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Volume 16, issue 14
Atmos. Chem. Phys., 16, 9505-9532, 2016
https://doi.org/10.5194/acp-16-9505-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 9505-9532, 2016
https://doi.org/10.5194/acp-16-9505-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Jul 2016

Research article | 29 Jul 2016

Spectroscopic evidence of large aspherical β-NAT particles involved in denitrification in the December 2011 Arctic stratosphere

Wolfgang Woiwode1, Michael Höpfner1, Lei Bi2,a, Michael C. Pitts3, Lamont R. Poole4, Hermann Oelhaf1, Sergej Molleker5, Stephan Borrmann5,6, Marcus Klingebiel6,b, Gennady Belyaev7, Andreas Ebersoldt8, Sabine Griessbach9, Jens-Uwe Grooß10, Thomas Gulde1, Martina Krämer10, Guido Maucher1, Christof Piesch1, Christian Rolf10, Christian Sartorius1, Reinhold Spang10, and Johannes Orphal1 Wolfgang Woiwode et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843, USA
  • 3NASA Langley Research Center, Hampton, VA 23681, USA
  • 4Science Systems and Applications, Incorporated, Hampton, VA 23666, USA
  • 5Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 6Institute for Physics of the Atmosphere (IPA), University of Mainz, Mainz, Germany
  • 7Myasishchev Design Bureau, Zhukovsky-5, Moscow Region, Russia
  • 8Institute for Data Processing and Electronics, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 9Jülich Supercomputing Centre (JSC), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
  • 10Institute of Energy and Climate Research (IEK-7), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
  • anow at: School of Earth Sciences, Zhejiang University, Hangzhou, China, 310027
  • bnow at: Atmosphere in the Earth System Department, Max Planck Institute for Meteorology, Hamburg, Germany

Abstract. We analyze polar stratospheric cloud (PSC) signatures in airborne MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft) observations in the spectral regions from 725 to 990 and 1150 to 1350cm−1 under conditions suitable for the existence of nitric acid trihydrate (NAT) above northern Scandinavia on 11 December 2011. The high-resolution infrared limb emission spectra of MIPAS-STR show a characteristic “shoulder-like” signature in the spectral region around 820cm−1, which is attributed to the ν2 symmetric deformation mode of NO3 in β-NAT. Using radiative transfer calculations involving Mie and T-Matrix methods, the spectral signatures of spherical and aspherical particles are simulated. The simulations are constrained using collocated in situ particle measurements. Simulations assuming highly aspherical spheroids with aspect ratios (AR) of 0.1 or 10.0 and a lognormal particle mode with a mode radius of 4.8µm reproduce the observed spectra to a high degree. A smaller lognormal mode with a mode radius of 2.0µm, which is also taken into account, plays only a minor role. Within the scenarios analyzed, the best overall agreement is found for elongated spheroids with AR  =  0.1. Simulations of spherical particles and spheroids with AR = 0.5 and 2.0 return results very similar to each other and do not allow us to reproduce the signature around 820cm−1. The observed “shoulder-like” signature is explained by the combination of the absorption/emission and scattering characteristics of large highly aspherical β-NAT particles. The size distribution supported by our results corresponds to ∼9ppbv of gas-phase equivalent HNO3 at the flight altitude of ∼18.5km. The results are compared with the size distributions derived from the in situ observations, a corresponding Chemical Lagrangian Model of the Stratosphere (CLaMS) simulation, and excess gas-phase HNO3 observed in a nitrification layer directly below the observed PSC. The presented results suggest that large highly aspherical β-NAT particles involved in denitrification of the polar stratosphere can be identified by means of passive infrared limb emission measurements.

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The analysis of spectral signatures of a polar stratospheric cloud in airborne infrared remote sensing observations in the Arctic in combination with further collocated measurements supports the view that the observed cloud consisted of highly aspherical nitric acid trihydrate particles. A characteristic "shoulder-like" spectral signature may be exploited for identification of large, highly aspherical nitric acid trihydrate particles involved in denitrification of the polar winter stratosphere.
The analysis of spectral signatures of a polar stratospheric cloud in airborne infrared remote...
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