TROPOSAT

The Use and Usability of Satellite Data for Tropospheric Research

Project Description

  Section 3

3. Project Description

3.1 Background

While the measurement of stratospheric gases was the primary goal of the satellite instruments and has received much public attention (for instance the observation of the ozone layer and the ozone hole), during the last decade it has become possible to measure reliably a comprehensive set of tropospheric trace gases and other parameters with satellites (Table 1). The newly-gained capability of observing tropospheric trace gas distributions (such as NO2, O3, SO2, HCHO, BrO, and water vapour) with space-borne sensors can truly be regarded as a revolutionary step in the technical development and will lead to a significant enhancement of our capability of investigating the chemistry and physics of the troposphere.[Fishman et al., 1990; Eisinger et al., 1998; Richter and Burrows, 1999]

Table 1. Satellite instruments, coverage of their measurements, species measured and the satellite platform. The list is not intended to be complete, but merely to illustrate the currently available instrumentation. [Burrows 2000]

Name

Target Species

Satellite Platform

Orbit

ATMOS, Atmospheric Trace Molecule Spectroscopy

O3, NOx, N2O5 ClO NO2, HCl, HF, CH4, CFCs, etc. (upper troposphere)

Space Shuttle Spacelab-3 (1985), ATLAS-1,2 and 3 (1992,1993, 1994)

inclined

ATSR Along Track Scanning Radiometer

Aerosols, clouds, sea surface temperature

ESA-ERS1, 2 (1991-present)

Polar, Sun Sync.

AVHRR Advanced Very High Resolution Radiometer (4/5 chan.)

Smoke, fire, clouds aerosols, vegetation

TIROS-N, NOAA-6 - 13 (1978- present)

 

BUV, Backscatter Ultraviolet Ozone Experiment

O3 (profiles)

Nimbus-4 (1970-1974)

Polar

GOME, Global Ozone Monitoring Experiment

O3, NO2, H2O BrO, OClO, SO2, HCHO, clouds, aerosol

ESA-ERS-2 (1995-present), METOP-1 - METOP-3 (2005/6 2010/11, 2015/16)

Polar, Sun Sync.

GOMOS, Global Ozone Monitoring by Occultation of Stars

O3, NO2, upper troposphere

ESA ENVISAT (2001 -)

Polar, Sun Sync.

IASI, Imaging Atmospheric Sounding Instrument

O3, CO, CH4, N2O, SO2

METOP-1 (2005/6)

Polar, Sun Sync.

IMG, Interferometric Monitor for Greenhouse Gases

O3, N2O, H2O, CH4, CO and CO2

ADEOS (1996-97), ADEOS-II (2001)

Polar, Sun Sync.

MERIS, Medium Resolution Imaging Spectrometer for Passive Atmospheric Sounding

H2O, clouds and aerosol

ESA-ENVISAT (2000)

Polar, Sun Sync.

MIPAS, Michelson Inferometer for Passive Atmospheric Sounding

O3, NOx, N2O5 ClONO2, CH4, CFCs, etc.; temperature (upper troposphere)

ESA ENVISAT (2000)

Polar, Sun Sync.

MOPITT, Measurement of Pollution in the Troposphere

Total column of CO;
CH4 + CO profiles

NASA AM-1 (1999)

 

ODUS, Ozone Dynamics Ultraviolet Spectrometer

SO2, NO2, BrO, OClO

GCOM-A1 Prog, Japan (2005)

inclined

OMI, Ozone Monitoring Instrument

O3, SO2, NO2,

NASA-EOS-CHEM (2004)

Polar, Sun Sync.

POLDER, Polarization and Directionality of the Earth's Radiance

Polarization, aerosols, clouds

ADEOS-1 (1996-97)

Polar, Sun Sync.

