Air Quality at Heathrow Airport 2020
Annual Report for 2020
| Authors | Sion Carpenter |
| Compilation date | 24 May 2021 |
| Customer | Heathrow Airport Ltd |
| Approved by | Nick Rand |
| Copyright | Ricardo Energy & Environment |
| EULA | http://ee.ricardo.com/cms/eula/ |
| Contract reference | ED12559 | Report reference | ED12559 Issue 1 |
Executive summary
This report provides details of air quality monitoring conducted around Heathrow Airport during 2020. The work, carried out by Ricardo Energy & Environment on behalf of Heathrow Airport Ltd (HAL), is a continuation of monitoring undertaken at Heathrow Airport since 1993. The aims of the programme are to monitor air pollution around the airport, to assess compliance with relevant national air quality objectives, and to investigate changes in air pollutant concentrations over time.
Automatic continuous monitoring was carried out at four locations on behalf of HAL, referred to as LHR2, London Harlington, Green Gates and Oaks Road. A new monitoring station, at Bath Road, was installed in November 2020, however due to power issues as a result of Covid-19 restrictions no data from this site will be presented in this report.
Data from these five continuous monitoring stations, as well as 21 other continuous monitors operated by Hillingdon, Hounslow, Slough, Spelthorne, and Defra are shared and summarised on heathrowairwatch.org.uk.
LHR2 is located on the northern apron, between the airport boundary and the northern runway (grid reference 508400 176750), London Harlington is located at the Imperial College Sports Ground (508299 177809), Green Gates is located near the north western airport perimeter (505630 176930), Oaks Road, on a residential location to the south west (505740 174500) and Bath Road on the northern perimeter of the airport (508280 176941).
All sites monitored oxides of nitrogen (nitric oxide and nitrogen dioxide) and Particulate Matter (PM10 and PM2.5). PM10 and PM2.5 data for all sites in 2020 was measured using FIDAS instruments.
Ozone measurements were undertaken at London Harlington and Black Carbon (BC) monitoring was undertaken at LHR2 and Oaks Road using aethalometer instruments.
The minimum applicable data capture target of 90% (from the European Commission Air Quality Directive) was achieved for all instruments at all stations.
The UK AQS hourly mean objective for NO2 is 200 μg m-3, with no more than 18 exceedances allowed each year. There were no excceedances of this objective at any of the sites during 2020.
The annual mean AQS objective for NO2 is 40 μg m-3, this was met at all sites.
PM10 may exceed the 24-hour mean limit of 50 μg m-3 no more than 35 times per year to meet the AQS objective. There was one exceedances at London Harlington during 2020, this AQS objective was therefore met for all HAL sites. The annual mean AQS target for PM10 is 40 μg m-3. This objective was met at all the monitoring stations.
The AQS objective for daily maximum on an 8 hour running mean is of 100 μg m-3 (not to be exceeded more than 10 days a year). Harlington exceeded the AQS objective for ozone on 20 days during 2020. This was driven by regional episodes during the spring and summer months.
Average concentrations of NO, NO2, PM10, PM2.5 and O3 at the Heathrow sites were generally comparable to those measured at urban background air pollution monitoring sites in London.
The pattern of monthly averaged concentrations throughout the year showed that concentrations of the primary pollutant NO were typically highest in the winter months. NO2, which has both primary and secondary components, showed a similar pattern. PM10 and PM2.5 showed a much less pronounced seasonal pattern, which is quite common for particulates in urban areas, with peaks in April, August and November coinciding with regional episodes. Ozone (measured at Harlington only) showed higher concentrations in the spring and summer. This is a typical seasonal pattern for ozone, which is formed from other pollutants in the presence of sunlight.
Wind speed and direction data measured at the LHR2 location were used to investigate effects on pollutant concentrations and potential sources at all four sites. Bivariate plots of pollutant concentration indicated that nearby sources, such as the perimeter road, were probably the main source of NO. There were also moderate NO concentrations at greater wind speeds from the south to west quadrant measured at LHR2. With regards to NO2, local activities appear to be the main source. There also appeared to be a contribution from the north west to south west quadrant at higher wind speeds, possibly indicating a major source further away. For both PM10 and PM2.5, high concentrations were dominated by low wind speeds and regional episodes which brought polluted air masses from the east. Bivariate plots of Black Carbon data indicate readings were higher under calmer conditions suggesting local emission sources were probably the main source.
