Variations in ionic ratios between reference sea water and marine aerosols

Abstract

Atmospheric particles were collected over the open sea (western Mediterranean, North Atlantic, and Norwegian Sea) by air filtration and cascade impactor sampling. The observed K/Na concentration ratios exhibited marked variation and were greater than the reference ratio in bulk sea water. These concentration ratios can be related to the existence of a chemical fractionation affecting the marine aerosols produced by bubbles bursting at the sea surface. Minute amounts of terrigenous dust are found in northern hemisphere marine air in the same range of concentrations as sea-salt particles and contain sodium and potassium of primary continental origin. The presence of this terrigenous dust in the samples raises questions as to the validity of the fractionation hypothesis. In strong support of this fractionation concept are data from our laboratory based on (a) laboratory and in situ experiments with artificially produced aerosols; (b) high-volume air sampling over the Antarctic Ocean with controlled low loads of hydro-insoluble dust; and (c) coastal region sampling where silicon, aluminum, iron, and potassium provide a corrective index for potassium-rich terrigenous dust concentrations. The Cl/Na ratios measured resemble the sea-water values, which could indicate that an enrichment of potassium takes place rather than a depletion of sodium. The Ca/Na ratios often follow the K/Na ratios and suggest identical enrichment of calcium. The enrichments seem to preferentially affect particles of small size (≤1–2 μm), and long-range transport of fractionated marine aerosols can be hypothesized. This is supported by K/Na, Ca/Na, and Mg/Na ratios as measured in another laboratory. These ratios, observed in ice from the central part of the East Antarctic Plateau and corrected for a terrigenous influence with the iron index, suggest a geochemical fractionation at the marine source. These data further suggest that fractionated marine aerosols can be transported far from their place of origin.