High resolution aircraft observations of ClO are used to estimate how much the local ozone loss rate due to the ClO dimer reaction is underestimated when small scale structure in ClO is not fully resolved. A local resolution error in the ozone loss rate, for a model with grid scale L, is defined in terms of the variance of the observations within data segments of length L. Errors in the northern and southern winter polar regions are similar in magnitude, and are largest for gridboxes near the vortex edge. At the edge of the Arctic vortex, the resolution error increases linearly from 5--35% as the grid scale increases from 100--500 km. Errors are much smaller inside the vortex where [ClO>≥0.7 ppbv, increasing from 2--10% over the same range of grid scales. ¿ 1998 American Geophysical Union