AIRBORNE POLAR EXPERIMENT

GEOPHYSICA AIRCRAFT IN ANTARCTICA (APE-GAIA):

A REMOTE SENSING CHEMISTRY MISSION*

B. Carli, U.Cortesi

Istituto Ricerca Onde Elettromagnetiche “Nello Carrara” (IROE-CNR)

Firenze, Italy

G. De Rossi

Ente per le Nuove tecnologie l’Energia e l’Ambiente (ENEA)

Roma, Italy

ABSTRACT

In the frame of the “Airborne Polar Experiment” (APE) a Russian military plane M55 has been converted in a high flying research platform for investigations of the polar lower stratosphere and upper troposphere. With the first campaign, carried out in the Arctic during the winter 1996-97, the M55-Geophysica aircraft was shown to be a reliable and powerful tool, to access geographical and altitude ranges, which can be only partially covered by other platforms. A new mission of the M55 is now planned over the Antarctic Peninsula, aiming at studying the chemical processes responsible for stratospheric ozone losses by means of a remote sensing payload. The APE-GAIA campaign (Geophysica Aircraft In Antarctica) will take place from the airport of Ushuaia (Argentina) from 15 September to 15 October 1999. The geographical location of the operative base (Ushuaia, 55°S is the southernmost airfield suitable for the M55) will permit to reach the borders of the polar vortex and to fly along its edge; the selected period is the most suitable for the study of ozone depletion and recovery chemistry. The scientific payload will be basically composed by remote-sensing instrumentation (middle and far-infrared FT spectrometers, UV-Visible spectrometer, lidars). In-situ analysis of O₃, H₂O and other trace gases will contribute to better constrain the remote observations. First priority issues of the 5 or 6 flights of the mission will be the latitude and altitude dependence of ozone loss cycles during the depletion phase and the mixing mechanisms between the polar vortex and the mid and low latitude air masses.

1.0 INTRODUCTION

The use of high-flying research aircraft for investigations in the upper troposphere and lowermost stratosphere can make important contributions to several key issues related to the depletion of the stratospheric ozone layer. Experimental data gathered by in-situ instruments and remote sounders operating onboard high altitude airplanes may, in fact, significantly improve our knowledge, especially with regard to the chemical and physical processes occurring at latitude and altitude ranges that cannot be easily reached by ground or balloon-based measurements and that are poorly covered even by satellite observations. The experience of the American ER-2, for instance in the large expeditions to Antarctica [AAOE in 1987 from Punta Arenas, Chile, Lat. 53°S (Tuck et al., 1987) and ASHOE from Christchurch, Australia, Lat. 45° S (Tuck et al..1997)] have clearly demonstrated the capabilities of this kind of platform, in terms of temporal and geographical coverage and operational flexibility. More recently, another high altitude research aircraft – the M55-Geophysica - has become available for scientific purposes, in the frame of the project “Airborne Polar Experiment” (APE), a co-operation between the Italian National Progamme for Antarctic Research (PNRA) and the Russian Myashischev Design Bureau (MDB), AviaEcoCenter (AEC).and Central Aerological Observatory (CAO) of Moscow.

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* Presented at the Fourth International Airborne Remote Sensing Conference and Exhibition/

21^st Canadian Symposium on Remote Sensing, Ottawa, Ontario, Canada, 21 – 24 June 1999.

As part of the APE program, the Russian reconnaissance aircraft M55 has been modified to accommodate several remote-sensing and in-situ sensors, specifically developed to investigate the chemical, dynamical and radiative mechanisms responsible for the spring-time ozone losses observed in the polar stratosphere (De Rossi et al., 1994). A first scientific mission of the M55-Geophysica was carried out in the Arctic region during the winter 1996/97 (APE-POLECAT campaign), aiming at studying the chemistry and microphysics of Polar Stratospheric Clouds, as well as the chemical and transport mechanisms around the polar vortex .Seven scientific flights were performed in the period from 23rd December 1996 to 14th January 1997, from the operative base of Rovaniemi (Finland, Lat 67°N), on the Arctic Polar Circle, towards the region between Spitzbergen and Greenland and over the Siberian Arctic Along with a number of scientific results which contributed to gain a deeper insight into several questions related to PSC formation, background aerosol loading and stratospheric chemical composition (Stefanutti, 1998a), the APE POLECAT mission obtained a great success from the technical point of view. In fact, it showed, for the first time, the capabilities of the Geophysica aircraft as a research platform equipped for atmospheric sounding; the M55 was operated during the entire period of the campaign in an efficient and reliable manner and provided a safe and comfortable environment for operation of the scientific payload (Stefanutti, 1998b)

The final goal of the “Airborne Polar Experiment” is, however, represented by the execution of a second measurement campaign, to be carried out by the M55-Geophysica in the Antarctic region. The APE-GAIA campaign (Airborne Polar Experiment – Geophysica Aircraft In Antarctica) has its primary objective in the remote-sensing of chemistry and transport across the southern polar vortex edge. The scientific missions will be performed, at the beginning of the austral Spring (September-October 1999), flying from the southernmost part of South America (Tierra del Fuego, Argentina) towards the Antarctic Peninsula, in order to penetrate the polar vortex and to explore the stratospheric air masses located at its borders.

