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A detailed spatial and temporal assessment of urban NH 3 levels and potential emission sources was made with passive samplers in six major Spanish cities (Barcelona, Madrid, A Coruña, Huelva, Santa Cruz de Tenerife and Valencia). Measurements were conducted during two different periods (winter–autumn and spring–summer) in each city. Barcelona showed the clearest spatial pattern, with the highest concentrations in the old city centre, an area characterised by a high population density and a dense urban architecture. The variability in NH 3 concentrations did not follow a common seasonal pattern across the different cities. The relationship of urban NH 3 with SO 2 and NO X allowed concluding on the causes responsible for the variations in NH 3 levels between measurement periods observed in Barcelona, Huelva and Madrid. However, the factors governing the variations in A Coruña, Valencia and Santa Cruz de Tenerife are still not fully understood. This study identified a broad variability in NH 3 concentrations at the city-scale, and it confirms that NH 3 sources in Spanish urban environments are vehicular traffic, biological sources (e.g. garbage containers), wastewater treatment plants, solid waste treatment plants and industry. The importance of NH 3 monitoring in urban environments relies on its role as a precursor of secondary inorganic species and therefore PM X . Further research should be addressed in order to establish criteria to develop and implement mitigation strategies for cities, and to include urban NH 3 sources in the emission inventories. [ABSTRACT FROM AUTHOR]