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We analyze polar stratospheric cloud (PSC) signatures in airborne MIPAS-STR(Michelson Interferometer for Passive Atmospheric Sounding – STRatosphericaircraft) observations in the spectral regions from 725 to 990 and 1150 to1350 cm−1 under conditions suitable for the existence of nitric acidtrihydrate (NAT) above northern Scandinavia on 11 December 2011. Thehigh-resolution infrared limb emission spectra of MIPAS-STR show acharacteristic “shoulder-like” signature in the spectral region around820 cm−1, which is attributed to the ν2 symmetric deformationmode of NO3− in β-NAT. Using radiative transfer calculationsinvolving Mie and T-Matrix methods, the spectral signatures of spherical andaspherical particles are simulated. The simulations are constrained usingcollocated in situ particle measurements. Simulations assuming highlyaspherical spheroids with aspect ratios (AR) of 0.1 or 10.0 and a lognormalparticle mode with a mode radius of 4.8 µm reproduce the observedspectra to a high degree. A smaller lognormal mode with a mode radius of2.0 µm, which is also taken into account, plays only a minor role.Within the scenarios analyzed, the best overall agreement is found forelongated spheroids with AR  =  0.1. Simulations of spherical particles andspheroids with AR  =  0.5 and 2.0 return results very similar to each otherand do not allow us to reproduce the signature around 820 cm−1. Theobserved “shoulder-like” signature is explained by the combination of theabsorption/emission and scattering characteristics of large highly asphericalβ-NAT particles. The size distribution supported by our resultscorresponds to ∼ 9 ppbv of gas-phase equivalent HNO3 at theflight altitude of ∼ 18.5 km. The results are compared with the sizedistributions derived from the in situ observations, a corresponding ChemicalLagrangian Model of the Stratosphere (CLaMS) simulation, and excess gas-phaseHNO3 observed in a nitrification layer directly below the observed PSC.The presented results suggest that large highly aspherical β-NATparticles involved in denitrification of the polar stratosphere can beidentified by means of passive infrared limb emission measurements

atmospheric chemistry and physics

Spectroscopic evidence of large aspherical &amp;lt;i&amp;gt;β&amp;lt;/i&amp;gt;-NAT particles involved in denitrification in the December 2011 Arctic stratosphere