The main results of an experimental study focusing on the formation and transport of photochemical pollution in the Madrid air basin are presented. This southern European, heavily populated urban area is located on an elevated plateau at a height of 700 m, near a mountain range with maximum heights of around 2,400 m. Daily and seasonal cycles of ozone 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 plume on polluted days (when values exceeded 90 ppb) has been estimated at around 40-50 ppb.

[1] An experimental characterization of biogenic emission from Quercus ilex ssp. rotundifolia in a forest near Madrid, Spain, was carried out in the early autumn of the years [2000][2001][2002][2003]. A dynamic branch enclosure technique was implemented to determine the monoterpene emission rates of this evergreen oak species during the 2000 and 2001 campaigns. Major compounds emitted during both measurement periods were limonene, a-pinene, b-pinene, sabinene, and myrcene. In the 2000 field campaign the light-and temperature-dependent model of Guenther et al. [1993] did not fit the data due to drastic reductions of emission rates (and leaf gas exchange related parameters) observed at high air temperature and low air humidity (high water vapor pressure deficit). This plant physiological activity depletion and the subsequent emission reduction were attributed to severe water soil deficit conditions, as precipitation was very scarce during the growing season. In contrast, during the 2001 field campaign, neither emission nor physiological activity showed strong decreases in hot days. A good fit of experimental data to Guenther model was achieved in this field campaign (r 2 = 0.90), and linear regression gave a standard emission factor (E S ) of 14.0 mg gdw À1 h À1 (gdw is grams dry weight). Soil moisture was presumably higher than during the 2000 campaign due to recent rain events. With the purpose of documenting the drought stress effect at canopy level, monoterpene

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