Turbulent Exchange: Aerosols, Bubbles And Gases

There is now a consensus that global climate is changing in response to increasing atmospheric concentrations of greenhouse gases. These gases have natural as well as man-made sources and sinks. For carbon dioxide the largest sink is the ocean, which absorbs between 30% and 50% of the CO2 generated by the burning of fossil fuel. The direction of the exchange of gases between atmosphere and ocean depends on the difference in gas concentration between the air and water, and on a number of physical processes that modify the rate of the exchange. The most important of these processes is turbulent mixing in both the air and water close to the surface. This increases with wind speed, but the relationship is complicated by other factors such as the waves state and the thermodynamic stability of the near-surface layers of both ocean and atmosphere. At high wind speeds wave breaking generates bubbles, mixing air down into the water column. The presence of bubbles increases the rate of gas exchange, but the detailed nature of the process is not fully understood and there is considerable disagreement about the exact form of the equations that describe the rate of gas transfer. This is largely a result of a lack of sufficiently detailed measurements.
Wave breaking and bubbles are also closely linked to the formation of sea-spray aerosol particles - these are important as cloud condensation nuclei. Aerosols are generated by the bursting of bubbles at the sea surface. The rate of aerosol
formation is often expressed as a function of whitecap fractions on the sea surface, but there is an uncertainty of about a factor of 10 in the production rate. This suggests that whitecap fraction alone does not control the production rate, but that
factors such as the size and number of bubbles produced by breaking waves may vary with other factors such as size or steepness of the wave.
This project is a UK contribution to a US research cruise that aims to examine the impact of wave breaking and bubble processes on air-sea gas exchange. We will measure whitecap fraction, wave state, wave breaking statistics, and bubble
properties beneath breaking waves. Measurements will be made from an 11-m spar buoy equipped with wave wires to measure the local wave height at high spatial resolution, a bubble camera to measure large bubbles near the surface, and
2 acoustical resonators to measure smaller bubbles deeper below the surface. A separate Waverider buoy will also be deployed to make longer term and independent measurements of the wave spectra. On the ship we will make direct measurements of aerosol fluxes via the eddy covariance technique, along with those of heat, water vapour, CO2, and momentum. Our partners from NOAA and the University of Hawi'i will measure fluxes of several different gases: CO2, CO, and DMS. The joint measurements of gas fluxes, and whitecap and bubble properties will allow the influence of bubbles on the flux to be evaluated directly against a variety of existing parameterizations.

The ultimate impact of this work will be through improvements to air-sea exchange parameterizations in climate models. This will improve the representativeness of wind-speed dependant gas transfer and sea spray aerosol production within climate simulations - both currently have large uncertainties associated with them. Air sea gas transfer controls the uptake of CO2 to the oceans - the largest single sink for atmospheric CO2. Improved representation of these processes in climate models will provide more accurate prediction of future atmospheric CO2 concentrations and hence of future climate. In the short term this will provide better guidance to policy makers on steps necessary to limit the environmental and human impacts of changing climate. Links with the Met Office/Hadley Centre provide a pathway to transfer our results directly to model development teams.
Another direct impact of this work is on satellite retrievals of ocean properties such as chlorophyll concentration in surface waters and whitecap fraction. Satellite-based measurements are the only practical means of making globally distributed observations of a wide range of parameters relevant to operational weather and oceanographic forecasts, and environmental monitoring. The provision of in situ measurements for development and validation of retrieval algorithms will help improve the quality of such retrievals in the future. We have collaborative links with several groups working in these areas and are working actively with them on validation projects. The sea spray source flux measurements are of relevance to applications in near-surface electro-magnetic and electrooptical propagation, and in particular to defence users. We have links with these communities through both the Met Office and DSTL. We also have a related joint project about to start with the Central Research Institute of Electric Power Industry in Japan to validate model predictions of sea-spray fluxes for a specialist application to forecasting of snow that may be a hazard to overhead power lines during land-falling storms.
Education and public outreach activities will include participation in the University of Southampton Express Roadshow (see impact plan), talks to school groups, and the maintenance of a fieldwork blog (including interactive comment with school classes, allowing pupils to ask questions of staff in the field during field campaign). These activities will aid the national needs to make rigorous science accessible to enable the UK to have a scientifically informed public base, and encourage and enthuse the next generation of scientists and engineers. A formal outreach plan has been drafted to aid communication with the media, general public, and schools.