There are a range of methods that can be used to monitor air pollutants. Each method has advantages and disadvantages. Regardless of the method, all data should be subject to robust quality assurance and control.


Air quality is monitored for the following reasons:

  • Public information: It allows local residents to identify current air pollution levels and air pollution trends in their local areas
  • Policy Development: It allows local authorities to identify local air pollution hotspots (i.e., places that are above the legal limit values for air pollution) and to drive local policies to address air pollution in those areas
  • Research needs: It provides useful data for the development of air quality research and for the validation of air quality models
  • Compliance: It allows regulating bodies and members of the public check if air quality standards and objectives are being achieved.
  • Air Quality Trends: It allows trends in air quality to be assessed to see if pollution levels are improving over time.
  • Validating air pollution dispersion models: It allows air quality dispersions models to be check and calibrated to ensure they provide a realistic representation of the real world.
  • National and local air quality management: It allows air quality to be regularly reviewed at both a national and local level plans to be made to manage air quality.
  • Evidence based approaches: there are various interventions that can help reduce air pollution and measuring air pollution before and after the intervention can help decide if the intervention is working or if it needs to change.


In Oxfordshire, air pollution is officially monitored and reported at about 350 locations. Three monitoring techniques are used by the local authorities in Oxfordshire to monitor air quality at those locations:


Automatic Monitoring

Automatic air quality monitoring stations (automatic monitors) produce typically hourly measurements which can be viewed by anyone in near to real-time. The data is corrected, maintained, and calibrated on a regular basis to ensure data is accurate and reliable. Automatic monitors draw in ambient air and measure the concentration of the pollutant in the sampled air. In Oxfordshire there are currently seven automatic monitoring sites, they all measure nitrogen dioxide, one also measures PM2.5).

Monitoring Station


Passive Monitoring (Diffusion Tubes)

Diffusion tubes absorb specific pollutants from ambient air, usually nitrogen dioxide. They can be used to indicate average pollutant concentrations, and therefore are useful for assessment against the long term (annual mean) UK objectives.










In Oxfordshire diffusion tubes are used for nitrogen dioxide monitoring. After the sample is collected, the diffusion tubes are transported to certified laboratories for analysis.


After receiving the results from the laboratory, diffusion tube data undergoes a correction procedure, to ensure the data is accurate and can be used for official reporting.


For any given year, the monthly results are averaged to produce the annual average for each monitoring location.  This average is then adjusted at the end of the year, using a correction factor (also called "bias adjustment factor") which is calculated by direct comparison of all the monthly means obtained by diffusion tubes for the year, with the ones obtained by an automatic monitor (which is the most accurate measurement technique) at a specific location.


Air Quality Sensors

Air quality sensors tend to be simple, small and low power devices. They can be battery powered, mains powered or powered by solar energy. They provide data with high time resolution such as 1 minute average or less. Depending on the sensors used, data management services, with regards to quality control of data, might be required to ensure the accuracy of the air quality sensors.










For official monitoring and reporting, local authorities are only allowed to use the automatic and passive monitoring methods described above.  This is because unless handled carefully, air quality sensor data can be misleading. Data quality and measurement uncertainty for sensors operating in real-world conditions is often unknown and therefore they have not been certified as a reference method for compliance measurements.


The following table provides an outline of the different advantages and disadvantages of the three monitoring methods used in Oxfordshire.


  Advantages Limitations
Automatic point monitoring
  • High resolution data
  • Data reported on air quality websites near real-time
  • Monitor pollutants 24 hours a day
  • It allows to identify daily trends and the impacts of short-term pollution episodes
  • Very expensive
  • Trained operator required
  • Regular service and maintenance required
  • Stations can be large and bulky so not suitable for every location
Passive Sampling (Diffusion Tubes)
  • Low-cost
  • Minimal operator training required
  • No power supply needed
  • It allows air pollution hot spots to be identified
  • Provides monthly averages, so cannot highlight identify high pollution episodes within the day
  • Require laboratory analysis
  • Requires annual bias adjustment which means collected data cannot be meaningfully used until the following year
  • Cannot be used to identify daily trends or short term air pollution episodes, as information on air quality levels arrive a lot later, which prevent people from acting on them
Air Quality Sensors
  • Low-cost compared to traditional air quality monitoring techniques
  • High time resolution means pollution episodes within the day can be identified
  • Require data management to ensure reasonable data quality
  • Without a comprehensive data correction and adjustment, the raw data may differ significantly from the 'true' pollution value
  • Data from sensors need to be checked against established methods such as automatic monitoring and data cannot be used for LAQM reporting