Geostatistical characterization of the nitrogen dioxide concentration in an urban area: Part I: Spatial variability and cartography of the annual concentration

Maps of the annual average of the NO₂ concentration in an urban area are generally built from temporary seasonal measurements, as in 2001 at Mulhouse (France). To improve the precision of the estimation, auxiliary information about the environment can be used. Instead of kriging the average of the two seasonal measurements (in “winter” and in “summer”), cokriging these seasonal values guarantees the consistency of seasonal and annual estimations. Even more, cokriging allows the use of “incomplete series” at the sites measured during only one season. Based on cokriging, an optimization of the sampling is studied, while alternating the seasonal measurements on a part of the sites.Using the linear model of coregionalization, the fitting of simple and cross-variograms reveals a remarkable spatial decomposition of the seasonal concentrations. The different spatial components are interpreted in terms of physico-chemical processes of production, transformation or destruction of the NO₂ in urban atmosphere.