Photochemical pollution under sea breeze conditions, during summer, at the Portuguese West Coast

Photochemical pollution was studied at three sampling places located on the Portuguese West Coast along the coast-to-continent breeze direction, during the summer period. A first basic campaign (June–July 2001) covered mainly the monitoring of trace gases, volatile organic compounds (VOCs) and the vertical profiles of O3. During a second auxiliary campaign (July 2002), the vertical profiles of pollutants (O3 and VOCs) at the west coast point were further assessed. The important role of marine breezes on the formation and transport of photochemical pollutants has been confirmed.Vertical profiles of O3 revealed a depletion of ozone at altitudes lower than 100–300 m during the night and the early morning and a good vertical mixture during the day in the sea breeze boundary layer. The analysis of vertical profiles of VOCs revealed that lower concentrations tended to exist at the top of the sea breeze layer. However some compounds, such as n-hexane, showed a higher concentration at higher levels. The concentration of O3 in the diurnal boundary layer, resulting from photochemical reactions of both the anthropogenic and biogenic precursors, gradually increased with the sea breeze inland, ranging from around 30 ppb at midday on the coast, to 70 ppb in the inland rural zone. However, in all sampling places, the photochemical pollution episodes at 80–100 ppb level took place under particular conditions of high photochemical activity and see breeze circulation. As revealed from analyses of propylene equivalent concentrations, the photochemical formation of O3 in the inland rural zone is strongly affected by biogenic VOCs (isoprene and monoterpenes), whereas a major contribution to the photochemical pollution with ozone in the coastal zone issued from anthropogenic VOCs (alkenes and aromatics).