Twenty years of tropospheric ozone variability measured in the western Mediterranean basin

Abstract

Understanding of the tropospheric mid-latitude ozone trends in the free troposphere depends on several forcing mechanisms varying in different directions: change of continental emissions, Sratosphere-Troposphere Exchange (STE), upward ozone flux from the boundary layer (BL) and long range transport. The western Mediterranean is a well suited region to study the change of European emission but also the role of long range transport of other continental emissions (Ravetta et al., 2007). Moreover, Wilson et al., 2012, show a difference of ozone trend in south Europe compared to north and central Europe but south Europe is much less represented. In this work, the tropospheric ozone variability is derived from regular vertical profiles obtained using two types of instruments during the time period 1990-2010 at Observatoire de Haute Provence in Southern France (44N, 6E) which is included in the NDACC network: ECC balloon borne ozonesondes and a UV DIAL lidar. The latter provides data in the altitude range 3-14 km. A ground based UV photometer is also used since 1998. Seasonal variations in 2-km layers in the troposphere (2-8 km) for four five years periods were analysed with 3-days three dimensional backward trajectories calculated for each vertical profile during twenty years. Four source regions are identified: [Europe (lat>45N)], [Eastern Mediterranean], [Spain/Africa/North America (lat40N)]. Considering lidar and ozonesondes data seperatly we cannot identify a clear bias due to the measurement method while differences are related to the sampling conditions. Using lidar and ECC observation together, we have shown that there is a good correlation between ozone variability and changes of the transport pattern in the free troposphere, but no significant trends are observed above 3 km. In the lower troposphere, there is, however, a significant downward trend which is linked to the evolution of the planetary boundary layer height and changes in continental emissions. We will show also a joint analysis of ozone layers and the climatology of aerosol and water vapor layers. Water vapor profiles are obtained from the VAÏSALA sondes flown with the ECC sonde since 1990. The aerosol profiles are provided by the CALIPSO mission since 2006 using a trajectory approach to get a better matching between OHP observations and satellite data