Ice-forming nuclei in air masses over the Gulf of Mexico

Abstract

Aerosol particles collected over the Gulf of Mexico during the period from 20 July to 30 August 1986 were examined for their ability to nucleate ice by condensation-followed-by-freezing. Ice-forming nuclei (IFN) in the 0.1–0.4 μm-diameter size range nucleated ice at a temperature of −4°C; their concentrations were between 2 and 10 m −3. Fractions of aerosol particles in that size range nucleating ice at the initial (the highest) temperatures were between 10 −8 and 10 −7. Peaks in the concentration of dimethyl sulfide (DMS) (0800 h) preceded peaks in ice-nucleating temperatures (1300 h) by 5 h; this is sufficient time for DMS molecules to be oxidized to sulfates and to produce mixed aerosol particles through coagulation of different-sized aerosol particles and absorption of sulfur-bearing gas molecules. Fractions of aerosol particles larger than 0.2 μm in diameter containing SO 2− 4 ions were larger than 0.90; most of the time they were 0.99–1.00. All IFN displayed characteristic features of mixed IFN, that is of marine origin (part of IFN concentration independent of temperature) and of continental origin (part of IFN concentration dependent on temperature).