Polar stratospheric cloud evolution and chlorine activation measured by CALIPSO and MLS, and modeled by ATLAS

摘要

We examined observations of polar stratospheric clouds (PSCs) by CALIPSO, andof HCl and ClO by MLS along air mass trajectories, to investigate thedependence of the inferred PSC composition on the temperature history of theair parcels and the dependence of the level of chlorine activation on PSCcomposition. Several case studies based on individual trajectories from theArctic winter 2009/2010 were conducted, with the trajectories chosen suchthat the first processing of the air mass by PSCs in this winter occurred onthe trajectory. Transitions of PSC composition classes were observed to behighly dependent on the temperature history. In cases of a gradualtemperature decrease, nitric acid trihydrate (NAT) and super-cooled ternarysolution (STS) mixture clouds were observed. In cases of rapid temperaturedecrease, STS clouds were first observed, followed by NAT/STS mixtureclouds. When temperatures dropped below the frost point, ice clouds formedand then transformed into NAT/STS mixture clouds when temperature increasedabove the frost point. The threshold temperature for rapid chlorineactivation on PSCs is approximately 4 K below the NAT existence temperature,. Furthermore, simulations of the ATLAS chemistry and transport box model along the trajectories were used to corroborate the measurements and show good agreement with the observations. Rapid chlorine activation was observed when an air mass encountered PSCs. Usually, chlorine activation was limited by the amount of available ClONO. Where ClONO was not the limiting factor, a large dependence on temperature was evident.