Freezing-induced acidification of sea ice brine

Highlights

• Brine in sea ice acidifies progressively with decreasing temperature.
• Sea water salinity controls the sea ice brine acidity more than the original pH.
• The minerals’ precipitation causes the reduced buffering capacity and acidification.
• In-situ probe method assessing microscopic (sea) ice brine acidity is introduced.

Abstract

The acidity of sea ice and snow plays a key role in the chemistry of the cryosphere; an important example lies in the photochemical catalytic release of reactive bromine in polar regions, facilitated at pHs below 6.5. We apply in-situ acid-base indicators to probe the microscopic acidity of the brine within the ice matrix in artificial sea water at a range of concentrations (0.35–70 PPT) and initial pHs (6–9). The results are supported by analogous measurements of the most abundant salts in seawater: NaCl, Na₂SO₄, and CaCO₃. In the research herein, the acidity is expressed in terms of the Hammett acidity function, H₂₋. The obtained results show a pronounced acidity increase in sea water after freezing at −15 °C and during the subsequent cooling down to −50 °C. Importantly, we did not observe any significant hysteresis; the values of acidity upon warming markedly resembled those at the corresponding temperatures at cooling. The acidity increase is attributed to the minerals’ crystallization, which is accompanied by a loss of the buffering capacity. Our observations show that lower salinity sea water samples (≤ 3.5 PPT) reach pH values below 6.5 at the temperature of −15 °C, whereas higher salinity ices attain such values only at −30 °C. The ensuing implications for polar chemistry and the relevance to the field measurements are discussed.

Veselý L., Štůsek R., Mikula O., Yang X. & Heger D., 2024. Freezing-induced acidification of sea ice brine. Science of the Total Environment 946: 174194. doi: 10.1016/j.scitotenv.2024.174194. Article.

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