Abstract
Abstract. 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.
Highlights
Polar stratospheric clouds (PSCs) are the key mediator in polar ozone depletion by enabling heterogeneous processes that release halogens from relatively stable reservoir species
4 Results 4.1 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) PSC classification The CALIOP PSC type classification algorithm developed by Pitts et al (2009) categorizes the total and perpendicular lidar backscatter signals into one of four statistical composition classes based on a depolarization and inverse backscatter relation
We have imposed an additional constraint (β⊥ ≥ 2.5 × 10−6 km−1 sr−1) on the MIX1 class to accept only PSCs which have significant perpendicular backscatter. This attempts to compensate for the increased noise on the depolarization ratio at low backscatter by placing STS-nitric acid trihydrate (NAT) mixtures dominated by liquid particles into the LIQ class and reserving the MIX1 class for PSCs with a significant nonspherical solid component
Summary
Polar stratospheric clouds (PSCs) are the key mediator in polar ozone depletion by enabling heterogeneous processes that release halogens from relatively stable reservoir species. Space-based attempts to determine the composition of PSCs from their spectral infrared signatures were made using instruments on the Upper Atmosphere Research Satellite (UARS) (Taylor et al, 1994; Massie et al, 1997; Hervig, 1999), which provided contemporaneous observations of temperature and gas species. These early investigations of PSC composition from UARS were subsequently improved upon by the Cryogenic Spectrometers and Telescopes for the Atmosphere (CRISTA) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instruments (Spang and Remedios, 2003; Hopfner et al, 2006b).
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