Estimating long-term population exposure to ozone in urban areas of Europe

Tropospheric ozone concentrations regarded as harmful for human health are frequently encountered in Central Europe in summertime. Although ozone form ation generally results from precursors transported over long distances: in urban areas local effects, such as reactions due to nearby emission source s, play a major role in determining ozone concentrations. Europe-wide mappi ng and modeling of population exposure to high ozone concentrations is subj ect to many uncertainties: because small-scale phenomena in urban areas can significantly change ozone levels from those of the surroundings. Currentl y the integrated assess ment modeling of European ozone control strategies is done utilizing the results of large-scale models intended for estimating the rural background ozone levels. This paper presents an initial study on how much local nitrogen oxide (NOx) concentrations can explain variations between large-scale ozone model results and urban ozone measurements, on on e hand, and between urban and nearby rural measurements, on the other. The impact of urban NO, concentrations on ozone levels was derived from chemica l equations describing the ozone balance. The study investigated the applic ability of the method for improving the accuracy of modeled population expo sure, which is needed for efficient control strategy development. The metho d was tested with NO, and ozone measurements from both urban and rural area s in Switzerland and with the ozone predictions of the large-scale photoche mical model currently used in designing Europe-wide control strategies for ground-level ozone. The results suggest that urban NOx levels are a signifi cant explanatory factor in differences between urban and nearby rural ozone concentrations and that the phenomenon could be satisfactorily represented with this kind of method. Further research efforts should comprise testing of the method in more locations and analyzing the performance of more wide ly applicable ways of deriving the initial parameters. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.