Urban buses turn into mobile air quality labs to map pollution street by street

• The study, published in IEEE Internet of Things Journal, demonstrates that low-cost PM2.5 sensors mounted on city buses can reliably capture real-time, high-resolution air pollution patterns across urban environments.
• The research is the result of collaboration between the Instituto de Física Interdisciplinar y Sistemas Complejos (IFISC, UIB-CSIC), Institute for Environmental Assessment and Water Research (IDAEA-CSIC), and collaborating institutions in the framework of the CSIC PTI Mobility.

A new study shows that public buses can be transformed into powerful tools for monitoring air quality, offering a detailed, dynamic picture of pollution levels across entire cities. By equipping buses with low-cost particulate matter sensors, researchers have demonstrated a scalable way to track harmful concentrations with unprecedented spatial and temporal resolution.

The research team deployed sensors on three urban buses in Valladolid, Spain, over a seven-month period. These mobile devices continuously measured fine particulate matter while circulating through different neighborhoods, generating more than one million data points. After calibration and validation against reference monitoring stations, the sensors achieved strong agreement with official measurements, confirming their reliability despite their lower cost.

“Mobile sensing allows us to move beyond the limitations of fixed stations and capture how pollution actually varies across the city in real time”, explains José Ramasco, researcher at IFISC (CSIC-UIB) and lead author from IFISC of the study published in IEEE Internet of Things Journal. “This approach reveals patterns that would otherwise remain invisible”.

Unlike traditional air quality stations, which provide highly accurate but spatially sparse data, the bus-mounted sensors enabled dense mapping of pollution levels along streets and transport routes. The results uncovered clear daily and seasonal trends. PM2.5 concentrations peaked during morning and evening hours, likely linked to traffic activity, and were consistently higher in winter due to atmospheric conditions that trap pollutants near the ground.

PM2.5 refers to microscopic particles in the air with a diameter of 2.5 micrometers or smaller, making them invisible to the naked eye but particularly harmful to human health. They originate mainly from traffic, industrial activities, and combustion processes, and due to their tiny size, they can penetrate deep into the lungs and even enter the bloodstream. As a result, they are associated with respiratory and cardiovascular diseases, as well as an increased risk of premature mortality, making them a key indicator of air quality in urban environments.

Crucially, the study also identified localized pollution hotspots, particularly near busy intersections, high-traffic corridors, and bus stops where vehicles frequently accelerate and brake. These fine-scale variations are difficult to detect with stationary monitoring networks alone but are essential for understanding real human exposure to air pollution. ”Being able to pinpoint pollution hotspots at the street level opens the door to more targeted and effective mitigation strategies”, the authors note. “Cities can use this information to design smarter traffic policies or guide citizens toward cleaner commuting routes” explains Ramasco.

The researchers highlight that integrating mobile sensor data with existing air quality monitoring networks can significantly enhance decision-making. While low-cost sensors require careful calibration and maintenance, their ability to provide continuous, high-resolution coverage makes them a valuable complement to traditional systems.

The study also outlines practical challenges, including sensor maintenance, occasional device failures, and data gaps caused by bus downtime. However, these limitations can be mitigated by deploying redundant sensors and ensuring robust system design. “Our findings show that existing public transport infrastructure can double as a city-wide environmental sensing network”, the researchers conclude. “This is a cost-effective and scalable solution for building healthier and more sustainable cities” concludes Ramasco.

Image: Valladolid bus station // Lourdes Cardenal

González-de-Castro, I., Viana, M., Ramasco, J. J., Maín-Nadal, A., and Moreno, T. (2026). Air Quality Mapping Using PM2.5 Sensors on Urban Buses. IEEE Internet of Things Journal. https://doi.org/10.1109/JIOT.2026.3672932

• CSIC
• UIB