| Authors | Louisa Kramer |
| Compilation date | 05 April 2024 |
| Customer | Epping Forest District Council |
| Approved by | Tom Green |
| Copyright | Ricardo |
| EULA | Ricardo Report EULA |
| Contract reference | ED16566 | Report reference | Report - Aug 22 – Sept 23 |
Between August 2022 and September 2023, on behalf of Epping Forest District Council, Ricardo deployed two South Coast Science Praxis/Urban air quality sensors at residential properties in Chipping Ongar and North Weald. The purpose of the monitoring was to understand any impact from steam and diesel locomotive operations between North Weald and Chipping Ongar on the Epping Ongar Railway. Residential back gardens within 25 metres of the stations at Chipping Ongar and North Weald respectively were chosen as locations for the sensors and they represent some of the nearest sensitive receptors to the railway. As an indicative measurement methodology, following best practice guidance, the sensors were characterised with an approved reference automatic air quality monitoring station at the start, middle and end of monitoring. For this reason, the sensors were not measuring at the locations between 17 January 2023 and 12 February 2023 and this data has been removed from the analysis. Pollutants measured were NO2, SO2, PM10 and PM2.5 and the results of the monitoring are detailed in this report.
This is a dynamic report which contains the data it displays embedded within and allows the reader a level of interaction with some of the findings. This enables a more easily navigated and streamlined report with a more engaging, intuitive reader experience.
The report is easily navigable using the floating table of contents on the left pane which tracks with the reader’s progress through the report and expands and collapses to a level of detail related to the subheadings used. The layout of the report is also dynamic, with some sections split into specific ‘tabs’ (e.g. per pollutant or per site) for ease of access to those sections.
The location of the sensor devices are shown on the map below. Further information on the monitoring sites can be obtained by clicking on the site markers on the map.
Figure 1: Location of air quality monitoring sites
As sensor measurements are an indicative measurement methodology, best practice guidance suggests that sensors are periodically characterised with nearby approved reference monitoring stations, and co-location correction factors applied. The co-location factor is applied during the ratification process for each sensor pollutant dataset.
NO2 is a gas that is mainly formed from combustion processes such as wood burning and vehicle emissions. When breathed in NO2 can irritate the throat and lungs and worsen symptoms in those with underlying respiratory issues. NO2 is also involved in the formation of photochemical smog and acid rain and may cause damage to crops and vegetation.
Particulate matter in the atmosphere has many man-made and natural sources and can vary widely in its physical and chemical composition, and size. For air quality PM is typically classified into two sizes - PM10 is the concentration of particles with an effective median diameter of 10 micrometers (μm) and PM2.5 the concentration of particles with an effective median diameter of 2.5μm. PM2.5 is sometimes known as “fine” particulate matter. PM10 can irritate the eyes, nose and throat, cause respiratory illness and deterioration in case of cardio-respiratory disease. PM2.5 is a particular concern to human health, as the particles are small enough to penetrate deep into the lungs and even the bloodstream.”
The main sources of SO2 in the atmosphere is from burning of coal or oil. SO2 can irritate the nose and throat, and cause tightening of the chest if breathed in. SO2 can also form secondary particulate matter in the atmosphere. In the UK SO2 has decreased dramatically over the past few decades, due to the phase out of coal use for energy production.
The tables below shows the legal limits for NO2, PM, and SO2, for local authorities in England as set out in the Air Quality Standards Regulations 2010. Also shown are the 2021 WHO guidelines.
The following tables present some basic pollutant statistics for the period selected in the report. Comparisons have been made with the WHO Guidelines and UK Objectives, however, it should be noted this is for information only, as the sensor units are not reference standard therefore they cannot be used for compliance of objectives. In addition, for the comparison with the annual mean limits and guidelines it should be noted that the period of averaging here is not representative of one calendar year.
The plots below show the time series of concentrations. Each pollutant is presented on a different tab and both sites are shown on each plot for comparison. The period covered is the complete data record to show trends over the medium and long term. A daily average resolution has been chosen as the most appropriate metric over a variety of different time windows. Zooming in on specific periods of the plot can be done by dragging a box over the section of the main plot frame or by using the sliders in the narrow plot frame below. To return to the default (all data) zoom level, double click the plot. Holding the mouse over the lines will highlight specific values and time stamp for that record for each station.