Several high pollution episodes occurred during 2020. At all sites, particularly high concentrations of PM10 and PM2.5 were recorded around 9th April, 12th August and 5th November. Local emissions, combined with trans-boundary emissions from continental Europe, in conjunction with weather conditions are the origin of these high concentration episodes. There was also a long running period of Moderate ozone concentrations between 7th and 12th August.
In the long term, annual mean concentrations of NO appear to show a general decrease over the past decade at LHR (although there is considerable year-to-year fluctuation). The trend for NO2 overall suggests a small decrease in concentrations over the last decade. The proportion of NOx measured as NO2 has increased over the last 10, there was a decrease in 2016 which has been reversed slightly since. A slight decrease in long term trends can be seen in the PM data as a result of new analysers being installed in 2014. Annual means are generally consistent with those measured at other sites in London, excluding PM10 and PM2.5 which recorded lower annual averages than the comparison sites located in London. The long term ozone profile is still one of a slow increase in concentrations, however, this trend has grown more pronounced over the last 3 years.
Covid-19 restrictions were found to have had a significant impact on NO2 annual means with all sites recording a 34%-42% decrease in 2020 compared to 2019. The sudden drop in all activity, including ATM’s is seen in the data in March. While PM has also seen a decrease the impact of the restrictions is less obvious in the data. This supports the fact that although the airport is a contributor to local air pollutant concentrations, there appears to be little to no relationship between air traffic movements and ambient pollutant concentrations, either on a seasonal or long-term basis. This indicates that variations in ambient concentration are mainly driven by other factors (such as variations in meteorological conditions and emissions from non-airport sources such as road transport and stationary combustion processes). Air quality in the wider region can also be significantly influenced by long-range trans-boundary air pollution.
1 Introduction
1.1 Background
Heathrow Airport is traditionally the world’s busiest 2-runway international airport, handling almost 81 million passengers in 2019 though this dropped to 22.1 million in 2020 (Travel Stats, 2020) due to Covid-19 restrictions. The airport is situated approximately 12 miles to the west of Central London, but within the general urbanised area of Greater London.
Airports are potentially significant sources of many air pollutants. Aircraft jet engines emit pollutants including oxides of nitrogen (NOx), carbon monoxide (CO), oxides of sulphur (SOx), particulate matter, hydrocarbons from partially combusted fuel, and other trace compounds. There are also pollutant emissions from the airside vehicles, and from the large number of road vehicles travelling to and from the airport each day. Also, Heathrow Airport is situated in an urban area, containing many domestic, commercial and industrial sources of pollution.
Heathrow Airport Ltd therefore carries out monitoring of ambient air quality at four sites around the airport: on the northern apron near the perimeter and northern runway (LHR2), and outside the airport boundary at Harlington, Green Gates and Oaks Road.
The following pollutants are monitored at these sites:
- Oxides of nitrogen (nitric oxide (NO) and nitrogen dioxide (NO2));
- Particulate matter (PM10 and PM2.5 fractions);
- Ozone (O3) (Harlington);
- Black Carbon (BC) (LHR2 and Oaks Road).
LHR2 also records meteorological data.
Ricardo Energy & Environment was contracted by Heathrow Airport Ltd (HAL) to carry out the required programme of air pollution measurements during 2020, the 28th continuous year of monitoring, and this report presents and summarises the fully validated and quality controlled dataset for the period 1st January to 31st December 2020.
In addition to this report, HAL has daily access to provisional data from its monitoring sites via their own Heathrow Airwatch website (Heathrow Airwatch, 2020) and data from the UK’s national air quality monitoring network, through the Defra UK Air Information Resource (UK-AIR) (“UK-AIR” 2019).
Data in the annual report have been processed according to the rigorous quality assurance and quality control procedures used by Ricardo Energy & Environment. These ensure the data are reliable, accurate and traceable to UK national measurement standards.
