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3. The high altitude platform and the APE-GAIA payload

The complete set of instrument which operated in the frame of the APE-GAIA campaign is listed in Table 1, along with some general information about the techniques and the products of the measurements. The location of the instruments onboard the M55 Geophysica aircraft is shown in Figure 1. The payload includes remote sensing and in-situ equipment for gaseous trace compounds, instruments aimed at characterising aerosols and cloud particles and devices for radiation measurements. From the point of view of the scientific objectives and of the measurement technique, the payload can be 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 (microphysics payload)

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The remote sensing chemistry instruments are the core of this payload as they provide simultaneous measurements of a large suite of chemical species. The two Fourier Transform Spectrometers MIPAS-STR and SAFIRE-A make limb and upward soundings to obtain column amounts above the aircraft and profiles below. Together they cover the main components of the Cl_y family (ClO, HCl and ClONO₂), MIPAS-STR almost covers the entire the NO_y family, whereas SAFIRE-A obtains information on Br_y. GASCOD-A uses the DOAS technique and measures total amounts of the trace constituents at zenith and nadir in the case of high sun, and vertical profiles in the case of sunrise and sunset. Important active species observed by this instrument are OClO and BrO. The three instruments also observe several long-lived source gases. Their redundant measurements allow validation of the individual measurements and thus an improvement of the quality and reliability of the APE-GAIA data. In situ instruments provide measurements of ozone, water vapour and long-lived tracers at flight altitude with high spatial resolution. From these measurements vertical distributions can be observed during dives of the aircraft. The horizontal variability of the atmospheric composition and the dynamics of the mixing can be derived during flights at constant altitude ensuring an important synergy with the more extended vertical maps that the remote observations obtain on the basis of the assumption of horizontal homogeneity. Aerosols and PSC particles are detected and characterised by lidars and scatterometers pointing at different directions and operating with different ranges and different longitudinal resolutions. Correlation of PSC detection with the measurements of atmospheric composition simultaneously made by the "chemistry payload" provides an opportunity for the direct analysis of the effects of PSC formation (denitrification, dehydration, heterogeneous chemistry). Finally, two instruments provide measurements of radiation fluxes for a more precise quantification of photochemical parameters.

		Name	Instrument Type	Prodotti della misura	Institute
Remote-sensing Chemistry and Radiation		GASCOD-A Gas Absorption Spectrometer Correlating Optical Differences ? Airborne	UV ? Visible spectroradiometer	Vertical profiles and column of O3, NO2, OClO, BrO. Actinic flux measurements	ISAO-CNR Bologna, Italy
		MIPAS-STR Michelson Interferometer for Passive Atmospheric Sounding ? STRatospheric	Mid Infrared Fourier transform spectrometer	Vertical profiles and column of minor atmospheric constituents	IMK-FZK Karlsruhe, Germany
		SAFIRE-A Spectoscopy of the Atmosphere using Far-InfaRed Emission-Airborne	Far Infrared Fourier transform spectrometer	Vertical profiles and column of minor atmospheric constituents	IROE-CNR Firenze, Italy
		SORAD SOlar RADiation	Piranometer	Net irradiance	CAO Moscow, Russia
In-situ Chemistry		ECOC ElectroChemical Ozone Cell	Elettrochemical Ozonesonde	Ozone	CAO Moscow, Russia
		FOZAN Fast OZone Analyzer	Chemioluminescent Ozonesonde	Ozone	CAO Moscow, Russia
		FISH Fast In-situ Stratospheric Hygrometer	Hygrometer	Water vapour	ICG-FZJ Juelich, Germany
		FLASH FLuorescence Airborne Stratospheric Hygrometer	Hygrometer	Water vapour	CAO Moscow, Russia
		HAGAR High Altitude Gas AnalyzeR	2 channel Gascromatograph	N2O, SF6, CO2, CFC-12, CFC-11, Halon-1211	IMG, Univ. of Frankfurt Frankfurt, Germany
Microphysics		ABLE AirBorne Lidar Experiment	Lidar	Particle density and optical properties from 2 to 15 km from the aircraft	Univ. of Rome Rome, Italy
		MAL-1 Microjoule Airborne Lidar	Lidar	Particle density up to 10 km above the aircraft	Observatory of Neuchatel Neuchatel, Switzerland
		MAL-2 Microjoule Airborne Lidar	Lidar	Particle density up to 10 km below the aircraft	Observatory of Neuchatel Neuchatel, Switzerland
		MAS Multiwavelength Aerosol Scatterometer	Laser scatterometer	Particle density up to 2 km from the aircraft	IFA-CNR Roma, Italy

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