Tropospheric ozone has an impact on air quality and acts as a greenhouse gas and therefore plays a role in public and environmental health, as well as climate change, linking the two subjects. In order to establish tropospheric ozone trends, more high-quality and high-frequency observations are needed (see G.2.10) and a rigorous error budget is required. Measurements of tropospheric ozone by means of the Differential Absorption Lidar (DIAL) technique are close to reference quality and may meet this need if development of traceable products can be realised.

Tropospheric ozone has an impact on air quality and acts as a greenhouse gas and therefore plays a role in public and environmental health, as well as climate change, linking the two subjects. Establishing processes and trends in tropospheric ozone, in particular in the free troposphere, above the mixed layer and below the stratosphere, is difficult due to a lack of data. Also, ozone soundings using balloon borne samplers are too scarce to capture the relatively high spatial and temporal variability in the troposphere.

One of the paramount needs for developing long-term ECV datasets for atmospheric monitoring is to calibrate measurements using SI traceable standards. For water vapour measured with the Raman lidar technique, a solution is represented by the calibration of water vapour profiles using reference calibration lamps, which are traceable to NMIs standards.

Aerosol lidar data can potentially be used to constrain uncertain model processes in global aerosol-climate models. Satellite-borne lidar data can be effectively assimilated to improve model skill but, currently, aerosol lidar data assimilation experiments are mainly limited to the assimilation of attenuated backscatter, which is a non-quantitative optical property of aerosol. There is much additional valuable data that could be utilised to improve data assimilation.

Raman lidars or multi-wavelength Raman lidars are undoubtedly an integral component of an aerosol global measurement infrastructure as they can provide quantitative range-resolved aerosol optical and microphysical properties. It is very important to carefully assess the value of the retrieval of advanced lidar systems and to study if the global coverage of the existing networks is sufficient to carry out adequate satellite-retrieval characterisation.

Limited availability of traceable uncertainty estimates limits the direct applicability of the majority of existing data to high-quality applications, such as satellite-data characterisation, model validation, and reanalysis. While a vast amount of data are available, the uncertainty of these data is - in a metrological sense - often only insufficiently specified, estimated, or even unknown.

The need for extensive and accurate metadata is ever increasing in both research and operations, enabling large-scale, distributed management of resources. Recent years have seen a growth in interaction between previously relatively isolated communities, driven by a need for cross-domain collaboration and exchange of data and products. However, metadata standards have generally not been able to meet the needs of interoperability between independent standardization communities.

The availability of user tools able to jointly visualize the current satellite and non-satellite observing capabilities for measuring ECVs at the global scale has never been provided in the past.

There currently exists no universally recognised approach for assessing quantifiable aspects of the measurement system maturity of existing observing networks. Although absolute measurement quality cannot be assured, fundamental properties of the measurement system that build confidence in its appropriateness and metrological verity can be assessed. The lack of an agreed international framework for such an assessment leads to heterogeneity in the approaches used to select the most suitable measurement series for any given application.