Particle shapes and infrared extinction spectra of nitric acid dihydrate (NAD) crystals: optical constants of the β-NAD modification
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Published:2023-06-20
Issue:12
Volume:23
Page:6789-6811
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Wagner RobertORCID, James Alexander D.ORCID, Frankland Victoria L., Möhler OttmarORCID, Murray Benjamin J.ORCID, Plane John M. C.ORCID, Saathoff Harald, Weigel RalfORCID, Schnaiter MartinORCID
Abstract
Abstract. Satellite- and aircraft-based mid-infrared measurements
of polar stratospheric clouds (PSCs) have provided spectroscopic evidence
for the presence of β-NAT (nitric acid trihydrate) particles.
Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory
experiments but not yet detected as a constituent of PSCs in atmospheric
measurements. As for the β-NAD modification, its formation was first
observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum
has been reported, but so far no optical constants have been derived that
could be used as input for infrared retrievals of PSC composition. In this
work, we show that β-NAD particles were efficiently formed in
isothermal heterogeneous crystallisation experiments at 190 K from
supercooled HNO3/H2O solution droplets containing an embedded
mineral dust or meteoric smoke particle analogue. An inversion algorithm
based on a T-matrix optical model was used to derive for the first time the
mid-infrared complex refractive indices of the β-NAD modification
from the measured extinction spectrum of the particles. In contrast to the
heterogeneous crystallisation experiments, the α-NAD phase was
formed when the HNO3/H2O solution droplets did not contain a solid
nucleus and crystallised homogeneously. Using a light-scattering detector
that recorded two-dimensional scattering patterns of the crystallised NAD
particles, we were able to determine predominant shapes of the α-
and β-NAD crystals. We found that α-NAD grew into elongated,
needle-shaped crystals, while β-NAD particles were compact in shape.
This agrees with previously reported images of α- and β-NAD
particles grown on the cryo-stage of an environmental scanning electron
microscope. While direct evidence for the existence of metastable NAD in the
polar stratosphere is still lacking, our experiments add to the wealth of
previous laboratory studies that have identified various conditions for the
rapid growth of metastable compositions. In the atmosphere, these could be
intermediate states that transform into thermodynamically stable NAT on
longer timescales in aged PSCs.
Funder
Helmholtz-Gemeinschaft FP7 Ideas: European Research Council Leverhulme Trust
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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