The composition of the upper troposphere and the stratosphere (UTS) plays a key role in the climate system. Our understanding of the interactions between dynamics, chemistry and climate in this region has been rapidly increasing over the last years thanks to combined observational and model based studies. In this session we invite studies of dynamical, transport and chemical processes determining the variability at all scales and long-term trends in the composition of the UTS. We particularly encourage studies bringing together recent in-situ and/or remote sensing observations and model simulations.