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This chapter reviews the current capabilities for ultraviolet and visible spectroscopic measurements of the Earth’s troposphere, and discusses what remains to be achieved in the short term to enable global, continuous measurements of atmospheric pollution from space to be undertaken. Challenges in instrumentation, spectroscopy, radiative transfer modeling, and retrievals are discussed. Current and planned satellite instruments with the capability to make tropospheric measurements in the ultraviolet and visible, with their measurement properties, spectral coverage, and target molecules, are presented. Measurement examples are taken from recent work done at the Harvard-Smithsonian Center for Astrophysics, together with our colleagues at a number of institutions. The examples include global tropospheric ozone (O) measurements from the nadir geometry; global tropospheric nitrogen dioxide (NO); bromine oxide (BrO) in the polar spring, and from salt lakes and volcanoes; global tropospheric formaldehyde (HCHO); and preliminary measurements of glyoxal (CHOCHO). Except for a few remaining developments, the field is shown to be sufficiently mature that global measurements of atmospheric pollution from space may be undertaken.

This paper summarises the status of the European Space Agency’s programme for observation of the composition of Earth’s atmosphere: the three large spectrometers GOMOS (Global Ozone Monitoring by Occultation of Strars), MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) onboard Envisat (ENVIronment SATellite) since 2002, the opportunities in the Earth Explorer research programme and in the Earth Watch programme for operational missions in cooperation with European Union ‘Global Monitoring for Environment and Security’ (GMES) or Eumetsat. Two studies on molecular spectroscopy commissioned by ESA (European Space Agency) and targetted at the needs of potential ESA space instrumentation are reviewed. One of them addresses mainly the temperature-dependent measurement of collisional linewidth parameters in the mm/sub-mm wave range and the setup of a spectroscopic dataset dedicated to a possible future limb-sounding instrument; the other aims to establish the feasibility of a high accuracy determination of absorption cross sections of water vapour in the near infrared for a potential future HO lidar instrument.

The European satellite ENVISAT has been successfully launched at 1 March 2002. Onboard ENVISAT the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) experiment has been carried to a sun-synchronous polar orbit. MIPAS is a mid-infrared high resolution (0.035 cm) limb sounder. The measured spectra in the range between 4.1 and 14.6 µm are processed in order to derive global distributions of temperature and about 25 trace constituents. In addition, the detected broadband spectra allow to determine properties of Polar Stratospheric Clouds (PSCs) and aerosol amount. The status of the MIPAS experiment since its launch will be described. The strength of the MIPAS experiment is demonstrated by the simultaneous detection of a larger number of stratospheric trace species including ClONO and ClO. In addition, it will be shown that essential properties of PSCs will be derived from corresponding spectra including the spatial distribution and temporal development of PSCs. MIPAS can also be used to study the troposphericstratospheric exchange (H2O distributions) and the mesosphericstratospheric exchange (NO/NO2 distributions). Further investigations deal with the analysis of highly resolved spectra in order to determine the concentration profiles of isotopes (e.g. of water vapour). These results yield complementary information on dynamics and transport in the atmosphere. The article will be concluded with future expectations on MIPAS results

The MIPAS instrument, operating on board of the Envisat satellite, with its broad band and high resolution measurements has posed a major challenge to operational retrieval techniques. The code developed for the operational analysis of MIPAS measurements has proved that near-realtime operation is possible and that a three-dimensional picture of the atmospheric composition can be retrieved from satellite observations. The features and performances of MIPAS retrieval code are briefly recalled.

The breakthrough in retrieval techniques obtained with MIPAS can be the basis for further significant improvements. The correlation that exists among the observations and among the target parameters are more rigorously and more efficiently accounted for if the retrieval is made handling simultaneously correlated observations and correlated target parameters. This is the case of two-dimensional retrieval and multi-target retrieval. Another direction for further improvements is that of the effect of model errors. These can be better accounted either with the use of the variance covariance matrix of these errors or with the simultaneous retrieval of the main model errors. These retrieval approaches can be very demanding in terms of computing resources, but they change the ultimate accuracy possible with remote sensing techniques and must be taken into account in the planning of future instruments for environment and climate.

The challenge of understanding our environment requires global measurements of increasing numbers of trace species and requires some priority to be given to new observations of classes of species which may play fundamental roles in climate and chemistry interactions. Such compounds include “new” ozone depleting substances (ODS), greenhouse gases (GHGs) and indirect controlling influences such as key chemical oxidants. For many of these gases, relevant spectral signatures had, until recently, either not been well characterised globally or never been observed in the atmosphere. The launch of high-resolution spaceborne spectrometers, such as the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT, is providing an unprecedented opportunity to examine spectra of the atmosphere and to determine new observable trace species. In this paper, MIPAS infra-red spectra of the upper troposphere are examined, methods for detecting weak spectral signatures in atmospheric spectra are discussed and key trace gas signatures demonstrated for HCFC-22 and ethane as examples of the potential for three-dimensional daily data of trace gases.

