14 Int. J. Environment and Pollution, Vol. 44, Nos. 1/2/3/4, 2011 Copyright © 2011 Inderscience Enterprises Ltd. Towards uncertainty mapping in air-quality modelling and assessment Bruce Denby*, Agnes Dudek and Sam Erik Walker Norwegian Institute for Air Research (NILU), P.O. Box 100, 2027 Kjeller, Norway E-mail: bruce.denby@nilu.no E-mail: Agnes.Dudek@dnv.com E-mail: sew@nilu.no *Corresponding author Ana Margarida Costa and Alexandra Monteiro CESAM & Department of Environment and Planning, University of Aveiro, 3810-193 Aveiro, Portugal E-mail: amcosta@ua.pt E-mail: alexandra@dao.ua.pt Sef van den Elshout DCMR Environmental Protection Agency Rijnmond, P.O. Box 843 3100 AV, Schiedam, The Netherlands E-mail: sef.vandenelshout@dcmr.nl Bernard Fisher Risk and Forecasting Science, Environment Agency, Kings Meadow House, Kings Meadow Road, Reading RG1 8DQ, UK E-mail: bernard.fisher@environment-agency.gov.uk Abstract: The aim of this paper is to promote the use of uncertainty mapping when spatial assessments of air quality are made. A large number of air quality maps are produced for scientific and policy purposes but rarely are corresponding maps of their uncertainty included. The need for such maps and the methods to produce them are described. Several uncertainty parameters are discussed but it is recommended to use the probability density function as the basis of the uncertainty estimates. Several examples are provided discussing indicative uncertainty, ensemble methods, comparisons with observations, spatial representativeness, uncertainty in exceedances and probability of exceedance. Keywords: air quality; assessment; modelling; uncertainty; data assimilation; Bayesian; probability; exceedance; mapping.