SAGE I-II Stratospheric Aerosol and Gas Experiment

O3, NO2, (H2O), aerosols (upper troposphere)

NASA- Atmospheric Explorer Mission (1979-81), Earth Radiation Budget Sat. (1984 - pres.)

inclined

SAGE III, Stratospheric Aerosol and Gas Experiment III

O3, OClO, BrO, NO2, NO3 aerosols

Meteor 3M (2001); International Space Station (2003?)

inclined

SBUV, Solar Backscatter Ultraviolet Ozone Experiment

O3 profiles

Nimbus-7 (1979-90)

polar

SCIAMACHY, Scanning Imaging Absorption Spectrometer for Atmospheric Cartography

O2, O3, O4, NO, NO2, N2O, BrO, OClO H2CO, H2O, SO2, HCHO, CO, CO2, CH4, clouds, aerosols, p, T, col. and profiles

ESA-ENVISAT (2001)

Polar, Sun Sync.

TES, Tropospheric Emission Spectrometer

Various incl. HNO3, O3, NO, H2O (col. and profiles)

NASA-EOS-CHEM (2004)

 

TOMS, Total Ozone Monitoring Spectrometer

O3

Nimbus 7 (1979-92)
ADEOS (1996-97)
Earth Probe (1996-)
Meteor (1992-94)

polar

As can be appreciated from the figures shown overleaf and on the cover, the detailed distribution of NO2 that it is now possible to obtain on regional and global scales is truly remarkable. The applications to model intercomparison and possibly to monitoring for policy purposes is self evident.

The retrieval of information about atmospheric trace gases from satellites relies on the knowledge and understanding of the absorption, emission and scattering of electromagnetic radiation in the atmosphere. Satellite sensors utilise several characteristic fingerprint spectral regions in emission or absorption (Table 1): (i) rotational transitions, observed primarily in the far infrared and microwave regions; (ii) vibrational-rotational transitions, observed in the infrared; (iii) electronic transitions, mainly observed in the UV, and visible spectral regions.

While the tropospheric data from present day satellite instruments have already been used with enormous success in many studies [Fishman and Brackett 1997; Wagner and Platt 1998; Eisinger and Burrows 1998; Hegels et al., 1998], they can still largely be regarded as explorative and preliminary.

In particular it has been the instrument - oriented groups that have conceived, suggested and supported the development of a satellite instrument, often for a decade or more, until it goes into orbit. It is now important that a larger part of our scientific community appreciate the possibilities so that they may contribute to specifications for future instruments as well as making full use of the data becoming available.

Satellite measurements have, like those made with any other technique, specific strengths (like extensive spatial - frequently global -, and temporal coverage) and weaknesses (such as frequently giving only data for a total column). These strengths and limitations must be better understood in order to use the data to the best effect in tackling current scientific problems.

Judging from the recent development of the field, it is clear that many new (and frequently unexpected) applications of satellites will be discovered once enough expertise is brought together. Examples are the discovery of huge amounts of BrO in the polar boundary layer during springtime by the GOME instrument, despite GOME being intended only to measure BrO in the stratosphere [Wagner and Platt 1998]. Also global mapping of the tropospheric distributions of NO2, HCHO and SO2 [Burrows et al., 1999; Leue et al. 1998; Richter et al. 1998b] are now close to reality.

3.2 The Scientific objectives of TROPOSAT

From the background given it is clear that exchange of knowledge, expertise, and - most importantly - ideas between scientists is a particularly important ingredient to the success of this proposal. Therefore a good fraction of the effort in this project will be devoted to the exchange of information, reviewing the problems, and the education of the scientists in areas that will certainly require the use of satellite data in the future.

In this sense TROPOSAT is not a usual EUROTRAC-2 project since the aim is to encourage the appropriate use of satellite data by scientists throughout EUROTRAC-2 and the community in atmospheric chemistry research. Similarly it hoped to make the agencies responsible for atmospheric policy development in Europe aware of the potential of satellites for providing, in the future, reliable data for monitoring the atmospheric environment.

In practice the scientific results of TROPOSAT will be new areas of application of satellite measurements in tropospheric research, and new and improved algorithms for these applications and validation of the results.

The subproject will (e.g. through the determination of the distributions of NOx, SO2, and aerosol) contribute directly to studies of photo-oxidants, aerosols and acidifying substances with EUROTRAC-2 and will thus participate in scientific tasks 1, 3 and 6 given in section 5 of the EUROTRAC-2 project description.

The work of TROPOSAT will be divided between four task groups which will study the topics outlined in the following sections.

