Impact of Ocean Acidification on the Air-Sea Exchange of Trace Gases

As atmospheric carbon dioxide continues to rise the pH of seawater will get progressively less alkaline. The effect of this will almost certainly become progressively more apparent as the century progresses. It is likely to affect marine organisms from corals to microscopic plankton, particularly those that form their structures with calcium carbonate as a major component. The possible effects on non-carbonate secreting organisms are harder to predict. In this Ph.D. proposal the student will study the effect of decreasing pH on the production of trace gases (dimethyl sulphide and its precursor dimethylsulphoniopropionate and a range of naturally produced organo-halogen gases) by marine microorganisms. These compounds are volatile and can cross the air-sea interface. In the atmosphere their oxidation products can lead to new particle formation or growth of existing particles to cloud condensation nuclei size. Because of this they can affect the formation of clouds and hence climate. In addition, these gases play important roles in controlling the oxidation capacity of the atmosphere by, for example, destruction of ozone and so affect air quality. The student will examine the effect of pH change on the biogenic production of the gases by microbes. In order to quantify the implications of changed gas fluxes under high CO2 conditions, the student will input estimates of current day emissions as well as future emissions into a state-of-the-art one-dimensional model of the atmospheric marine boundary layer (the MISTRA model). This will allow assessment of the potential impact of pH-induced changes in air-sea fluxes of trace gases on particle formation and the oxidation capacity of the atmosphere. The approach we propose to studying this system is by a mixture of field work using floating and fixed mesocosms in which the pH is adjusted to mimic future conditions and in vitro studies using pH-altered natural seawaters and cultures of specific organisms. In addition, since there is growing evidence that lowered pH of seawater leads to significant changes in the type and composition of marine organic matter, the student will test whether this brings about alteration in the rate of transfer of gases across the air-sea interface. This will be done in the mesocosm studies by adding minute amounts of the tracer pair sulphur hexafluoride and 3-helium, changes in whose concentration ratio over the time-course of the experiments will give estimates of air-sea gas exchange rates. If this effect (which has never been tested previously) proves significant then the implications would be far wider than just for the gases proposed for study here, e.g. on rates of man-made carbon dioxide uptake by the oceans and calculation of air-sea fluxes of other climate relevant gases. Finally, the student will use a one-dimensional model to assess the importance of pH-induced changes in the air-sea exchange of trace gases for particle formation and air quality. What is proposed here would add an additional dimension to the NERC/Defra Ocean Acidification research programme by not only studying one important impact of seawater pH change on processes in the oceans but also by linking effects in the oceans to the wider fields of climate and atmospheric chemistry.