1.2 Aims and objectives
The aim of this monitoring programme is to monitor concentrations of several important air pollutants around the airport. The results of the monitoring are used to assess whether applicable national air quality objectives have been met, and how pollutant concentrations in the area have changed over time. Additionally, meteorological data were used to investigate the importance of various sources of pollution.
It is important to note that the pollutants measured in this study will have originated from a wide variety of sources, both local and long range. Not all of these sources will be directly connected with the airport.
Monitoring data collected at Heathrow are compared in this report with:
- Relevant UK air quality limit values and objectives.
- Corresponding results from a selection of national air pollution monitoring sites.
- Statistics related to airport activity.
In addition, periods of relatively high pollutant concentrations are examined in more detail.
1.3 UK Air Quality Strategy
Within the European Union, controls on ambient air quality are covered by Directive 2008/50/EC (“Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on Ambient Air Quality and Cleaner Air for Europe” 2008), and its update EU Directive 2015/1480 (“COMMISSION DIRECTIVE (EU) 2015/ 1480 - of 28 August 2015 - Amending Several Annexes to Directives 2004/ 107/ EC and 2008/ 50/ EC of the European Parliament and of the Council Laying down the Rules Concerning Reference Methods, Data Validation and Location of Sampling Points for the Assessment of Ambient Air Quality (Text with EEA Relevance)” 2015), known as the Air Quality Directive. This consolidated three previously existing Directives, which set limit values for a range of air pollutants with known health impacts. The original Directives were transposed into UK law through The Environment Act 1995 which placed a requirement on the Secretary of State for the Environment to produce a national Air Quality Strategy (AQS) containing standards, objectives and measures for improving ambient air quality.
The Environment Act 1995 also introduced the system of local air quality management (LAQM). This requires local authorities to review and assess air quality in their areas against the national air quality objectives. Where any objective is unlikely to be met by the relevant deadline, the local authority must designate an air quality management area (AQMA). Local authorities then have a duty to carry out further assessments within any AQMAs and draw up an action plan specifying the measures to be carried out, and the timescales, to achieve the air quality objectives. The legal framework given in the Environment Act has been adopted in the UK through the UK AQS. The most recent version of the AQS was published by Defra in 2007, and the currently applicable air quality objectives are summarised in Appendix 1 of this report. Figure 1 shows a map of Hillingdon AQMA.
Figure 1: Map of Hillingdon AQMA
2 Air quality monitoring
2.1 Pollutants Monitored
2.1.1 Nitrogen Oxides (NOx)
Combustion processes emit a mixture of oxides of nitrogen, NO and NO2 - collectively termed NOx.
NO is described as a primary pollutant (meaning it is directly emitted from source). NO is not known to have any harmful effects on human health at ambient concentrations. However, it undergoes oxidation in the atmosphere to form the secondary pollutant NO2.
NO2 has a primary (directly emitted) component and a secondary component, formed by oxidation of NO. NO2 is a respiratory irritant and is toxic at high concentrations. It is also involved in the formation of photochemical smog and acid rain and may cause damage to crops and vegetation.
Of the NOx emissions (including NO2) considered to be airport-related, over 50 % arise from aircraft during take-off and landing, with around two-thirds of all emissions occurring at some distance from airport ground-level.
Based on 2019 calendar year emissions data from the 2021 submission of National Atmospheric Emissions Inventory (NAEI) data to the EU, civil aircraft taking off and landing (up to a height of 1000m) was estimated to contribute 1.5% to the total reported UK emissions of NOx. The Air Quality Expert Group (AQEG) (Air Quality Expert Group, 2004) has stated that:
Around a third of all NOx emissions from the aircraft (including ground-level emissions from auxiliary power units, engine testing etc., as well as take-off and landing) occur below 100 m in height. The remaining two-thirds occur between 100 m and 1000 m and contribute little to ground-level concentrations. Receptor modelling studies show the impact of airport activities on ground-level NO2 concentrations. Studies have shown that although emissions associated with road traffic are smaller than those associated with aircraft, their impact on population exposure at locations around the airport are larger.
