No significant trend can be observed.For instance, a very weak annual increasing rate of 1.9 ± 5.6 ng m −3 yr −1 is calculated for sulfate in summer, but the regression line slope was found to be not statistically different from zero at the P > 95 % confidence level.As shown in Fig. 1, both nssSO 4 and 210 Pb levels in bulk aerosol collected at Concordia steadily increase from September to November and decrease from February to April.In Antarctica, the seasonal change of 210 Pb, which is useful to trace the long-range transport of continental submicron aerosol is characterized by summer maximum mainly driven by (1) strong inversion layer in winter particularly at inland sites and (2) seasonal change in the efficiency of the meridional long-range transport (Elsässer et al., 2011).At Concordia, the seasonal 210 Pb amplitude (a factor of 3 from May-August to November-February) is weaker than that of nssSO 4 (more than a factor of 10, Fig. 2).Since 210 Pb and nssSO 4 are both  , 2006-2015) and the coastal site of Neumayer (NM, 1983(NM, -1995) ) and Dumont d'Urville (DDU, 1991(DDU, -1996)) Jourdain and Legrand (2002) present in the atmosphere as submicron aerosol, this difference cannot be attributed to different atmospheric lifetime.Instead, this difference implies a strong seasonal change of sulfur emissions, particularly from September to November and February to April.That is supported by satellite observations showing that concentrations of chlorophyll at high latitudes in the southern surface ocean are increased and decreased at spring and fall equinoxes, respectively (Fig. 2).
In the following we discuss the respective abundance of the two sulfur species at Concordia, their seasonal cycle, and variability over the nine year-round records (Sect.3.1).We then focus discussions on the striking drop of the MSA to nssSO 4 ratio observed during mid-summer over inland Antarctica (Sect.3.2).Finally, in Sect.3.3 we examine the importance of non-biogenic sources of sulfate for inland Antarctica over the course of the year.

