Nearly all gaseous air pollution analysers approved for use in the UK have the capability of running an internal zero and span check each night.
Here at King’s, overnight calibration data is used extensively in both daily validation and in ratification. Overnight calibrations are vital to identify instrument response changes such as after a power cut, or inadvertently at a calibration or repair visit.
Without overnight calibrations, such response changes may not be identifiable, which leads to incorrect data scaling and in turn, the dissemination of inaccurate air quality information to the public.
In the past year, we have seen an increasing number of new gaseous analysers being installed at sites on the LAQN which do not have overnight zero and span capability. Additionally, the maintenance of overnight zeros and spans for existing instruments appears to be given lower priority than previously. This effect is leading to a reduction in assured data quality at affected sites and increases the likelihood of greater data loss in the future.
During ratification, overnight calibration information is used to assess instrument stability and identify points at which an unexpected change in response is likely to have occurred. Without these, the whole of the data back to the previous calibration may be in jeopardy. During periods of instrument drift, overnight calibrations are used to support the information from fortnightly calibrations, enabling us to assess the rate of the drift and rule out cylinder contamination as shown in Fgiure 5. In some cases, the additional information from overnight calibrations can allow us to validate measurements that might otherwise have to be excluded.
The use of overnight calibrations for gaseous analysers is recommended in the EU ambient air quality directive (2008/50/EC) which specifies the requirements of air quality measurement methods via the CEN standards (eg. NO2 and NOx: EN 14211:2005). Where possible, we aim to achieve the same quality of data on the LAQN as on the UK network which reports measurements to the European Commission. A lack of overnight calibrations reduces our ability to meet these standards.
Given this, we recommend air quality officers to enquire about the availability and support for overnight calibrations when purchasing a new analyser or renewing a maintenance contract. Although this involves an additional cost at the outset, we believe that omitting the maintenance of this function is a false economy both in terms of data capture and data quality over the analyser’s life.
If you have any further questions or would like any advice on this matter when purchasing new equipment or taking out a maintenance contract, we would be happy to discuss this further.
Item date 27/08/2015