Application of Long-Range Transport Models in the Framework of Control Strategies: Example of Photochemical Air Pollution

Abstract

Dispersion models, if properly designed and evaluated, can give substantial support to the development of control strategies. This holds essentially true in view of the large scale adverse effects attributed to air pollution impacts, and of the complex non-linear source-receptor relationships. As a typical example photochemical air pollution, with ozone as a major pollutant, is caused by the combined emissions of nitrogen oxide (NO_x) and volatile reactive hydrocarbons (RHC). Photochemistry during multiday episodes, which regularly occurs in northwestern Central Europe is determined by transport, chemical conversion and deposition over extended areas. In the dutch-german PHOXA project (Photochemical Oxidant and Acid Deposition Model Application in the Framework of Control Strategy Development) calculations have been performed for several photochemical episodes using the Regional Transport Model (RTM III). The RTM-III-model is an Eulerian grid-model based on the numerical solution of a 3 1/2 layer multi-component diffusion equations. The chemical mechanism used is the Carbon Bond-IV mechanism. The model evaluated for a 5 day episode in 1980 shows a reasonable agreement between measured and calculated O₃-concentrations.

Irrespective still existing insufficiencies in the emissions input data bases, sensitivity scenarios give the following preliminary indications: Hydrocarbon control would lower peak ozone concentrations everywhere in the modeling region. NO_x centro1 decreases higher ozone levels in most cases, ozone increase is also possible, however, depending on the history of an air mass which arrives at a receptor area. A combined NO_x /RHC control seems to be most favourable in reducing high ozone levels.