Satellite, aircraft and surface-based observations play complimentary roles in addressing the challenges formulated in the GAW mission that require integrated global atmospheric chemistry observations. The GAW Programme will address the challenge of implementing the WMO Integrated Global Observing System by initially developing pilot projects focussing on ozone/UV and aerosols.
The backbone of GAW observations is the surface-based in situ and remote sensing network of stations and sampling sites. These are classified as Global, Regional, or Contributing stations. All stations and networks supporting GAW are expected to contribute data of known quality to one of the GAW World Data Centres and to document tracability of observations for a particular variable to the GAW Primary Standard.
Global or Regional GAW stations are operated by WMO members. A Contributing station is one that is operated by a WMO partner network or organization that contributes data of known quality to one of the GAW World Data Centres and that is linked to the GAW Primary Standard for a particular variable. A Contributing Network is one that has signed a letter of agreement (LoA) with WMO. Any such agreement should contain a list and the characteristics of the stations that will be included in the GAW network as Contributing stations. Some of the stations within these networks are also classified as Global or Regional GAW stations.
Observations of atmospheric parameters supported by Quality Assurance of data/observations are stored and disseminated by the World Data Centres.
In the last decade, airborne and space-based observations have begun to contribute significantly to the characterization of the upper troposphere and lower stratosphere, in particular with regards to ozone, solar radiation, aerosols, and certain trace gases. Some aircraft measurements remain in continuous danger of being terminated because of insufficient funding. A new generation of satellite sensors have begun operation, in some cases adding to relatively long measurement series, in other cases beginning new measurement series. During the last years, expertise has been built up to use satellite information to explore the composition of the lower and middle troposphere. Data access has been improving, but is still not optimal.