Details of Monitoring Site QA/QC

Daily automatic calibration Zero air is generated by passing air through scrubbers and passed through the reaction cell. Span gas is generated by a permeation tube and passed to the reaction chamber to give the span calibration response. The daily automatic calibrations are used as a check on the instrument performance and drift.

Fortnightly analyser inspection and manual calibration

Manual calibrations are carried out on a fortnightly basis using zero air generators and calibration gases traceable to national standards. The analyser is taken out of service, and the inlet filter is changed prior to connecting the calibration gases. The zero air and span gases are run through the analyser and the responses noted together with the instrument gain factor. The output of the analyser is then adjusted to the correct certificated value and the new instrument gain factor noted. The calibrations are used to rescale the generated raw data from the analyser, using calculated instrument span factors. This work is carried out by the Environmental Research Group (ERG) at Imperial College, London, who also handle the data from the stations on behalf of Bexley Council. Any anomalies noted during these fortnightly calibrations are followed up with a service call out, and additional checks are made on the data for that period. TEOM filters are changed when required in accordance with the manufacturer’s instructions. Inlet heads are cleaned at this time.

Six Monthly Checks

All stations are serviced in accordance with the manufacturer’s recommendations at 6 monthly intervals. In addition, 6 monthly calibration checks are made by a third party (Ricardo) Data Ratification Measurements from analysers have to go through a number of checks before they are considered 'ratified'. The first stage of ratification occurs automatically as data is downloaded from the analysers. Data is checked by a series of protocols and then scaled using results from manual calibrations. Measurements appearing on the hourly bulletin ('current air quality') has undergone automatic checks only. The second stage occurs each day as air quality analysts manually check the data, confirm any automatic checks and flag up any faults that require attention. Measurements appearing on the daily bulletin ('Yesterday's Air Quality') and the seven-and-30-day graphs will have undergone this second stage. The third and fourth ratification stages occur in the coming months as more information becomes available. Data can be viewed over a longer time periods and results from fortnightly manual calibrations, equipment services and equipment audits can be considered. Measurements cannot be considered 'final' until all stages of the ratification process are complete. The time lag is usually between six month and a year.

PM₁₀ Monitoring Adjustment

“The Volatile Correction Model” is used to correct TEOM measurements made at the Falconwood Roadside site. All statistics and readings from this site where quoted are TEOM x 1.3.

Measurements from BX1 Slade Green, BQ6 Belvedere West and BX2 Belvedere are made using FIDAS instruments. These are reference equivalent and do not require any post measurement manipulation of the data.