High levels of photochemical pollutants, like ozone, frequently affect south European countries and the preoccupation with this subject substantially increased during the last decade. In the west coast of Portugal, where the human activities are concentrated, several episodes of photochemical pollution have been verified. This coastal zone is strongly influenced by the nearby Atlantic Ocean, with frequent sea/land breeze circulation.

Aiming to contribute to a better understanding of these mesoscale phenomena a field campaign was carried out in a coastal region, named Aveiro, from 25th June to 2nd July of 2001, which covered the last ozone episode noticed in the region. Measurements of all the main meteorological parameters and of ozone and its precursors concentrations were taken at surface and in altitude.

In addition, two mesoscale modeling systems (MEMO-MARS and MAR IV), with three different chemical mechanisms (EMEP, KOREM and CB-IV) were applied to the study area, for the two most representative days of the field campaign. The main purpose was to investigate the origin and formation of the measured high air pollution levels, to evaluate these systems, and to analyze the influence of the chemical mechanism type in the simulations. The MAR IV system integrates two models: the Systems Applications International Mesoscale Model (SAIMM), and the photochemical Urban Airshed Model (UAM). UAM is a three-dimensional Eulerian model that contains the photochemical Carbon Bond Mechanism (CB-IV). This version uses over 80 reactions and over 30 species to solve chemical kinetics, and explicitly treats the isoprene.

In what concerns the other mesoscale system, the meteorological non-hydrostatic MEMO model was applied jointly to the photochemical dispersion MARS model. Two chemical mechanisms were analyzed: EMEP, which describes the tropospheric gas-phase chemistry with 66 species, 139 photochemical reactions, and 13 categories of VOC emissions; and the KOREM mechanism, which is a simplest one, including 39 chemical reactions and 20 reactive pollutants.

Analysis of the measured values during the campaign and of the simulation results allowed characterizing the verified ozone episode. Both modeling systems presented a reasonable performance (from the meteorological point of view and also from the photochemical air pollution aspect). Concerning the chemical mechanisms preliminary conclusions point to a non-significant improvement of results associated to a more complete description of photochemical reactions.