Southern Ocean Seaspray Aerosol Flux Experiment

Atmospheric aerosol / small particles with sizes ranging from 1nanometre up to 100s of micrometres / play a significant role in many important processes related to global climate. Of particular note are their direct effect on the radiation balance within the atmosphere - where they can act both to reflect incoming solar radiation, and hence as a cooling factor on climate / and their indirect effect via their interactions with clouds, where they act as condensation nuclei for cloud droplets to form on. Over the open oceans the single most important source of aerosol is sea spray; sea salt aerosol are the major scatterer of solar radiation, and a major source of condensation nuclei for marine stratocumulus clouds. Marine stratocumulus are one of the largest sources of uncertainty in climate models due to their great sensitivity to environmental conditions; relatively small changes in the temperature, humidity, or depth of the marine boundary layer, or of the availability of aerosol to act as condensation nuclei, can have a significant impact on the spatial extent, lifetime, and reflectivity of the clouds. Small errors in the representation of any of these factors within climate models can result in major errors in the prediction of future climate. The best available parameterizations of the generation of sea spray aerosols have a scatter of about a factor of 10. These sea-spray source functions are almost all derived indirectly from measurements of the mean aerosol size spectra averaged over periods of the order of minutes to hours; the flux of aerosol being inferred from the change in mean concentration over time and assumptions about the balance of generation and remocal processes. An alternative approach has been to try to scale up laboratory measurements of the aerosol generated by bursting bubbles within a single bubble plume, or whitecap, to the average whitecap coverage under a range of wind conditions in the field. The most robust method of measuring the flux is via the eddy covariance technique, in which the high frequency fluctuations of the vertical wind component and aerosol concentration are measured and the flux estimated from their correlation over time. Until recently the instrumentation required to make such measurements did not exist. A new instrument / CLASP - designed and built at Leeds has both the capability to measure aerosol concentrations fast enough for eddy covariance flux estimates to be made, and be small enough to locate close to a sonic anemometer without causing an unacceptably large flow distortion. Measurements made on the foremast of a research ship during cruises in 2006 and 2007 have shown CLASP to be robust and capable of operating unattended for extended periods. The project proposed here will install two CLASP units on a new Extreme Air-Sea Interaction (EASI) buoy, developed by the University of Miami, Florida, for a measurement campaign in the Southern Ocean. The EASI buoy is an ideal platform for making such measurements: the instrumentation can be sited close to the surface (~6 m), while remaining safe from immersion by waves due to the wave-following performance of the buoy. During a 4-week deployment, measurements of the turbulent flux of sea spray aerosol will be obtained under a wide range of conditions, along with those of wind stress, heat, moisture, and details of the wind-driven wave field. The results will help to develop new and improved parameterizations of sea-spray aerosol production for use in climate models, and thus improve the fidelity of predictions of future climate.