Exchange of ammonia at the sea surface—a preliminary study

Abstract

The rate of deposition of ammonia to sea water was observed in laboratory chamber experiments. Similar experiments using demineralised water and dilute sulphuric acid were also carried out, and information on the surface resistance for ammonia at the sea water surface was determined from the results. Estimates of the resistance for transfer through the air to the sea surface were added to the surface resistance in order to derive an estimation of the mean deposition velocity of ammonia to sea water around the British Isles of approximately 5 mm s -1, with an uncertainty range of 2–15 mm s -1. Despite the limitations of these preliminary experiments, the chamber method has been demonstrated successfully for ammonia. Further refinements should increase confidence and reduce uncertainty in the rates and direction of exchange of ammonia with the sea surface. The derived deposition velocity, and its uncertainty range was used in a modelling study to determine the likely fate of ammonia in air passing over the North Sea in both wet and dry conditions. In a dry eastward trajectory, about 40% of the ammonia emitted from the central lowlands of Scotland was deposited before reaching the coast. Some 1.5–4% was lost to the sea surface, and approximately 55% was advected to the continent. In wet conditions, however, 80% of the emissions were deposited over Scotland before reaching the coast, and the remainder was lost to the sea, chiefly in rain. These calculations assumed that the sea will act as a sink for ammonia in dry conditions rather than a source. In order to investigate this, the equilibrium concentration at the surface was calculated to be 0.07 μg m -3 for typical North Sea conditions, and measurements of atmospheric concentrations over the North Sea were made during a short ship-board campaign which showed air concentrations of ammonia in the range of 0.06–2.4 μg m -3. These and other measurements indicate that on occasions the North Sea may be a source, but more often represents a sink for ammonia.