Upper tropospheric composition during Iberian cut-off lows as seen by IAGOS and CAMS

For thirty years, the European Research Infrastructure  IAGOS has been equipping commercial aircraft with instruments to monitor the composition of the atmosphere on long-haul flights around the world.  The aircraft measure  a number of chemical species,  meteorological parameters and cloud particles at cruise altitude in the upper troposphere/lower stratosphere and during landing and take-off at many international airports.   The in-situ data on chemical composition of the atmosphere collected by IAGOS is used in the routine validation of the forecasts and analyses from the Copernicus Atmosphere Monitoring Service (CAMS).  This  evaluation by IAGOS now covers  the  CAMS global and regional forecasts for ozone, carbon monoxide, and nitrous oxides, and the CAMS global greenhouse gas forecasts for carbon dioxide and methane.  We describe recent cut-off lows that led to severe flooding over Spain in Autumn 2024, characterised by the dynamical fields in the ERA-5 meteorological re-analysis. IAGOS measurements of the trace gases, ozone, water vapour and carbon monoxide in the cut-off low allowed us to identify stratosphere to troposphere transport. We describe the differences between the CAMS forecasts and IAGOS observations of these trace gases during this event. In addition,  we use a new tool based on  FLEXPART using ERA-5 winds and METEOSAT third generation's (MTG)  lightning imager to determine the origin of elevated NOx observed at altitude.