Intercomparison of IAGOS-CORE, IAGOS-CARIBIC and WMO/GAW-WCCOS Ozone Instruments at the Environmental Simulation Facility at Jülich, Germany

Abstract. As part of the Quality Assurance (QA) plan of the In-service Aircraft for a Global Observation System (IAGOS), IAGOS-CORE and IAGOS-CARIBIC UV-photometer instruments have been compared with the dual-beam UV- Ozone (O3) PhotoMeter (OPM) of the World Calibration Center of Ozone Sondes (WCCOS) at the Forschungszentrum Jülich in an environmental simulation chamber. The WCCOS is established since about 30 years ago as part of the WMO-GAW measurement quality program of the global ozonesonde network, in which the OPM instrument serves as the ozone reference standard. In the simulation chamber, pressure, temperature, and ozone concentration can be controlled at quasi-realistic flight conditions between the Earth surface (∼ 1000 hPa) and ∼ 35 km altitude (5 hPa). During the intercomparison, different ascent/descent and cruise altitude profiles of ozone, pressure and temperature have been simulated between the surface and ∼ 12.5 km altitude (200 hPa). In general, the two O3 instruments P1-O3 (IAGOS-CORE) and CAR-O3 (IAGOS-CARIBIC) showed good agreement with the OPM reference standard within 5 %–6 %. At a pressure of 400–500 hPa the agreement was even within 2 %. The observed differences are small but systematic and reproducible during this experiment. CAR-O3 showed a small, pressure-independent deviation of −2 ± 1.5 % compared to the OPM. P1-O3 revealed O3 deviation to the OPM which changes with pressure of about +2 % at 1000 hPa to −3 % at 400 hPa, which might be an artefact on the experimental set-up and subject for further investigations. This intercomparison is a first step of the long-term goal to make the global ozone sonde data (GAW-NDACC-SHADOZ-GRUAN) and IAGOS-O3 (CORE: P1-O3, CARIBIC: CAR-O3) data traceable to one common reference, the OPM instrument of WCCOS. Recommendations are made for further regular (every two to three years) intercomparison of the operational instruments to ensure external consistency in general and specifically towards the synergy of IAGOS-O3 and ozonesonde data. An important gap in such intercomparison studies is the lack of a reference ozone instrument operated at reduced pressures at any National Metrological Institute in the world. For observation networks measuring vertical ozone profiles, it is essential to close this gap to enable the traceability of ozone measurements from different platforms to one reference standard. This is crucial to harmonize long-term ozone records to detect any changes of ozone in the free atmosphere.