Task group 1: Development of algorithms for the retrieval of tropospheric species and parameters

Useful satellite measurements rely - to a much larger extent than typical conventional measurements - on sophisticated algorithms to extract the desired quantities from the raw data. For space observations tropospheric signals are usually very weak and frequently shielded by clouds. They are also affected by the variable aerosol load and ground albedo. However the first algorithms developed for a novel generation of space borne instruments with increased spectral resolution (e.g. GOME aboard ERS-2, ILAS on ADEOS, MIPAS and SCIAMACHY on ENVISAT) have recently been shown to be capable of determining the tropospheric column densities of several tropospheric species including BrO, NO2, SO2, HCHO, and H2O. In task group 1 it is proposed to move towards higher precision and towards expanding he range of applied evaluation techniques. The methods for the determination of the tropospheric information are based on a variety of physical principles.

  1. Cloud shielding: If clouds shield the troposphere, the measurements of tropospheric species are decreased for cloudy conditions (e.g. BrO, Wagner and Platt [1998], Koelemeier et. al., [1999]).
  2. Albedo/wavelength effect: Measurements at different wavelengths have a different sensitivity for the troposphere because of the wavelength dependence of the ground albedo and the atmospheric Rayleigh-scattering (e.g. NO2, Richter et al. [2000]).
  3. Temperature effects: For several atmospheric species the temperature dependence of the absorption cross section causes slightly different absorption structures for species in the cold stratospheric and warm troposphere (e.g. NO2, Richter [1997]).
  4. Different temporal/spatial patterns: For several species the lifetime in the troposphere is significantly smaller than in the stratosphere. Small scale spatial patterns can be attributed to the troposphere (e.g. BrO, NO2, Wagner and Platt [1998], Leue [1999]).
  5. Combination of limb/nadir observations: Nadir observations are typically sensitive for the total atmospheric column, while limb measurements typically yield the stratospheric column. From the difference the tropospheric contribution can be assessed. By applying this method the novel SCIAMACHY sensor will determinate the tropospheric column densities for nearly all target species without using further assumptions.

The aim of task 1 is thus to exploit the methods described fully - and to explore new ones -for the quantitative determination of tropospheric species from space. The work will focus on development and refinement of algorithms for:

Task group 2: Use of satellite data for understanding atmospheric processes

The main objective of task 2 is to demonstrate how satellite data can be combined with model results and data from terrestrial measurement to improve our qualitative and quantitative interpretation and understanding of dynamic and chemical processes in the troposphere. Satellite data are ideally suited to supply initialisation, boundary conditions, and test data for chemical transport models on regional scales and coupled global chemistry-climate models.

Using inverse modelling techniques, satellite data also offer the possibility of identifying and quantifying the emission sources and sink processes of specified pollutants; they should be particularly useful for species for which there is coarse spatial-temporal coverage from existing in-situ measurements and poorly known sources. Lightning as a source of NOx is an example here; the present estimates for its contribution in the upper troposphere vary by more than an order of magnitude.

Because of their global and long-term availability, satellite data can support two further types of studies. First, they can provide wide-ranging supplementary data to support field studies, intensive measurement campaigns and observations at individual and groups of stations. Second, they are also suitable for long-term observations over periods of years, and thus, they are especially useful for the extrapolation, to longer time and global spatial scales, of the results from process studies, thus improving our understanding of the budgets of many trace substances.

In TROPOSAT the activities will include:

  1. case studies with chemical transport models including inversion of transport processes, e.g. with a Lagrangian tracer models;
  2. validation of chemistry-climate models;
  3. comparison and interpretation of model results and individual observations with satellite data.

The studies within the task group will address the following scientific problems.

Task group 3: Synergistic use of different instrumentation and platforms for tropospheric measurements

The troposphere over Europe is part of the larger atmosphere which sustains life on Earth. Thanks to the requirements of the increasing population the atmosphere has undergone, and will certainly undergo, substantial changes in its composition and in its physical state. It is a great scientific challenge for us (and our successors) both to understand the atmosphere and to follow the changes taking place in detail.

Satellites with their frequent global coverage will be of great assistance in this task but clearly they cannot do everything. We will, therefore, continue to need terrestrial measurements, atmospheric measurements made with balloons and aircraft, and the chemical transport models required to encompass and understand the results. Thus use of multiple observations and models provide a synergistic improvement to the measurements from any one technique. For example satellite data will allow a better understanding of measurements made along the track of a single aircraft; and profiles obtained from individual sonde measurements should add a dimension to column densities obtained from satellites.