Figure 2: Time series plot of daily average NO2 concentration.
Figure 3: Time series plot of daily average PM10 concentration.
Figure 4: Time series plot of daily average PM2.5 concentration.
Figure 5: Time series plot of daily average SO2 concentration.
These plots show concentrations over different time intervals such as diurnal, day of week and month of year. The plot showing seasonal variation will show only the months during the period covered in this report. The topmost frame shows the concentrations as they vary by hour of the day and day of the week. The hour of the day variation is summarised on its own in the lower left pane and the variation by day of the week is shown in the lower right pane. These plots often help explain variations in concentration according to the emissions activity associated with them.
Figure 6: Time variation of hourly NO2 concentrations.
Figure 7: Time variation of hourly PM10 concentrations.
Figure 8: Time variation of hourly PM2.5 concentrations.
Figure 9: Time variation of hourly SO2 concentrations.
The plots below show daily variation in concentrations by pollutant and site (one on each tab) across the period of the report, as laid out in a calendar style. This allows intuitive viewing of day to day headline trends in the wider context of the period. The background colours shown for each day relate to the concentration. The actual concentrations, along with the wind speed, can also be seen by hovering the mouse on the cell.
Also shown is timetable when diesel and steam trains are active on the line (shaded in grey).
Figure 10: Timetable of train times
Figure 11: NO2 calendar plot: North Weald.
Figure 12: NO2 calendar plot: Chipping Ongar.
Figure 13: PM10 calendar plot: North Weald.
Figure 14: PM10 calendar plot: Chipping Ongar.
Figure 15: PM25 calendar plot: North Weald.
Figure 16: PM25 calendar plot: Chipping Ongar.
Figure 17: SO2 calendar plot: North Weald.
Figure 18: SO2 calendar plot: Chipping Ongar.
The plots below are box and whisker plots to show the distribution in concentrations for each monitoring station, during days at the weekend when trains are running and when no trains are running.
The boxes demarcate the lower quartile, median and upper quartile. The whiskers extend to the maximum and minimum values within median ± 1.5 times interquartile range (IQR). Values outside the median ± 1.5 times IQR are generally considered as outliers.
To compare the boxplots in more detail, you can use your mouse to zoom into the plots.
Figure 19: Boxplot for hourly NO2 concentrations.
Figure 20: Boxplot for daily PM10 concentrations.
Figure 21: Boxplot for daily PM2.5 concentrations.
Figure 22: Boxplot for hourly SO2 concentrations.
The map below is overlaid with a specific air quality data analysis graphic called a bivariate polar plot. The polar plots provide information on pollutant source identification, depending upon the wind speed and wind direction. Bivariate polar plots help us to understand the origin of air pollution sources in the areas where air quality is being monitored continuously, in this instance making use of the sensor monitoring at North Weald and Chipping Ongar. This is made by integrating the air quality readings that are being measured, with measurements of wind speed and wind direction. The wind speed and wind direction data is obtained from nearby Stansted Airport from the NOAA Integrated Surface Database
The vertical and horizontal axes of the polar plots give you an indication of the wind direction, in a similar way as a wind rose or a compass would, with the cardinal points N (North), S (South), W (West) and E (East) being displayed. The circular dashed rings give you an indication of the wind speed. This starts at zero (0 m/s) at the “bullseye” central point, and slowly increases outwards in all directions.
Wind Direction – Gives you a better idea of the direction where the potential air pollution source is coming from.
Wind Speed – Gives you a better understanding of where this air pollution source is located: if very close to your monitoring location or if it comes from further way.
Polar plots also display different colours. The darker red colour of the surface corresponds to higher concentrations whereas the blue colours correspond to lower pollutant concentrations.
When using the below map, please note that the region covered by the individual polar markers is not significant – the radial axes represent wind speed, not distance from the measurement site. Instead, focus on the directions from which the air associated with the highest concentrations appear to be arriving. Only data from the days when a train is running on the line between the two stations are included in the dataset analysed here.
Figure 23: Polar plot for NO2.
Figure 24: Polar plot for PM10.
Figure 25: Polar plot for PM2.5.
Figure 26: Polar plot for SO2.
