Spatial distributions of soluble salts in surface snow of East Antarctica

3.1. Chemical characteristics of soluble particles from coastal to inland regions in Antarctic snow

In going from the coast (ca. 1000 m a.s.l.) to inland (ca. 3800 m a.s.l.), the fraction of Cl-containing particles to total particles decreases and the fraction of S-containing particles increases (Fig. 2). This latter fraction remains consistent among all summer and multiyear samples. As most of the particles contain S or Cl, sulphate and chloride salts are major compounds of the non-volatile particles. For such particles having sulphate or chloride salt, Na is the major metal. This result shows that the major compounds of sulphate and chloride salts are Na₂SO₄ and NaCl, respectively.

Fig. 2

The particles labelled ‘other’ in Fig. 2 have neither S nor Cl. These other particles are likely to be mainly NaNO₃ (see later discussion), whereas those that contain Si are likely silicate minerals.

From 69°S (coastal region) to 73°S (ca. 3000 m a.s.l.), about half of all salt-containing particles have NaCl. But from 74°S (ca. 3300 m a.s.l.) to 80°S (inland region), most salt-containing particles instead have Na₂SO₄ (Fig. 2). This result is consistent among all summer and multiyear samples. Thus, it suggests that, from the coast to inland, the sea-salt (NaCl) sulphatisation reaction (Legrand et al., 1988b) is as follows:

Consider now the ion concentrations at the same locations. Table 2 shows that the major ion species are Na⁺, Cl⁻, $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ and $\text{N}{\text{O}}_{\text{3}}^{-}$ from 69°S to 80°S. In agreement with the particle analyses, these ion results show that Na is the major metal and the other three metals are available in minor quantity. Also, the major ions are consistent with the major salts NaCl, Na₂SO₄ and NaNO₃. The results are also consistent with Hara et al. (2014), who seldom found ammonium salts, instead found that some of the major aerosol compositions on the same traverse route are Na₂SO₄ and NaNO₃. NaNO₃ is thought to be formed by (Kerminen et al., 2000)

Thus, NaNO₃ comes at the loss of NaCl. Because Cl⁻+$\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ $\text{N}{\text{O}}_{\text{3}}^{-}$ >> Na⁺ (Table 2), some of the Cl⁻, $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ and $\text{N}{\text{O}}_{\text{3}}^{-}$ form acids (HCl, H₂SO₄ and HNO₃) and some form salts (NaCl, Na₂SO₄ and NaNO₃).

Consider the Cl⁻/Na⁺ ratio. NaCl is the major component of sea salt, and H₂SO₄ originates from marine biological activity. NaCl and some H₂SO₄ in this area come from the Southern Ocean via the troposphere, whereas the remaining H₂SO₄ and HNO₃ originate from the stratosphere, passing from inland to the coast (Suzuki et al., 2002). The concentration of Cl⁻ until 70°S (ca. 2000 m a.s.l.) is larger than that of the other two anions (Table 2). In particular, the Cl⁻/Na⁺ ratio (1.19) of the 69°07′ S sample is almost the same as the sea-salt ratio (1.18). On the other hand, the Cl⁻/Na⁺ ratios of the other 14 inland samples are higher (Table 2). This suggests that most sea salts in these inland-snow samples had been modified to Na₂SO₄ or NaNO₃.

3.2. Sea-salt modification of single particles from coastal to inland regions in Antarctic snow

To further examine sea-salt modification, consider the ternary diagrams of Na, Cl and S atomic ratios in single particles. The bottom-right diagram in Fig. 3 shows how we interpret the diagrams: the yellow circle marks the sea-salt ratio of the three elements. If sea salt reacts with nitric acid, the plot moves on the purple dotted line to the purple circle. The purple circle means that all Cl is lost and the particle consists only of NaNO₃. In a similar way, if sea salt reacts with sulphuric acid, the plot moves on the blue dotted line to the blue circle. The blue circle means that the particle consists only of Na₂SO₄ without NaCl. Similarly, the green dotted line and circle represent the reaction that forms CH₃SO₃Na. Most of the particle compositions cluster near the purple or blue dotted lines and circles, indicating that the sea salt reacts to form mainly NaNO₃ and Na₂SO₄.

Fig. 3

The scatter in these ternary diagrams exceeds that from previous aerosol analyses on the same traverse route (Hara et al., 2014). Since only a limited number of particles are actually used for EDS, sample heterogeneity may have contributed to the observed scatter. Otherwise, the increase in scatter likely comes from the larger variability in particle trajectories. The previous aerosols were sampled in a short period and thus trace just one trajectory of sea salt in one air mass. In contrast, the present results come from snowpack, in which the snow composition comes from both snow precipitation and dry deposition after the snowfall. Moreover, the summer samples consist of several snowfall events, each one likely following a different trajectory. So, soluble salts in snowpack are considered to contain a range of aerosol compositions over a duration that includes various air masses.

