The concentration, size distribution and production rate of large salt nuclei over the oceans

Abstract The sampling and measurment of large condensation nuclei (equivalent salt contents greater than 2 × 10 −14 g) on board of Weather Ships in the N. Atlantic, 200 mi from land, reveal the existence of two distinct types of size spectra. It seems that the nuclei which predominate when waves are breaking (Type I distribution) are the residues of spray droplets, but that those which occur with wind speeds of less than 7 m sec −1 have a (Type II) distribution which closely resembles that of continental aerosol. In winds of up to 15 m sec −1 the measured concentrations of large salt nuclei rarely exceeded 10/cm 3 . From the measured concentration and size distribution of Type I nuclei, it is deduced that in winds of 15 m sec −1 the rate of production at the sea surface of salt nuclei with mass greater than 10 −13 g is about 40 cm −2 sec −1 ; very similar values have been obtained from laboratory experiments in a wind‐wave tunnel. The corresponding rate for nuclei with m >2 × 10 −14 g would be about 100 cm −2 sec −1 . Laboratory studies show that the bursting of air bubbles of diameters between 0.3 mm and 4 mm at the surface of sea water produces small numbers of rather large drops (diameters 50‐500μ), the majority of which would fall back quickly into the sea. The large salt nuclei found in the atmosphere are probably the residues of smaller drops produced by disintegration of the bubble caps.