The fundamentals on non-local thermodynamic equilibrium (non-LTE) in the atmosphere are briefly described as well as for which atmospheric emissions, in which regions and for which instruments (viewing geometry, spectral coverage and resolution), non-LTE processes are more important. The retrieval of atmospheric species from non-LTE infrared emissions and the different options for performing non-LTE retrievals are discussed. The IMK/IAA non-LTE retrieval scheme for inverting atmospheric species from the MIPAS spectra and its application to the retrievals of the CO and NO abundances and non-LTE parameters related to the CO levels emitting near 4.3 µm are described. Finally, a non-LTE parameterization of the CO 4.3 µm daytime outgoing radiation in the nadir view is described and its potential application to low altitude atmospheric temperature sounding from nadir infrared instruments is discussed.

Optical measurements of atmospheric minor constituents are performed using spectrometers working in the UV-visible, infrared and microwave spectral ranges. In particular recently the satellite ENVISAT has been launched with three spectrometers on board, SCIAMACHY and GOMOS working in the UV-visible spectral region and MIPAS working in the thermal infrared. The analysis and interpretation of the atmospheric spectra require good knowledge of the molecular parameters of the species of interest as well as of the interfering species. This is true not only in the spectral domain used to retrieve the species (Thermal infrared for MIPAS for example) but also in the other spectral domains used by other instruments: Meaningful comparisons of profiles retrieved by various instruments using different spectral domains require indeed that the spectral parameters are consistent in these spectral domains. To illustrate these points we will concentrate on three molecules namely nitric acid, formaldehyde and ozone. For HNO we will show the difficulty to measure line intensities in the laboratory and we will describe how a comparison of MIPAS profiles with those obtained by another instrument operating in a different spectral range (Far infrared) may be used to validate the HNO line parameters in the mid-infrared. For the measurement of atmospheric formaldehyde concentrations, mid-infrared and ultraviolet absorptions are both used by ground, air or satellite instruments. It is then of the utmost importance to have consistent spectral parameters in these various spectral domains. Consequently the aim of the study performed at LISA was to intercalibrate formaldehyde spectra in the infrared and ultraviolet regions. The experiments were performed by acquiring simultaneously UV and IR spectra at room temperature and atmospheric pressure using a common optical cell. The reactor contains two multiple reflection optical systems interfaced to a Fourier transform infrared spectrometer and to an UV-visible absorption spectrometer. The results are discussed and compared with previous ones. In the mid-infrared range, the 10 µm ozone band is very strong and is the most widely used to derive concentration profiles. In the UV region, various bands are currently used for spectroscopic remote-sensing of ozone. In this paper we present two sets of results:

The analysis of spectra of the terrestrial atmosphere recorded using remote sensing techniques requires reference spectroscopic information, measured in the laboratory. This article describes laboratory measurements of reference absolute absorption intensities for atmospheric trace species using Fourier transform spectroscopy in the infrared spectral range. Emphasis is put on measurements of quantitative information for chemically unstable species.

The global modeling of high-resolution spectra of linear molecules which our group performs is aimed to generate high-resolution spectroscopic databanks for the atmospheric and high-temperature applications. The method of effective operators which is used for the modeling is briefly discussed. The main goal of our modeling is to attain the near experimental accuracy of the line position and line intensity calculations in a wide spectral range from microwave to visible. The recent results achieved in the modeling of high-resolution spectra of CO, NO, and CH molecules are presented.

We present CDSD-296 a version of the Carbon Dioxide Spectroscopic Databank aimed at processing signals from the satellite based sensors. The databank contains line parameters (positions, intensities as well as HITRAN air- and self-broadened halfwidths and coefficients of temperature dependence of air-broadened halfwidths) of the four most abundant isotopic species of CO. The reference temperature is =296 K, the intensity cutoff is =10 cm/(molecule cm) and the spectral range is 405–12784 cm. The databank was generated within the framework of the method of effective operators and based on the global fittings of parameters of the models to observed data collected from the literature. Calculated line positions were systematically replaced where possible by the differences between experimental term values derived from recalibrated observed line positions with the help of the combination Ritz principle. The databank includes statistically justified confidence intervals for each line position and intensity.