Variation of ion ratios with size among particles in tropical oceanic air¹

1 Hawaii Institute of Geophysics Contribution No. 164.

Abstract

During the summer of 1966, size fractions of sea-salt particles were collected in the marine atmosphere using a six-stage cascade impactor mounted on a tower 14 meters above the windward shoreline near Hilo, Hawaii. The particle sizes collected varied from essentially all particles of unit density with radii greater than 10 microns on the first stage to all particles with radii of approximately 0.3 microns on the sixth stage. The use of the tower assured that no local surf spray would be collected by the impactor while sampling during onshore trade wind flow. Iodine, bromine, and chlorine analyses were performed on eight sets of samples by neutron activation. The I/Cl ratio showed a regular increase by a factor of 50 to 100 from the largest to the smallest particles. On the smallest particles this ratio was approximately a thousand times that in sea water. The Br/Cl ratio showed a minimum at intermediate particle sizes, with a majority of the values below the sea water ratio. Most of the iodine mass was found on particles with radii of approximately 0.6 microns, whereas the majority of the chlorine and bromine was on particles with radii between 1.25 and 2.5 microns.

This information provides a possible method for tracing the role of sea-salt nuclei in precipitation processes, and may be used to gain new insight into the cycle of the halogens between the atmosphere and the sea.