Almost all particles containing Cl are sea-salt products. Even in the 69°07′ S sample, sea-salt modification occurs in many particles. In the 69°07′ S sample, the Cl⁻/Na⁺ ratio (1.19) is consistent with the sea-salt ratio. Nevertheless, the sea-salt particles in snow may react with acid (HNO₃ or H₂SO₄) and retain the same Cl⁻/Na⁺ ratio. According to Hara et al. (2014), atmospheric aerosols mainly consist of sea salt in spring and mainly modified sea salt in summer. As the snow was sampled in late November (late spring), there are two possible reasons for the spring–summer difference. One is post-depositional effects may cause the difference; sea salt deposited at the 69°07′ S site and then acid (dry deposition) reacted with the deposited sea salt via reactions (1) and (2). As a result, the products are Na₂SO₄ (or NaNO₃) and HCl with the sea-salt ratio of the Cl⁻/Na⁺. The second reason is if the two products of Na₂SO₄ (or NaNO₃) and HCl deposit on the same level on snowpack, modified sea-salt particles can be obtained under the sea-salt ratio in bulk snowpack. It also may be a mix of the two reasons.

Below 3000 m a.s.l. (73°S), about half of the sea-salt particles reacted with nitric acid, whereas above 3300 m a.s.l. (74°S), the sea-salt particles instead mainly reacted with sulphuric acid (Fig. 4). Indeed, from 76°S to 77°S (near Dome Fuji at 3700 to 3800 m a.s.l.), almost all sea-salt particles reacted with sulphuric acid. This finding contrasts with Hara et al. (2014), who found that the inland sea-salt particles in atmospheric aerosol instead mainly reacted with nitric acid. The difference between these results may be due to (1), with snow, the acids can continually deposit, thus increasing in amount, and (2), after depositing in or on the snow, the nitric acid may volatilise via the equilibrium reaction 2NaNO₃+H₂SO₄ → Na₂SO₄+2HNO₃.

Fig. 4

Below 3000 m a.s.l. (<73°S), about half of the Na-containing particles partly change, gaining either Na₂SO₄ or NaNO₃ (light blue and purple in Fig. 4). But further inland, above 3300 m a.s.l. (more than 74°S), the fraction with two Na compounds decreases to about one-tenth. Here, almost all Na-containing particles modify fully to either Na₂SO₄ or NaNO₃ (dark blue and purple in Fig. 4) at the expense of NaCl. These results suggest that sea-salt modification occurs progressively on the Antarctic ice sheet as the salt moves inland, with the reactions becoming complete by 3300 m a.s.l. (74°S). As described above, sea-salt particles mainly reacted with sulphuric acid above 3300 m a.s.l. (more than 74°S). Therefore, Na₂SO₄ is a major soluble salt in snow of the inland region of Antarctica.

Concerning spatial correlations between ion species (Table 2), some are correlated along the entire traverse, and some are not. A plot of Na⁺ and Cl⁻ (not shown) has a strong correlation r², with r² exceeding 0.90 (n=14) along the entire traverse. But for Na⁺ and $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$, no correlation exists in the coastal region (from 69°S to 74°S), whereas a relatively low correlation of 0.69 (n=7) occurs in the inland area (from 74°S to 80°S). Thus, inland, the correlation between Na⁺ and $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ is less than that between Na⁺ and Cl⁻. These trends agree with previous findings (e.g. Suzuki et al., 2002).

Na⁺ and Cl⁻ are considered as having the same origin (sea salt; r²=1), and following eqs. (1) and (2), sea-salt modification increases linearly with the distance from coast to inland. Due to the linear modification through the traverse, the strong correlation between Na⁺ and Cl⁻ remains even after the sea-salt modification on inland Antarctic snow. On the other hand, in the inland area, Na⁺ and $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ are considered as having different origins. Moreover, the ion balance shows $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$>Na⁺, indicating that H₂SO₄ exists. Nevertheless, despite these factors, most Na⁺ instead exists as Na₂SO₄. This finding suggests that using only the ion correlation does not predict the main chemical form in all cases.

3.3. Quantification of sea-salt sulphatisation from coastal to inland regions in Antarctic snow

Due to its importance in the inland region, we now focus on sea-salt sulphatisation by examining the NaCl/Na₂SO₄ ratios. With the sublimation–EDS method, we first estimate the mass of Na, Cl and S and then plot the resulting number and mole ratios of NaCl/Na₂SO₄. Figure 5 shows that, for each set of compounds, the mole ratio nearly equals the number ratio. The main factors that influence the calculated number and mole ratios are the atomic ratio and diameter of each particle. As the particle diameters vary, the equality of the two NaCl/Na₂SO₄ ratios may be due to the averaging over all the particles on a given filter sample. But regardless of the cause, the result shows that one can use the number ratio to estimate the mole (or mass) ratio of these compounds.