This paper provides a short presentation of the APE-GAIA campaign. The key scientific objectives are reported (paragraph 2.0), along with a detailed description of the stratospheric platform (paragraph 3.0) and the payload selected for the scientific mission (paragraph 4.0). A general outline of mission planning is also included and other aspects, related to auxiliary activities (modelling, correlative measurements, measurements during the transfer flights) are discussed (paragraph 5.0).

2.0 SCIENTIFIC OBJECTIVES

In the last ten years our understanding of the ozone budget in the stratosphere has significantly improved and it is now widely accepted that depletion of stratospheric ozone in the polar regions is the result of the confinement of high latitude air masses inside the polar vortex, and of the heterogeneous chemistry reactions which are activated on the particles of polar stratospheric clouds. Nevertheless, we are still far away from a deep knowledge of the phenomenon: large uncertainties still exist, in particular, on the role played by the catalytic cycles of ozone destruction and on the interactions between the polar vortex and the mid-latitudes air masses. In order to further investigate the questions related to chemistry and transport, the collection of new and more accurate experimental data is vital. The need is particularly strong for the mid-high latitudes of the austral hemisphere, to which the more recent campaigns have devoted less attention. A new measurement campaign is needed also for the range of altitudes between the tropopause and the lower stratosphere, which is only partially observable from satellites and ground-based stations. It is evident that measurements carried out from an airborne platform, such as the M55-Geophysica, fully equipped with chemical and microphysical sensors and capable of flying at 20 km altitude, can yield fundamental results in the observation of the phenomenon. The objectives of the APE-GAIA campaign have been defined in the light of these considerations. The priority assigned in this mission to the study of chemistry, together with the new observation capabilities offered by remote sensing measurement techniques, have identified the beginning of the southern spring as the optimum period for observations. This transition period between the depletion phase (August-September) and the recovery phase (October-November) is preferable, compared to the activation phase (May-July), which is more suitable for microphysics observations (in particular, because of the higher probability of formation of PSCs). In the depletion phase, key aspects are both the study of the most important catalytic cycles involved in ozone chemistry, as a function of latitude and altitude, and the possibility of extending the measurements to those compounds for which the observation data base is very limited (for example, HBr and HOBr in the bromine family).

Similar interest exists for observations conducted at the beginning of the recovery phase, because of the analysis of the reconversion processes of the active chlorine species into the reservoir compounds ClONO₂ and HCl. Finally, another aim of the observations is to clarify in what extent and at what altitude the mixing of polar air masses with mid latitudes air occurs, when the vortex breaks down in late Spring and Summer. A better understanding of the phenomenon is critical to assess in what measure the mid-latitude ozone losses in the austral hemisphere are due to transport or to the dilution of the vortex, rather than to other mechanisms.

3.0 THE M55-GEOPHYSICA AIRCRAFT

The M55-Geophysica is an all-weather single-seater stratospheric aircraft capable of operating in all climates, both day and night, for about 5 hours up to an altitude of 21km, even in critical meteorological conditions (temperatures down to -80°C, strong cross winds at takeoff/landing for such a high-altitude aircraft). These characteristics, together with the possibility of housing a scientific payload up to 1500kg inside its bays (the main bay is over 5m long and can accommodate bulky instruments) make the M55-Geophysica an ideal platform for research in the upper troposphere and lower stratosphere.

Figure 1 - The M55-Geophysica high-altitude research platform

Since the beginning of the co-operation between Italy and Russia in 1995, the M55-Geophysica has undergone several transformations in order to house the scientific payload and to increase its efficiency and flexibility as scientific laboratory. With this perspective, the bays of the aircraft have been modified according to a modular criterion, and have been equipped with standard interfaces with the instruments (attachment points, electrical connections, viewing windows, and servicing hatches). In addition, a series of dorsal bays has been built onto the fuselage of the aircraft, in order to lodge other instruments .

