The action of wind stress on the ocean surface results in the mechanical production of sea-spray aerosol (SSA), consisting of a suspension in air of particles that are directly produced at the sea surface mainly by bursting of whitecap bubbles. SSA differs from sea-salt aerosol in the sense that the particles may be enriched by other substances such as organic matter. SSA is the strongest source of natural aerosols in the global climate system, making up between 30- 70% of all natural aerosols. SSA also plays a major role in the Earth’s radiative budget by scattering incoming solar radiation (the direct aerosol effect) and by the modification of cloud microphysics and radiative properties (the indirect aerosol effect). SSA is a natural component of atmospheric aerosol, having no direct anthropogenic source. However, it is important to quantify the emission processes and atmospheric burden of SSA in order to understand how SSA may respond to anthropogenic climate change, to put anthropogenic aerosol emissions into context, and to understand the cloud microphysical processes where SSA contributes a major fraction of cloud condensation nuclei. The sea-spray source function (SSSF) is used to describe the production of the number of aerosol particles per unit surface area and per second. Estimates of global annual mass emission of sea salt with current chemical transport models and global climate models (CTMs and GCMs, respectively), using various parameterizations of the sea-spray source function, range over nearly two orders of magnitude. This large uncertainty has consequences for the evaluation of effects of sea-spray on climate, air quality and atmospheric chemistry. The OceanFlux Sea-Spray Aerosol project focussed on developing a more accurate SSSF by making use of EO-based data sets of oceanographic physical and biological parameters, combined with in-situ aerosol measurements. In particular, EO-based wave, temperature and ocean-colour information was used. The resulting OSSA SSSF parameterization was incorporated into the aerosol-climate model ECHAM-HAMMOZ to investigate the direct and indirect radiative effects of sea spray aerosol particles. Model projections of aerosol emissions were also validated against aerosol optical depth retrieved from EO (PARASOL). The OSSA SSSF provides aerosol fluxes which are on the lower side (380-1200 Tg/yr) of those calculated using many other source functions developed in the last decade, however, it also results in particle number and mass concentrations that are closer to the ones measured in real ambient conditions in three marine and coastal sites. The simulated sea spray aerosol contribution to the indirect radiative effect was positive (0.3 W/m2 ), in contrast to previous studies. This positive effect was ascribed to the tendency of sea salt aerosol to suppress both the in-cloud supersaturation and the formation of cloud condensation nuclei from sulphate. However, due to a strong negative direct effect, the simulated effective radiative forcing (total radiative) effect was −0.2 W/m2. The project delivered a scientific roadmap listing key questions for further research, under different themes related to processes, impacts and observation systems, with achievable targets for the next 3-5 years.
Publications:

• Ovadnevaite, J., Manders, A., de Leeuw, G., Ceburnis, D., Monahan, C., Partanen, A.-I., Korhonen, H., and O'Dowd, C. D.: A sea spray aerosol flux parameterization encapsulating wave state, Atmos. Chem. Phys., 14, 1837-1852, doi:10.5194/acp-14-1837- 2014, 2014
• Partanen, A.-I., Dunne, E. M., Bergman, T., Laakso, A., Kokkola, H., Ovadnevaite, J., Sogacheva, L., Baisnée, D., Sciare, J., Manders, A., O'Dowd, C., de Leeuw, G., and Korhonen, H.: Global modelling of direct and indirect effects of sea spray aerosol using a source function encapsulating wave state, Atmos. Chem. Phys., 14, 11731-11752, 2014, doi:10.5194/acp-14-11731-2014