Every month we collect data on air quality in Oxford as part of our statutory duty to monitor air pollution. This data is then used to produce our annual air quality status reports.
The following reports show the latest raw NO2 data figures from our diffusion tubes, before it has been verified and corrected for bias following detailed technical guidance from DEFRA.
Download our Local Air Quality Management diffusion tube network analysis reports
Download our Low Traffic Neighbourhood and Traffic Filter diffusion tube network analysis reports
How we monitor air quality data
In Oxford we currently monitor air quality levels by using diffusion tubes in 137 locations. Diffusion tubes are a passive monitoring technique that absorbs specific pollutants from ambient air (usually nitrogen dioxide (NO2). This technique can be used to calculate average pollutant concentrations over a year.
Every month, we replace exposed diffusion tubes with new ones and send the used tubes to an external accredited laboratory for monthly mean analysis and calculation.
Correcting diffusion tube data and accounting for bias
The use of diffusion tubes is a standard practice that local councils are required to follow - see Local Air Quality Management Technical Guidance. However, these can often report large over or under-estimates - also known as bias - with their initial readings.
Diffusion tube monitoring is considered an indicative monitoring technique, as its raw measurements (before any bias adjustment factor is applied) is linked to uncertainty levels of ± 25% - see Diffusion Tubes for Ambient NO2 Monitoring: Practical Guidance.
Historically, Oxford's diffusion tubes tend to overestimate readings, which is why we take bias adjustment into consideration when reviewing the annual data before publication.
We identify diffusion tube bias by conducting a local annual co-location study. This involves placing diffusion tubes in the same locations as an automatic monitoring station (which is considered the most accurate monitoring method) and comparing the results from both sources.
After comparing the two datasets, we calculate a local bias adjustment factor, which represents the adjustment that needs to be made to the raw annual mean diffusion tube data obtained for each monitoring location.
Only after this bias correction has been applied, can the diffusion tube data be considered ratified, and ready for submission to DEFRA for external appraisal and for official reporting and publication. These are then published in our annual reports.
Without doing this you cannot provide an accurate picture of air quality within an area. It is therefore important to note that raw and unverified diffusion tube data, cannot be used to make any conclusions about air quality within a certain location.
You can find more detailed information on how to interpret air quality data (including how to compare air quality measurement results against the current air quality limit values) in our air quality factsheet.
