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 throughout much of the column. Whereas the detection of aerosol layers and their vertical extent requires only simple single wavelength backscatter lidars, the derivation of extinction coefficient profiles and of a series of derived aerosol properties requires advanced lidar setups and techniques such as high-spectral resolution lidars (HSRL, Shipley et al., 1983) or Raman lidars (Ansmann et al., 1992). The estimation of aerosol microphysical properties and mass concentration requires at minimum a one-wavelength Raman lidar, though the error affecting these estimations can be dramatically reduced if a multi-wavelength Raman lidar system is used. This highlights the relevance of having an enhanced number of multi-wavelength Raman lidars globally if they are to be used to characterise satellite measurements that aim to discern such properties.

Such lidars also have a  potential role as anchor reference stations for the study of the impact of aerosols on weather and climate more generally. The availability of multi-wavelength Raman lidar measurements also ensures that ground-based instruments can deliver wavelength conversion information for different aerosol and cloud types to relate the current and future space-borne measurements performed by different satellite missions at different wavelengths (for example, CALIPSO at 532 nm and the future EarthCARE mission at 355 nm). In addition, space-based measurements have the advantage of obtaining global spatial coverage, but long-term ground-based observations can provide a critical contribution to distinguish natural and anthropogenic aerosols from satellite data.

Multi-wavelength Raman lidars could be considered to be the future backbone of a larger network incorporating simpler lidar instruments and/or ceilometers, and so be able to have a denser global spatial coverage. In this process, it is very important to carefully assess the value of the retrieval of advanced lidar systems and to study if the coverage of the existing networks globally is sufficient to carry out a sufficiently accurate aerosol study.

Steps towards automatic or semi-automatic usage of the most advanced lidars are needed to reduce the traditional intensive manpower typically required to operate these systems. In this sense, the effort spent over the last year by the biggest aerosol lidar networks (EARLINET, MPLnet) to develop automatic lidar data processing chain must be acknowledged.

The working groups of lidar network representatives involved in the Aerosol SAG (Scientific Advisory Group) of the WMO-GAW programme has recently started working to address (on voluntarily basis) specific harmonisation issues on the global scale.