Seasonal cycle of MSA and nssSO 4 levels at Concordia
In winter (June-September), the levels of MSA and nssSO 4 at Concordia remain as low as 0.6 ± 0.4 and 6.4 ± 2.2 ng m −3 , respectively (Table 1).The relative contribution of marine biogenic vs. non-biogenic sources to the sulfate budget over the Antarctic plateau in winter will be discussed in Sect.3.3.From winter to November, MSA and nssSO 4 levels exhibit a similar increase by a factor of 9 to 10 (Table 1).This increase is larger than the ones seen at coastal sites (a factor of 6 to 7 for MSA and close to a factor of 5 for nssSO 4 ; Table 1).The increase of MSA and nssSO 4 from September to November at the coast was attributed by Minikin et al. (1998) to the recovery of the marine biota in the Southern Ocean.The larger increase of the two sulfur species from winter to spring at Concordia compared to coastal sites is likely related to weakening of the inversion layer at Concordia at the end of winter that also contributes to the increase there.As seen in Table 1, both at coastal sites and Concordia the R MSA ratio remains close to 0.08 in winter.Such a low value of R MSA is discussed in terms of source region of biogenic sulfur in Sect.3.3.2.The maximum of nssSO 4 in January seen in multipleyear records available at the coastal sites of NM and DDU (Fig. 3) is also observed in the multiple-year nssSO 4 record at Concordia (Fig. 2).As seen in Table 1, the nssSO 4 levels are coastal sites are enhanced by around a factor of 2 from November to January.At Concordia, the nssSO 4 levels are consistently increasing from 63.6 ± 22.5 ng m −3 in November to 100.4 ± 25.3 ng m −3 in January.A larger increase of MSA compared to the nssSO 4 is observed from November to January at the coast (more than a factor of 3; Table 1) leading to a large increase of R MSA .Given the latitudinal dependence of R MSA characterized by high values at very high latitudes (> 60 • S), such a difference in the recovery of MSA and nssSO 4 in January is expected since the activity of marine biota at latitudes higher than 60 • S peaks at that time (Fig. 2).Note the lower value of R MSA in January at DDU compared to NM (Table 1) that will be discussed in Sect. 4.
At Concordia, a quite different picture emerges for MSA with levels peaking before and after sulfate (in November and March; Fig. 2).Even more dramatic is the difference of R MSA between Concordia and coastal sites with values dropping at 0.05 in January at Concordia (Fig. 2) versus around 0.2 at DDU and 0.4 at NM (Fig. 3).Given the relative abundance of MSA with respect to nssSO 4 at the coast, with a mean level  of 100 ng m −3 of nssSO 4 at Concordia in January we would expect between 21 and 41 ng m −3 of MSA at that time (i.e., 4 to 8 times higher than the mean observed level of 5.2 ng m −3 ; Table 1).
A few previous studies already pointed out the occurrence of R MSA as low as 0.1 or less at inland Antarctic sites in mid-summer.As seen in Table 2, most of observations were restricted to a few weeks in December and/or January except the one conducted at Kohnen (located at 2890 m a.s.l.The presence of very large nssSO 4 particles at DDU in summer (dashed black line in panel f) is due to sulfate from ornithogenic soils present at the site (Jourdain and Legrand, 2002).Note that at DDU the impactor was run using only 11 stages, missing the smallest particles.
in Dronning Maud Land) by Weller and Wagenbach (2007) where a composite annual cycle based on discontinuous sampling done over 2.5 years was obtained.In addition, Preunkert et al. (2008) and Becagli et al. (2012) reported continuous samplings done at Concordia over the year 2006.However, none of them examined in detail the change of R MSA over the course of summer (from spring to summer and fall) and its variability from year to year.Here we may first invoke a different size distribution of nssSO 4 and MSA leading to a change of their respective abundance during transport between the ocean and central Antarctica.Indeed, several studies (Rankin and Wolff, 2003;Kerminen et al., 2000) pointed out an enrichment of MSA with respect to nssSO 4 in supermicron compared to submicron particles at coastal Antarctic sites in summer.In this way, the decrease of R MSA at Concordia in December and January could be explained by (1) a larger abundance of MSA with respect to nssSO 4 in large than in small particles at the coast and (2) a transport of marine air mass between the coast and Concordia becoming less efficient in December and January compared to October and March.An alternative possibility involves a selective formation of sulfate with respect to MSA under mid-summer conditions in inland Antarctica.As discussed by Davis et al. (1998), the R MSA ratio in Antarctica is strongly influenced by the respective importance of liquid-and gas-phase sulfur chemistry.Briefly, the OH oxidation of DMS produces SO 2 (abstraction pathway), DMSO, and SO 2 (addition pathway).DMSO is further oxidized by OH either in the gas phase or in the aqueous or aerosol phase.Legrand et al. (2001) reported that the heterogeneous DMSO oxidation efficiently produces MSA in summer at the coast.The efficiency of this heterogeneous process has been confirmed by kinetic studies (Bardouki et al., 2002).Therefore, due to a larger presence of aerosol, it is expected that the oxidation of DMSO would produce much more MSA in the atmospheric boundary layer compared to the buffer layer above.That was supported by field observations made by Davis et al. (1998) at the Palmer site showing a rapid increase of DMSO when vertical downward transport brought buffer layer air mass within the boundary layer.These observations imply a longer lifetime of DMSO in the buffer layer than in the boundary layer, following a strong weakening of the heterogeneous reaction of DMSO caused by a far lower aerosol surface (and liquid water) available there.From that, Preunkert et al. (2008) proposed that the weakening of marine advection in December-January compared to March associated with an ongoing oxidation of SO 2 into sulfate in the buffer layer, where the heterogeneous chemistry of DMSO is very limited, would account for the drop of R MSA observed in mid-summer at Concordia.Note, however, that this hypothesis was based on observations that were limited to 1 year.

