SESSION OVERVIEW

Topic 2: What is the relative importance of chemical versus dynamical processes in governing the chemical composition of the extratropical UTLS?

Session convenors: Kathy Law & Peter Haynes
(kathy.law@aero.jussieu.fr & phh@damtp.cam.ac.uk)

Analysis of observational datasets has shown that a transition layer exists between the stratosphere and troposphere which exhibits characteristics of both regions. The extent to which dynamical and/or chemical processes are influencing the composition of this region still remains to be quantified. A better characterization of the 3D spatial (latitudinal, longitudinal, and altitudinal) and seasonal distribution of the region strongly perturbed by STE is an important step for identifying the relative importance of the dynamical processes involved. Better understanding is required about dynamical processes which result in irreversible transport and mixing of air masses of different origin, and their representation in models needs to be improved. Chemical processes can also influence the chemical composition of the UTLS. Precursors transported into the UTLS can lead to in-situ production of ozone, radicals (eg. HOx) and possibly new aerosol production. The overall impact of in-situ chemistry compared to other sources (eg. production from in-situ emissions) and the role of mixing between air masses of different chemical composition remains uncertain. The possibility that short-lived halogenated species are being transported into the lowermost stratosphere where they could be having an impact on stratospheric ozone levels also needs to be addressed.