Pervasive wireless sensor networks offer many mission oriented opportunities. In this paper we evaluate the potential benefits of combining fixed and mobile sensor nodes as part of a mission-oriented, participatory sensor network deployed on an urban environment that has the objective of detecting the extent of the emission of a substance of interest (e.g. pollutant). We evaluate the system both by assuming simple mobility models for sensors and more realistic models that seek to reduce operating costs. Copyright © 2012 ACM.