Size-segregated composition of sulfur aerosol at Concordia in summer
The previously mentioned enrichment of MSA with respect to nssSO 4 seen in supermicron compared to submicron particles at the coast in summer is also observed at DDU, a coastal site located in margin regions facing the Indian and Pacific oceanic sectors from which most of the marine air mass reaching Concordia comes.As shown in Fig. 4, the second supermicron mode of MSA in aerosol at DDU remains centered around 1-2 µm and differs from the coarse sea-salt aerosol mode observed at 6-8 µm by Jourdain andLegrand (2001, 2002).The contribution of the second mode to the total MSA mass remains rather weak (less than 18 %).At Concordia in summer, the size distribution is very similar to the one observed at DDU with no significant decreasing contribution of the supermicron particles to total MSA mass between the two sites (Fig. 4).Furthermore, as seen in Fig. 5, the size distributions of the two sulfur species typically observed at Concordia indicate no significant decreasing contribution of the supermicron mode to the total MSA mass in summer compared to other seasons (16 % in winter, 18 % in spring and summer, and 10 % in fall).That suggests an absence of selective deposition of MSA during transport between the coast and the inland Antarctic plateau in midsummer.The weak contribution of supermicron particles to the total MSA mass, as observed at the coast, likely limits the segregation between MSA and nssSO 4 during the transport between ocean and Concordia.It is also interesting to notice that the dominant presence of MSA in submicron sulfuric acid particles does not confirm the statement generally presented in previous studies that, after its formation in the gas phase, in contrast to sulfuric acid, MSA is more easily incorporated in larger, less acidic particles (Jefferson et al., 1998).Since DMSO is soluble in acidic solution, we may in fact expect that its solubilization in hydrated acidic aerosol followed by a rapid oxidation into MSA explains the presence of this latter in sulfuric acid particles.
Figure 6 shows that the large drop of R MSA observed on bulk aerosol in summer (Fig. 2, see also Fig. 8 in Sect.3.2.3.) is consistently revealed by impactor data (open triangles in Fig. 6).It also shows a far more pronounced decrease of R MSA in submicron than micron particles.An example of sponding to the March-November time period (Fig. 7a and b) show that R MSA is very poorly related to the nssSO 4 content (R 2 of 0.01 and 0.06 for submicron and micron particles, respectively).Conversely, the higher is the MSA content the higher is R MSA (R 2 of 0.55 and 0.47 for submicron and micron particles, respectively).In fact, as seen in Fig. 7c and d, when distinguishing samples with a high and low nssSO 4 content (higher and lower than 100 ng m −3 ), a strong relationship is found between MSA and R MSA .Note that most of samples containing less than 100 ng m −3 of nssSO 4 correspond to the November-December period, whereas those containing more than 100 ng m −3 of nssSO 4 to the January-March period.In this way, the drop of R MSA from November to December and from February-March to January is examined separately.In both cases, the R MSA drop is mainly due to a decrease of MSA.For instance, in submicron particles R 2 equal to 0.86 is calculated for samples containing less than 100 ng m −3 of nssSO 4 (0.84 for those having more than 100 ng m −3 of nssSO 4 ).The same is seen for supermicron particles (R 2 equal to 0.89 and 0.83 for samples containing less and more than 100 ng m −3 of nssSO 4 , respectively).

Chemical signature of air mass experienced a summer drop of MSA concentrations
The year-round record of R MSA (HV filter data) was examined at the light of different parameters related to the history of air mass present during sampling at Concordia (Fig. 8).This was done by using 10-day backward trajectory as well as chemical characteristics (i.e., sodium, MSA, nssSO 4 , and ozone) and air temperature at the site.We restrict the discussion to the last 5 years, for which the chemical records are the most continuous (Fig. 8).Sodium is here used to evaluate the importance of marine advection from spring to fall and its interannual variability.As discussed by Legrand et al. (2009), ozone at Concordia exhibits a seasonal cycle characterized by a maximum in July followed by a decrease until October and the occurrence of a secondary maximum in November-January (Legrand et al., 2009), similar to what is observed at the South Pole (Crawford et al., 2001).Whereas it is expected that such very remote regions experience winter accumulation of O 3 transported from other regions followed by photochemical destruction in spring and summer, the occurrence of a secondary maximum in November-January is surprising.That was attributed to a photochemical ozone production induced by the high NO x levels generated by the photo-denitrification of the Antarctic snowpack (Davis et al., 2001).
The sodium record reported in Fig. 8f does not support the assumption that the drop of R MSA values coincides with a weakening of marine air advection.Indeed, whereas there are in general much more sodium (late October-early November) just before the drop of R MSA (see the grey areas in Fig. 8c), the recovery of high R MSA values in fall (February-March) is never accompanied by a recovery of sodium levels.For example, R MSA jumped from < 0.10 mid-February 2014 to 0.40 at the end of February 2014, whereas the sodium levels remained over the whole period between 1 and 3 ng m −3 .Consistently with what is observed on impactor (Fig. 7), HV data indicate that the drop of R MSA in mid-summer at Concordia is related to a decrease of MSA rather than an increase of nssSO 4 levels (Fig. 8d).The lack of a recovery of marine air advection in fall and the fact that low R MSA values are more related to low MSA rather than high nssSO 4 levels do not support the preceding assumption of a chemistry favoring sulfate during transport in mid-summer as resulting from a weakening of marine advection and a chemistry of DMSO promoting formation of sulfate in the absence of heterogeneous chemistry.Note also that the temperature record (Fig. 8e) does not support the possibility of a drop of MSA caused by evaporative loss from aerosol.The fact that the drop of R MSA values in mid-summer at Concordia is mainly due to the disappearance of MSA in the fine aerosol and not to an increase of sulfate permits us to reject the assumptions of (1) a selective deposition of MSA with respect to sulfate or (2) a preferential production of sulfate with respect to MSA during transport between the coast and the inland Antarctic plateau.As seen in Fig. 8, the only significant change that coincides fairly well with the drop of R MSA in mid-summer is the occurrence of the secondary maximum of ozone mixing ratio that is attributed to a local photochemical activity driven by NO x emissions from the snowpack of the Antarctic plateau.To illustrate the timing and the amplitude of the photochemical ozone production we have reported in Fig. 8a the de-seasonalized ozone record.It is seen that the sudden appearance of low R MSA values that generally occurred at the beginning of November and ended in February (see the grey areas in Fig. 8) coincides with the periods over which the excess of ozone related to the strong photochemical activity took place.The link between low R MSA values and high photochemical activity is also seen in the interannual variability, with particularly low R MSA values in November-December 2011 and 2012 (∼ 0.03) compared to November-December 2013 and 2014 (0.08) corresponding to larger excess ozone (10.5 ppbv in November-December 2011 and 2012 versus 8.0 ppbv in November-December and 2014).As discussed by Legrand et al. (2016), the more time spent by the air mass above 3200 m elevation prior to its arrival at Concordia, the higher the ozone mixing ratio was.That can be seen in Fig. 8b when comparing the fraction of time spent by the air mass above 3200 m a.s.l. in November-December 2011 and 2012 with November-December 2013 and 2014 (6 days instead of 4-5 days).

