Removal of sea salt hydrate water from seawater-derived samples by dehydration

Aerosol particles produced from bubble bursting of natural seawater contain both sea salts and organic components. Depending on the temperature, pressure, and speed of drying, the salt components can form hydrates that bind water, slowing evaporation of the water, particularly if large particles or thick layers of salts undergo drying that is nonuniform and incomplete. The water bound in these salt hydrates interferes with measuring organic hydroxyl and amine functional groups by Fourier transform infrared (FTIR) spectroscopy because it absorbs at the same infrared wavelengths. Here, a method for separating the hydrate water in sea salt hydrates using freezing and then heating in warm, dry air (70 °C) is evaluated and compared to other methods, including spectral subtraction. Laboratory-generated sea salt analogs show an efficient removal of 89% of the hydrate water absorption peak height by 24 h of heating at atmospheric pressure. The heating method was also applied to bubbled submicrometer (Sea Sweep), generated bulk (Bubbler), and atomized seawater samples, with efficient removal of 5, 22, and 39 μg of hydrate water from samples of initial masses of 11, 30, 58 μg, respectively. The strong spectral similarity between the difference of the initial and dehydrated spectra and the laboratory-generated sea salt hydrate spectrum provided verification of the removal of hydrate water. In contrast, samples of submicrometer atmospheric particles from marine air masses did not have detectable signatures of sea salt hydrate absorbance, likely because their smaller particles and lower filter loadings provided higher surface area to volume ratios and allowed faster and more complete drying.