It is not always possible to measure for a whole year to obtain an annual mean for a pollutant. Sometimes instrument faults or problems with data quality can also lead to missing data and a full year's measurements are not achieved. Defra technical guidance (TG16, Feb 18) provides a method for estimating an annual mean that should be used if available data capture is below 75%. This process is termed annualising.
The tool below allows you to do this for any LondonAir measurement site. Simply select the measurement site and period that you wish to annualise and then select between two and four nearby background measurement sites to act as a reference for the annualisation. The tool matches available measurements from the site being annualised with measurements from each background site to produce a ratio. TG16 provides the following guidance on the choice of reference sites for the annualisation,
"Identify two to four nearby, long-term, continuous monitoring sites... The data capture for each of these sites should be at least 85%. These sites should be background (Urban Background, Suburban or Rural) sites to avoid any very local effects that may occur at Urban Centre, Roadside or Kerbside sites, and should, wherever possible lie within a radius of about 50 miles."
This annualisation tool only selects background sites with at least 85% data capture for the corresponding year for use in the calculation.