A destruction of MSA over the Antarctic plateau under conditions
The preceding observations of a drop of R MSA driven by a decrease of MSA level in submicron particles around the beginning of November and its recovery in February, simultaneous with the high photochemical activity at mid-summer at Concordia, suggest the occurrence of a (photo)chemical destruction of MSA taking place in submicron particles at that time.Under conditions encountered in the marine atmosphere, in contrast to its fast heterogeneous production, a significant (but slow) in cloud destruction of MSA is suspected to take place (Von Glasow and Crutzen, 2004;Barnes et al., 2006;Hoffmann et al., 2016).After its production, MSA present in air masses traveling over inland Antarctica will encounter more oxidative conditions, especially when air masses remained for a few days over the high plateau, and is thus characterized by high ozone mixing ratio (see discussions in Legrand et al., 2016).Assuming both the fact that aerosol particles spend about 3 h per day as cloud droplets and an it is out of the scope of this paper to identify the involved chemical mechanisms leading to a destruction of MSA in submicron sulfuric acid particles over central Antarctica.We can note, however, that over the Antarctic plateau, while the chance of aerosol experiencing aqueous-phase chemistry in cloud droplets is far lower than in the marine boundary layer, the production of OH aq from the reaction of ozone with O − 2 would be favored compared to conditions encountered in the marine boundary layer due to far more acidic conditions (Ervens et al., 2003).Note also that the dissolution of H 2 O 2 that also contributes to the budget of OH aq would be 2 orders of magnitude higher at Antarctic temperatures than at temperatures encountered in the marine boundary layer.Feely et al., 1988), a remote site located at 5220 m a.s.l. in Bolivia.At this site, a typical sulfate level of 250 ng m −3 can be assumed (see Minikin et al., 1998, and references therein).Note that this value is similar to the one reported by Huebert and Lazrus (1980) for the free troposphere (240 ng m −3 , at 5-6 km elevation over the Pacific ocean).Assuming a sulfate concentration of 250 ng m −3 for the continental free troposphere of the southern hemisphere, and applying a dilution factor of 18 based on 210 Pb data (407 µBq m −3 at Chacaltaya compared to 23 µBq m −3 at Concordia), we calculate a mean sulfate concentration of 14 ng m −3 .The 210 Pb activities at Concordia (Fig. 2) show an enhancement by a factor of 2.5 from June-September to November-February, leading to an estimated concentration of continental sulfate of 8 and 20 ng m −3 , respectively.These values may be overestimated since an influence of the city of La Paz (3600 m a.s.l., located only at 25 km away from the Chacaltaya site) on the sulfate concentration remains here possible.Note also that other continents, such as Australia, certainly contribute to the long-range transport of continental 210 Pb and sulfate towards Antarctica, in particular in the case of East Antarctica (Heimann et al., 1990).