Out of the multitude of possibilities Task group 3 will address the following issues.

An important aspect in task 3 is that the different contributions cover a broad scale of the integrated uses of satellite and non-satellite data to study tropospheric processes on different spatial and temporal scales. The experience and the know-how achieved during the project are the first steps towards the establishment of an "Integrated Observation System" as part of an integrated research approach to explore the troposphere.

Task Group 4: Development of Validation Strategies for Tropospheric Satellite Data Products

The novel algorithms allowing the observation of tropospheric composition from space require thorough testing and verification and so strategies for the geophysical validation of tropospheric satellite data products need to be developed and applied. Strategies proposed are: collection of comparative measurements (ground-based, aircraft, and other satellite measurements); intercomparison of these measurements with the satellite data; comparison of the satellite data with model results; analysis of different retrieval methods; analysis of different measuring techniques; use of data assimilation methods.

Among the validation activities envisaged are the following.

Attempts at validation using model results include comparing the SCIAMACHY CO and CH4 tropospheric columns with results from the 3D chemistry transport model, TM3. The MOPITT CO and CH4 data will be compared with the Oslo University GCM model.

Underpinning and Derived Activities

The main task group activities will require supporting research, also we expect to undertake some derived activities.

a. Development of appropriate data assimilation techniques

Satellites can observe the Earth with nearly complete global coverage every day, and also provide continuous observations of particular regions. They also offer the only practical method of obtaining valuable data over much of the world, especially the oceans and remote land areas. It is therefore prudent to make sure that we can use these data as effectively as possible.

The aim of data assimilation is the combination of our theoretical understanding encapsulated in a model, with measurements (which may be sparse), and any a priori information that we might already have (e.g. climatologies). In this way we can analyse and/or forecast the state of the atmosphere at any given time or place. Assessment of model and observation errors will guide model and instrumental algorithm improvement [e.g. Lary 1999]. Data assimilation techniques enable us to exploit information elegantly that is simply not available by using models or observations independently. Due to the complexity of the system, and the spatial inhomogeneity of the observations, the analysis of chemical trace species has received little attention in comparison with the analysis of meteorological variables. Satellite observations of multiple species have only become available relatively recently, while meteorological satellite observations were among the first to be made.

Data assimilation analysis can provide guidance on how, when, and where additional measurements are necessary (see also task 3) and point to possible redundancy. It can answer the question which, how many, and where instruments should be employed. Software already available includes Optimum interpolation, 4D-Var, and the Kalman filter data assimilation tools.

b. Specification of requirements for new satellite instruments

Measuring chemical species in the troposphere from space is at the forefront of current science and technology and a clear need is for the development of new observing techniques, concepts, and strategies for space borne instruments. In order to define future requirements for satellite measurements of the troposphere, there is a need to understand the fundamental science, policy and also the technological limitations. As questions arise about the changing nature of the troposphere, it is essential to develop and to take advantage of these new methods and techniques and to integrate them with both scientific and policy requirements. Within the next few years there will be opportunities to propose new satellite missions, some of which are likely to address the requirements of measuring the troposphere from space.

TROPOSAT will provide a European forum for development of concepts for instruments and platforms aimed at observing tropospheric parameters from space. Specifically, new concepts such as GEO-SCIA, observing the troposphere from a geostationary orbit, require an understanding of the scientific and technological advantages and limitations of making diurnal rather than sun-synchronous measurements from space. New options are likely to be available for space borne instrumentation with the growing commercial use of space and the use of small satellites as in the IRIDIUM and GLOBALSTAR networks, which will require smaller and lighter instrumentation developed over shorter time scales. Industry and the scientific community should be encouraged to participate at an early stage in the definition of and priority for new satellite instrumentation for measuring the troposphere and so be allowed to influence the future prospects for future tropospheric measurements and science. Within the bounds of EUROTRAC-2, TROPOSAT will develop a European wide approach for specifying the requirements for new instrumentation and techniques.

