LAQM.TG(22) provides guidance on estimating PM2.5 from PM10 measurements. For local authorities with only PM10 monitoring stations available, it is possible to estimate PM2.5 concentrations by using a local or national factor. Refer to LAQM.TG(22) for details on applying a local factor.
Estimation of PM2.5 data from PM10 data should only be used to give an indication of PM fractions where only one of the two metrics is available, or where estimates on modelled PM2.5 can be derived from modelled PM10.
Using the National Factor
Where no appropriate local automatic monitoring sites measuring both PM10 and PM2.5 are available, a nationally derived factor can be used to estimate PM2.5 concentrations. Two separate factors are calculated on an annual basis for Background and Roadside sites by analysing hourly data for all AURN sites which measure both PM10 and PM2.5 concentrations for years 2010 to current day. PMCoarse was calculated by subtracting the PM10 concentration by the PM2.5 concentration for the hours when both size fractions are measured. The calculated average PMCoarse split is then used to estimate PM2.5 concentrations by subtracting the PM10 concentration by the calculated average PMCoarse split.
It is not recommended to calculate PM2.5 from PM10 at Industrial sites due to their unique site-specific characteristics. The factors can only be applied to estimate PM2.5 concentrations at Background and Roadside sites.
Current Version of National Factor
This table will be updated annually to provide the national factor for each year following 2021.
For all years prior to 2021, please continue to use the previous methodology as specified in LAQM.TG(16), by multiplying the ratio of 0.7 to the PM10 concentration.
The following provides an example of the estimation of PM2.5 using the nationally derived correction factor. For this example, the 2021 National Factor for Roadside sites has been used. The recorded annual mean concentration at a roadside site in 2021 was 30.1μg/m3. The PM2.5 concentration at this site can be estimated as follows:
Step 1: Subtract the annual mean PM10 concentration by the nationally derived correction factor:
30.1 – 5.7 = 24.4
Step 2: Estimated annual mean PM2.5 = 24.4μg/m3