In-situ measurements of synoptic scale polar stratospheric clouds in the Arctic stratospheric vortex: Microphysical properties of exceptionally large HNO3 containing particles

In January 2010 and December 2011 synoptic scale polar stratospheric cloud (PSC) fields were probed during seven flights of the high altitude research aircraft M-55 Geophysica inside the Northern hemispheric winter polar vortex. During more than 12 flight-time hours PSC particle size distributions in a diameter range between 0.46 µm and 40 µm were recorded simultaneously by up to four different optical in-situ instruments. Three of these particle instruments are based on the detection of forward scattered light by single particles. The fourth instrument is a grey scale optical array imaging probe. Optical particle diameters of up to 35 µm were detected with particle number densities and total particle volumes exceeding previous arctic measurements. Also, gas phase and particle bound NOy were measured, as well as other variables. The remote sensing particle instrument MAL (Miniature Aerosol Lidar) showed the synoptic scale of the encountered PSCs. The particle mode below 2 µm in size diameter has been identified as Supercooled Ternary Solution droplet mode. The PSC particles in the size range above 2 µm in diameter are considered to consist of nitric acid hydrates or ice, and the particle's high HNO3 content was confirmed by the NOy instrument. Assuming a particle composition of nitric acid trihydrate (NAT), the optically measured size distributions result in particle-phase HNO3 mixing ratios by far exceeding available stratospheric values. In particular, with respect to the denitrification by sedimentation of large HNO3-containing particles, presumably NAT, our new measurements raise questions concerning composition, shape and nucleation pathways. The measurement uncertainties and potential artefacts are discussed with respect to possible overestimations of measured particle sizes and volumes. We hypothesize that either a strong asphericity in particle shape or the particle chemical composition (e.g., water-ice coated with NAT) could explain our observations.