This session focuses on the variability of the tropospheric and stratospheric chemical composition on seasonal to decadal timescales. An emphasis is put on the scientific value of long-term measurement time series. The Earth's atmosphere is a sensitive system whose evolution in time depends on many factors. Long-term measurements and related model studies contribute to an improved understanding of the mechanisms which control interannual to decadal variability of atmospheric greenhouse gases and air pollutants. The work documents the changing atmospheric composition in the context of global change.

Researchers are invited to present novel scientific results from mid- and long-term observational time series (more than 10 years) from various in situ measurements networks such as Global Atmosphere Watch (GAW), European Monitoring and Evaluation Programme (EMEP), Network for the Detection of Atmospheric Composition Change (NDACC), Southern Hemisphere Additional Ozonesondes (SHADOZ), Advanced Global Atmospheric Gases Experiment (AGAGE), National Oceanic and Atmospheric Administration (NOAA), other in situ measurements and vertical soundings, regular airborne (e.g. MOZAIC) and other campaigns as well as remote sensing data . Data relevant to tropospheric and stratospheric composition, in particular to ozone depletion and climate change as well as firn data on the earlier atmospheric composition are welcome. We also invite contributions from long-term modeling studies with a focus on tropospheric or stratospheric composition changes, for example in the context of IGAC Atmospheric Chemistry and Climate or SPARC CCMVAL initiative. Results of the multi-model simulations in the framework of other international projects (TFHTAP, AC&C, GEOMON, QUANTIFY, etc.) are also very welcome.