UV photolysis of ClOOCl and the ozone hole.

The photochemistry of the ClO dimer (ClOOCl) plays a central role in the catalytic destruction of polar stratospheric ozone. In spite of decades of intense investigations, some of its laboratory photochemical data had not reached the desired accuracy to allow a reliable simulation of the stratospheric ozone loss until recently. Inevitable impurities in ClOOCl samples have obstructed conventional measurements. In particular, an absorption measurement of ClOOCl in 2007, which gave much lower cross sections than previous studies, implied that the formation of the ozone hole cannot be explained with current chemical models. Scientists have wondered whether the model is insufficient or the data is erroneous. Efforts aiming to resolve this controversy are reviewed in this paper, which emphasizes newly developed experiments to determine two critical photochemical properties of ClOOCl--its absorption cross section and product branching ratio--including the first reported product branching ratio at 351.8 nm photolysis.