1Max Planck Institute for Chemistry (MPI), Particle Chemistry Department, Mainz, Germany
2Institute for Physics of the Atmosphere (IPA), University of Mainz, Germany
3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institute of Atmospheric Physics, Oberpfaffenhofen, Germany
4Eidgenössische Technische Hochschule Zürich, Zürich, Switzerland
5Technical University Darmstadt, Darmstadt, Germany
6Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Neuchâtel, Switzerland
7Laboratoire Temps-Fréquence, Institut de Physique, Université de Neuchâtel, Switzerland
8Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
9Institut für Energie- und Klimaforschung (IEK-7), Forschungszentrum Jülich GmbH, Jülich, Germany
10Institute of Energy and Environmental Technology e.V. (IUTA), Duisburg, Germany
11Institute of Atmospheric Science and Climate (ISAC-CNR), Rome, Italy
*Now at: School of Earth Sciences, The University of Melbourne, Melbourne, Victoria, Australia
Received: 06 Apr 2014 – Discussion started: 13 May 2014
Abstract. 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 within the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interaction) and the ESSenCe (ESSenCe: ESA Sounder Campaign) projects. Particle size distributions in a diameter range between 0.46 and 40μm were recorded by 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 grayscale optical array imaging probe. Optical particle diameters of up to 35μm were detected with particle number densities and total particle volumes exceeding previous Arctic measurements. Also, gas-phase and particle-bound NOy was measured, as well as water vapor concentrations. The optical characteristics of the clouds were measured by the remote sensing lidar MAL (Miniature Aerosol Lidar) and by the in situ backscatter sonde MAS (Multiwavelength Aerosol Scatterometer), showing the synoptic scale of the encountered PSCs. The particle mode below 2μm in size diameter has been identified as supercooled ternary solution (STS) droplets. The PSC particles in the size range above 2μm in diameter are considered to consist of nitric acid hydrates, and the particles' 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 exceeding available stratospheric values. Therefore the measurement uncertainties concerning probable overestimations of measured particle sizes and volumes are discussed in detail. We hypothesize that either a strong asphericity or an alternate particle composition (e.g., water ice coated with NAT) could explain our observations. In particular, with respect to the denitrification by sedimentation of large HNO3-containing particles, generally considered to be NAT, our new measurements raise questions concerning composition, shape and nucleation pathways. Answering these would improve the numerical simulation of PSC microphysical processes like cloud particle formation, growth and denitrification, which is necessary for better predictions of future polar ozone losses, especially under changing global climate conditions. Generally, it seems that the occurrence of large NAT particles – sometimes termed "NAT rocks" – are a regular feature of synoptic-scale PSCs in the Arctic.
Revised: 09 Sep 2014 – Accepted: 12 Sep 2014 – Published: 14 Oct 2014
Molleker, S., Borrmann, S., Schlager, H., Luo, B., Frey, W., Klingebiel, M., Weigel, R., Ebert, M., Mitev, V., Matthey, R., Woiwode, W., Oelhaf, H., Dörnbrack, A., Stratmann, G., Grooß, J.-U., Günther, G., Vogel, B., Müller, R., Krämer, M., Meyer, J., and Cairo, F.: Microphysical properties of synoptic-scale polar stratospheric clouds: in situ measurements of unexpectedly large HNO3-containing particles in the Arctic vortex, Atmos. Chem. Phys., 14, 10785-10801, doi:10.5194/acp-14-10785-2014, 2014.