Abstract
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 from 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. This may explain why statistical relationships between models and ozonesonde measurements are far less satisfactory than shown in previous studies for surface measurements at all seasons. The lowest bias between model-calculated ozone profiles and the ozonesonde 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 ozonesonde measurements and the individual models deteriorates further. At selected sites calculated contributions to ozone levels in the free troposphere from intercontinental transport are shown. Intercontinental transport is identified based on differences in model calculations with unperturbed emissions and emissions reduced by 20% by region. Intercontinental transport of ozone is finally determined based on differences in model ensemble calculations. 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 was 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.
Highlights
While local and regional emissions sources are the main cause of air pollution problems worldwide, there is increasing evidence that many air pollutants are transported on a hemispheric or global scale, see TF HTAP (2007) and references therein
Under the framework of this task force a set of coordinated multi-model studies to address hemispheric transport issues have been defined. These multi model experiments were set up to give a first assessment of the source receptor relationships between the main source regions in the Northern Hemisphere in order to contribute to the revision of the Gothenburg protocol
Model-calculated ozone profiles are compared to measurements focusing on a subset of four sites located in North America (Goose Bay, and Trinidad Head), Europe (Uccle) and Asia (Yakutsk)
Summary
While local and regional emissions sources are the main cause of air pollution problems worldwide, there is increasing evidence that many air pollutants are transported on a hemispheric or global scale, see TF HTAP (2007) and references therein. Model-calculated ozone profiles are compared to measurements focusing on a subset of four sites located in North America (Goose Bay, and Trinidad Head), Europe (Uccle) and Asia (Yakutsk). Transport events of air pollutants from Asia to the western parts of North America typically occur 1-2 times per month (Liang et al, 2004) Such export events can have substantial impacts on concentrations in the free troposphere above the downwind continent. With the use of trajectories, or tracer transport models, we can assess the likely origin and advection path of the pollutants This information can enhance our understanding of the measurements and model calculations at the sonde sites, help explain differences between model results and provides information on the predictability of ozone at different sites and height levels
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