Photochemical ozone production in the Eastern Mediterranean

A 7 year time series (1997–2004) of surface ozone at Finokalia, Crete, in the Eastern Mediterranean, was analysed to investigate the factors that control the diurnal variability of ozone, and evaluate seasonally distributed ozone production/destruction rates in the area. The observed diurnal evolution of related chemical/physical parameters indicates that ozone morning built-up is driven by photochemistry while during summer the entrainment from the free troposphere is the dominant process in the afternoon. The observed similar behaviour of ozone maxima and Radon-222 minima supports that entrainment from the free troposphere affects the ozone diurnal pattern. Ozone nighttime depletion is mainly attributed to deposition and to a lesser extent to chemical reactions. On an annual basis the role of local photochemistry is found to be limited (−1–1.7 ppbv d−1) contributing by less than 4% to the observed ozone levels. During summer the enhanced ozone destruction via deposition and chemistry are almost balanced by the chemical production and the entrainment of ozone rich air masses from the free troposphere that maximizes in summer (4–6% of the observed ozone levels). Chemical box model simulations also indicate low net chemical production in the area throughout the year that results from high chemical production and destruction terms. Especially during summer photochemical ozone depletion over the area is revealed both by model results and observations (0.5–1.0 ppbv d−1).