1 Spatial interpolation of ambient ozone concentrations from sparse monitoring points in Belgium Hooyberghs Jef *a , Mensink Clemens a , Dumont Gerwin b , Fierens Frans b a Flemish Institute for Technological Research (VITO),Boeretang 200, B-2400 Mol, Belgium b Interregional Cell for the Environment (IRCEL), Kunstlaan 10-11, B-1210 Brussels, Belgium ABSTRACT Due to scientific interest on the one hand and political and regulatory obligations on the other hand the monitoring of ozone in the troposphere is an important issue. To this end, in Belgium as in many other countries, a fixed network of monitoring stations is operated. In order to estimate the ozone concentrations over the whole territory, a model is needed to spatially complement the sparse measurements. This paper describes the development of an interpolation scheme which is aimed at fast operational use. The model uses the population density as auxiliary data to remove a spatial trend due to titration by nitric oxide. The residuals are interpolated by kriging. As a benchmark the inverse distance weighting interpolation method is used with and without the detrending. The proposed model systematically improves the interpolation and makes a significant difference when estimating human exposure to ozone. It is generic in design, easy to implement and flexible to changes in the monitoring network. * Corresponding author: Jef Hooyberghs, VITO, Boeretang 200, B-2400 Mol, Belgium jef.hooyberghs@vito.be , fax: +32 (0)14 32 27 95 The work described in this text is published in J. Environ. Monit., 2006, 8, 1129 - 1135