Temperature thresholds for polar stratospheric ozone loss

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 HNO 3 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 H 2 O, potential temperature, and the sulphate aerosol loading, but unlike T NAT is not dependent upon HNO 3 . 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 .