Cirrus clouds and their impact on trace gas budgets in the upper troposphere and lower stratosphere (UTLS) are one of the least understood factors modulating climate change. Nucleation and growth of cirrus cloud ice particles can result in vertical redistribution of the most important greenhouse gas water vapor (H2O) due to gravitational settling of the cloud ice particles. Furthermore, cirrus cloud ice particles are capable of trapping and redistributing nitric acid (HNO3) and other trace gases. Accurate simulations of cirrus clouds and their impact on trace gas budgets in the UTLS pose a challenge for numerical weather prediction (NWP) models and global climate-chemistry models. Using measurements by the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) during the combined HALO (High Altitude and LOng range research aircraft) mission POLSTRACC/GW-LCYCLE/SALSA and model simulations, ICECREAM investigates cirrus-cloud impacts on the chemical composition of the UTLS at high latitudes.