3.3
For the first time, 10 Be concentrations are documented in the atmosphere of the high East Antarctic plateau (Fig. 9).The seasonal cycle characterized by a winter minimum and a January-February maximum is similar to what was observed at the coastal site of Neumayer by Elsässer et al. (2011) but with a far stronger winter-summer amplitude (a factor of 10 at Concordia instead of 2-3 at NM).This difference is likely reflecting (1) the particularly strong inversion layer in winter at inland Antarctica and (2) higher summer concentrations at  3200 m a.s.l.than at the sea level.From the observation of a mean winter concentration of 10 Be of 0.6 × 10 4 atoms m −3 at Concordia (Fig. 9) and considering the 10 Be concentration of 10 7 and 0.5 × 10 7 atoms m −3 observed between 11 and 19 km elevation at 65 • N by Raisbeck et al. (1981) and Field et al. (2006), respectively, we derive a dilution factor in the range of 800-1700 between the lower stratosphere and the atmosphere at Concordia in winter.Lazrus et al. (1979) measured worldwide background (non-volcanic) sulfate mixing ratios from 0.1 ppbm at 11 km elevation to 0.5 ppbm at 19 km elevation.Considering a mean sulfate mixing ratio of 0.3 ppbm for the lower stratosphere, and the typical dilution factor observed for 10 Be between the lower stratosphere and the atmosphere at Concordia, we estimate that stratospherictropospheric exchange may account for 0.4 ng m −3 of sulfate in winter at Concordia.As shown in Table 3, similarly to what was previously shown for NM (Wagenbach, 1996), an increase of 7 Be / 210 Pb and 10 Be / 7 Be ratios from winter to summer is seen at Concordia, suggesting a 2 times stronger downward transport from the stratosphere in summer than in winter there.Beryllium data do not account for sulfate transported downward from the lower stratosphere during sedimentation of polar stratospheric clouds that occurs without stratospheric-tropospheric air mass exchange.Measurements of 35 S that offer the possibility to estimate stratospheric input of sulfur were done on aerosol collected at Concordia, showing that as upper estimates 3.3 ng m −3 of sulfate in winter and 16.5 ng m −3 in summer/fall come from the lower stratosphere (Hill-Falkenthal et al., 2013).
Table 3. Monthly data of atmospheric 10 Be and 7 Be at Concordia along with tracers of arrival of stratospheric aerosols ( 10 Be / 7 Be as atom ratio and 7 Be / 210 Pb as activity ratio).The preceding discussions have shown how uncertain estimates of the contribution of non-biogenic source of sulfate over the Antarctic plateau remain based on radionuclide data, except for the downward transport from the stratosphere as traced back using beryllium ( 10 Be and 7 Be).

