Semi-blind intercalibration and intercomparison field campaign of ground-based UV-Visible DOAS instruments with external referee. Complemented by mobile DOAS systems and an exhaustive range of measurements of aerosols, ozone and other UV-Visible trace gases. Primary goals: (i) to intercalibrate instruments and assess mutual consistency of their measurements, (ii) to get NDACC/ACTRIS-CREGARS certification for new instruments, (iii) to assess and improve FRM maturity of the measurements for the validation of the Air Quality and Ozone satellite constellations. Primary target species: atmospheric NO2 and O3; secondary targets: HCHO, CHOCHO and aerosols; others: HONO, BrO and H2O.