Pollution events over the East Mediterranean: Synergistic use of GOME, ground-based and sonde observations and models

The behaviour of ozone (O3) and two important precursors, nitrogen dioxide (NO2) and formaldehyde (HCHO), over the East Mediterranean in spring from 1996 to 2002 is studied in order to characterise the buildup of tropospheric O3. The vertical distribution of O3 observed over Crete during the Photochemical Activity and Solar Ultraviolet Radiation (PAUR II) campaign in May 1999 has been used for validation of satellite-derived data. Retrievals of O3 columns from measurements of backscattered radiation by Global Ozone Monitoring Experiment (GOME) are compared with Total Ozone Mapping Spectrometer (TOMS), balloon, Systeme d’Analyse par Observation Zenithale (SAOZ) and LIDAR observations. The total O3 vertical columns vary between 270 and 402 DU and correlate well with changes in air circulation patterns. The total observed variability in tropospheric O3 is about 25 DU. Chemical box model calculations associate the GOME-observed NO2 and HCHO tropospheric columns with a potential of daily photochemical enhancement in the tropospheric O3 columns of about 0.8–1 DU over Crete and estimate the daily potential of regional photochemical buildup within upwind polluted air masses at about 2–8 DU. A Langrangian analysis attributes at most 10–20 DU of tropospheric O3 to stratosphere–troposphere exchange (STE). The remainder is attributed to long-range transport of O3 from industrial regions in Central Europe. From 1996 to 2002, in May no significant inter-annual variation in the tropospheric NO2 and HCHO columns over Crete has been observed by GOME suggesting no detectable increase in regionally produced tropospheric O3.