Months
The initial motivation to conduct simultaneous measurements of MSA and sulfate in Antarctic air or snow was to separate the marine biogenic source of non-sea-salt sulfate from others like volcanic emissions, anthropogenic sources, and terrestrial sources.It was expected that the examination of the relationship between nssSO 4 and MSA would help.However, several previous studies pointed out that the quantification of the non-biogenic sulfate sources by examination of the y intercept of the relationship between nssSO 4 and MSA is complicated by the fact that R MSA is seasonally dependent and varies with the MSA concentration range; see for instance Legrand and Pasteur (1998) and Piel et al. (2006).Figure 10 illustrates how poor the correlation between the two sulfur species is at Concordia ([nssSO 4 ] = 6.0 [MSA] + 14 with R 2 = 0.33; Table 4).As recommended by Ayers (2001), we here used a bivariate regression (so called reduced major axis regression).As clearly shown by Fig. 10a, the scattering of the correlation is largely due to summer samples (R 2 = 0.37 for summer samples as shown with red points).Since, as previously discussed, a destruction of MSA takes place in summer at Concordia, leading to unusually low R MSA values (even lower than in winter), we have scrutinized the correlation only considering data corresponding to the rest of the year (i.e., considering only black points shown in Fig. 10a).For these samples, which cover the spring to fall time period, a far better correlation is observed ([nssSO 4 ] = 3.5 [MSA] + 7 with Table 4).However, the significant change of R MSA from  spring to fall still leads to an overestimation of non-biogenic sulfate when considering the y intercept of the linear regression line.In the following, we therefore scrutinize separately spring, fall, and winter data (red triangles, black triangles, and blue circles in Fig. 11, respectively).In winter, both nssSO 4 and MSA exhibit low concentrations (Fig. 2).Nevertheless, as suggested by increases of 210 Pb, the isolation of the high Antarctic plateau from the free Antarctic troposphere sometimes breaks down, often resulting from arrival of warm air associated with a marine intrusion.As seen in Fig. 12, under these conditions, the increase of 210 Pb is accompanied by an increase of sulfate, suggesting that the lower troposphere at Concordia was temporary filled with winter free-tropospheric air.Interestingly, Fig. 2 shows that the highest mean winter levels of nssSO 4 occurred in 2011 (8.0 ± 3.9 ng m −3 ) and 2013 (8.1 ± 5.6 ng m −3 ), when the 210 Pb levels were also the highest of the record (22-24 µBq m −3 ).Conversely, the lowest nssSO 4 mean winter level is observed in 2008 when the level  4) corresponds to a R MSA value of 0.066 ± 0.004.Such a R MSA value below 0.10 in winter is typically observed in the remote marine boundary layer at low-to mid-southern latitudes in winter (from April to September): 0.077 at 40 • S (Cap Grim; Ayers et al., 1991), 0.037 at 29 • S (Norfolk; Saltzman et al., 1986), and 0.026 at 22 • S (New Caledonia; Saltzman et al., 1986).The y intercept of the linear relationship (0 ± 1 ng m −3 ; Table 4) suggests that in winter, when marine biogenic emissions are located far away from the Antarctic continent, the contribution of non-biogenic source to nssSO 4 remains at the best limited to around 1 ng m −3 (for a total nssSO 4 concentration of 6 ± 4 ng m −3 ).
In spring, the linear relationship between nssSO 4 and MSA ([nssSO 4 ] = 5.1 [MSA] + 9 with R 2 = 0.89; Table 4) suggests an increase of R MSA compared to winter (0.20 instead of 0.07 in winter).This increase of R MSA from winter to spring likely reflects the enhanced contribution of DMS emissions from marine area located south of 50 • S as shown by the increase of oceanic chlorophyll (Fig. 2).In contrast to winter, the linear relationship between nssSO 4 and MSA observed in spring suggests the existence of a non-biogenic sulfate source accounting for a few ng m −3 .The non-linearity of the relation between nssSO 4 and MSA tends, however, to overestimate the y intercept (Fig. 11a); a y intercept of around 3 ng m −3 is obtained when samples with less than 5 ng m −3 of MSA are considered.
In fall, the relationship between nssSO 4 and MSA becomes even less linear than in spring, rendering rather inaccurate the use of the y intercept in evaluating the nonbiogenic sulfate contribution (Fig. 11b).However, Fig. 11b suggests that, if significant, the non-biogenic sulfate concentration remains well below 5 ng m −3 at that time.The slope of the relationship between nssSO 4 and MSA in fall (2.9 ± 0.2, Table 4) indicates a further increase of R MSA compared to spring (0.34 instead of 0.20 in spring).Comparing November and March, Fig. 2 indicates that a decrease of oceanic chlorophyll concentrations have started between 50 and 60 • S whereas those at latitudes higher than 60 • S are maintained or slightly higher in March than in November.That can explain the higher R MSA values in March compared to November.Note that such a higher R MSA in March compared to November is also observed at coastal sites (Fig. 3).Jourdain and Legrand (2002), they were calculated after having subtracted the contribution of ornithogenic soils to the sodium and sulfate levels.
To minimize the uncertainties related to the variability of R MSA between the different samples, we also examined the relationship between non-sea-salt sulfate and MSA concentrations on the 12 stages of impactor run.This approach reduces the uncertainty linked to variability of R MSA value over time.A few impactor runs show a y intercept that was icantly different from zero, reaching 1 ng m −3 in winter and 1 to 4 ng m −3 at other seasons.
It can therefore be concluded that, whatever the season, marine biogenic emissions of DMS dominate the atmospheric budget of sulfate over inland Antarctica.If it exists, the contribution of non-biogenic sulfate source remains limited to 1 ng m −3 in winter and possibly reaches a few ng m −3 particular from spring to fall.Previous discussions on radionuclide data gained at Concordia suggest the long-range transported sulfate from continent in spring and/or summer and downward transport from the lower stratosphere particularly in summer and/or fall as non-biogenic sources of sulfate.

