Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 2 2006 10.5194/acpd-6-1915-2006 http://www.atmos-chem-phys-discuss.net/6/1915/2006/ http://www.atmos-chem-phys-discuss.net/6/1915/2006/acpd-6-1915-2006.html http://www.atmos-chem-phys-discuss.net/6/1915/2006/acpd-6-1915-2006.pdf 1915 1951 2006-03-21 A Lagrangian analysis of the impact of transport and transformation on the ozone stratification observed in the free troposphere during the ESCOMPTE campaign A. Colette G. Ancellet L. Menut S. R. Arnold Service d’A&eacute;ronomie/Institut Pierre-Simon Laplace, Centre National de la Recherche Scientifique, Universit&eacute; Pierre et Marie Curie, 4, place Jussieu, P.O. Box 102, 75 252 Paris Cedex 05, France Laboratoire de M&eacute;t&eacute;orologie Dynamique/Institut Pierre-Simon Laplace, Centre National de la Recherche Scientifique, Ecole Polytechnique, 91 128 Palaiseau Cedex, France Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK The ozone variability observed by tropospheric ozone lidars during the ESCOMPTE campaign is analyzed by means of a hybrid-Lagrangian modeling study. Transport processes responsible for the formation of ozone-rich layers are identified using a semi-Lagrangian analysis of mesoscale simulations to identify the planetary boundary layer (PBL) footprint in the free troposphere. High ozone concentrations are related to polluted air masses exported from the Iberian PBL. The chemical composition of air masses coming from the PBL and transported in the free troposphere is evaluated using a Lagrangian chemistry model. The initial concentrations are provided by a model of chemistry and transport. Different scenarios are tested for the initial conditions and for the impact of mixing with background air in order to perform a quantitative comparison with the lidar observations. For this meteorological situation, the characteristic mixing time is of the order of 2 to 5 days depending on the initial conditions. Ozone is produced in the free troposphere within most air masses exported from the Iberian PBL at an average rate of 0.2 ppbv h<sup>-1</sup>, with a maximum ozone production of 0.4 ppbv h<sup>-1</sup>. Transport processes from the PBL are responsible for an increase of 13.3 ppbv of ozone concentrations in the free troposphere compared to background levels; about 45% of this increase is attributed to in situ production during the transport rather than direct export of ozone.