Abstract: The relationship between springtime mid-latitude cyclones and background ozone at two rural monitoring sites on the west coast of Europe — Mace Head, Ireland and Monte Velho, Portugal — is explored using a combination of observations and three reanalyses: 1) the European Centre for Medium-Range Weather Forecasts’ (ECMWF) ERA-Interim reanalysis, 2) the Monitoring Atmospheric Composition and Climate (MACC) reanalysis and 3) NASA’s Modern-Era Retrospective Analysis for Research and Application…