OceanFlux Sea Spray Aerosol (OSSA): A new Formulation for Production Fluxes and Implications for Climate Studies
de Leeuw, Gerrit¹; Ovadnevaite, Jurgita²; Manders-Groot, Astrid³; Norris, Sarah⁴; Brooks, Ian M.⁴; Dunne, Eimear⁵; Korhonen, Hannele⁵; Sogacheva, Larisa⁵; Lappalainen, Hanna⁶; Pinnock, Simon⁷; O'Dowd, Colin^2
1FMI & Dept. Physics, Univ Helsinki, FINLAND; ²NUIG, IRELAND; ³TNO, NETHERLANDS; ⁴Univ. of Leeds, UNITED KINGDOM; ⁵FMI, FINLAND; ⁶Dept. Physics, Univ Helsinki, FINLAND; ⁷ESA-ESRIN, ITALY

The amount of sea spray aerosol (SSA) produced per area of sea surface and per unit of time is described by the sea spray source function (SSSF), i.e. a parameterization of the SSA production flux formulated in terms of meteorological and oceanographic parameters. The most common of these parameters is the wind speed and in some formulations also the sea surface temperature is used to provide quantitative information on the production of SSA particles. In addition, information on enrichment in organic matter is obtained from satellite-derived ocean chlorophyll data. The scientific objective of the ESA Oceanflux Sea Spray Aerosol (OSSA) project is to support the SOLAS programme by further exploring the use of satellite data to obtain information on the production of SSA particles and apply the results in a climate model to determine direct and indirect radiative effects of SSA. For the formulation of the OSSA SSSF two data sets have been used spanning a size range from 0.01 - 6 µm (r80, i.e. particle radius at relative humidity of 80%). The smaller particles were measured at Mace Head using a scanning mobility particle sizer (up to about 0.5 µm) and the fluxes of particles larger than 0.2 µm were measured using eddy covariance techniques applied from a ship in the open North Atlantic Ocean. Flux formulations were sought in terms of wave parameters using ECMWF WAM model data. The WAM model assimilates Jason and Envisat altimeter and SAR wave height data. The reason for the use of model data rather than directly using satellite-observed wave data is the better coverage in space and time which is essential since only a limited number of high-quality in situ data is available for SSSF development. In our formulation of the source function the wave parameter used is the Reynolds number RHw, i.e. formulated in terms of the viscosity of sea water. Thus effects of wave age are accounted for as well as wind speed and water temperature. The results, i.e. an SSSF for each data set, appeared to fit excellently in the overlapping size range. Thus an SSSF has been formulated as the sum of five lognormal modes spanning all sizes relevant for climate studies. For each of these modes the production flux has a different wind speed dependence reflecting different production mechanisms. The initial validation versus independent measurements shows that the SSA mass is reasonably well reproduced. Next steps are the implementation of the OSSA source function in the ECHAM-SALSA climate model, together with an existing formulation for the organic mass fraction. Runs will be made to determine the direct and indirect radiative effects of SSA, together with a number of sensitivity studies.

The OSSA project is financially support by ESA as part of the Support to Science Element (STSE) program.