Airborne Polar Experiment Research Articles

Emission Fourier transform spectroscopy for remote sensingof the Earth's atmosphere

Fourier transform emission spectroscopy can make an important contribution in the observation of the Earth's atmosphere and in the investigation of atmospheric physics and chemistry. In this paper, we report the measurement performances and the result obtained by a Fourier transform spectrometer, named SAFIRE-A (Spectroscopy of the Atmosphere using Far Infrared Emission - Airborne), operating in the far infrared spectral region from limb sounding observation of the atmospheric emission from a high altitude aircraft. After a short review of the advantages of FTS emission limb sounding in comparison with competing techniques and observation geometries, a detailed description of the instrument is given with the result obtained during the APE-GAIA (Airborne Polar Experiment - Geophysica Aircraft In Antarctica) fi eld campaign carried out over the Antarctic Peninsula.

Measurements from ground and balloons during APE-GAIA – A polar ozone library

Many long-term monitoring sites in Antarctic regions, which deploy ground-based stratospheric remote sensors and fly radiosondes or ozonesondes on balloons, supported the Airborne Polar Experiment in September and October 1999. Support consisted of supplying data to the campaign in real time, and in some cases by increasing the frequency of measurements during the campaign. The results will strengthen scientific conclusions from the airborne measurements. But results from these sites are allowing important scientific studies of new aspects of the ozone hole in their own right, because like the aircraft and its campaign, many sites traverse the vortex edge and are close to the largest source of lee waves, or measure infrequently observed trace gases such as HNO 3. Examples of such studies are the behaviour and value of NO 2 in midwinter, ozone filamentation with no apparent horizontal advection, the frequency and amplitude of gravity waves over the Antarctic Peninsula, mixing in the lowest stratosphere in Antarctic spring, the mechanism and frequency of HNO 3 enhancement above the ozone peak in midwinter, and trends in UV dose in southern South America.

Polar stratospheric clouds observed during the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE‐GAIA) campaign

In this work, observations of polar stratospheric clouds (PSC) carried out during one flight of the stratospheric research aircraft Geophysica deployed in the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE‐GAIA) campaign in September–October 1999 are presented. An analysis of data from the two lidars and a backscatter sonde on board the Geophysica aircraft is presented, coupled with temperature measurements. The observations are analyzed with the aid of a mesoscale model to provide air mass thermal histories and a microphysical box model to simulate PSC formation. The results obtained from this comparison are discussed in view of the theories for PSC formation processes.

Off-axis measurements of atmospheric trace gases by use of an airborne ultraviolet-visible spectrometer.

An airborne UV-visible spectrometer, the Gas Analyzer Spectrometer Correlating Optical Differences, airborne version (GASCOD/A4pi) was successfully operated during the Airborne Polar Experiment, Geophysica Aircraft in Antarctica airborne campaign from Ushuaia (54 degrees 49' S, 68 degrees 18' W), Argentina in southern spring 1999. The instrument measured scattered solar radiation through three optical windows with a narrow field of view (FOV), one from the zenith, two from the horizontal, as well as actinic fluxes through 2pi FOV radiometric heads. Only a few airborne measurements of scattered solar radiation at different angles from the zenith are available in the literature. With our configuration we attempted to obtain the average line-of-sight concentrations of detectable trace gases. The retrieval method, based on differential optical absorption spectroscopy, is described and results for ozone are shown and compared with measurements from an in situ instrument as the first method of validation.

High-Resolution Stratospheric Tracer Fields Reconstructed with Lagrangian Techniques: A Comparative Analysis of Predictive Skill

Numerical experiments and statistical analyses are conducted to determine the skill of different Lagrangian techniques for the construction of tracer distributions. High-resolution potential vorticity (PV) maps are calculated from simulations of the 1996/97 arctic winter stratospheric dynamics using two different numerical schemes—reverse domain filling trajectories (RDF) and contour advection with surgery (CAS)—and data from three meteorological agencies (NCEP, the Met Office, and ECMWF). The PV values are then converted into ozone (O3) concentrations and statistically compared to in situ O3 data measured by the electro chemical ozone cell (ECOC) instrument during the Airborne Polar Experiment (APE) using cross correlation, rms differences, and the Kolmogorov–Smirnov (KS) test. Results indicate that while Lagrangian techniques are successful in increasing the presence of lower-scale tracer structures with respect to the plain meteorological analyses, they significantly improve the statistical agreement between the simulated and the measured tracer profiles only when there is clear evidence of filaments in the measured data. This better fit is most clearly seen by using the KS test, rather than cross correlation. It is argued that this difference in the performance of Lagrangian techniques can be partly related to the treatment of mixing processes in the framework of the Lagrangian schemes. Statistical analyses also show that the temporal rather than the spatial resolution of the input meteorological fields, used to advect tracers, enhances the predictive skill of the Lagrangian products. The best overall performance is obtained with the Lagrangian product (not gridded) based on high-resolution reverse trajectories calculated along a flight track, in particular when the simulation is initialized with ECMWF data. Other products, such as CAS initialized with ECMWF and 3D-gridded RDF initialized with the Met Office data, show fairly good performances, thus with lower statistical confidence.

