Air quality monitoring is essential to protect public health and support environmental policies, particularly in urban areas of developing countries. This paper presents the design and implementation of a cost-effective Mobile Internet of Thing (M-IoT) system for real-time monitoring and mapping of air quality (AQI), with a case study conducted in the downtown of San Salvador, El Salvador. The system integrates portable IoT nodes equipped with MEMS and MOx sensors capable of detecting TVOCs, CO2, and environmental parameters such as temperature and humidity. Data are collected in 10-second intervals and transmitted via GSM (2G) to a cloud-based IoT platform for processing and visualization. The system uses GIS heatmaps to provide a dynamic spatial representation of air quality throughout the urban landscape. The results of the field tests revealed good to excellent air quality in pedestrian areas, with moderate levels of pollution near high-traffic zones. The system performed reliably throughout the test, demonstrating its potential as a scalable and low-cost solution for monitoring air quality in resource-constrained environments.