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The Air Quality Standards (Wales) Regulations 2007

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5.  The following criteria are to be used for checking validity when aggregating data and calculating statistical parameters—

ParameterRequired proportion of valid data
(1)

In cases where all possible measured data are not available, the following factor must be used to calculate AOT40 values: AOT40 (estimate) = AOT40 measured × (total possible number of hours ÷ number of measured hourly values). The reference in this formula to the total possible number of hours is to the hours within the time period of AOT40 definition (that is, 8:00 to 20:00 Central European Time from 1 May to 31 July each year, for vegetation protection and from 1 April to 30 September each year for forest protection).

1 hour values75% (45 minutes)
8 hour values75% of values (6 hours)
Maximum daily 8 hours mean from hourly running 8 hours averages75% of the hourly running 8-hour averages (18 8 hours per day)
AOT4090% of the 1-hour values over the time period defined for calculating the AOT40 value(1)
Annual mean75% of the 1-hour values over summer (April to September) and winter (January to March, October to December) seasons separately
Number of exceedances and maximum values per month90% of the daily maximum 8 hours mean value (27 available daily values per month) 90% of the 1-hour values between 8:00 and 20:00 Central European Time
Number of exceedances and maximum values per yearFive out of six summer months over the summer season (April to September)

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