Air quality sensors were deployed between August 2022 and September 2023 to better understand the effect of steam and diesel train activities on the Epping Ongar Railway. The sensors were deployed at residential properties close to the North Weald and Chipping Ongar station terminals. To characterise the sensor response and following best practice quality assurance and quality control, the sensors were co-located at a reference automatic air quality monitoring station at the start, middle and end of the monitoring period. As the sensors were removed from site for co-location between 17 January 2023 and 12 February 2023, no data between these dates was included in the analysis.
The results, highlighted in Table 4, show two exceedances in 15-minute mean SO2 concentrations at Chipping Ongar, however, this represents far fewer than the 35 exceedances permitted in the UK Air Quality Objectives for this pollutant. These two 15-minute SO2 exceedances occurred on Thursday 20 April 2023. The maximum hourly SO2 concentration of 273 µgm-3 recorded on this day is still below the UK Air Quality hourly mean SO2 objective of > 350 µgm-3.According to the Epping Ongar Railway timetable, there were no trains operating on this day, however that doesn’t rule out the possibility of operations which were not timetabled or an alternative pollution source. There were no exceedances measured in NO2, PM10 or PM2.5 at Chipping Ongar across the entirety of the monitoring period.
At the North Weald sensor, across the entire monitoring period, there were three exceedances of the 24-hour mean PM10, which is far fewer than the 35 exceedances permitted in any one year in the UK Air Quality Objectives for this pollutant. Two of these exceedances occurred on the 14-15 February 2023 when there were no trains timetabled as running. These exceedances could be attributed to a wider regional particulate pollution episode during this period. With a daily mean PM10 concentration of 51.4 µgm-3, the third exceedance was recorded on the 16 December 2022. The Epping Ongar Light Fantastic was running during this period. Across the entire period, there were no exceedances of the UK Air Quality Objectives measured in NO2, SO2 or PM2.5 at North Weald. The monitoring period means for NO2, PM10 and PM2.5 are all below the annual mean UK objectives at both the North Weald and Chipping Ongar monitoring sites.
There were numerous special events on the Epping Ongar Railway during the course of the monitoring period. Given SO2 is predominately produced from the combustion of coal, increases in this pollutant could be expected during the annual steam gala which took place between the 23-25 June 2023. The maximum measured 15-minute mean SO2 concentration of 132 µgm-3 was measured at 10:00 on 23 June 2023 at North Weald. By comparison, the highest 15-minute mean SO2 concentration during the same period at Chipping Ongar was 115 µgm-3 measured at 11:45 on 24 June 2023. These concentrations are well below the 15-minute mean SO2 objective of 266 µgm-3.
The annual autumn diesel gala between 22-24 September 2023 saw the highest concentrations of NO2 in September 2023 for both locations. This would be expected as primary NO2 emissions are particularly important from diesel combustion. The maximum hourly NO2 concentration over this period, measured at North Weald on 22 September 2023 was 88.3 µgm-3, which is well below the hourly mean NO2 objective of 200 µgm-3.
The boxplots in Section 4.5 show a direct comparison of pollutant concentrations during days at the weekend when trains were timetabled, with days when no trains are timetabled. At both locations, there is some indication median concentrations are slightly lower for PM2.5 and SO2 during weekends when no trains are running, and for PM10 at Chipping Ongar. There is also some evidence for a greater number of high concentration outliers for all pollutants on days when trains are running compared to when they are not. However, it should be noted that meteorological variations may also have some impact on the pollutant concentrations.
Finally, the polar plots in section 4.6 provide an indication of the relative strength and direction of sources of pollution on days when trains were timetabled as running. At North Weald, the directional source for all pollutants is generally to the south and west of the sensor which indicates the railway, and particularly the North Weald Station, being the likely source. Likewise, at Chipping Ongar the polar plots indicate a source to the south and east of the sensor which again points towards Ongar Station.
To conclude, this report highlights that there are measurable effects of the Epping Ongar Railway on nearby residential properties in North Weald and Chipping Ongar however despite a few individual exceedances all pollutants measured during the monitoring period, were within the UK Air Quality Objectives.
| Name | Tom Green |
| Address | Ricardo, Gemini Building, Harwell, Didcot, OX11 0QR, United Kingdom |
| Telephone | 01235 753091 |
| Tom.Green@ricardo.com |