This policy brief explores the role and potential of citizen science in supporting the implementation of the Directive, with particular attention to the contribution of citizen-generated data to air quality assessment, monitoring and policymaking. Drawing on experiences from the Horizon Europe CitiObs project, the brief illustrates how citizen observatories and low-cost sensor networks can complement official monitoring systems, improve spatial coverage, identify local pollution hotspots and strengthen public participation in air quality management. Examples include De Luchtclub in Rotterdam, AtmoSud and AirCarto in France, and citizen-led industrial pollution monitoring in Kristiansand, Norway. The brief also highlights CitiObs tools and approaches, including S-MESH and FILTER, that support the integration, quality assurance and use of citizen-generated environmental data.