The traceability of ground-based microwave radiometer (MWR) estimates and their uncertainty requires the traceability of MWR calibration to SI standards. Currently, no SI standard is available for MWR at any national/international measurement institute. Thus, full SI-traceability of ECVs from MWR is currently not feasible. However, at least one national measurement institute is currently developing SI standards for MWR. It is expected that SI-traceable standards for MWR will be available in the next few years.

Temperature lidars provide important information for trend detection in the middle atmosphere (connected to trends in the ozone layer). These are measured using lidar systems that often also measure the ozone layer. The lidar technique to measure temperature is sensitive to the presence of aerosol, which is an important contribution to the error budget. In addition, lidar techniques exist to measure temperature profiles in the troposphere using the pure-rotational Raman (PRR) technique that can be used in the presence of aerosol.

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.