The possibility that stratospheric chlorine is converted from reservoir to reactive forms by heterogeneous reactions on background sulfate aerosols is examined. Tightly constrained photochemical models have been used to calculate [ClO> for the morning and the afternoon conditions observed on January 24, 1989 outside the Arctic polar vortex by instruments on the NASA ER-2 aircraft. Calculations involving gas-phase chemistry only and calculations with heterogeneous chemistry including HCl evaporation from the aerosol both produce ClO abundances that agree with observations; calculations in which HCl removal from the aerosol is controlled by heterogeneous reactions produce ClO abundances that are 3 to 3.5 times larger than observations. These results suggest that HCl evaporation must be included in a model of the chemistry of background aerosols. As a result, heterogeneous chemistry on background aerosols is inefficient for the direct conversion of chlorine to its reactive forms throughout most of the stratosphere. However, the background aerosols can be effective at converting NOx to HNO3, and thus by homogeneous chemistry, a ClO enhancement of a factor of two at midlatitudes can result. ¿ American Geophysical Union 1990