The Remote Sensing of Atmospheric Constituents from Space
ACCENT-TROPOSAT-2 (AT2): An ACCENT Integration Task

The second half of the twentieth century was marked by the realisation that air pollution was not only of local and regional importance but that it was indeed a global phenomenon, with local actions giving rise to the depletion of stratospheric ozone, the inter-continental transport of pollutants and the appearance of pollutants in hitherto pristine parts of the world.

The physical, chemical and biological processes, which determine the composition of the atmosphere, the conditions at the Earth’s surface and its climate, comprise a complex system having many non-linear interactions and feedback. To assess accurately our current knowledge of the Earth-atmosphere system, detailed global information about the amounts and distributions of key atmospheric constituents and parameters is required.

Thanks to principally European efforts, it is now possible to obtain global information for both the stratosphere and, most recently, for the troposphere, using remote sensing instrumentation on-board orbiting satellites. In particular, passive remote sensing in the ultraviolet, visible and infrared spectral regions has provided, for the first time, information about the tropospheric column amounts of trace gases and aerosols, enabling comprehensive global views of the atmosphere to be built up. The principal research groups have already developed algorithms for retrieving tropospheric information from the complex spectroscopic data streams, and have produced the first global maps of the tropospheric distributions of NO2, O3, SO2, HCHO, SO2, BrO and aerosols.

These activities have demonstrated how such data might be used, either alone or in combination with information from other satellites or from ground stations to determine:

-        the distributions of NO2, O3 and SO2 both globally, and regionally;

-        regional source strengths;

-        the effects of forest fires and biomass burning;

-        the intercontinental transport and transformation of NO2 and aerosol;

-        the spatial distribution of PM2.5 over Europe;

-        the presence of BrO around the Arctic and Antarctic sea ice in Spring (which corresponds with events having nearly zero surface ozone concentrations);

-        the plumes of noxious gases resulting from volcanic eruptions;

-        photochemical oxidation in areas of fossil fuel burning and biomass burning using plumes of formaldehyde as an indicator.

ACCENT-TROPOSAT-2 (AT2) is an ACCENT integration task which addresses the need for global information about trace atmospheric constituents. AT2 is building upon and improving on the efforts initiated within TROPOSAT, the subproject of EUROTRAC-2, to facilitate the generation of tropospheric data products, to encourage their use for research and in the medium term, and indicate their potential use in the development of environmental policy.

Specifically AT2 will deal with the growing need to co-ordinate and integrate global, regional and local data sets. It aims to ensure that the data obtainable from the new generation of satellite instruments, (e.g., ESA-ESR2/GOME, NASA-TERRA/MOPITT, ESA-ENVISAT/SCIAMACHY, MIPAS and MERIS, NASA-AURA/TES and OMI and EUMETSAT-ESA-Metop/GOME-2 and IASI) are fully used to determine two- and three- dimensional distributions and time series of trace constituents, aerosols and pollutants in the troposphere.

The field is still in its initial phase and there remain many scientific problems, associated with the retrieval of tropospheric distributions of chemically active species, the generations of profiles of their concentrations in the atmosphere, to be solved. These will be facilitated by the use of limb sounding with SCIAMACHY, and the utilisation of the daily observations to generate a continuous picture of concentrations of trace species in space and time. A substantial effort is required in the near future to ensure that the potential of the data is fully realised.

AT2 is organised into three task groups, which will investigate and study:

1.       The development and improvement of algorithms for the retrieval of tropospheric data. The task group is divided into three groups specialising in aerosols, infra-red measurements and UV/Visible measurements.

2.       The synergistic use of models and observations to improve our understanding of tropospheric chemistry and dynamics.

3.       The development of validation strategies for tropospheric satellite data products using existing data.

Each task group consist of a number of principal investigators each of whom has agreed to contribute to the work of the integration task.

In addition, two further activities will be undertaken by groups of the principal investigators:

-        to develop an updatable source list of tropospheric data products for use by the wider community;

-        to contribute to the outreach activities of ACCENT by developing an educational package on Remote Sensing from Space for use in Universities.

The aims of the project and its constituent parts are being pursued through regular workshops and meetings, inter-laboratory exchanges and the publication of regular reports.

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