Temperature thresholds for chlorine activation and ozone loss in the polar stratosphere

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 the formation of polar stratospheric clouds (PSCs), i.e. without any significant uptake of HNO3 from the gas phase. Using reaction rates on cold binary aerosol in a model of stratospheric chemistry, a chlorine activation threshold temperature, T-ACL, is derived. At typical stratospheric conditions, T-ACL is similar in value to T-NAT (within 1-2 K), the highest temperature at which nitric acid trihydrate (NAT) can exist. T-NAT is still in use to parameterise 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 and potential temperature, but unlike T-NAT is not dependent upon HNO3. Furthermore, in contrast to T-NAT, T-ACL is dependent upon the stratospheric sulfate aerosol loading and thus provides a means to estimate the impact on polar ozone of strong volcanic eruptions and some geo-engineering options, which are discussed. A parameterisation of T-ACL is provided here, allowing it to be calculated for low solar elevation (or high solar zenith angle) over a comprehensive range of stratospheric conditions. Considering T-ACL as a proxy for chlorine activation cannot replace a detailed model calculation, and polar ozone loss is influenced by other factors apart from the initial chlorine activation. However, T-ACL provides a more accurate description of the temperature conditions necessary for chlorine activation and ozone loss in the polar stratosphere than T-NAT.