A Forum for Exchange of Expertise

The exchange of knowledge, expertise, and ideas between scientists throughout the tropospheric community is a primary goal of this proposal. Therefore a good fraction of the effort in this project will be devoted to the exchange of information, view of the problems, and education of the scientists in the areas of research, which are important for the successful use of satellite data.

Equally important is to ensure that the "satellite instrument community" (i.e. the scientists and groups working on the design of satellite instruments and the development of evaluation methods) are fully aware of the needs and requirements of the atmospheric scientists.

Besides the organisation of workshops open to the general scientific community there will be special workshops on selected subjects (like image processing or the properties of the satellite instruments of interest to the atmospheric research).

3.3 Potential applications to environmental policy development

As already indicated it is now possible to measure tropospheric concentrations of many trace constituents of environmental importance directly from space. It is already possible to observe boundary layer concentrations of some species and it is to be expected that, as algorithms and techniques are improved, concentrations of many further species at low altitudes will be observable. In addition, satellite measurements are likely to provide the only way to observe developments in the upper troposphere on a continuous basis. Satellite measurements are in their very essence reliable since, when launched, they are largely free from external influence. Thus, in the long term, satellite measurements are ideal for the determination of trends.

The results obtained on a regional scale, albeit presently mainly as column densities of limited resolution, will provide an excellent test and validation framework for the present chemical transport models which underpin European environmental strategy.

In the future it will be certainly possible to undertake some environmental monitoring tasks using satellite data. Also TROPOSAT already includes several contributions aimed at improving the emission data bases used for air quality modelling and management.

The various features commend themselves for future environmental monitoring and it is essential that the agencies responsible for atmospheric policy development in Europe have knowledge of and direct access to the rapid developments expected in this field. EUROTRAC-2 provides such access and thus TROPOSAT will contribute directly to the expressed objective of contributing to the formulation and abatement strategies and the improvement of future air quality in Europe.

3.4 Quality Assurance Plan

Quality assurance will be considered at two levels within the project:

  1. The quality assurance of satellite data products (e.g. tropospheric concentrations of chemical species) will be a central activity of this project, as will the development and application of strategies for satellite data validation in task group 4.
  2. Conversely, validated satellite data will provide excellent bases for the validation of regional and global chemical transport models and also confirmation of the significance of measurements made with ground networks and during campaigns. Activities of this sort are envisaged in several of proposals for task group 2.

3.5 Data bases

The primary satellite data are available by arrangement from the space agencies. Most of the data products, consisting of concentrations and profiles in the troposphere, are produced by the major groups themselves. It is intended to make these available, when validated, on the appropriate web pages. To accelerate the work of TROPOSAT, data exchange within the subproject and with other groups will take place following bilateral agreements between the would-be user and the data supplier.

A handbook (giving details of data available, current algorithms, contacts and sources etc., see section 3.8), also available via the web, will act as a data base, providing information about data available and links to the appropriate sites. Access protocols will be devised for the data since it is essential that the groups that produce it have appreciable priority as, in many cases, they have been engaged in the preparational work for a decade or more.

3.6 Operational plan and time schedule

Although the project will last only two and half years, there has been so much interest that there are enough investigators to at least attempt and, in many cases, pursue in detail many of the topics mentioned in this proposal. The project has been organised into four task groups and the leaders, who are members of the steering group, will be expected to develop their own work plans to ensure that the main aims are achieved. As the time is short, TROPOSAT will have five project meetings: A kick off meeting followed by three mid-term progress meetings, one of which will be held at the Symposium in 2002, and a final meeting. The task groups will probably have additional meetings.

The group expects to publish its principal results in the scientific literature, in the two annual reports and in the final report.

3.7 Collaboration foreseen with other subprojects

There are common interests with the work in the observational subproject, TOR-2, and the modelling subproject, GLOREAM. Several of our principal investigators are already investigators in one or other of these subprojects and will provide a natural link. There is also an interest from LOOP in obtaining regional information pertinent to the field campaigns already held or planned. The possibility of using inverse modelling to identify emission sources from satellite data will be of direct interest to GENEMIS with which we share a principal investigator.

As the project develops we would like to make presentations at appropriate workshops and encourage joint efforts, particularly with respect to the use of satellite data, for the validation of satellite measurements and for the synergistic use of platforms and measurement sites. These have yet to be discussed with the subproject coordinators of the projects concerned.

