Methodology for calculating progress towards the PM2.5 targets

Question 1

1. Background

Accepted Answer

The Environmental Targets (fine particulate matter) (England) Regulations 2023 set the Population Exposure Reduction Target (PERT) and define how compliance should be assessed. Interim targets are assessed in the same way.

This description sets out the method in the Regulations in a more accessible way and provides additional detail on the process of applying it.

1.1 Data

Annual mean PM2.5 concentrations and annual data capture rates used for calculating the PERT are available within the Statistical Tables that accompany this release. The statistical tables provide further information on the monitoring stations used in the calculations.

Question 2

2. Calculating the annual mean concentration

Accepted Answer

Compliance with the AMCT is assessed as follows:

Obtain all available verified PM2.5 concentrations data in England from the AURN.
For each monitoring station, calculate the percentage of hours in the year for which there is verified data, rounded to the nearest percent. Filter out stations which do not have verified data for 85 per cent or more of the hours in the year (i.e., do not include stations that do not meet the data capture threshold).
Where stations capture both daily and hourly measurements, the hourly measurements should be used for the assessment.
Calculate the average concentration over the year for each station that met the data capture threshold, and round this to the nearest whole number of µg m^-3.
Compare these stations against the target. All stations need to have annual mean concentrations of 10 µg m^-3 or below in 2040 for the AMCT to be met.

Question 3

3. Calculating the Base Year (PEI base)

Accepted Answer

Population exposure is represented by the Population Exposure Indicator (PEI), calculated using measurements of annual mean concentration at representative monitoring stations. The PEI for the base year is calculated as follows:

Download hourly and daily PM2.5 data for England stations for the base year (2018) and two preceding years (2017 and 2016) using the UK-AIR website’s data selector tool or the openair R package (better suited for analysts). The data contains both automatic hourly measured PM2.5 (the majority) and gravimetric daily measured PM2.5 for the small number of stations which use these techniques. For the small number of stations where there are two types of monitoring instrument, one producing hourly and one producing daily measurements, the annual mean based on the automatic (hourly) dataset should be given priority and used in preference to the gravimetric (daily) value. The gravimetric value should only be included where data capture for the automatic hourly monitoring technique is less than 85 per cent.
Remove stations which are not categorised as urban background or suburban background at the year of measurement. New monitoring stations are added over time and others decommissioned, so not all stations in this list will be operational in every year. Monitoring station classifications can also change (e.g., the monitoring stations ‘Warrington’ and ‘Middlesborough’ changed classification from industrial stations to urban background stations in 2022).
Remove all monitoring stations that were not ‘open’ throughout each baseline year. For example, when considering PM2.5 concentrations in 2016, the stations used in calculations should have been fully operational throughout the whole of 2016.
For each urban and suburban background station, calculate the percentage of hours in each year for which there are verified data, round to the nearest whole number. Remove monitoring stations that do not meet the 85 per cent data capture threshold.
Calculate the annual mean concentration for each of the remaining stations using both the ‘hourly’ and ‘daily’ measured data. Do this for each of the three years.
For each year, calculate the average concentration across all of the monitoring stations.
Calculate the average concentration across each of the three years and round to two decimal places to produce the base year PEI.

Question 4

4. Calculating Yearly Increments

Accepted Answer

The yearly increments (change) in PEI are calculated for every subsequent year until the target deadline (or most recent year with validated data if assessing progress), using the following steps:

Download hourly and daily PM2.5 data for England stations for the year and three preceding years using the UK-AIR website’s data selector tool or the openair R package (better suited for analysts). The data contains both automatic hourly measured PM2.5 (the majority) and gravimetric daily measured PM2.5 for the small number of stations which use these techniques. For the small number of stations where there are two types of monitoring instrument, one producing hourly and one producing daily measurements, the annual mean based on the automatic (hourly) dataset should be given priority and used in preference to the gravimetric (daily) value. The gravimetric value should only be included where data capture for the automatic hourly monitoring technique is less than 85 per cent.
Remove stations which are not classified as urban background or suburban background for all four years.
Remove stations which were not operational throughout the whole of the four-year period.
For each remaining monitoring station, calculate the percentage of hours in each year for which there are verified data to produce the annual data capture percentage.
Filter out stations which do not meet the 85 per cent data capture threshold for three out of the four years. The remaining monitoring stations are used in the calculation of both the reference indicator (RI) and the PEI for that particular year. There will be a different RI and PEI pair for each year, calculated using a different set of monitoring stations for each year.
For each of the four years, calculate the annual average concentration from the hourly or daily data for each monitoring station. Then average the values of all monitoring stations for the year that meet the 85 per cent data capture threshold (rounded to the nearest percent). This provides an England concentration for each year, calculated using only qualifying monitoring stations.
Calculate the 3-year average for the first three years (e.g., 2016, 2017 and 2018) to obtain the RI. Then calculate the 3-year average for the latter three years (e.g., 2017, 2018, 2019) to obtain the PEI.
Calculate the difference between the RI and PEI calculated in Step 7 and round to two decimal places. This is the yearly increment in PEI in µg m^-3 (e.g., increment in PEI for 2019 equals the RI for 2019 minus the PEI for 2019).

Once the yearly increments are calculated for each year up to 2040 (or the latest year with complete, verified data if assessing progress), the following steps are taken:

Sum all the yearly increments in PEI in µg m^-3 from 2019 to 2040 (or latest year with verified data) to produce the change in sum.
Divide the change in sum by the base year’s PEI and multiply by 100 to produce the percentage change in population exposure from 2018. Round to the nearest percent.
For the PERT to be met this percentage must be 35 per cent or greater in 2040.

Annual mean concentrations and data capture percentages for each station are published on UK-AIR, rounded to the nearest percent. However, this rounding may introduce differences in the calculations, so the raw (hourly or daily) data should be used to calculate the PEI.

The following equation summarises the calculation described above, where n is the year for which the increment is being calculated and is between 2019 and 2040.