Six low-cost air quality sensors were installed on the front roof area of three buses to enable real-time monitoring of air quality across the city of Valladolid (Spain) over a 7-month period, capturing variability in meteorological conditions and emission sources. Prior to deployment, the sensors were placed at a reference station in Barcelona (Spain) for validation and calibration. Measurements deviating by more than 30% from reference values were discarded, and correlation coefficients (R²) were calculated. After the monitoring campaign, the calibration procedure was repeated. The results (R2 = 0.85) suggest that bus-mounted sensors can effectively support real-time detection of urban air quality changes and contribute to detailed air quality mapping. Integrating data from reference air quality monitoring networks (AQMNs) with low-cost sensor (LCS) systems can strengthen evidence-based policymaking and help refine regulatory frameworks aimed at reducing urban air pollution.