Fallout of chlorine 36 to the Earth's surface in the southern hemisphere

Chlorine 36 is a radioactive isotope produced in the atmosphere by the cosmic-ray spallation of Ar-40. It has many applications as an environmental tracer which require an understanding of Cl-36 in modern deposition. Data are currently available only for high latitudes in the northern hemisphere and for Antarctica. In the present work, the first data for Cl-36 i, modern deposition for the southern hemisphere are presented. Excluding the tropical data, the latitude dependence of this fallout is found to follow the form predicted by Lal and Peters, and its magnitude is within 40% of their predictions. In tropical Australia, however, a substantial excess of Cl-36 is observed in rainfall from sites that derived most of their precipitation from the summer monsoon. If the high-latitude northern hemisphere data are representative of the entire hemisphere, then the southern hemisphere fallout appears to be 2-3 times less than in the northern hemisphere. It is suggested that this may be due to enhanced stratosphere-troposphere exchange north of the equator promoted by the greater area of landmass, while the symmetric thermal structure of the southern atmosphere dampens this exchange.