Previous rounds of review and assessment within the LAQM process have not highlighted any cases where airports appear to have caused exceedances of air quality objectives for particulate matter measured as PM10. Therefore, in the context of LAQM, the key pollutant of concern from airports is NO2. Local authorities whose areas contain airports with over 10 million passengers per annum must take these into account in their annual review and assessment of air quality.
2.1.2 Particulate matter
Airborne particulate matter varies widely in its physical and chemical composition, source and particle size. The terms PM10 and PM2.5 are used to describe particles with an effective size with a median aerodynamic diameter of 10 and 2.5 nm respectively. These are of greatest concern with regard to human health, as they are small enough to penetrate deep into the lungs. They can cause inflammation and a worsening of the condition of people with heart and lung diseases. In addition, they may carry surface absorbed carcinogenic compounds into the lungs. Larger particles, meanwhile, are not readily inhaled, and are removed relatively efficiently from the air by sedimentation.
The main sources of airborne particulate matter in the UK are combustion (industrial, commercial and residential fuel use). The next most significant source is road vehicle emissions. Based on 2019 NAEI data, less than 0.1% of UK total PM10 emissions are believed to originate from civil aircraft taking off and landing.
Previous rounds of review and assessment within the LAQM process have not highlighted any cases where airports appear to have caused exceedances of air quality objectives for particulate matter measured as PM10.
2.1.3 Ozone (O3)
Ozone (O3) is not emitted directly into the atmosphere in significant quantities, but is a secondary pollutant produced by reaction between nitrogen dioxide (NO2) and hydrocarbons, in the presence of sunlight. Whereas nitrogen dioxide (NO2) contributes to ozone formation, nitrogen oxide (NO) destroys ozone and therefore acts as a local sink. For this reason, ozone levels are not as high in urban areas (where NO is emitted from vehicles) as in rural areas. Ozone levels are usually highest in rural areas, particularly in hot, still, sunny weather conditions giving rise to “summer smog.”
2.1.4 Black Carbon (BC)
Black Carbon (BC) is the strongest light-absorbing component of particulate matter. It is a primary aerosol, emitted directly at the source, as a result of incomplete combustion of fossil fuels (automobile exhaust, industrial and power plant exhaust, aircraft emissions, etc.) and biomass burning (burning of agricultural wastes, forest fires). Therefore, much of atmospheric BC is of anthropogenic origin. Exposure to BC is of great concern with regard to human health due to its small size, typically finer than PM2.5. It has been linked to health impacts such as cardiopulmonary morbidity and mortality, cancer and respiratory diseases. Reductions in exposure to particles containing BC will consequently reduce such adverse health impacts.
2.2 Monitoring sites and methods
Automatic monitoring for a full year was carried out at four sites during 2019. These are referred to as LHR2, London Harlington, Green Gates and Oaks Road. The location descriptions of the sites fall into the category “other” as defined by the Defra Technical Guidance on air quality monitoring LAQM.TG(09) (“Local Air Quality Management - Technical Guidance LAQM.TG (16)” 2016), (i.e. any special source-oriented or location category covering monitoring undertaken in relation to specific emission sources such as power stations, car-parks, airports or tunnels).
The pollutants that were monitored at each monitoring site are shown in Table 1. The LHR2 site has been in operation since 1993; the Harlington site commenced in 2003. The Green Gates and Oaks Road sites were originally set up for monitoring in connection with the Terminal 5 Construction Impact Assessment in 2001, but were retained at the conclusion of this project, as part of the ongoing monitoring programme from 2007 onwards. A fifth site was installed in November 2019, on Bath Road, however due to Covid-19 restrictions the McDonalds site which provides power to the hut has only been open intermittently. In order to prevent instrument damage due to being repeatedly powered on and off, and after discussion with Heathrow, the site instruments have been turned off until uninterupted electricity can be supplied. Figure 2 shows a map of the locations of all monitoring sites used in this study. The map can be zoomed in and out and more information on the monitoring sites can be obtained from clicking on the marker.
