The air quality in the modern cities and urban areas is strongly affected by chemical pollutants such as toxic gases, volatile organic compounds and particulate matter. They are monitored by governmental agencies using regulatory monitoring stations which are highly accurate, but also very expensive, bulky, and maintenance demanding. There is a compulsory need to monitor air quality at high spatio-temporal resolution in smart cities for public health protection and environmental sustainability. The low-cost and low-accuracy sensors, properly calibrated, are usually deployed in stationary and mobile nodes for urban air quality monitoring. A simple indicator of the current status of urban air pollution is the Air Quality Index (AQI) used to communicate the pollution level under time-changing trend of a specific pollutant. In this study, continuous measurements have been performed in the city of Bari (Southern Italy) by electrochemical gas sensors (NO₂, O₃, CO), optical particle counter (OPC) for particulate matter (PM₁₀), NDIR infrared sensor (CO₂), including microsensors for temperature and relative humidity. The sensors have been installed in stationary nodes located in urban sites and in a mobile node mounted on a public bus moving in the urban routes. AQI data gathered by the low-cost sensors have been compared with reference instrumentations as a case-study of citizen science.