Urban air pollution is a major problem in many cities globally. Prolonged exposure to particulate matter in the air is harmful to health. Government agencies and industries use expensive EPA-rated Air Quality Monitoring Systems (AQMS) with specialized and expensive instruments. However, the air quality data provided by agencies is based on delayed averages and is region-based. The AirQ platform is a smart and cost-effective solution for air quality monitoring. The AirQ device is portable, low-cost and provides real-time and location-specific air quality data. It supports wireless data communication technologies including WiFi, 4G, Bluetooth, LoRa, etc. We have developed an ontology-based backend to support data management and the implementation of smart data analytics algorithms. The real-time air quality data and alerts are provided by the AirQ Dashboard and Smartphone App. The AirQ platform can be used to build large-scale and high-density air quality monitoring networks for Smart Cities, with devices deployed at many locations such as schools, childcare centers, hospitals, community centers, bus stops, parks, etc.