Phase transitions of sea‐salt/water mixtures at low temperatures: Implications for ozone chemistry in the polar marine boundary layer

Abstract

Abstract. We present laboratory experiments employing differential scanning calorimetry as well as flow cell microscopy to study the microphysics of aqueous NaC1 and sea-salt solutions and droplets at temperatures below 273 K. The freezing and melting points of ice and other precipitates were determined in NaC1 and sea-salt bulk samples as well as in emulsion samples. Using flow cell microscopy, we have determined the deliquescence and efflorescence relative humidities of NaC1 and sea-salt droplets at temperatures between 249 and 273 K, extending the existing room temperature data to polar conditions. Our measurements suggest that sea-salt aerosols will most likely be liquid most of the time under polar marine boundary conditions. In addition, we show that sea-salt aerosols or seawater spray deposited on the polar ice pack will remain partly liquid down to 230 K, with concentrations of C1-and Br-increasing by more than an order of magnitude upon cooling when compared to normal seawater concentrations. This is likely to enhance the rate at which heterogeneous bromine activation reactions occur in the sea-salt deposits. Such reaction rate enhancements with decreasing temperatures are currently not implemented in chemical models, and might help explain the fast bromine activation and subsequent ozone destruction observed during ozone depletion events in the polar marine boundary layer in spring. , 1994]. Information on the microphysics of sea-salt aerosols and patches of sea salt deposited on the ice pack under these extrcme polar conditions is still lacking. In this paper we present results on the microphysics of NaC1 and sea-salt solution droplets at low temperatures. We have preformed calorimetric freezing experiments with emulsified droplets as well as flow cell microscopy of deposited droplets to investigate the behavior of sea-salt aerosols under polar conditions. In addition, we have conducted bulk freezing experiments to mimic the behavior of patches of sea-salt aerosols and seawater deposited on the polar ice pack. Experimental Procedure CalorimetryWe have employed differential scanning calorimetry (DSC) to study phase transitions occurring in bulk and emulsion samples of aqueous NaC1 and sea-salt solutions. The experimental setup used in this paper has been described in detail previously [Chang et al., 1999;Koop et al., 1999]. Solutions of NaC1 were prepared by adding deionized water to NaC1 crystals (Mallinckrodt, >99%). The NaC1 crystals were dried in an oven at a temperature of about 380 K for at least 2 hours prior to preparing the solutions. Sea-salt solutions were prepared by diluting an aqueous stock solution of a synthetic sea-salt mixture (Instant Ocean ©, Aquarium Systems). Since sea salt is a very hygroscopic salt mixture, the composition of the stock 26,393

Direct observation of ice nucleation events on individual atmospheric particles

“…42,43 As shown in Movie S5 (ESI †), both NaCl deliquescence and water condensation occurred uniformly across the sample area, indicating homogeneous water vapor distribution across the 0.2 mm 2 sample area probed in the experiments presented here.…”

Ions at the Air/Water Interface

“…Field as well as laboratory measurements suggest that the photochemical release of Br 2 and BrCl is responsible for the destruction of the surface layer ozone in the Arctic at polar sunrise [53][54][55]. Similar bromine chemistry involving, albeit, lower mixing ratios of Br 2 and BrCl, is anticipated also in mid-latitudes [56].…”

Introducing the bromide/alkalinity ratio for a follow-up discussion on "Precipitation of salts in freezing seawater and ozone depletion events: a status report", by Morin et al., published in Atmos. Chem. Phys., 8, 7317–7324, 2008

“…Using a typical value of RH = 80 %, a NaCl solution of m(Na + (aerosol)) ≈ 5 mol/kg is reached (Tang et al, 1997). According to Koop et al (2000), the deliquescence/efflorescence curves of sea salt hardly depend on temperature, suggesting that the water activity of sea salt solutions of fixed molality also remains nearly constant with changes in temperature. Thus, like Sander et al (2006), we assume a NaCl molality of 5 mol/kg in aerosol particles for RH = 80% at all temperatures.…”