Most sensitive analytical scientific equipment is housed, calibrated and operated in laboratories under stable conditions. Not so, air quality monitors. These analysers run 24/7, in all seasons, sucking polluted urban air into their delicate spectroscopic detectors. Unsurprisingly, this has an effect on performance with the response of the analysers decreasing over time. This decrease is corrected for by regularly assessing the analyser's response against a known concentration of calibration gas. But what happens if the calibration cylinder becomes contaminated?

This question is carefully considered in a new study, published in Atmospheric Environment and co-authored by King's and the National Physical Laboratory (NPL).

King’s have worked closely with NPL for over ten years to ensure that monitoring equipment on the London Air Quality Network (LAQN) is audited and tested to the highest practical standards. NPL is the only organisation UKAS accredited to certify calibration cylinders in the field in the UK. The cylinders used to calibrate NOX analysers contain a known small fraction (about 450 parts per billion) of NO in nitrogen. The study has generated results from over 1000 NO cylinder tests, providing a unique opportunity to assess concentration changes in a large population of calibration cylinders for the first time.

The data shows that in many cylinders, a small percentage of the NO was found to have oxidised to NO2.

The study demonstrates that due to this phenomenon, in certain circumstances, this oxidation will lead to important underestimates in reported NO2 concentrations, especially at kerbside and roadside locations. EU standard methods (CEN 14211:2012) for checking sites and processing data are followed by King’s and NPL, but the consequence of fractional NO cylinder oxidation has not been fully assessed before. Instances of NO2 underestimation are likely to be widespread both historically and internationally.

The results from this study suggests that standard methods need to be improved to better account for this fractional oxidation. These would encourage the use of more stable calibration cylinders and regulator systems, and require better checks on the calibration cylinders in-service.

This study shows the importance of regular calibrations and audits and is a powerful example of how King's, working in partnership with the local authorities who form the LAQN and partner research organisations can provide important insights for the measurement, health and policy communities in the UK and beyond. We thank the boroughs of London who participate in the network for their continued support that enables research of this kind to happen.

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Item date 27/02/2015