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Vortex-wide detection of large polar stratospheric cloud particles in the Arctic winter stratosphere

Depletion of gas-phase HNO3 observed by Aura MLS at 31 and 46 hPa (left two columns, light green and blue contours) is consistent with the presence of large aspherical NAT particles (which efficiently sequester HNO3) detected by MIPAS at 20 km (right column, dark blue triangles).

Nitric acid (HNO3) containing polar stratospheric clouds (PSCs) play key roles in chemical ozone destruction. Not only do they facilitate conversion of chlorine to ozone-destroying forms, but also if the PSC particles grow large enough, they gravitationally settle to lower altitudes, thereby denitrifying the stratosphere and prolonging chlorine-catalyzed ozone loss.

A new algorithm allows vortex-wide detection of large (equivalent median radii ≥ 3 μm) aspherical nitric acid trihydrate (NAT) PSC particles from infrared limb spectra taken by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite instrument. Formation of such large NAT particles leads to efficient sequestration of HNO3 from the gas phase (condensed HNO3 > 10 ppbv). The rate, magnitude, and spatial extent of gas-phase HNO3 depletion indicated by Aura Microwave Limb Sounder (MLS) measurements, which are unaffected by the presence of PSCs, are consistent with the widespread populations of large NAT particles detected by MIPAS.

Although solid NAT particles were first observed in the stratosphere about 20 years ago, knowledge of their characteristics remains limited and state-of-the-art model parameterizations still fail to reproduce their observed rapid particle growth, high condensed HNO3 content, and long persistence times. This new method of detecting large NAT particles will provide data to test and refine models, reducing uncertainties in projections of chemical ozone loss in the polar lower stratosphere.

Technical description of figure:

Maps during Arctic winter 2011/12 showing the onset of denitrification and detection of NAT particles with volume‐equivalent radii ≥3 μm. (left and middle columns) MLS gas‐phase HNO3 observations (colored contours) at approximately 22 and 20 km, respectively. (right column) MIPAS measurement locations at 20 km (open gray circles), PSCs detected using the sensitive but unspecific cloud index method (Spang et al. [2004], open green circles), populations of small NAT particles in high volume densities (Höpfner et al. [2006], filled red triangles), and populations of large aspherical NAT particles (detected using the new algorithm, open blue triangles). ECMWF temperature isolines are shown at 200 (orange), 195 (~TNAT, magenta), and 190 K (~2 K above Tice, cyan). The inner and outer black solid circles in the panels mark latitudes of 60N and 80N.

Scientific significance, societal relevance, and relationships to future missions:

See previous slide for scientific significance. Extensive denitrification and chemical ozone loss occur every winter in the Antarctic and have been observed in recent extreme winters in the Arctic. Accurate representation of PSC formation is essential for reliable model depiction of denitrification, which in turn is essential for reliable simulations and projections of the stability of the stratospheric ozone layer. Although vertically resolved limb measurements of O3 will be continued by the Ozone Mapper and Profiler Suite Limb Profiler (OMPS-LP) scheduled for launch on the Joint Polar Satellite System (JPSS)-2 spacecraft in 2022, no concrete plans for future measurements of stratospheric gas-phase HNO3 or infrared limb spectra (by NASA or other agencies) currently exist.

Data Sources:

Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) infrared limb spectra, accessible via‐community/applyfor‐data/fast‐registration. Aura Microwave Limb Sounder (MLS) version 4.2 measurements of HNO3 available from European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA‐Interim temperatures (

References: Woiwode, W., Höpfner, M., Bi, L., Khosrawi, F., & Santee, M.L., Vortex‐Wide Detection of Large Aspherical NAT Particles in the Arctic Winter 2011/12 Stratosphere, Geophys. Res. Lett., 46, doi:10.1029/2019GL084145, 2019.