Mapping Annual Nitrogen Dioxide Concentrations in Urban Areas

Abstract

Urban background nitrogen dioxide (NO 2 ) is measured using passive samples, exposed during several consecutive fortnights in winter and in summer. Because of unavoidable technical incidents, the total number of “annual” measurements collected is limited to a few tens, which is not sufficient for estimating precise maps. NO 2 comes mainly from the combustion of fossil hydrocarbons. Auxiliary variables like emission inventories, population density or land use, giving an approximate description of those emitters, may be entered in the mapping process as additional information. For two French cities (Mulhouse and Montpellier) with different geographic context, the relationships between seasonal NO 2 concentrations and auxiliary variables are thoroughly examined. A high correlation between seasonal concentrations is shown, as the difference of spatial structures consistency for winter and summer concentrations. The usual cross validation method brings out the interest of cokriging the annual concentration from the seasonal measurements, with auxiliary variables as external drift. This approach ensures the consistency of seasonal or yearly concentration estimations and allows a greater precision by the use of all available measurements.