FORMATION AND TRANSPORT OF THE MADRID OZONE PLUME

The main results of an experimental study focusing on the formation an d transport of photochemical pollution in the Madrid air basin are pre sented. This southern European, heavily populated urban area is locate d on an elevated plateau at a height of 700 m, near a mountain range w ith maximum heights of around 2,400 m. Daily and seasonal cycles of oz one were documented during a one-year survey at three semi-rural sites located 30 km away from the urban center. Maximum hourly values of up to 140 ppb were measured, and the ozone generated within the urban pl ume on polluted days (when values exceeded 90 ppb) has been estimated at around 40-50 ppb. A meteorological characterization of these smoggy days pointed out the influence of thermally induced local wind flows on the concentration daily cycles at the measuring sites, denoting a p referred advection of the urban plume. Moreover, during intensive summ er field campaigns, the use of meteorological and ozone sondes, as wel l as an instrumented aircraft, revealed some features about the horizo ntal and vertical distribution of the polluted air masses, as well as their evolution within the planetary boundary layer. Ozone plumes have been detected up to 100 km away from the city, usually mixed in a lay er that reaches a height of 1,000-1,500 m in the afternoon. On some oc casions, ozone-enriched layers have been detected as high as 4,000 m d uring morning hours,suggesting possible tropospheric injection induced by topography-driven flows or convective mesoscale systems that are u sually present in the center of the Iberian Peninsula in the summer.