Ozone measurements during the Airborne Polar Experiment: Aircraft instrument validation, isentropic trends, and hemispheric fields prior to the 1997 Arctic ozone depletion

The first deployment of the ECOC electro chemical ozone cell (ECOC) instrument onboard the high‐altitude research aircraft, the Geophysica M‐55, took place from Rovaniemi, northern Finland, between December 23, 1996, and January 14, 1997. The ECOC data were compared against contemporaneous data from a network of balloon‐borne ozone sondes. The comparison was carried out in potential vorticity‐potential temperature (PV, Θ) coordinates, using meteorological analyses from the European Centre for Medium‐Range Forecasts. The comparison showed that ozone mixing ratios measured by ECOC are lower than those measured by ozonesonde by a small but statistically significant bias of (−5.7±2.8)% at the cruising altitudes of the aircraft, 15 to 19 km. After establishing and removing the average bias, ECOC and ozonesonde data were analyzed together to follow the development of ozone distributions in the early winter Arctic stratosphere. The analysis showed no evidence of chemical ozone depletion at the cruising altitudes of the aircraft, that is, between 435 and 490 K. The absence of chemical depletion is in agreement with polar statospheric cloud (PSC) observations, which showed no PSCs at aircraft cruising altitudes, although from January 5 onwards, PSCs were observed above cruising altitudes. Results from a three‐dimensional chemical transport model reproduce the basic features of the reconstructed ozone fields. However, the model does not capture the observed ozone increase during the campaign, due to weak modeled ozone vertical gradients, and indicates small ozone depletion of about 3% inside the vortex at 480 K by mid January.

Stratospheric aerosol measurements in the Arctic winter of 1996/1997 with the M-55 Geophysika high-altitude research aircraft

In-situ aerosol measurements were performed in the northern hemispheric stratosphere up to altitudes of 21 km between 13 November 1996 and 14 January 1997, inside and outside of the polar vortex during the Airborne Polar Experiment (APE) field campaign. These are measurements of particle size distributions with a laser optical particle counter of the FSSP-300 type operated during 9 flights on the Russian M-55 high-altitude research aircraft Geophysika. For specific flights, the FSSP-300 measurements are compared with balloon-borne data (launched from Kiruna, Sweden). It was found that the stratospheric aerosol content reached levels well below the background concentrations measured by the NASA operated ER-2 in 1988/89 in the northern hemisphere. During the APE campaign, no PSC particle formation was observed at flight altitudes although the temperatures were below the NAT condensation point during one flight. The measured correlations between ozone and aerosol give an indication of the subsidence inside the 1996/97 polar vortex. Despite the lower aerosol content in the winter 1996/97 compared to the 1989 background, the heterogeneous reactivity of the aerosol (as calculated from the measured data with additional model input) is comparable. This is due to the dependency of the reactive uptake coefficients on the atmospheric water vapor content. Under the described assumptions the reaction rates on the background aerosol are significantly smaller than for competing gas phase chlorine activation, as can be expected for stratospheric background conditions especially inside the polar vortex.

The M-55 Geophysica as a Platform for the Airborne Polar Experiment

Abstract The authors describe the Russian Stratospheric Aircraft M-55 Geophysica, an important new platform for earth observation, and describe some technical details of its inaugural mission. The M-55 has successfully conducted scientific test flights in Pratica di Mare, Rome, in November 1996, and the first Airborne Polar Experiment (APE 1) from 19 December 1996 to 16 January 1997 from Rovaniemi in northern Finland. Three test flights were carried out at Pratica di Mare, and seven scientific mission flights during APE 1, when “quasi-Lagrangian” flight paths (flights in the wind direction, assuming the stratosphere to be stationary over the flight period) and lee wave flight paths were employed. Combined sorties of the M-55 Geophysica and the DLR Falcon were performed, the latter acting as a pathfinder for the former, guiding it to small regions of intense polar stratospheric cloud activity. These small cloud patches are associated with intense atmospheric wave activity over the Scandinavian mountains an...

