While the figure of ambassador is being increasingly called upon in the field of environmental health, its scope remains fuzzy and its success factors have been little studied. This article presents the results of a qualitative study performed over three years on a French citizen-sensor scheme for air quality. The scheme draws on volunteer citizens to measure fine particles by means of micro-sensors. Volunteers are also tasked with raising awareness in their entourage about environmental issues with the aim of changing people’s behaviour. We investigated this strategy and sought to identify the conditions that enable citizens to become effective clean air ambassadors. The scheme’s intervention logic was first reconstructed and a literature review of similar projects was conducted. Then, three surveys were carried out with the scheme’s volunteers. Each survey consisted of an observation of the volunteers’ practices and individual interviews (70 in all) in order to understand these citizen-sensors motivations and experiences, and characterize how they fulfilled their role. We concluded that, for citizen sensing, the scope and role of ambassador should be reconsidered insofar as these citizens serve as peer leaders and mediators. In this respect, we try to define the success factors for citizen-sensing-based ambassadors programmes.