The decline in stratospheric ozone at northern midlatitudes in wintertime may be caused by chlorine photochemistry that has been enhanced by heterogeneous reactions. We examine the possibility that the heterogeneous reaction of N2O5 on sulfate aerosols is the cause of this decadal ozone decline by comparing ClO and O3 measurments made in the lower stratosphere during February, 1988, with results from a two-dimensional model. At midlatitudes, the abundances, latitudinal, and seasonal gradients of the observed ClO are similar to the results of a model with heterogeneous chemistry, but are in strong disagreement with the results from the model with only gas-phase chemistry. At low latitudes, agreement is best with the results of the model with only gas-phase chemistry. These limited observations indicate that the amount of reactive chlorine is being ehanced, and that heterogeneous chemistry is a likely cause, but the details of the heterogeneous chemistry and the possibility of other mechanisms still need to be considered. ¿American Geophysical Union 1991