The main objective of this campaign is to intercompare the NO₂ measuring instruments that can be used for validation of tropospheric NO₂ from satellites. The campaign should result in an estimate of the accuracy of the NO₂ tropospheric columns and profiles for the participating instruments under different atmospheric conditions (clouds/aerosols) and viewing geometries. Many different in-situ and remote sensing instruments will participate. Apart from NO₂, other parameters will be measured and intercompared, among which O₃, aerosol, HCHO, CHOCHO, and BrO.