At present the M55-Geophysica is, together with the U.S. aircraft ER-2 (a modified version of the U-2 reconnaissance aircraft, managed by the NASA and used for scientific purposes) one of the two airborne platforms operating world wide for stratospheric research. While the ER-2 has a longer flight endurance than the M55-Geophysica, the latter has superior characteristics with respect to scientific payload capacity, power supply, manoeuvrability, and less dependence on ground meteorological conditions.

Table 1 – Characteristics of the M55-Geophysica aircraft

4.0 THE SCIENTIFIC PAYLOAD

The priority assigned to gas-phase chemistry observations guided the choice of the APE-GAIA payload.. The core of the scientific payload selected for the Antarctic campaign consists, therefore, of remote sensing devices for simultaneous measurement of a large suite of chemical species. In situ measurements of ozone, water vapour and other tracers will provide independent values for these specific compounds. The higher spatial resolution, and the possibility provided by these sensors of measuring the horizontal variability of the atmospheric composition, will guarantee an important synergy with the remote observations. Finally, the measurements of aerosols and polar stratospheric cloud particles obtained by in situ devices and lidars provides the possibility of studying heterogeneous chemistry

The complete set of instruments that will be flown on the M55 aircraft during the Antarctic mission is summarised in table 2. Figure 2 shows the accommodation of the APE-GAIA payload onboard the M55. A short description of different sensors is provided in the following paragraphs, where the payload is being considered as divided into remote sensing instruments for the measurement of the chemical composition of the atmosphere (remote sensing chemistry payload), in-situ instruments for the measurement of the chemical composition of the atmosphere (in-situ chemistry payload), and instruments for the study of the aerosols and cloud particles (microphysics payload).

Figure 2 - Accommodation of the APE-GAIA scientific payload

Table 2 - APE-GAIA payload

4.1 REMOTE SENSING CHEMISTRY PAYLOAD

The remote sensing chemistry payload includes the combination of two Fourier transform spectrometers, SAFIRE-A and MIPAS-STR, operating in the Far and Medium Infrared respectively, and the GASCOD-A UV-Visible spectrometer. SAFIRE-A (Spectroscopy of the Atmosphere using Far InfraRed Emission - Airborne) and MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft) are two emission instruments that perform passive measurements by using the limb sounding technique, and are capable of observing simultaneously a large number of constituents involved in the ozone depletion processes. The measurement strategy of both instruments is a combination of limb and upward atmospheric measurements. By upward sounding the vertical column amount above the aircraft is obtained, whereas the limb sounding enables retrieval of profiles of the species below the flight level. Table 3 shows the list of the observable species and highlights the strong complementarity of the two instruments

Table 3 - Observable species for SAFIRE-A and MIPAS-STR

The MIPAS-STR almost covers the entire NO_y, together they cover Cl _y (e.g. both HCl and ClONO₂), whereas SAFIRE-A has its strength in HO_x and can obtain additional information on Bry species. Also various types of tracers and source gases can be observed. Redundant measurements will improve the quality of the corresponding trace gas distributions and allow to validate the calibration of both instruments.

GASCOD-A uses the differential optical absorption spectroscopy (DOAS) technique. It operates in the UV-visible spectral region and enables the detection of the trace gases listed in Table 4. The deliveries of GASCOD-A are total amounts of the trace constituents at zenith and nadir as well as vertical profiles.

Table 4 - Observable species for GASCOD-A

4.2 IN-SITU CHEMISTRY PAYLOAD

The detailed picture provided by the remote sensing instruments can be better constrained with the aid of some specific in-situ measurements of relevant species. The in-situ chemistry instruments are listed in Table 5.

Table 5 - Species observed by in-situ chemistry payload

ECOC (Electro-Chemical Ozone Cell) is the Electrochemical ozonometer based on the electrochemical concentration cell (ECC) ozonesonde. The air sample is pumped through the ECC where ozone molecules are completely absorbed by the potassium iodide solution which causes an electric current to flow through the external circuit connected to the cell electrodes. Each ozone molecule produces a two electron flow in the outer circuit. Thus measurement of the electric current allows to determine a number of ozone molecules pumped into the ECC in one second. This number can be converted to ozone pressure using a pump productivity and air temperature at the pump output.

The measurement system sensitivity threshold does not exceed 1 ppb. The time constant is determined by the electrochemical cell inertia and ranges 30 s, which corresponds to a 4-5-km and less than 300-m spatial resolution over the horizontal and vertical, respectively, for measurements during the ascent and descent of the aircraft. Atmospheric ozone concentration can be measured in-situ within the range of 0 - 4E+12 cm^-3, with an accuracy not worse than 5 % .