Figure 2: Locations of the Heathrow air monitoring sites
Figure 3 shows the LHR2 monitoring site. This is located on an area of the old apron between the northern runway and the northern perimeter road, 14.5 m from the kerb and 179 m from the runway centre. The prevailing wind direction is from the south west and hence this site, situated to the north east of the airport, was selected to monitor air pollutants arising from the airport area. The EU limit values and AQS objectives only apply to locations where public exposure may occur. As LHR2 is located within the airport perimeter, where members of the public do not have access, these limits do not apply.
Figure 3: Heathrow LHR2 air quality monitoring site
Figure 4 shows the Harlington site. This was established to measure air pollution concentrations in residential areas close to the airport. The site is located in the grounds of the Imperial College Sports Ground, approximately 1 km north of LHR2 and 300 m from the western edge of Harlington. Since 1st January 2004, the site has been part of the Defra Automatic Urban and Rural Network (AURN), and meets the Air Quality Directive siting criteria. Because the site is part of the national network, it is classified according to the site types defined in the Air Quality Directive: its classification of Urban Industrial reflects the presence of the airport.
Figure 4: London Harlington air quality monitoring site
Figure 5 shows the Green Gates site. This site is close to Bath Road, which runs along the northern perimeter of the airport.
Figure 5: Green Gates air quality monitoring site
Figure 6 shows the Heathrow Oaks Road site. This site is located in a residential area near to the south western boundary of the airport and is classified as an urban industrial site. Both Green Gates and Oaks Road meet the Directive criteria for urban industrial sites.
Figure 6: Oaks Road air quality monitoring site
Figure 7 shows the Heathrow Bath Road site. This is a new site, installed in November 2019. The site is located on Bath Road on the northern perimeter of the airport and is classed as a roadside site.
Figure 7: Bath Road air quality monitoring site
2.3 Automatic monitoring
The following techniques were used for the automatic monitoring of NOx (i.e. NO and NO2), PM, O3 and Black Carbon (BC):
- PM10 and PM2.5 - Fine Dust Analysis Systems (FIDAS);
- NO, NO2 - Chemiluminescence;
- O3 - UV absorption analyser;
- BC - Aethalometer.
Further information on these techniques is provided in Appendix 2 of this report. These analysers provide a continuous output, proportional to the pollutant concentration. This output is recorded and stored every 10 seconds, and averaged to 15-minute mean values by internal data loggers. The analysers are connected to a modem and interrogated through a GPRS internet device to download the data to Ricardo Energy & Environment. Data are downloaded hourly. The data are converted to concentration units at Ricardo Energy & Environment then averaged to hourly mean concentrations.
3 Quality assurance and data capture
3.1 Quality assurance and Quality control
In line with current operational procedures within the Defra Automatic Urban and Rural Network (AURN)(“QA/QC Procedures for the UK Automatic and Urban Rural Air Quality Monitoring Network (AURN)” 2009), full intercalibration audits of the HAL air quality monitoring sites take place at six-monthly intervals with audit of the ozone analyser every 3 months. In addition all analysers are serviced every 6 months at which time an inlet clean is also undertaken. Full details of these UKAS-accredited calibrations, together with data validation and ratification procedures, are given in Appendix 3 of this report. In addition to instrument and calibration standard checking, the air intake sampling systems were cleaned and all other aspects of site infrastructure were checked.
Following the instrument and calibration gas checking, and the subsequent scaling and ratification of the data, the overall accuracy and precision figures for the pollutants monitored at Heathrow are summarised in Table 2.
4 Results and discussion
4.1 Summary statistics
Overall data capture statistics along with summary statistics for the five monitoring sites are provided in Tables 3 to 7 below. The data capture statistic represents the percentage of valid data measured for the whole reporting period. A data capture target of 90% is recommended in the European Commission Air Quality Directive4 and Defra Technical Guidance is 85% LAQM.TG (16) (“Local Air Quality Management - Technical Guidance LAQM.TG (16)” 2016). This is particularly important at Harlington, as data from this site feed into the Automatic Urban and Rural Network (AURN), the UK’s main network used for compliance reporting against the Ambient Air Quality Directives.
