The direction and magnitude of the ammonia flux across the air-sea interface depends on the relative concentrations of ammonia in the atmosphere and ocean. It has been shown that in the remote marine environment the net ammonia flux is from sea to air (Quinn et al. 1990). Far from land, the ocean is the only significant source of ammonia, where it is produced by the breakdown of organic nitrogen-containing compounds and in zooplankton excreta. The direction of ammonia flux across the air-sea interface is therefore from sea to air. In coastal areas, however, terrestrial inputs of ammonia to the atmosphere raise concentrations to levels that outweigh ocean-produced marine ammonia concentrations, leading to a downward flux of NH₃.

Whilst the direction of the flux has been determined for certain areas, is is not known whether the net air sea flux of ammonia is into or out of the ocean on a global scale. We are also lacking in knowledge of the seasonal cycle of ammonia flux. The aim of this PhD studentship is to increase understanding in these areas through a combination of fieldwork, modelling and analysis of other data sets.

Hypothesis

We propose that, as one moves away from continental source regions a decrease in air-sea flux and an eventual change in flux direction will be observed (figure 1). In the context of the North Sea, the southern part of which has been identified as a sink of atmospheric NH₃ (Asman et al. 1994), one might expect to see decreasing and possibly reversing fluxes with increasing latitude. Study of the position of the 'zero point' may be a useful tool in estimating the net ammonia flux over the North Sea and other areas.

Fieldwork

To quantify the air-sea flux of ammonia we measure the seawater and atmospheric ammonia concentrations, as well as meteorolgical conditions (especially wind speed) that also affect the rate of transfer. Seawater ammonia concentrations are calculated from the total ammonium concentration [NH₄⁺_(sw)] and the pH. [NH₄⁺_(sw)] is measured using the OPA fluroimetry method (Holmes et al. 1999). This method is also used to analyse the atmospheric ammonia samples we collect. These samples have been collected using filter packs (acid-coated paper filters which collect the ammonia from the air which is drawn through them using a vacuum pump) on the first cruises of the project, but are not ideal because of the long sampling period necessary to measure significant amounts of ammonia. For this reason we are developing a rotating annular denuder which should give us an atmospheric ammonia concentration every 1-2 hours.

Preliminary data from cruises in the North Sea and the NE Atlantic show, as predicted, the fluxes out of the sea are more common away from coastal areas and downward fluxes seem to be more common near to the coast. Upward fluxes were even found in the NE Atlantic in December, when one might expect ammonia production in the ocean to be low.

References

Asman, W. A. H., R. M. Harrison, et al. (1994). "Estimation of the net air-sea flux of ammonia over the Southern Bight of the North Sea." Atmospheric Environment 28: 3647-3654.
Holmes, R., A. Aminot, et al. (1999). "A simple and precise method for measuring ammonium in marine and freshwater ecosystems." Canadian journal of fisheries and aquatic science 56: 1801-1808.
Quinn, P. K., T. S. Bates, et al. (1990). "Interactions between the Sulfur and Reduced Nitrogen Cycles Over the Central Pacific Ocean." Journal of Geophysical Research 95(D10): 16405-16416.