FOZAN (Fast OZone ANalyzer) is a chemioluminescence Ozone Analyser, based in the chemioluminescence reaction between ozone present in an air flow and a laser dyes sensor such as Rodamine B, Rodamine 6G and Kumarine 153. Range of concentration is (1-999)mgr/m³.

HAGAR (High Altitude Gas chromatograph for Atmospheric Research) is a two-channels gas chromatograph with electron capture detection (GC/ECD). It allows the measurement of CFC-11, CFC-12, N₂O and SF₆. with precision equal to ~1% of tropospheric value for all species. The time resolution: of measurement for all species is less than 2 minutes (hopefully every minute).

FISH (Fast In-situ Stratospheric Hygrometer) is a stratospheric hygrometer based on the Lyman-alpha photofragment fluorescence technique. The instrument performs measurements of the stratospheric water vapour content (total water and gas phase) in the range: 500 - 0.2 ppmv (precision < 4 %, accuracy 0.2 ppmv ) with a time resolution of 1 s. The instrument is calibrated in the laboratory against a frost point hygrometer at real stratospheric conditions.

4.3 MICROPHYSICS PAYLOAD

The microphysics payload includes lidars (ABLE and MAL) and in-situ instruments for aerosol measurements (FSSP-300, MAS and mini-COPAS).

ABLE (AirBorne Lidar Experiment) is a Nd-Yag high energy lidar for the measurement of aerosols and PSC. It will operate in either a dual wavelength configuration 532 nm or 355 nm emissions or at 532 nm with dual polarisation detection capacity. It will detect aerosols, PSC and tropospheric clouds.

MAL (Microjoule Airborne Lidar) is a microjoule lidar operating with a laser diode at 900 nm. The operative range is 0 to 3.4 km from the aircraft. It can measure aerosols and PSCs and the brightness of the sky at that wavelength. MAL aerosol measurements can only be obtained at either dusk or night-time.

FSSP-300 (Forward Scattering Spectrometer Probe) can measure concentration and size distribution of the aerosol and PSCs. It is able to detect particles and count particles larger than 0.4 micron. The size distribution is between 0.4 and 23 microns. In conjunction with MAS is able to give information on the refractive index of the particle, hence make hypotheses on the particle composition.

MAS (Multiwavlength Aerosol Scatterometer) is able to measure the particle density and optical properties at 532, 690 and 820 nm of aerosol and PSCs. It can give information on the phase of the particles on the basis of depolarisation measurements. In conjunction with FSSP-300 is able to furnish information on the refractive index of the particle, hence make hypotheses on the particle composition.

Mini-COPAS (Mini COndensation PArticle detection System) can measure the particle total concentration. It can detect particles with sizes larger than 0.01 micron. It has equipped with two measuring channels: one for the total concentration and the second for concentration of particles with no-volatile core. It is meant to complement the FSSP-300 for particles smaller than its detection limit.

5.0 THE ANTARCTIC CAMPAIGN

The Antarctic campaign will take place from the operative base of Ushuaia in Tierra del Fuego (Argentina) from 15 September to 15 October 1999. The choice of the site was determined in the first place by its favourable geographic location: Ushuaia (Latitude: 54° 48’ S, Longitude: 68° 19’ W) is the southernmost airport in the world able to accommodate the M55-Geophysica, and thus the nearest to the Antarctic continent. Considering that the polar vortex generally extends to a latitude between 60° S and 70° S, and that the operative radius of the M55 is about 15° latitude, departing from Ushuaia the aircraft will be able to reach the periphery of the vortex, explore its edges, and penetrate its interior. The flight profiles of the M55-Geophysica will mainly respond to the optimum requirements for remote sensing observations: they will be, as far as possible, flight profiles at maximum (20 km) and constant altitude. Occasionally dives will be made to the troposphere, in order to allow in situ measurements. A total of 30 hours of flight are scheduled, distributed over 5 or 6 missions.

5.1 METEOROLOGICAL FORECASTS AND MODELLING STUDIES

An important component of the mission will be represented by the meteorological forecasts and modelling activity. Both tropospheric and stratospheric forecasts are required for the planning of the scientific flight missions. A good knowledge of ground meteorology is mainly needed for aircraft safety reasons and a minimum of 72 hours forecasts on ground (winds, cloud coverage. probability of precipitation, etc.) is necessary. Special care must be taken in evaluating the incidence of strong winds in the operative region, since one of the most critical limitations for the aircraft operation is the maximum speed of cross-wind that can be tolerated by the M55 (equal to 20 Knots). Stratospheric and global tropospheric forecasts, on the other hand, will be used essentially for the scientific planning.