In 2020, data capture for all pollutants at all 4 sites attained data capture greater than 90%. Table 8 highlights the details of any significant data gaps.
NO2
PM2.5
PM10
O3
Black Carbon
Data Gaps
Significant data gaps for periods > 24h for the stations are shown in Table 8.
4.2 Time series plot
Below are time series plots of concentrations of pollutants at the five sites. The particulate plots present daily mean concentrations whilst for NO2 the plot is presented as hourly concentrations and for ozone as hourly rolling 8-hour mean concentrations. There is one tab per pollutant with all data from the five sites displayed on the chart. Hovering the cursor over the graph will highlight the trace for each monitoring site. It is possible to zoom in on a section of the graph using the sliders below the chart.
All sites show the highest peak of PM10 and PM2.5 in April with significant high periods also seen in January, August and November. Hourly NO2 was highest in January to March before Covid-19 restrictions came into force, but along with Black Carbon does show a general seasonal pattern of higher winter levels and low levels during the summer months. Ozone also shows a typical seasonal pattern with the highest concentrations recorded in the summer months. These patterns and peaks are discussed further below.
NO2 hourly
PM2.5
PM10
O3 rolling 8-hour mean
Black Carbon
4.3 Rolling annual mean
Figures 7 to 9 present the monthly rolling annual mean for NO2, PM10 and PM2.5. Rolling annual mean measurements from nearby LA stations are included for additional information.
NO2
Figure 8: NO2 12-month rolling mean for the Heathrow sites from 2008.
PM2.5
Figure 9: PM2.5 12-month rolling mean for the Heathrow sites from 2008.
PM10
Figure 10: PM10 12-month rolling mean for the Heathrow sites from 2008.
4.4 AQ index distribution
The plots below illustrate the distribution of AQ index values for each site by pollutant. The plots show the number of days that each site reported concentrations are in each index. The plots show the number of days that each site reported concentrations in each index. More information on the AQ Index is available in Appendix 1 and from UK-Air. The NO2 AQI is based on hourly mean NO2 concentrations and ozone on the running 8-hourly mean. PM10 and PM2.5 AQIs are based on the daily mean.
During 2020, no site recorded any days with the AQI for NO2 higher than the Low banding. London Harlington recorded a single day of PM10 and PM2.5 in the Moderate banding, Oaks Road and LHR2 both recoded a single day of PM2.5 in the Moderate band. Green Gates did not record any days in the Moderate band for any pollutant. Ozone, measured only at London Harlington, recorded 20 days of Moderate pollution.
NO2
Figure 11: Distribution of AQI for NO2.
PM2.5
Figure 12: Distribution of AQI for PM2.5.
PM10
Figure 13: Distribution of AQI for PM10.
O3
Figure 14: Distribution of AQI for O3.
4.5 Comparison with air quality objectives
The details of UK air quality standards and objectives specified by Defra are provided in Appendix 1.
During 2020 there was an exceedance of the O3 8-hour rolling mean limits specified by Defra.
The annual mean AQS objective for NO2 is 40 μg m-3. This was met at all four monitoring stations with annual means of 19.2 μg m-3, 25.2 μg m-3, 16.8 μg m-3 and 20.1 μg m-3 for Green Gates, LHR2, Oaks Road and London Harlington respectively.
The AQS objective for hourly mean NO2 concentration is 200 μg m-3 which may be exceeded up to 18 times per calendar year. There were no hourly mean NO2 measurements exceeding 200 μg m-3 at any of the sites during 2020.
The short term AQS objective for PM10 is a maximum of 50 μg m-3 for 24h mean periods, not to be exceeded more than 35 times a year. All sites were well within the yearly maximum permitted number of exceedances of 35, thus all meeting the AQS objective for 24 hour mean PM10. However, there was one exceedance of the 50 μg m-3 24h mean value at London Harlington with a concentration of 50.8 μg m-3. The annual mean AQS objective for PM10 is 40 μg m-3. All sites measured average annual values ranging between 11 and 14 μg m-3, this objective was therefore met.