Fig. 5

This NaCl/Na₂SO₄ ratio is high in the coastal region and low in the inland region. If the sea salt is unmodified, the NaCl/Na₂SO₄ ratios of sea salt should be 39.1 and 14.7 for open sea and sea ice origins, respectively (Iizuka et al., 2012b). However, below 2000 m a.s.l. (from 69°S to 70°S), the mole ratio is much lower, ranging from 1 to 10, indicating that [NaCl] exceeds [Na₂SO₄]. Then, from about 2500 m a.s.l. (71°S), almost all mole ratios are below 1, indicating that [Na₂SO₄] exceeds [NaCl]. So, around 2000 to 2500 m a.s.l. (70°S to 71°S), the major components of soluble salt change from NaCl to Na₂SO₄. Above 3300 m a.s.l. (more than 74°S), most of the mole ratios are below 0.1, indicating that [Na₂SO₄] is more than 10 times higher than [NaCl]. In particular, above 3700 m a.s.l. near Dome Fuji (from 76°S to 77°S), the mole ratio is below 0.01, indicating that [Na₂SO₄] is more than 100 times higher than [NaCl]. These results show that sea-salt sulphatisation proceeds inland from the coast, changing 1000-fold between reactant NaCl and product Na₂SO₄. The lowest ratio, being near the peak of the ice sheet, suggests that the sulphatisation depends on the altitude of the ice sheet. The peak in altitude may be a region of downflow from the stratosphere to the troposphere, a downflow that brings sulphur compounds from the stratosphere (e.g. Delmas and Boutron, 1980; Legrand and Delmas, 1986). Past research shows that winter snow in this region contains more Na⁺ and Cl⁻, but less $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ than the summer snow (e.g. Iizuka et al., 2004; Hoshina et al., 2014). We did not detect such a winter–summer difference, as the multiyear samples were consistent with the summer samples. This consistency may arise from a post-depositional effect of sea-salt sulphatisation in winter snow. But to test this idea, winter precipitation and surface snow along the same or similar route would need to be analysed.

We previously proposed the IDM to determine the concentration of salt compounds for the Dome Fuji ice core (Iizuka et al., 2008). How well does it work along the present traverse? Table 2 shows [NaCl] and [Na₂SO₄], and Fig. 5 shows the mole ratio of NaCl/Na₂SO₄ by the IDM. The IDM assumes that Na⁺ more readily combines with $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ than with Cl⁻. As a result, NaCl should be negligible where the equivalent [$\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$]>[Na⁺] is inland from 70°S. However, the particle analysis with the sublimation–EDS method detects some NaCl in this inland region. We have found that the sea-salt sulphatisation proceeds in the area below 3000 m a.s.l. (from 70°S to 73°S), yet some NaCl remains, suggesting that the IDM assumption does not apply well to this area. Another area with this same issue is the Talos Dome site. Holocene ice at Talos Dome has a certain amount of NaCl yet is mismatched with the IDM (Iizuka et al., 2013). Thus, like the Talos Dome site, the IDM method might not accurately determine sea-salt sulphatisation in the coastal region.

On the other hand, the inland area above 3300 m a.s.l. (more than 74°S), the sublimation–EDS method gives [NaCl]/[Na₂SO₄] values below 0.1, indicating less discrepancy between sublimation–EDS and IDM. This means that IDM can be applied inland and suggests that almost all Na⁺ does not form NaCl. Similarly, Holocene ice in the Dome Fuji ice core has negligible NaCl, yet closely matches the IDM (Iizuka et al., 2012a, 2012b; Oyabu et al., 2014). Like the Dome Fuji site, the IDM method seems to accurately determine sea-salt sulphatisation in the inland region.

Why is ~3300 m a.s.l. (74°S) the threshold latitude at which sea-salt sulphatisation becomes complete? Stohl and Sodemann (2010) suggested a greater likelihood, compared to a subpolar region, of air from the stratosphere reaching the lowest 500 m above the surface in the Antarctic Plateau during summer and autumn. We find that $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ has a minimum near 3300 m a.s.l. (74°S; see Table 2). The minimum likely occurs here because this area lies away from both the coastal source of H₂SO₄ and the stratospheric source in the Antarctic Plateau (above 3700 m a.s.l.) (Suzuki et al., 2003). The sea-salt sulphatisation should proceed in the area from 1000 to 3300 m a.s.l. (69°S to 74°S), due to the high levels of tropospheric $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$, then becoming complete from 3300 to 3800 m a.s.l. (74°S to 80°S), where additional $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ enters from the stratosphere. In fact, the [NaCl]/[Na₂SO₄] values are lowest at the Antarctic Plateau (Fig. 5), where the contribution of stratospheric materials is highest (e.g. tritium concentration as shown in Kamiyama et al., 1989). This additional $\text{S}{\text{O}}_{\text{4}}^{\text{2}-}$ may accelerate the sea-salt sulphatisation. The 3300 m a.s.l. corresponds to a transition of snow surfaces between smooth (inland plateau type) and rough (katabatic wind zones) (Ageta et al., 1989). The surface snow type may reflect an effective territory of chemicals from the stratosphere. Further study is needed to test this hypothesis; for example, using winter measurements of the area and other areas.