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.
Highlights
Nitric acid trihydrate (NAT) particles are known to be involved in denitrification and vertical downward transport of HNO3 in the Arctic and Antarctic winter stratosphere and thereby affect polar ozone chemistry (e.g., Waibel et al, 1999; Carslaw et al, 2002.; Grooß et al, 2014). β-NAT is the only nitric acid hydrate known to be thermodynamically stable in condensed state under the conditions of the polar winter stratosphere at temperatures below ∼ 195 K (Hanson and Mauersberger, 1988)
The MIPAS-STR observations associated with the Arctic polar stratospheric cloud (PSC) flight on 11 December 2011 show a characteristic “shoulder-like” signature in the spectral region around 820 cm−1, which is attributed to the ν2 symmetric deformation mode of NO−3 of β-NAT
The observed signature is explained by the absorption/emission and scattering characteristics of large highly aspheric β-NAT particles
Summary
Nitric acid trihydrate (NAT) particles are known to be involved in denitrification and vertical downward transport of HNO3 in the Arctic and Antarctic winter stratosphere and thereby affect polar ozone chemistry (e.g., Waibel et al, 1999; Carslaw et al, 2002.; Grooß et al, 2014). β-NAT is the only nitric acid hydrate known to be thermodynamically stable in condensed state under the conditions of the polar winter stratosphere at temperatures below ∼ 195 K (Hanson and Mauersberger, 1988). As discussed by Grothe et al (2004), the spectroscopic data of NAD used in these studies closely correspond to the α-NAD modification Another metastable high-temperature modification β-NAD has been identified by Grothe et al (2004) in laboratory experiments at temperatures above ∼ 200 K, which decomposes into β-NAT and NAM (nitric acid monohydrate) at considerably higher temperatures. We use high-resolution MIPAS-STR spectra and collocated in situ observations during the high-altitude aircraft M-55 Geophysica (Stefanutti et al, 1999) ESSenCe (ESA Sounder Campaign 2011, Kaufmann et al, 2013) flight on 11 December 2011 inside an optically thin PSC to study the mid-infrared optical characteristics of large β-NAT particles in detail. We model the PSC signatures in the MIPAS-STR observations and constrain the simulations with collocated in situ particle measurements.
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