While no AQS objective exists for PM2.5, there is an annual mean objective of 25 μg m-3, although this is a non-mandatory compliance target to be met by 2020. The annual mean for this pollutant for all monitoring locations was between 6 and 8 μg m-3. This is less than one third of the average concentration target limit for 2020.
O3 was measured at Harlington only. The AQS objective for daily maximum on an 8 hour running mean is of 100 μg m-3 (not to be exceeded more than 10 days a year). Harlington exceeded the AQS objective for ozone 158 times over 20 days during 2020. Of these, 8 days occurred between 7th August and 13th August. These exceedances were common at monitoring sites across background and rural sites in the south east and are discussed further in Section 4.6.
Black Carbon was measured at LHR2 and Oaks Road. The highest hourly mean registered was 16.9 μg m-3 and 15.3 μg m-3 for LHR2 and Oaks Road respectively. The UK Government does not have specific policies to address black carbon and other short lived climate forcers, and therefore, no comparison to a limit can be made. As a proportion of particulate matter is black carbon, action to reduce particle emissions will reduce this pollutant.
4.6 Time variation plot
Figure 15 to Figure 19 below show the variation of monthly, weekly, daily and hourly pollutant concentrations during 2020 at Heathrow Green Gates, Heathrow LHR2, Heathrow Oaks Road, and London Harlington.
Seasonal variation
Seasonal variations can be observed in the ‘month’ plots of Figures 15 - 19. Elevated concentrations were registered for PM10 , Figure 17, and PM2.5, Figure 16, at all four sites in April. The April peaks were driven by the two episodes with the highest concentrations during 2020 which are discussed in more detail below.
NO2 concentrations, shown in Figure 15, at all sites follow the same seasonal cycle, with low concentrations observed in summer and high concentrations in winter. This seasonal cycle is typical for urban areas when highest levels of primary pollutants tend to occur in the winter months, when emissions may be higher, and periods of cold, still weather reduce pollutant dispersion. This year the seasonal cycle has been made more pronounced by the introduction of Covid-19 lockdown restrictions from April onwards.
O3 concentrations measured at Harlington, Figure 18, continue to follow a typical seasonal variation for this pollutant, with higher concentrations being measured during the summer months. At low/mid latitudes, high O3 concentrations are generally observed during late spring and/or summer months. This is partly due to predominant anti cyclonic conditions (characterized by warm and dry weather systems) which increase the number of photochemical reactions in the atmosphere, responsible for the increase of ground level ozone production. In addition, the convective fluxes created during hot summer days can also be responsible for an increase of O3 (stratospheric intrusion). The hot air generated at ground level due to high temperatures is lighter and tends to ascend, being replaced by colder stratospheric air masses coming from above, dragging stratospheric O3 to ground level as a consequence.
BC data, shown in Figure 19, was recorded at LHR2 and Oaks Road sites. The seasonal variation of this pollutant shows in general elevated levels of BC during the winter months. BC is directly related with the incomplete combustion of fossil fuels, it’s likely that during winter and colder periods fuel emissions associated with heating and reduced pollutant dispersion might be the main causes of elevated concentrations of this pollutant.
Diurnal variation
The diurnal variation analyses viewed in the ‘hour’ plots below show typical urban area daily patterns for NO2 at all sites in Figure 15. Pronounced peaks can be seen for these pollutants during the mornings, corresponding to rush hour traffic at around 07:00. Concentrations tend to decrease during the middle of the day, with a broader evening road traffic rush-hour peak building up from early afternoon.
O3 concentrations in Figure 18 always increase during daylight hours due to the photochemical reactions of NO2, VOCs and CO. In the evening and overnight, O3 gets consumed by a fast reaction with NO (NO titration). The absence of sunlight prevents the photolysis of the O3 precursors and formation of ozone.
The diurnal patterns for PM10 and PM2.5 are determined by two main factors and are shown in Figures 16 and 17. The first is emissions of primary particulate matter, from sources such as vehicles. The second factor is the reaction that occurs between sulphur dioxide, NOx and other chemical species, forming secondary sulphate, nitrate and other particles. Evidence of some morning and afternoon road traffic rush-hour peaks for PM10 and PM2.5 can be seen at all four sites, but these were less pronounced than those for oxides of nitrogen. Oaks raod shows a big spike around midday, this was caused by a single very large spike in the data. This was investigated during ratification and could not be discounted due to instrument error and so is considered a real locla event.
