There have been two recent changes made to improve the data capture of the corrected TEOM measurements. The VCM relies on measurements of volatile PM made by nearby FDMS instruments.

Previously the 3 nearest sites within a 130 km radius from the TEOM were chosen, using an algorithm that weighted distance with % data capture and % ratified data. The mean volatile PM (or purge measurement) was then used in the VCM correction. There were occasions when all three of these instruments were not working and there was therefore no purge measurement available to apply in the correction. To eliminate as many of these periods as possible, the VCM now uses the two nearest FDMS purge measurements AND an average of all the other FDMS purge measurements within 130 km. Using the two closest FDMS instruments maintains the element of proximity in the VCM while taking the average of the remaining available measurements maximises data capture.

The VCM also relies on temperature and pressure measurements to convert the TEOM measurements, which are reported at standard temperature and pressure, back to ambient temperature and pressure. Previously, the temperature and pressure measurements were chosen using an algorithm that weighted distance from the TEOM site with % data capture and % ratified data. However, there were occasions when temperature and pressure measurements were not available from this site and the VCM correction could not be applied. To eliminate these data losses, temperature and pressure measurements from the average of sites within a 260 km radius are now used to fill the gaps where no measurements are available from the chosen site. These changes will improve the data capture of measurement generated by the VCM. They will make very little change to measurements already generated by the VCM.

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Item date 18/09/2009