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

AbstractDuring the early morning hours on April 29, 2000, a time series of ozone observations from several stations showed that a uniqueand exceptional ozone episode occurred in Madrid city and surrounding areas, whereby monitoring stations reported ozoneconcentrations up to 1190 mg/m 3 . In order to investigate this phenomenon, two different air quality modeling approaches are usedhere:U The FLEXTRA trajectory model was initially used, suggesting that an intrusion of stratospheric air occurred over the Madridarea and brought stratospheric air down to 1000e2000 m AGL. The local circulation system, not resolved by the FLEXTRAtrajectories, subsequently brought some of this stratospheric air to the surface. However, the maximum ozone concentrationthat could be explained by this process is much less than the observed one.U The OPANA Air Quality Modeling System was also employed to study air quality over the Madrid community and citydomains. Results suggested that the main wind direction returned to Madrid after 180( wind change direction 1 to 2 h beforethe ‘‘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 correspondencewith the sequence of observations.Preliminary conclusions show that the exceptional meteorological conditions on such a night could be reason for the occurrenceof high values. Additional technical circumstances (such as technical incidences in some monitoring stations) also suggest that someinstruments did not work properly under these high concentrations and, thus, real ozone concentrations may have been lower thanthose measured. 2004 Elsevier Ltd. All rights reserved.