For many years, air quality surveillance has relied on fixed reference monitoring stations: an infrastructure that is as accurate as it is limited. While their high reliability has made them the regulatory benchmark, their considerable cost has also restricted their deployment. This is a well-known contradiction. We have excellent data where stations are in place, yet vast urban and industrial areas remain without properly characterised air quality.

Rather than replacing official monitoring stations, near-reference sensor monitoring complements them by enabling the deployment of dense, well-distributed networks equipped with professional-grade sensors capable of generating traceable and technically robust data. As these systems are calibrated alongside reference stations and aligned with the CEN/TS 17660 standard, they achieve uncertainties below 25% for gases and 50% for particulate matter.

An industrial facility does not end at its fenceline. What is emitted within its boundaries (particulate matter, gases, volatile organic compounds) crosses that boundary and enters the air breathed by neighbouring communities. Yet, in most industrial facilities, this boundary is still managed through spot measurements, periodic reports and estimates based on emission factors.

Perimeter monitoring addresses this gap by deploying sensors along the outer boundaries of the facility, where fugitive emissions mix with the wind before reaching residential areas. A well-designed network not only detects what is being emitted, but also where it is coming from.

The Valdemingómez landfill site, one of Europe’s largest waste treatment facilities, had long faced a chronic problem: unpleasant odours generated by its biomethane plants and active landfill cells. These odours reached entire neighbourhoods in the south-east of the capital, where residents’ complaints were the only available signal.

The results speak for themselves: 601 complaints were recorded in 2025, the lowest figure in the entire statistical series, compared with the 4,806 registered in 2018. Managing odours without measuring them means managing blind.

Nitrogen oxides (NOx) are, in fact, a family of gases. The one released directly during combustion is nitric oxide (NO). However, as soon as it comes into contact with atmospheric oxygen and sunlight, it is transformed into nitrogen dioxide (NO₂), the reddish-brown gas that tints the skyline of cities with heavy traffic. This NO₂ reacts with water vapour to form nitric acid, one of the main contributors to acid rain. One pollutant, three different problems.

Their ability to transform rapidly is what makes NOx particularly difficult to manage. These gases do not remain where they are emitted. They travel, react and are converted into other compounds before any fixed sensor has detected them at source.