Volatile Correction Model

FAQ's (Frequently Asked Questions)

  • Q: Have there been any recent changes to the way the VCM works?
  • A: 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.
  • Q: Why do I have to rerun correction made before 28th April 2009?
  • A: A small error in the model equation has been identified and corrected on 28th April 2009. This error concerned the temperature and pressure correction applied to the reported TEOM concentrations. This resulted in an underestimation of the TEOMVCM concentration of approximately 4% at background locations; an analysis of the impact of this error on the 2007 AURN measurements can be found here

    Concentrations calculated using the VCM before this date should therefore be recalculated. To help with this process the correction xls spreadsheets that have been generated using the VCM Web Portal have been regenerated and can be downloaded here. Unfortunately, the details for all the corrections made are not available to us, specifically, if you made more than one correction in a visit to the web portal then only the last correction made will have been saved.

    We apologise for the inconvenience caused. This error affects the application of the VCM to TEOM measurements through the VCM web portal. It does not affected the underlying basis of the VCM and its proven equivalence to the EU reference method.
  • Q: Is the model still valid if there is only one FDMS within 130 km?
  • A: The original model development showed that the model could be considered equivalent when only one FDMS was available see First VCM Report. www.airquality.co.uk
  • My uncorrected TEOM measurements are sometimes higher than the VCM measurements. Is this correct? And if so why?
  • A: The TEOM applies a correction in its own software so that it complies with U.S. EPA regulations. This increases the 'raw' TEOM concentration by 3% then adds 3 μg m-3 (more info).

    Additionally, the TEOM reports measurements to U.S. standard temperature (25 °C) and pressure (1013 mbar) rather than the ambient conditions required by the EU; this also the effect of decreasing ‘raw’ TEOM concentrations when ambient temperature is below 25 °C (more info).

    During the application of the VCM, these software corrections are removed, hence reducing the TEOM concentration prior to the estimated mass of volatile particulate being added. If the TEOM software corrections were greater than the VCM correction, the VCM corrected TEOM will report a lower mass than the original output from your TEOM instrument. This result is therefore not unexpected and will be most prevalent when the concentration of volatile particulate is low and when the TEOM concentrations are elevated.


    If we consider an uncorrected TEOM daily mean of 50 μg m-3 made at a daily mean temperature of 20 °C and daily mean pressure of 1013 mbar; fairly typical UK summer conditions. The TEOM software corrections will increase the concentrations as follows:
    3% correction+1.5 μg m-3
    3 μg m-3 correction+3.0 μg m-3
    Reporting to ambient conditions-0.9 μg m-3
    Total+3.6 μg m-3
    Therefore, if the VCM correction is less than 3.6 μg m-3 then the TEOM will measure a higher concentration than the VCM corrected TEOM.
  • Q: Can I use the VCM to correct my PM2.5 measurement?
  • A: The VCM has not yet been proven to work for PM2.5. However, a study to examine this has been commissioned and is due to be completed in 2009. Subject to funding this could be included in the web portal.
  • Q: Can I correct my historic TEOM measurements?
  • A: FDMS measurements only began routinely in the UK at the end of 2003 as part of the LAQN with some additional measurements available from Defra’s Equivalence Programme at four sites between 2004 and 2006. UK-wide measurements only began in 2007. Therefore, if your TEOM site is within 130 km of London then a VCM correction will be available. If you are outside of that area, the geographical and temporal coverage is limited. If in doubt, please attempt a correction.
  • Q: I chose to download the spreadsheet to my computer but it didn't download?
  • A: Please check your local network and browser security settings (ask your IT department if applicable) to ensure you can download .xls (spreadsheet) files. Also check that you know where the default folder is for saved files e.g. "My Documents". The actual download time will vary depending on the size of the spreadsheet and your Internet connection speed.
  • Q: I chose to email the spreadsheet to my email address but it didn't arrive?
  • A: Please check your spam and bulk mailboxes for an email from vcm@erg.kcl.ac.uk and ensure that your mailbox security settings allow you to receive .xls (spreadsheet) files.

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