ACTRIS topical centre for reactive trace gases in situ measurements, IMT Lille Douai unit, is taking part in a s-b-s intercomparison campaign from 8 July to 31 October 2019, involving 5 NOx instruments at the atmospheric observatory “Pic du Midi”, which is part of the Pyrenean Platform for Observation of the Atmosphere (P2OA – “PicoNOx” is organized together with IMT Lille Douai, Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), and Laboratoire d’Aérologie – Observatoire Midi Pyrénées (LA – OMP). The objectives of “PicoNOx” are to evaluate the performance of the instruments under low-pressure and clean air conditions at a high-mountain observatory at 2 877 m, and to build best practices and instrumental knowledge. The research groups are participating with 5 NOx instruments covering latest models used as well as a high-performance calibration unit. The intercomparison campaign will improve the ACTRIS Standard Operating Procedure.