Coastal Antarctica in summer
As seen in Table 5, in summer there is a systematic difference in the sulfur aerosol composition at sites facing the Atlantic sector compared to those facing the Indian sector.Whereas a mean R MSA value close to 0.3 is observed at all sites in March, a summer R MSA maximum of 0.4 in January at NM and Halley (Ha) (Atlantic sector) when the R MSA value remains close to 0.2 at DDU and Mawson (Indian sector).The corresponding relatively weak abundance of MSA compared to nssSO 4 in January at DDU and Mawson compared to NM and Ha may be related to a destruction of MSA that acts more efficiently in the Atlantic than Indian margin regions.Indeed the level of various oxidants was found to be unusually high at DDU compared to the situation at Ha.For instance, Kukui et al. (2012) reported a mean OH concentration of 2.1×10 6 radical cm −3 at DDU versus 0.39×10 6 radical cm −3 at Ha (Bloss et al., 2010).Note that OH concentra-tions at DDU are still in the range of those observed at Concordia (3.1×10 6 radical cm −3 ; Kukui et al., 2014).Concerning ozone, Legrand et al. (2016) compared records from NM and Ha with those of DDU and Syowa, a site also facing the Indian sector.They found that in December, more frequently high ozone values (17 % above 25 ppbv, 33 % above 22 ppbv) are observed at DDU compared to NM (1 % above 25 ppbv, 5 % above 22 ppbv) and Ha (< 1 % above 25 ppbv, 1 % above 22 ppbv).For the case of Syowa, an intermediate situation is observed with 2.5 % of values above 25 ppbv, and 12 % of values above 22 ppbv.These differences were attributed to the fact that ozone-rich air masses present in summer over the inland Antarctic plateau are more efficiently transported to DDU and Mawson than to NM and Ha.Since the nearsurface airflow between the Antarctic plateau and the coastal regions is largely controlled by the topography of the underlying ice sheets and the vicinity of low-pressure systems on the coast of the Antarctic continent, the transport of air mass from inland Antarctica to margin regions is far more important at DDU and Mawson than at NM and Ha (Parish and Bromwich, 2007).These differences in the oxidative property of the atmosphere may lead to larger destruction of MSA and therefore to a decrease of the R MSA ratio at DDU and Mawson compared to NM and Ha, as seen in Table 4.

Implications for the R MSA ratio in Antarctic ice
In the present-day (2006-2015) aerosol at Concordia, we observe an annual mean level of MSA and nssSO 4 of 4 and 38 ng m −3 , respectively, with a grand average R MSA value of 0.11.Assuming a similar air-snow relationship for MSA and nssSO 4 aerosol and referring to the mean level of sulfate observed in the Holocene ice (0-10 kry BP) at DC (100 ppb; Wolff et al., 2006), we would expect a corresponding level of MSA in ice of 12 ppb.The lower MSA value observed in the ice deposited during the Holocene at DC (from 1 to 5 ppb; Saigne and Legrand, 1987) indicates that, in addition to the previously discussed destruction of MSA in the atmowww.atmos-chem-phys.net/17/14055/2017/Atmos.Chem.Phys., 17, 14055-14073, 2017 sphere of central Antarctica, post-depositional effects also contribute to the low R MSA value seen in ice at that site.
Though year-round MSA data are not available at the South Pole, the drop of R MSA values seen at Concordia during mid-summer is also observed at the South Pole (see Table 2), supporting the relevance of an annual R MSA of 0.11 for the atmosphere at the scale of the whole Antarctic plateau.If confirmed, a R MSA value of 0.11 in the atmosphere at the South Pole is consistent with R MSA observed in south polar snow layers, 0.13 over the last century (Legrand and Feniet-Saigne, 1991), and from 0.08 to 0.15 over the last millennium (Feniet-Saigne, 1984).That also suggested that, if they occur, post-depositional effects remains rather limited at that site.
At Vostok, whereas R MSA values ranging around 0.15 were observed in the upper 2 m of snow (Wagnon et al., 1999), an averaged value of 0.05 is found in ice deposited over the last 10 kyr BP (Legrand et al., 1991).Therefore, in contrast to the case of the South Pole where atmospheric process can alone explained the relatively low R MSA values, at sites characterized by lower snow accumulation rates like Vostok and Concordia (2 and 3 g cm −2 yr −1 , respectively, instead of 8 g cm −2 yr −1 at the South Pole), there are also post-depositional destruction or release of MSA within the snowpack.Further measurements including gas-phase MSA would be needed at Concordia to conclude on the causes of post-deposition decrease of MSA in snow (re-emission into the gas-phase or in situ chemical destruction).