3.8 Plans for the assessment and integration of the results

The algorithm group form a natural focus in providing data for modelling, validation and the synergistic use of platforms. Also it is hoped to produce a review article of the work of the field drawing together the work and pointing to the future.

This will be supplemented by a handbook giving details of data available, current algorithms, contacts and sources etc. The purpose is to facilitate the entry of new colleagues into the field and ensure that that everyone with an interest has up to date information about satellite possibilities.

It is hoped to supplement the hand book with a web site so that the information can be kept up to date in the future.

3.9 Literature

Burrows, J.P., 1999, "Current and future passive remote sensing techniques used to determine atmospheric constituents", in: Developments in Atmospheric Sciences 24: Approaches to Scaling Trace Gas Fluxes in Ecosystems, Ed A. F. Bouwman Elservier Amsterdam pp 315-347. ISBN: 0-444-82934-2.

Burrows J.P. (2000), Current and future passive remote sensing techniques used to determine atmospheric constituents, in preparation.

Eisinger M. and Burrows J.P. (1998), Tropospheric sulfur dioxide observed by the ERS-2 GOME Instrument, Geophys. Res. Lett. 25, 4177-4180.

Fishman, J., Watson, C. E., Larsen, J. C. and Logan, J. A. (1990), Distribution of Tropospheric Ozone Determined From Satellite Data, J. Geophys. Res. 95, 3599-3617.

Fishman, J.; Brackett, V. G. (1997), The climatological distribution of tropospheric ozone derived from satellite measurements using version 7 Total Ozone Mapping Spectrometer and Stratospheric Aerosol and Gas Experiment data sets, J. Geophys. Res. 102, 19275-19278.

GOME Users Manual (1995), SP-1182, European Space Agency, publications division, ESTEC, Noordwijk, The Netherlands, F. Bednarz, Ed., ISBN 92-9092-327-x

Guyenne T.D. and Readings C., Editors (1993), GOME. Global ozone monitoring experiment interim science report, Published by ESA Publication Division ESTEC, Noordwijk.

Hegels E., Crutzen P.J., T. Klüpfel, Perner, D. and Burrows P.J. (1998), Global distribution of atmospheric bromine monoxide from GOME on Earth-observing satellite ERS 2, Geophys. Res. Lett. 25, 3127-3130.

Helten, M., H.G.J. Smit, W. Sträter, D. Kley, P. Nedelec, M. Zöger and R. Busen, Calibration and performance of automatic compact instrumentation for the measurement of relative humidity from passenger aircraft, J. Geophys. Res. 103, 25643-25652, 1998.

Koelemeijer, R. B. A.; Stammes, P.(1999), Effects of clouds on ozone column retrieval from GOME UV measurements, J. Geophys. Res. 104, 8281-8294.

Lary, D. 1999 private communication.

Leue C., Wagner T., Wenig M., Platt U. and Jähne B., (1999), Determination of the tropospheric NOX source strength from GOME data, Proc. ISAMS Conf., Jan. 18-22, Noordwijk.

Marenco, A., V. Thouret, P. Nedelec, H.G.J. Smit, M. Helten, D. Kley, F. Karcher, P. Simon, K. Law, J. Pyle, G. Poschmann, R. Von Wrede, C. Hume and T. Cook, Measurement of ozone and water vapour by Airbus in-service aircraft: The MOZAIC airborne program, An Overview, J. Geophys. Res. 103, 25631-25642, 1998.

Meerkoetter, R., Wissinger, B. and Seckmeyer, G. (1997), Surface UV from ERS-2/GOME and NOAA/AVHRR data: a case study, Geophys. Res. Lett. 24, 1939-1942.

Richter A., Wittrock F., Eisinger M. and Burrows J.P. (1998), GOME observation of tropospheric BrO in northern hemispheric spring and summer 1997, Geophys. Res. Lett. 25, 2683-2686.

Wagner T. and Platt U. (1998), Mapping of Polar Tropospheric BrO by GOME, Earth Observation Quarterly, 58, March, 21-24; Observation of Tropospheric BrO from the GOME Satellite, Nature 395, 486-490.

 


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