City buses in Valladolid have become a key tool for understanding the true air quality in the city. A research project has taken advantage of the daily routes of several bus lines to install mobile sensors that measure, in real time, the concentration of the finest and most dangerous polluting particles.
This initiative, developed by centers of the Spanish National Research Council (CSIC) , has allowed the collection of more than one million air quality readings over several months. These data have been used to generate highly detailed , street-by-street maps showing how pollution changes according to the time of day, the season, and traffic intensity.
A CSIC project that uses buses as mobile laboratories
The work has been promoted by the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) and the Institute of Interdisciplinary Physics and Complex Systems (IFISC, UIB-CSIC) , within the framework of the CSIC's PTI Mobility initiative. The initial idea is simple but powerful: to take advantage of the continuous movement of urban buses to collect environmental data in areas where fixed stations do not reach.
For a period of six to seven months, the scientific team installed mobile PM2.5 fine particle sensors on three city buses in Valladolid. These vehicles traveled daily through different neighborhoods, along regular public transport routes and lines, continuously generating data while performing their normal service.
In total, the devices installed on the Valladolid bus fleet recorded more than one million air quality measurements . Each bus trip thus became a kind of mobile sampling campaign, capable of capturing very localized variations that would be much more difficult to detect with a network of fixed stations.
The project was funded through the Next4mob program of the Spanish State Research Agency , which is part of the Ministry of Science, Innovation and Universities. The scientific work has been published in the specialized journal IEEE Internet of Things Journal , which details both the methodology and the results obtained in the Castilian capital.
What are PM2.5 particles and why are they a concern in cities?
The monitoring system has focused on PM2.5, microscopic suspended particles with a diameter of 2,5 micrometers or less. These fragments are so tiny that they are invisible to the naked eye, but they can penetrate deep into the respiratory system.
These fine particles originate largely from road traffic, certain industrial activities , and various combustion processes. Their size allows them to reach the alveoli of the lungs and, in some cases, enter the bloodstream, increasing the risk of health problems in the population continuously exposed to them.
By measuring the spatial and temporal distribution of these particles in detail, researchers can better approximate the actual exposure of residents . It's not just about knowing a daily or annual average value, but about seeing how levels spike in certain streets, time periods, or under specific weather conditions.
How do the mobile sensors installed on buses work?
The devices installed in the vehicles are low-cost sensors specialized in measuring PM2.5 . Unlike traditional fixed stations, which are much more complex and expensive, these devices are compact, consume little energy, and can be easily integrated into the interior of a bus without interfering with its operation.
During daily operation, sensors continuously record the concentration of particles in the air, associating each measurement with a specific time and location. Over days and weeks, this accumulation of data allows for the creation of highly detailed maps of urban pollution , practically at the street level.
Before validating the results, the scientific team carried out a calibration and validation process against reference stations from the official atmospheric monitoring network. Following this comparison, a high degree of agreement was found between the readings from the mobile sensors and those from the much more expensive fixed equipment.
According to the researchers, despite their lower price, these sensors have demonstrated sufficient reliability to complement traditional networks with their data. Their main advantage is not so much absolute precision, but rather the ability to provide broad and dynamic spatial coverage without the need to install dozens of additional static stations.
Pollution patterns: peak hours, winter, and hotspots
Once the data collected by the buses was processed, scientists identified very clear patterns in the distribution of PM2.5 particles within the city. The behavior of the pollution is closely linked to the daily rhythm of urban mobility.
The data show that fine particulate matter concentrations increase significantly during the morning and afternoon rush hours, coinciding with peak traffic times. This pattern is consistently repeated throughout the study period, reflecting the direct impact of vehicle traffic on pollution levels.
Higher levels are also observed during the winter months . Certain atmospheric conditions, such as temperature inversions or stable air near the ground, favor the accumulation of pollutants and hinder their dispersion. This results in a sustained increase in PM2.5 levels during the colder months.
In addition to these general trends, the mobile monitoring system made it possible to locate particularly problematic geographical points . These include major intersections, high-traffic corridors, and areas near bus stops where vehicles must repeatedly brake and accelerate.
These small-scale variations, which can produce significant differences in the exposure of people passing through the same area, are difficult to capture with conventional fixed networks . The mobility of sensors mounted on buses, however, allows us to focus on them and quantify their importance.
Advantages over fixed stations and usefulness for urban policies
Traditional air quality monitoring stations offer highly accurate measurements, but only at very specific locations . Their number is limited, and therefore the picture they provide of the city is necessarily incomplete. In contrast, buses equipped with sensors form a mobile network that fills in the gaps between these stations.
Thanks to this approach, researchers have been able to create high-resolution pollution maps for Valladolid, showing how PM2.5 concentrations change along public transport routes. This information can be very useful for local authorities when planning mobility measures and traffic management.
Among the potential applications, the scientific team mentions the option of designing smarter traffic policies , adjusting routes, frequencies, or traffic restrictions based on detected pollution levels. It could also guide citizens toward less polluted travel routes , something especially relevant for pedestrians and cyclists.
Teresa Moreno, a researcher at IDAEA-CSIC and coordinator of the study, emphasizes that this method of measuring air quality is much closer to the everyday reality of urban exposure , as it provides data under real-world mobility conditions. José Ramasco, from IFISC, underlines that mobile monitoring allows for the identification of patterns that would otherwise remain hidden in the averages provided by fixed stations.
The research team insists that the most robust strategy involves integrating mobile sensor data with existing monitoring networks . This is not about replacing traditional stations, but rather complementing them with an additional layer of information that better captures the spatial and temporal diversity of pollution in the city.
Technical challenges, scalability and future of this model in Europe
Although the system has proven to be economical, flexible, and scalable , the researchers also point to a number of practical challenges that will need to be addressed for its large-scale deployment. These include equipment maintenance, occasional device failures, and interruptions in data collection when buses are out of service.
To mitigate these problems, the study proposes using more robust technical designs and redundant sensor networks . This way, the failure of a specific piece of equipment would not result in a significant loss of information, as other sensors could cover the same area or time period.
Another important aspect is the periodic calibration of low-cost sensors . Although experience in Valladolid indicates that they can provide very consistent data compared to official stations, it is necessary to regularly review and adjust their operation to avoid deviations over time.
Beyond the specific case of this Castilian city, the model presents interesting possibilities for other cities in Spain and Europe . Any municipality with a well-established public transport network could adapt this approach, equipping part of its fleet with environmental sensors and thus generating highly detailed pollution maps without the need for large investments in fixed infrastructure.
With increasing pressure to improve air quality and meet European health and environmental targets, proposals such as turning buses into mobile laboratories can play a significant role. Valladolid has served as a real-world urban laboratory in this case, demonstrating how public transport can provide valuable information for more refined, data-driven environmental management.
This project highlights that using daily bus routes to measure particles like PM2.5 allows for a much more detailed understanding of where, when, and why pollution is concentrated in a city, providing concrete tools for administrations to adjust their mobility policies, reduce citizens' exposure, and move towards healthier urban environments.
