How do I process short-term NO2 diffusion tube monitoring survey results to obtain an annual mean NO2 concentration estimate?
In situations where a short-term diffusion tube monitoring survey has been completed in the present year, or spans across the previous and present calendar years, data will need to be adjusted to the previous year to aid the annualisation and bias adjustment processes. Note, this will apply when a monitoring survey in the current year is only partially complete and the remaining months in the current year will need to be forecast.
For the purposes of describing this FAQ, it has been assumed that monitoring was undertaken in 2018 (in this case the ‘previous year’) and 2019 (in this case the ‘present’ or ‘current year’).
It should be borne in mind that the below process does of course introduce an additional level of uncertainty in the annual mean concentration estimates, which should not be overlooked when interpreting the concentration results or the outputs from dispersion models that have been verified using monitoring data processed in this manner.
When your Collected Monitoring Data is Only in the Present Year
To make this adjustment for a roadside site, Roadside NO2 Projection Factors will need to be applied to the data to compensate for the general trend of reducing concentrations in future years. This should be specific to the year you wish to adjust to, i.e. use the ratio of the roadside NO2 projection factors for 2018 and 2019 to adjust your period mean for 2019 back to 2018. Do bear in mind that these projection factors are location and fleet composition specific.
If the monitoring location is classified as a background site, then instead of the above, the ratio of the appropriate grid square from Defra’s Background Maps for the two years of interest should be used to adjust the 2019 period data to a 2018 estimate. This compensates for the general trend of reducing background concentrations in future years.
Applying either of the above methods will derive an adjusted period mean concentration for 2018.
Following Step 1, if data capture for the adjusted period mean concentration for 2018 is lower than 75% after the above adjustments have been undertaken, annualisation of the adjusted period mean should be then be undertaken using 2018 continuous monitoring data in the usual manner following the guidance in Defra’s Technical Guidance LAQM.TG(22). An annual mean concentration estimate for 2018, i.e. the previous year, will then be obtained.
Following Step 2, a bias correction factor should then be applied to the adjusted period mean concentration for 2018.
An annual mean concentration estimate for 2018 will then be obtained.
When your Collected Monitoring Data Spans Across the Previous and Present Calendar Years
The same Step 1 process as described above can be repeated when a monitoring survey spans across the previous and present calendar years, however the roadside NO2 projection factors/background map derived ratio will only need to be applied to the portion of data collected within the latter (i.e. current) year. For example, if the monitoring period spans September 2018 to January 2019, the derived projection factor/background map ratio would only need to be applied to the data obtained during January 2019.
This adjusted data can then be combined with the monitored data collected between September to December 2018 inclusive to derive an adjusted period mean concentration for 2018.
Step 2 and Step 3, as described above, can then be applied, as required.
Monitoring of NO2, using a diffusion tube, was completed at a roadside site between July 2018 – April 2019. The road at which the diffusion tube is located experiences <10% HDVs within the overall fleet composition.
In order to derive an annual mean estimate from the monitoring results which spans across the previous and present calendar years, roadside NO2 projection factors are applied to the portion of data obtained within the current year (i.e. January – April 2019). This 2018/19 Roadside Projection Ratio is calculated by dividing the applicable 2018 roadside projection factor against the 2019 factor. This calculated ratio equals 1.36.
2018 Roadside Projection Rest of UK (HDV <10%) = 0.966
2019 Roadside Projection Rest of UK (HDV <10%) = 0.932
2018/19 Roadside Projection Ratio = 0.966 / 0.932 = 1.036
The 2018/19 Roadside Projection Ratio is then applied to the data obtained during 2019, as displayed below, in order to adjust the 2019 monthly period means back to 2018 estimates.
From the above adjusted 2018 dataset, an adjusted period mean concentration for 2018 can then be calculated. In this example, the adjusted period mean concentration for 2018 is 21.7µg/m3.
In this example, no annualisation is required as the data capture is 83%, which is greater than the 75% minimum criteria below which annualisation is required.
This adjusted period mean concentration for 2018 result should then be bias adjusted using the relevant bias adjustment factor for 2018 in the standard manner as detailed in Defra’s Technical Guidance LAQM.TG(22), i.e. apply either a locally derived or national bias adjustment factor for the appropriate preparation method and laboratory. In this worked example, a bias factor of 0.92 was applied, deriving a bias adjusted annual mean concentration estimate for 2018 of 20.0µg/m3.
The derived annual mean concentration estimate for 2018 in this worked example is therefore 20.0µg/m3.