CRISTA-NF observations of polar stratospheric clouds and trace gas volume mixing ratios in the Arctic winter stratosphere

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The distributions of many trace gases such as HNO3, O3, and ClONO2 inside the polar vortex are influenced by polar stratospheric clouds (PSCs). NAT (Nitric Acid Trihydrate) particles, which are one type of PSC particles, can grow to sizes, which are large enough to cause a sedimentation of the particles and, thus, a redistribution of NOy. The ozone loss in denitrified air masses lasts longer, since the deactivation of chlorine is slowed down. These processes need to be understood and considered in a model to simulate the distribution of the important trace gases as precisely as possible. Especially the nucleation and growth of NAT particles is a key point, since these processes are parametrized in models based on observations but not yet understood completely. Even improved parametrizations lead to simulation results that still differ from observations at some points (e.g. size distribution of NAT, NOy redistribution).Measurements by the airborne infrared limb sounder CRISTA-NF (CRyogenic Infrared Spectrometers and Telescope for the Atmosphere - New Frontiers) will be used to gain more insight into the relevant processes. CRISTA-NF measures altitude profiles of thermal emissions of atmospheric trace gases in the mid-infrared. These measurements allow the derivation of 2-dimensional cross-sections of volume mixing ratios (VMRs) of several trace gases (e.g. HNO3, CFC-11, O3, ClONO2) and, additionally, the detection and discrimination of different PSCs (NAT, STS (Supercooled Ternary Solution), and ice). Small NAT particles (radii > 3 µm) cause a spectral feature, which is used for detection. New findings show that this feature can be shifted and the magnitude of the shift depends on the size distribution of the particles. The existing detection method does not consider this shift and the method will be updated to reduce misinterpretations. Additionally, the new method will enable the derivation of information about the particle size distribution of small NAT particles. Furthermore, the radiance influence due to PSCs will be considered in the retrieval process to significantly improve the derivation of trace gas VMRs in the presence of PSCs.Simulations by the chemistry and transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) will be used in the project. Comparisons between the CRISTA-NF observations and simulation results will be used to study the important processes in more detail. Detailed comparisons enable the analysis of different aspects such as the influence of a possible temperature bias or temperature fluctuations on the nucleation of NAT, the influence of model resolution (temporal and spatial), and especially the nucleation of and the HNO3 uptake by NAT and STS particles at the same time including the consequences for size distributions and NOy redistribution.