Aircraft lidar observations of an enhanced type Ia polar stratospheric clouds during APE‐POLECAT

Polar stratospheric clouds (PSCs) which do not fit into the standard type Ia/Ib scheme were measured by the airborne lidar OLEX (Ozone Lidar Experiment) on board the Deutsches Zentrum für Luft‐ und Raumfhart (DLR) Falcon during the Airborne Polar Experiment and Polar stratospheric clouds, Leewaves, Chemistry Aerosol and Transport (APE‐POLECAT) campaign. In contrast, the standard classification is satisfied by almost all observations for four winters at Ny Ålesund, Spitsbergen, which is one of the most comprehensive data sets of ground station lidar measurements presently available. The cloud observed by the Falcon south of Spitsbergen on December 31, 1996, was a 400‐km long type I cloud with backscatter ratio S = 2.5 and aerosol depolarization δA = 15%, which is clearly distinct from the Ny Ålesund 4 year record. Using a combination of microphysical and optical modeling, we investigate the possible evolution of this cloud assuming either in situ freezing of ternary HNO3/H2SO4/H2O droplets as nitric acid trihydrate, or the formation of the clouds in mountain waves over the east coast of Greenland, as suggested by a mountain wave model. Best agreement with the observations was obtained by assuming mountain‐wave‐induced cloud formation, which yields nitric acid trihydrate particles with much higher total mass than achieved by assuming synoptic‐scale freezing. Our analysis suggests that this rare type of PSC, which we term type Ia‐enh, is characterized by nitric acid hydrate particles rather close to thermodynamic equilibrium, while the more common type Ia PSCs appear to contain much less mass than representative of equilibrium.

SAFIRE-A: Spectroscopy of the Atmosphere Using Far-Infrared Emission/Airborne

A new instrument named SAFIRE-A (Spectroscopy of the Atmosphere using Far-Infrared Emission/Airborne), which can operate on high-altitude platforms, has been developed for the study of the atmospheric composition through limb-scanning emission measurements. The instrument is a polarizing Fourier transform spectrometer that operates in the far infrared with a resolution of 0.004 cm−1. SAFIRE-A uses efficient photon noise limited detectors and a novel optical configuration, which provide a cold pupil and field stop as well as cold narrow bandpass filters to enhance its sensitivity. The instrument was successfully operated on an M-55 stratospheric research aircraft in the polar regions during the winter 1996–97 Airborne Polar Experiment. The instrument design, aircraft integration, and performances attained in the field campaign are described and discussed. The atmospheric emission spectrum is measured with an rms noise accuracy of 0.5 K (measured in brightness temperature) in each spectral element near 20 cm−1 with a 30-s measurement time.

A Diode Laser Spectrometer for the In Situ Measurement of the HNO3Content of Polar Stratospheric Clouds

A new instrument realized for measuring the HNO3 concentration in air is described. The device is a midinfrared absorption spectrometer based on a tunable diode laser and a multipass absorption cell. The instrument is specifically designed for airborne operation on board the M-55 Geophysica, in the frame of the Airborne Polar Experiment project, taking into account all the related environmental and operational constraints. The device is part of a complete measurement package for the measurement of the chemical content of the polar stratospheric clouds and other atmospheric aerosols. Furthermore, it can be used as a stand-alone detector of molecular trace gases. The primary purpose of the instrument is to perform in situ diagnostic measurements in the upper troposphere‐lower stratosphere. Design criteria include a new optical setup, one much less sensitive to the vibration and thermal stresses with respect to the conventional diode laser spectrometers. Furthermore, the authors developed a novel detection scheme for quicker acquisition and better signal-to-noise ratio. This paper reports calibration and testing measurements, including a detection lower limit both for the HNO 3 and ammonia. This last gas is used as a wavelength and absorption reference.

A Chemiluminescent Analyzer for Stratospheric Measurements of the Ozone Concentration (FOZAN)

Abstract Within the framework of the Airborne Polar Experiment, a new ozone analyzer based on chemiluminescent technology has been developed for high-altitude measurements. The instrument was tested on board the M-55 Geophysica aircraft during some flights over Italy, reaching an altitude of about 20 000 m and a temperature of about −75°C. This paper presents the chemiluminescent characteristics of the sensor, the electronic design of the instrument, and a comparison between the results obtained during a test flight with those derived from a contemporary balloon sounding. The instrumental performances of the analyzer were found to be suitable for stratospheric applications.

Chemistry and aerosol measurements on the Geophysika stratospheric research aircraft: The airborne polar experiment

Since the early nineties efforts have been made to transform the Russian high altitude aircraft M-55 Geophysika into a flying laboratory suited for stratospheric research aimed at environmental issues such as ozone depletion, the assessment of the influence of commercial airtraffic on the atmosphere and other topics concerned with upper tropospheric and lower stratospheric physics and chemistry. As a result of these efforts the Airborne Polar Experiment (APE) is presently being planned and prepared (Stefanutti et. al., 1995). The first field campaign with Geophysika is scheduled to take place in the 1996/97 winter season from an airport in northern Sweden or Finland and a southern hemispheric experiment is projected for mid 1997 or mid 1998.