Temperture histories impact on PSC formation

Polar Stratospheric Clouds (PSCs) are composed of H₂SO4/HNO₃/H₂O aerosols, called supercooled ternary solution (STS); solid nitric acid trihydrate (NAT) particles and, ice particles. These clouds are important due to the critical role they play in the destruction of ozone over the poles. The role of PSCs in ozone destruction has been understood since the 1980's however the mechanisms involved in PSC formation, particularly NAT formation, is still not well understood. Typically formation of PSCs in climate models is handled by using a temperature threshold as a proxy. A commonly used threshold is the equilibrium temperature for the existence of NAT, T_NAT; however when the area with temperatures below T_NAT are compared with observations of PSC it is found that this measure significantly over-estimates the abundance of PSC.

This research uses back trajectories stemming from the site of PSC observations to investigate the effect that the temperature history has on PSC formation. The trajectories are calculated using a simple Lagrangian model on an isentropic surface. The observational PSC data is obtained from the CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization) instrument during the June to September period from the years 2007-10 and for latitudes between 55^oS and 82^oS, and totals in excess of 7 million individual measurement profiles. A four day back trajectory is calculated for each observation along the 450 K isentropic level. Temperature and wind data is from the MERRA (Modern Era Retrospective-analysis for Research and Applications) reanalysis and H₂O and HNO₃ concentrations (required for calculating equilibrium temperatures) are from Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) version 3.3.

It was found that different PSC types had different characteristics in their temperature histories. The mean temperature history associated with ice observations cooled by around 3 K over the previous 24 hours to slightly below the water ice frost point at the time of observation. However, in trajectories associated with the presence of NAT the temperature has on average stayed 4-5 K below T_NAT for the entire four days of the trajectory. Analysis of the CALIOP data shows that the presence of ice is associated with an increase in the local abundance of high NAT density observations. This supports the theory of NAT formation by nucleation on existing ice particles; however there remains a significant portion of low density NAT observations that cannot be accounted for by this mechanism.