Presence of an atmospheric oxalate source in the intertropical zone―its potential action in the atmosphere

Abstract Two series of aerosol collections were made at ground level in several sites in Ivory Coast, in January and June 1986. These two periods are respectively characteristic of the dry and rainy seasons in Western Africa. Oxalate-containing particles are detected with an average concentration of 99.5 ppt. only during the dry period. The dry season aerosol is composed of a mixture of materials produced by the Saharan terrigenous sources and burning vegetation. The oxalate is most likely produced by the combustion of vegetal biomass. The coincidence of the presence of oxalate in the dry season aerosol, and the ice-nucleating properties of this same aerosol shown by Bertrand (1977), suggests that the oxalate is in the form of calcium oxalate, which is an active ice nucleus at −7°C. We present this as a possible source of ice nuclei for the formation of the ice phase in the convective clouds.