Bulletin No. 9 shows an important change from previous issues. Its new title, "Envirorxnental Assessment of ozone layer modification and its impact", reflects the uncertainty that exists with respect to future budgets of trace gases with the potential to modify the ozone layer. Aitnough the chemistry and physics of the atmospheric processes which control ozone are now better understood than ever before, the ability to predict accurately future atmospheric concentrations of chlorine species, carbon dioxide, methane, nitrous oxide and other oxides of nitrogen has not yet been achieved.