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

Abstract

Abstract. We examined observations of polar stratospheric clouds (PSCs) by CALIPSO, and of HCl and ClO by MLS along air mass trajectories, to investigate the dependence of the inferred PSC composition on the temperature history of the air parcels and the dependence of the level of chlorine activation on PSC composition. Several case studies based on individual trajectories from the Arctic winter 2009/2010 were conducted, with the trajectories chosen such that the first processing of the air mass by PSCs in this winter occurred on the trajectory. Transitions of PSC composition classes were observed to be highly dependent on the temperature history. In cases of a gradual temperature decrease, nitric acid trihydrate (NAT) and super-cooled ternary solution (STS) mixture clouds were observed. In cases of rapid temperature decrease, STS clouds were first observed, followed by NAT/STS mixture clouds. When temperatures dropped below the frost point, ice clouds formed and then transformed into NAT/STS mixture clouds when temperature increased above the frost point. The threshold temperature for rapid chlorine activation on PSCs is approximately 4 K below the NAT existence temperature, T NAT . 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 2 . Where ClONO 2 was not the limiting factor, a large dependence on temperature was evident.

Vortex-wide chlorine activation by a mesoscale PSC event in the Arctic winter of 2009/10

“…6d-f) shows very good agreement for the first activation phase. Nakajima et al (2016) also reported very good agreement of model and observations during this phase. HCl along the trajectories deviates from the observations after a couple of days.…”

A quantitative analysis of the reactions involved in stratospheric ozone depletion in the polar vortex core

“…A detailed discussion of the rationale behind these choices can be found in Wohltmann et al (2013) The treatment of conditions where both NAT and STS clouds are allowed to form has changed compared to Wohltmann et al (2013). Since mixed NAT/STS clouds are commonly observed (e.g., Pitts et al, 2011), they can now form in the model to allow for a more realistic behavior; see Nakajima et al (2016) for details.…”

Chlorine partitioning near the polar vortex edge observed with ground-based FTIR and satellites at Syowa Station, Antarctica, in 2007 and 2011

“…Inner and outer edges (at least 5 • apart from each other) of the polar vortex were defined by local maxima of the isentropic potential vorticity gradient with respect to equivalent latitude only when a tangential wind speed (i.e., mean horizontal wind speed along the isentropic potential vorticity contour; see Eq. 1 of Tomikawa and Sato, 2003) near the vortex edge exceeds a threshold value (i.e., 20 m s −1 ; see Nash et al, 1996, andTomikawa et al, 2015).…”