Modelling studies will support the experimental activities both in the forecast and in the post-mission data analysis phases. Several tools are being optimised to this purpose: A global trajectory model (TM), that calculates backward and forward trajectories using global winds and temperature analyses, can be used to provide global tracking of atmospheric tracers or to carry out a detailed investigation of the local dynamics. Global trajectory analyses can be also performed to follow the polar vortex dynamics using a reverse domain filling trajectories (RDFT) approach. A 3D Chemical Transport Model (CTM) will be used for a few days chemical prediction or for short diagnosis of atmospheric events The high resolution mesoscale model MM5 can be used to provide lee waves and tropospheric forecasts. Extended chemical (photochemical box model and chemical transport model) and dynamical analyses will be performed after the measurements period. Coupled run and inter-comparisons between RDFT and CTM calculation could be used to study the global structure an evolution of the intrusion/filamentation processes partly sampled by the instruments and to try to quantify the extent of mixing of latitude air into the polar vortex.

5.2 CORRELATIVE MEASUREMENTS

In parallel with M55-Geophysica flights, a series of ground-based and balloon measurements will be made from different sites located in Tierra del Fuego and on the Antarctic Peninsula, in order to validate the observations conducted during the APE-GAIA campaign. Ground-based ozone, UV radiation and lidar measurements will be carried out from Ushuaia; at about the same time of the M55 flights, five or six balloon launches are scheduled from Ushuaia for testing the instrumentation of the STRATEOLE project and could be exploited for measurements correlated to the APE-GAIA observations. On the Antarctic Peninsula, real-time measurements of total ozone and NO₂ will be performed from the base of the British Antarctic Survey in Rothera (67° 34’ S; Long. 68° 07’ W), along with radiosondes launching. Ozone soundings and measurements of UV profile will be carried out also from the Argentinean bases in Marambio (Lat. 64º 14' S. Long. 56º 37' W) and Belgrano II (Lat. 77º 52' S; Long. 34º 37' W).

5.3 MEASUREMENTS DURING THE TRANSFER FLIGHT

A further opportunity offered by the APE-GAIA campaign will be the possibility to carry out scientific observations during the return flight of the M55-Geophysica from South America to Europe. The possibility of performing scientific measurements at low latitudes during the transfer flight is of great scientific interest. This will make it possible to explore the upper troposphere and lower stratosphere over a wide range of latitudes, and to study the interactions between the polar and middle latitude stratosphere. The observations will therefore concentrate on measuring Cl _y species, radicals responsible for ozone removal (ClO, BrO, HO₂, NO₂), and tracers (HF, N₂O).

This objective, rich with scientific potential, is, however, very demanding from the logistical point of view. First of all, in order to minimise the drawbacks of possible damage to the scientific payload due to the operation in the tropical environment, it is advisable to perform scientific operation during the transfer flight from South America to Europe, at the end of the Antarctic campaign,. Furthermore, due to the limited range of the M55 requires the transfer flights to be completed in at least four legs: the proposed route is indicated in figure 3.

Figure 3 - The route of the APE-GAIA transfer flight

The scientific payload will be operated during the four legs of the transfer flight. However, access to the instruments will be possible only at the intermediate airport of Recife (Brazil) and all other stopovers will be used just for refuelling and change of the pilot. This exacts additional requirements on the scientific payload, as an operation autonomy of at least 14 hours (2 successive flights with an intermediate stopover) and a data storage capacity sufficient for recording two successive flights with no unloading of intermediate data.

REFERENCES

1. DeRossi G., Puccetti G., Puccini M., “Airborne Polar experiment – technical activities for the integration on M55 Myashischev aircraft of a scientific instruments payload”, Proceedings of the First International Airborne Remote Sensing Conference and Exhibition, Strasbourg, France; 11-15 September 1994.

2. Stefanutti L. et al., “APE-POLECAT - Rationale, Road Map and Summary of Early Results”, Journal of Geophysical Research, in press.

3. Stefanutti L. et al., “Special issue on the Airborne Polar Experiment”, Journal of Atmospheric and Oceanic Technology, in press

4. Tuck A.F. et al., “The Planning and Execution of the ER-2 and DC-8 Aircraft Flights Over Antarctica, August and September 1987”, J. Geophys. Res., 94, pp. 11181-11222, 1989,

5. Tuck A.F. et al., “Airborne Southern Hemisphere Ozone Experiment/ Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/ MAESA): A road map”, J. Geophys. Res., 102, pp. 3901-3904, 1997.