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| Titel |
A-train CALIOP and MLS observations of early winter Antarctic polar stratospheric clouds and nitric acid in 2008 |
| VerfasserIn |
A. Lambert, M. L. Santee, D. L. Wu, J. H. Chae |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 6 ; Nr. 12, no. 6 (2012-03-23), S.2899-2931 |
| Datensatznummer |
250010943
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| Publikation (Nr.) |
 copernicus.org/acp-12-2899-2012.pdf |
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| Zusammenfassung |
| A-train Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)
and Microwave Limb Sounder (MLS) observations are used to investigate
the development of polar stratospheric clouds (PSCs) and the gas-phase
nitric acid distribution in the early 2008 Antarctic winter.
Observational evidence of gravity-wave activity is provided by
Atmospheric Infrared Sounder (AIRS) radiances and infrared
spectroscopic detection of nitric acid trihydrate (NAT) in PSCs is
obtained from the Michelson Interferometer for Passive Atmospheric
Sounding (MIPAS). Goddard Earth Observing System Data Assimilation
System (GEOS-5 DAS) analyses are used to derive Lagrangian
trajectories and to determine temperature-time histories of air
parcels. We use CALIOP backscatter and depolarization measurements to
classify PSCs and the MLS measurements to determine the corresponding
gas-phase HNO3 as a function of temperature. For liquid PSCs
the uptake of HNO3 follows the theoretical equilibrium curve
for supercooled ternary solutions (STS), but at temperatures about 1 K
lower as determined from GEOS-5. In the presence of solid phase PSCs,
above the ice frost-point, the HNO3 depletion occurs over
a wider range of temperatures (+2 to −7 K) distributed about the
NAT equilibrium curve. Rapid gas-phase HNO3 depletion is first
seen by MLS from from 23–25 May 2008, consisting of a decrease in the
volume mixing ratio from 14 ppbv (parts per billion by volume) to
7 ppbv on the 46–32 hPa (hectopascal) pressure levels and
accompanied by a 2–3 ppbv increase by renitrification at the 68 hPa
pressure level. The observed region of depleted HNO3 is substantially
smaller than the region bounded by the NAT existence temperature
threshold. Temperature-time histories of air parcels demonstrate that
the depletion is more clearly correlated with prior exposure to
temperatures a few kelvin above the frost-point. From the combined data
we infer the presence of large-size NAT particles with effective radii
>5–7 μm and low NAT number densities
<1 × 10−3 cm−3. This denitrification event is observed
close to the pole in the Antarctic vortex before synoptic temperatures
first fall below the ice frost point and before the widespread
occurrence of large-scale NAT PSCs. An episode of mountain wave
activity detected by AIRS on 28 May 2008 led to wave-ice formation in the rapid
cooling phases over the Antarctic Peninsula and Ellsworth Mountains,
seeding an outbreak of NAT PSCs that were detected by CALIOP and
MIPAS. The NAT clouds formed at altitudes of 18–26 km in a polar
freezing belt and appear to be composed of relatively small particles
with estimated effective radii of around 1 μm and high NAT number
densities >0.2 cm−3. This NAT outbreak is similar to an event
previously reported from MIPAS observations in mid-June 2003. |
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