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IGACO-O3 Activities

Activity Cluster A: Stratospheric Ozone

Stratospheric ozone is currently well covered by a combination of ground, airborne and space observations. For total ozone measurements at ground, the two most widely used instruments are the Dobson and Brewer spectrophotometers. Great success in improving data quality of the measurements has been achieved since the early 1980s through the activities carried out in the SAG Ozone of the GAW programme and several related scientific teams from WMO member institutions and supporting agencies. The current satellite fleet is providing good data on ozone and related chemical constituents. Especially, total ozone measurements from space are consistent and can be considered mature and they can be used as a homogeneous global dataset for quality control of the ground-based network.

However, even with the great progress achieved already, Dobson, Brewer and satellite measurements show distinct differences in seasonal variations of ozone. Therefore, this information needs to be further distributed in the community. Further comparisons and studies are needed that would address instrumental characteristics and spectral temperature dependences of the ozone cross sections in order to understand the remaining instrumental differences. This approach is essential for the creation of a homogeneous data set comprising of data from different instruments.

Activity Cluster B: Ozone at the Upper Troposphere and the Lower Stratosphere (UTLS)

The upper troposphere/lower stratosphere (UTLS) is an important altitude in the
atmosphere where a variety of processes such as transport from the higher stratosphere, fast transport from the planetary boundary layer (such as by convection) and in-situ chemical production take place. Ozone is an important greenhouse gas and its effect on the radiation balance of our planet maximizes in the UTLS altitude.

Chemical ozone production at the UTLS depends on the ozone precursor concentrations mainly up-lifted from the planetary boundary layer and the emissions of the rapidly growing civil aircraft, released at their cruise altitude which is close to tropopause at mid-latitudes. Ozone at UTLS is also a key parameter for data assimilation which is used to predict or forecast tropospheric chemical air composition and ozone data are used in the retrieval algorithms of tropospheric satellite measurements for other species such as nitrogen dioxide. Ozone concentrations at UTLS also contain valuable information for weather forecast models.

Obtaining reliable ozone observations at the UTLS is a challenging task, because ozone concentrations tend to be very low close to the tropopause. The only continuous information on the long-term changes of ozone at UTLS originates from a very few ozone sonde stations operated at northern mid-latitudes since the late 1960s but the data quality of these measurements is difficult to assess. Measurements performed by ground-based LIDAR instruments have limited vertical resolution at the tropopause and satellite measurements are restricted by vertical resolution and large ozone variability in the UTLS, the best resolution and accuracy achieved with specific limbviewing instruments. Measurements from civil aircraft can provide reliable ozone measurements close to tropopause. Such continuous series are available since 1994 from the project MOZAIC, but presently the data coverage is not global and important parts of the northern mid-latitudes, namely the Pacific region, is missing.

Activity Cluster C: UV Radiation

UV radiation at the Earth's surface is a function of wavelength, solar zenith angle, ozone (and some other trace gases), cloud, aerosol, albedo, altitude and Earth-Sun distance. It is thus more variable than ozone and any change or trend in UV can result from changes in any or several of the influencing atmospheric variables. Changes in ozone produce a spectral signature in the resulting UV changes, with greatest change at the shortest wavelengths, whilst other atmospheric changes have a less wavelength dependent influence on the UV. In terms of potential climate change, possible changes in, for example, cloudiness may have an equal or more significant influence on UV than changes in ozone.

Standard instruments for UV measurement have been defined by the WMO-SAG UV instrumentation group and recommendations for their characterization and operation have been given. Spectroradiometers provide spectrally resolved data and multifilter radiometers provide data from several wavebands, while broadband radiometers produce a measure of the integrated total irradiance, weighted with an instrument response. The most common instrument response is one
similar to the erythemal action spectrum. Nonetheless, the erythemally effective UV, and hence the UV index can be derived from the great majority of UV instruments. The UV index is therefore taken as the single common factor that should be obtained from the data at every site. However, time and wavelength resolved data from a site is of much greater value for most applications and it is recommended that complete datasets rather than derived products should be submitted to any data centre.

Activity Cluster D: Services

Access to data with known quality is a key point for all users from research scientist to policymakers. New applications also are expected to emerge in forecasting ("chemical weather"), which sets new requirements on availability of data in near real time (NRT). NRT data also needs to be supported by ancillary data, such as meteorological observations.

One of the key points discussed in the first consultation meeting in Anavyssos in May 2006 was access to data. Registration bureaucracy, varying protocols, and data scattered in all too numerous places around the world were seen as bottlenecks holding back success in science. As a result, it was agreed that a dedicated meeting should be held to discuss improvements over the existing system. This meeting was held in March 2007, hosted by EMPA in Dübendorf, Switzerland. The discussions in the meeting covered a wide scope of goals: access and submission, how to further improve the overall quality of the data (including ancillary data, metadata and error estimates on total ozone and profile data), as well improving homogeneity between data sets. Services for derived products were also discussed: data visualisation and data availability in long term were also on the agenda. Feedback from users to providers and, in general, open communication between data providers and users were considered extremely important.

Activities in this section form a starting point. Many of these activities also have much wider interest than only IGACO-O3/UV, and some of them will probably in the long run be coordinated with other IGACO Foci and the World Data Centre for Remote Sensing of the Atmosphere (WDC-RSAT).