A review of sea-spray aerosol source functions using a large global set of sea salt aerosol concentration measurements
Sea-spray aerosols (SSA) are an important part of the climate system because of their effects on the global ra- diative budget – both directly as scatterers and absorbers of solar and terrestrial radiation, and indirectly as cloud conden- sation nuclei (CCN) influencing cloud formation, lifetime, and precipitation. In terms of their global mass, SSA have the largest uncertainty of all aerosols. In this study we re- view 21 SSA source functions from the literature, several of which are used in current climate models. In addition, we propose a new function. Even excluding outliers, the global annual SSA mass produced spans roughly 3–70 Pg yr−1 for the different source functions, for particles with dry diameter Dp<10 µm, with relatively little interannual variability for a given function. The FLEXPART Lagrangian particle disper- sion model was run in backward mode for a large global set of observed SSA concentrations, comprised of several station networks and ship cruise measurement campaigns. FLEX- PART backward calculations produce gridded emission sen- sitivity fields, which can subsequently be multiplied with gridded SSA production fluxes in order to obtain modeled SSA concentrations. This allowed us to efficiently and simul- taneously evaluate all 21 source functions against the mea- surements. Another advantage of this method is that source- region information on wind speed and sea surface tempera- tures (SSTs) could be stored and used for improving the SSA source function parameterizations. The best source functions reproduced as much as 70%of the observed SSA concentra- tion variability at several stations, which is comparable with “state of the art” aerosol models. The main driver of SSA production is wind, and we found that the best fit to the ob- servation data could be obtained when the SSA production is proportional toU3.5 10 , whereU10 is the source region averaged 10mwind speed. A strong influence of SST on SSA produc- tion, with higher temperatures leading to higher production, could be detected as well, although the underlying physi- cal mechanisms of the SST influence remains unclear. Our new source function with wind speed and temperature de- pendence gives a global SSA production for particles smaller than Dp<10µm of 9 Pg yr−1, and is the best fit to the ob- served concentrations.