Abstract. Low stratospheric temperatures are known to be responsible for heterogeneous chlorine activation that leads to polar ozone depletion. Here, we discuss the temperature threshold below which substantial chlorine activation occurs. We suggest that the onset of chlorine activation is dominated by reactions on cold binary aerosol particles, without formation of polar stratospheric clouds (PSCs), i.e. without significant uptake of HNO₃ from the gas-phase. Using reaction rates on cold binary aerosol, a chlorine activation threshold temperature, T_ACL, is derived. At typical stratospheric conditions, T_ACL is similar in value to T_NAT the highest temperature at which nitric acid trihydrate (NAT) can theoretically condense to form PSCs. T_ACL is still in use as parameterization for the threshold temperature for the onset of chlorine activation. However, perturbations can cause T_ACL to differ from T_NAT: T_ACL is dependent upon H₂O, potential temperature, and the sulphate aerosol loading, but unlike T_NAT is not dependent upon HNO₃. A parameterization of T_ACL is provided here, allowing it to be calculated over a comprehensive range of stratospheric conditions. Although considering T_ACL as a proxy for chlorine activation can be no substitute for a detailed model calculation, T_ACL provides a more accurate description of the temperature conditions necessary for polar ozone depletion than T_NAT and can readily be used in place of T_NAT.