WP2220 Si-traceable system development: This work package focussed on the development of procedures to retrieve traceable total column ozone from solar UV spectra using spectroradiometers. The standard instruments used world-wide for this purpose are Dobson and Brewer spectroradiometers that are operated in their particular networks, and are traceable to well defined reference instruments. Even though these two networks show an excellent homogeneity among their members, there remain unresolved differences among these two instrument types that need to be resolved in order to produce consistent measurements among these two networks, and in view of serving as vicarious references for satellite based retrievals of total column ozone. WP2230 Ground-based instrument calibration: This work package dealt with the traceable retrieval of aerosol optical depth (AOD) from narrowband solar filter radiometers. A precision filter radiometer (PFR) from PMOD/WRC was calibrated and characterised in the laboratory. The retrieval of AOD was then performed using solar irradiance measurements in conjunction with top of the atmosphere solar spectra to calculate the atmospheric transmission. This methodology was then compared to the traditional Langley-based method. Additionally, a CIMEL sunphotometer from AERONET-Europe was characterised in the laboratory for spectral responsivity and angular response, in view of validating the measurements of radiance and irradiance using traceable measurements to the SI. Finally, a field campaign was performed at the AERONET-Europe calibration site at Observatoire de Haute Provence to perform colocated measurements with a travel standard PFR, to provide traceability of the AERONET-Europe network to the WMO reference operated at PMOD/WRC.