A modelling study of an extraordinary night time ozone episode over Madrid domain

During the early morning hours on April 29, 2000, a time series of ozone observations from several stations showed that a unique and exceptional ozone episode occurred in Madrid city and surrounding areas, whereby monitoring stations reported ozone concentrations up to 1190 μg/m 3. In order to investigate this phenomenon, two different air quality modeling approaches are used here: ✓ The FLEXTRA trajectory model was initially used, suggesting that an intrusion of stratospheric air occurred over the Madrid area and brought stratospheric air down to 1000–2000 m AGL. The local circulation system, not resolved by the FLEXTRA trajectories, subsequently brought some of this stratospheric air to the surface. However, the maximum ozone concentration that could be explained by this process is much less than the observed one. ✓ The OPANA Air Quality Modeling System was also employed to study air quality over the Madrid community and city domains. Results suggested that the main wind direction returned to Madrid after 180° wind change direction 1 to 2 h before the “episode”, bringing back ozone generated the day before (typical weekend day with high traffic density). On the other hand, convergence of winds along the South-West North-East axis over the Madrid community showed an important correspondence with the sequence of observations. Preliminary conclusions show that the exceptional meteorological conditions on such a night could be reason for the occurrence of high values. Additional technical circumstances (such as technical incidences in some monitoring stations) also suggest that some instruments did not work properly under these high concentrations and, thus, real ozone concentrations may have been lower than those measured.