Atmospheric photochemistry models have been used to predict the sensitivity of the ozone layer to various perturbations. These same models also predict concentrations of chemical species in the present day atmosphere which can be compared to observations. Model results for both present day values and sensitivity to perturbation depend upon input data for reaction rates, photodissociation rates, and boundary conditions. This paper develops a method of combining the results of a Monte Carlo uncertainty analysis with the existing set of present atmosphere species measurements. The method is used to examine the range of values for the sensitivity of ozone to chlorine perturbations that is possible within the currently accepted uncertainty ranges for input data. It is found that model runs which predict ozone column losses much greater than 10% as a result of present fluorocarbon fluxes produce concentrations and column amounts in the present atmosphere which are inconsistent with the measurements for ClO, HCl, NO, NO2 , and HNO3 . ¿American Geophysical Union 1987