1Norwegian Meteorological Institute, Oslo, Norway
2NILU, Kjeller, Norway
3Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, NJ, USA
4National Center for Atmospheric Research, Boulder, CO, USA
5Laboratoire des Sciences du Climat et de l'Environnement, CEA/CNRS/UVSQ/IPSL, Gif-sur-Yvette, France
6Lancaster Environment Centre, Lancaster University, Lancaster, UK
7Centre for Atmospheric Science, University of Cambridge, Cambridge, UK
8European Commission, DG-Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy
9Center for research in Earth and Space Science, York University, York, Canada
10ICG-2, Forchungszentrum-Jülich, Jülich, Germany
11NASA Goddard Space Flight Center, Baltimore, MD, USA
12Grad. School of Environ. Studies, Nagoya University, Nagoya, Japan
13Office of Policy Analysis and Review, Environmental Protection Agency, Washington DC, USA
14Environment Directorate General, European Commission, Brussels, Belgium
15Central Aerological Observatory, Moscow, Russia
16Royal Meteorological Institute of Belgium (R. M. I. B.), Brussels, Belgium
17Environmental Canada, Downsview, Canada
18Central Weather Bureau, Taipei, Taiwan
19NOAA/ESRL, Boulder, CO, USA
*now at: National Institute of Water and Atmospheric Research, Lauder, New Zealand
Abstract. A multi-model study of the long-range transport of ozone and its precursors from major anthropogenic source regions was coordinated by the Task Force on Hemispheric Transport of Air Pollution (TF HTAP) under the Convention on Long-range Transboundary Air Pollution (LRTAP). Vertical profiles of ozone at 12-h intervals in year 2001 are available from twelve of the models contributing to this study and are compared here with observed profiles from ozonesondes. The contributions from each major source region are analysed for selected sondes, and this analysis is supplemented by retroplume calculations using the FLEXPART Lagrangian particle dispersion model to provide insight into the origin of ozone transport events and the cause of differences between the models and observations.
In the boundary layer ozone levels are in general strongly affected by regional sources and sinks. With a considerably longer lifetime in the free troposphere, ozone here is to a much larger extent affected by processes on a larger scale such as intercontinental transport and exchange with the stratosphere. Such individual events are difficult to trace over several days or weeks of transport. As a result statistical relationships between models and ozone sonde measurements are far less satisfactory than for surface measurements at all seasons. The lowest bias between model calculated ozone profiles and the ozone sonde measurements is seen in the winter and autumn months. Following the increase in photochemical activity in the spring and summer months the spread in model results increases and the agreement between ozone sonde measurements and the individual models deteriorates further.
At selected sites calculated contributions to ozone levels in the free troposphere from intercontinental transport are presented. Intercontinental transport is identified based on differences in model calculations with unperturbed emissions and emissions reduced by 20% by region. With emissions perturbed by 20% per region calculated intercontinental contributions to ozone in the free troposphere range from less than 1 ppb to 3 ppb, with small contributions in winter. The results are corroborated by the retroplume calculations. At several locations the seasonal contributions to ozone in the free troposphere from intercontinental transport differ from what has been shown earlier at the surface using the same dataset. The large spread in model results points to a need of further evaluation of the chemical and physical processes in order to improve the credibility of global model results.