Precipitation of salts in freezing seawater and ozone depletion events: a status report

S. Morin (1, 2) , G. M. Marion (3) , R. von Glasow (4) , D. Voisin (1, 2) , J. Bouchez (5) , J. Savarino (1, 2)

1 UJF - Université Joseph Fourier - Grenoble 1
2 LGGE - Laboratoire de glaciologie et géophysique de l'environnement
3 Desert Research Institute
4 School of Environmental Sciences [Norwich]
5 IPGP - Institut de Physique du Globe de Paris

In springtime, the polar marine boundary layer exhibits drastic ozone depletion events (ODEs), associated with elevated bromine oxide (BrO) mixing ratios. The current interpretation of this peculiar chemistry requires the existence of acid and bromide-enriched surfaces to heterogeneously promote and sustain ODEs. In a recent study, Sander et al. (2006) have proposed that calcium carbonate (CaCO₃) precipitation in any seawater-derived medium could potentially decrease its alkalinity, making it easier for atmospheric acids such as HNO₃ and H₂SO₄ to acidify it. We performed simulations using the state-of-the-art FREZCHEM model, capable of handling concentrated electrolyte solutions, to check the preliminary results of Sander et al. (2006). We show that the alkalinity of brine is indeed reduced to about half and a third of the initial alkalinity of seawater, at 263 K and 253 K, respectively. Such levels of alkalinity depletion have been shown to speed-up the onset of ODEs (Sander et al., 2006; Piot and von Glasow, 2008a), suggesting that carbonate precipitation could well be a key phenomenon linked with ODEs, in polar regions but also in other cold areas, such as altitude salt lakes. In addition, the evolution of the Cl/Br ratio in the brine during freezing was computed using FREZCHEM, taking into account Br substitutions in Cl–containing salts.

Domaines: Océan, Atmosphère

S. Morin, G. M. Marion, R. von Glasow, D. Voisin, J. Bouchez, et al.. Precipitation of salts in freezing seawater and ozone depletion events: a status report. Atmospheric Chemistry and Physics Discussions, 2008, 8 (3), pp.9035-9060. ⟨hal-00328327⟩