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

Abstract

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 1350 cm−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 820 cm−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 4.8 µm reproduce the observed spectra to a high degree. A smaller lognormal mode with a mode radius of 2.0 µm, 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 820 cm−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 ∼ 9 ppbv of gas-phase equivalent HNO3 at the flight altitude of ∼ 18.5 km. 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.

Interannual variations of early winter Antarctic polar stratospheric cloud formation and nitric acid observed by CALIOP and MLS

“…The morphology of NAT particles is still an open question, as is the compactness of the particles Woiwode et al, 2014Woiwode et al, , 2016. Light scattering studies have repeatedly shown that detailed particle morphology cannot be deduced from the depolarization; e.g., Nousiainen et al (2012) investigated simple and complex shapes with size parameters (ratio of the particle circumference to the wavelength) in the range 2-12 and real refractive indices in the range 1.55-1.603 representative of silicate particles, and noted similar depolarization ranges for the fifteen different shapes (regular and irregular) that were analyzed.…”

Widespread polar stratospheric ice clouds in the 2015/2016 Arctic winter – Implications for ice nucleation

“…In addition combined lidar or in-situ observations from aircraft (e.g. Fahey et al, 2001;Northway et al, 2002) could help to answer the question on the abundance of the upper NAT branch, which might include aspherical NAT particles as observed by Molleker et al (2014) and Woiwode et al (2016). Generally this branch is of importance for denitrification.…”

“…As a rough estimate, a 10-µm sized ice crystal sediments about 1 km in a day (Fahey et 10 al., 2001). For aspherical particles the sedimentation rates are even lower (Woiwode et al, 2014;Woiwode et al, 2016;Weigel et al, 2014;), hence generally our simplified calculations cover the altitude range of sedimenting PSC particles.…”

Vortex‐Wide Detection of Large Aspherical NAT Particles in the Arctic Winter 2011/12 Stratosphere

“…Simulations of highly aspherical β-NAT particles are based on T-Matrix calculations (Bi et al, 2013) and are used to define the criteria of our method. As in Woiwode et al (2016), we adopt elongated spheroids with an aspect ratio of 0.1. Note that highly oblate spheroids with an aspect ratio of 10 produced similar results in the previous study.…”

“…Ice is likely to play an important role in NAT nucleation (Weiss et al, 2016). While state-of-the-art parameterizations of large NAT particles are successful in reproducing major patterns of denitrification and the state at the end of the winter, large discrepancies remain in direct comparisons with rare Arctic particle field observations (Grooß et al, 2014;Molleker et al, 2014;Woiwode et al, 2016;Zhu et al, 2015). Depending on the different models and scenarios, simulated particle sizes are significantly smaller and/or number densities significantly lower than observed.…”