Conclusions
Load and composition of sulfur-derived aerosol (methanesulfonate and non-sea-salt sulfate) at inland East Antarctica are documented from multiple year-round records of bulk aerosol samplings and, for the first time in central Antarctica, the size-segregated composition of aerosol (0.03-20 µm diameter).A striking difference in the seasonality of sulfur aerosol composition, characterized by a MSA to nssSO 4 ratio reaching a minimum in December-January over the Antarctic plateau (0.05) and a maximum at the coast (up to 0.40 at sites facing the Atlantic oceanic sector), is clearly established.We find that the low value of R MSA in mid-summer at Concordia is due to a drop of MSA concentrations that occurs in the small particles (0.3 µm diameter) of sulfuric acid aerosol.The drop of MSA coincides with periods of high photochemical activity as indicated by the presence of ozone locally photochemically produced, strongly suggesting the occurrence of an efficient chemical destruction of MSA over the Antarctic plateau in mid-summer.The examination of the relationship between MSA and nssSO 4 levels indicates a non-biogenic sulfate level that does not exceed 1 ng m −3 in fall and winter and remains below 5 ng m −3 in spring.Thanks to atmospheric 10 Be, 7 Be, and 210 Pb data gained at Concordia, this weak level of non-biogenic sulfate over the Antarctic plateau is discussed with respect to the contribution of the stratosphere and of the long-range transport of sulfate from continents.The observed increases of R MSA values from winter to spring and spring to fall reflect change over the course of the year of marine source regions contributing to the sulfur load inland Antarctica, with marine emissions mainly located at temperate latitudes in winter and a progressive recovery of high-latitude DMS emissions in spring-summer-fall.The findings demonstrate that the relatively low R MSA observed in the ice deposited over the plateau compared to that at coastal Antarctica reflects at the first degree the atmospheric behavior of sulfur-derived aerosol, although, at least at sites with low snow accumulation rate, a loss of MSA from the snowpack due to either chemical destruction or re-emission occurs there.

Figure 2 .
Figure 2. Monthly mean concentration of nssSO 4 along with 210 Pb level (a), MSA (c), and MSA / nssSO 4 ratio (d) in bulk aerosol collected at Concordia from January 2006 to January 2016.(b) Monthly mean chlorophyll concentration in the Southern Ocean (2002-2011) (MODIS-Aqua satellite data as reprocessed by Johnson et al., 2013).Vertical bars denote year-to-year variability.

Figure 4 .
Figure 4. Mean size-segregated mass composition of sulfur aerosol (MSA and non-sea-salt sulfate) at Concordia in winter (a, b) and summer (c, d), at Concordia and DDU in summer (e, f).Vertical bars reported in panels (a)-(d) denote sample-to-sample variability.The presence of very large nssSO 4 particles at DDU in summer (dashed black line in panel f) is due to sulfate from ornithogenic soils present at the site(Jourdain and Legrand, 2002).Note that at DDU the impactor was run using only 11 stages, missing the smallest particles.

Figure 7 .
Figure 7. Relationship between the MSA / nssSO 4 ratio and the nssSO 4 level in small (a) and large (b) particles collected on the 12-stage impactor in summer (from November to March).Panels (c) and (d) are the same for the relationship between the MSA / nssSO 4 ratio and the MSA level, distinguishing samples containing more (open triangles) or less (red points) than 100 ng m −3 of nssSO 4 .

Figure 8 .
Figure 8.(a) Ozone mixing ratio corresponding to HV aerosol sampling time (red line), a sinusoidal fit of the ozone seasonal cycle (green line), and the amount of ozone present in excess (turquoise line) in summer are reported (see Sect. 3.2.3).(b) Excess ozone (turquoise line) together with the 10-day backward trajectory (arrival at 0 m a.s.l.) at Concordia (see details in Legrand et al., 2017).The red curve in panel (b) is the fraction of time spent above 3200 m a.s.l. by the air masses arriving at Concordia.Panels (c) and (d) show MSA / nssSO 4 mass ratio and MSA along with nssSO 4 observed on HV samples, respectively.(e) Air temperature at Concordia.(f) Sodium on HV samples.The grey area denotes the fast decreases of R MSA that coincide fairly well with the local O 3 photochemical production.

Figure 10 .
Figure 10.Correlation of nssSO 4 with MSA concentrations observed at Concordia on HV bulk aerosol samples.(a) All data (red circles highlight mid-summer samples); (b) spring to fall samples.

Figure 11 .
Figure 11.Correlation of nssSO 4 with MSA concentrations observed at Concordia on HV bulk aerosol samples collected in spring (a), fall (b), and winter (c).The paneled-in figures highlight the correlation at MSA concentrations close to zero.

Table 2 .
MSA / nssSO 4 ratios (R MSA ) observed at inland Antarctic sites in mid-summer and in February/March (if available).
Figure 9. Annual cycle of 10 Be concentrations in 2008 at Concordia.Vertical bars (in red) refer to AMS uncertainties (see Sect. 2).

Table 4 .
Slope and y intercept (± standard error estimate) of the linear regression between nssSO 2− 4 and MSA levels as a function of season at Concordia.

Table 5 .
MSA / nssSO 4 ratios (R MSA ) observed at coastal Antarctic sites in January and March: Neumayer and Halley facing the Atlantic ocean and Dumont d'Urville and Mawson the Indian ocean.The values are slightly higher than those reported by Legrand and Pasteur (1998) (0.16 ± 0.01 in January and 0.25 ± 0.04 in March) since, following *  Mar 1991-1996    *