BC diurnal variation, Figure 19, appears to follow the same trend pattern of NO2 with two peaks measured at the same periods (07:00 and 20:00) suggesting a strong primary component from vehicle exhaust.
Weekly variation
The analyses of the weekly variation for NO2, PM, BC and O3 show that a similar type of diurnal pattern occurs for all the days of the week. NO2 and BC early morning and late afternoon rush hour peaks are in general much more pronounced during the working week. O3 concentrations are highest on Saturday and Sundays as a result of a decrease in ozone precursors (NOx and VOCs).
NO2
Figure 15: Temporal variation NO2.
PM2.5
Figure 16: Temporal variation PM2.5.
PM10
Figure 17: Temporal variation PM10.
O3
Figure 18: Temporal variation O3.
BC
Figure 19: Temporal variation BC.
4.7 Source investigation
In order to investigate the possible sources of air pollution being monitored around Heathrow Airport, meteorological data measured at LHR2 was used to add a directional component to the air pollutant concentrations.
Figure 20 to Figure 27 show bivariate plots, ‘’pollution roses’’ of hourly mean pollutant concentrations against the corresponding wind speed and wind direction. These plots should be interpreted as follows:
The wind direction is indicated as in the wind rose above (north, south, east and west are indicated).
The wind speed is indicated by the distance from the centre of the plot: the concentric circles indicate wind speeds in 5 ms-1 intervals.
The pollutant concentration is indicated by the colour.
These plots therefore show how pollutant concentration varies with wind direction and wind speed.
NO2
NO2:Figure 20 shows the main source of NO2 at Oaks Road, Green Gates and London Harlington are close to the monitoring site, with the highest concentrations occurring at low wind speeds. Such conditions will have allowed NO2 emitted from nearby sources (vehicles from nearby roads and within the hotel car parks) to build up, reaching high concentrations. LHR2 however, while showing a local source, recorded higher emmisions at moderate to high wind speeds from the north west down to south. This can then be broken down into several sections. Winds from the west brought pollutants from the nearest roads (Bath Road and Northern Perimeter Road and associted junctions) in the more southernly directions the airports departures and arrivals area along with the Central Terminal Area (CTA) can be found.
At higher wind speeds, the airport activities look to be the main source at Oaks Road and Green Gates with elevated concentrations occurring at low and moderate wind speeds from a north easterly and south easterly direction respectively. These might be the result of activities around the airport terminal buildings. Part of this NO2 may also be created by the reaction between airport emissions of NO with ozone, travelling at increased wind speeds to create a faster reaction.
London Harlington is the site furthest from the airport and is situated near the urban area of Harlington. As is to be expected the main source of NO2 occures close by. At moderate to high winds however further sources are seen to the south, with potential sources being the airport and Bath road, and south west where the Heathrow Airport Spur road comes down from the M4.
Figure 20: NO2 Polar Plot for Heathrow sites
NO
NO:Figure 21 shows that NO concentrations are more heavily influenced by local emissions sources. At Oaks Road, Green Gates and London Harlington almost all higher concentrations are associated with emission of pollutants located close to the site. This will be traffic emissions from the perimeter and other roads. LHR2 shows an additional signature from the west to south west indicting some emissions from the airport and both Bath Road and the Northern Perimter Road.
Figure 21: NO Polar Plot for Heathrow sites
PM2.5
PM10 and PM2.5:Figure 22 and Figure 23 show that the sites have very similar, almost identical, plots with high concentrations occuring under calm conditions or when the wind is from a north easterly direction. This is in contrast to the NO and NO2 plots and is mostly due to the impact of transbounday pollution events.
To investigate further Figure 24 and Figure 25 present the variation in concentrations as hourly averages plotted by month. This shows that the highest concentrations were measured in April, August and November coinciding with the regional episodes as discussed in Section 4.6. The polar plots are therefore picking up these regional eposides hence the plots are all very similar in nature. –>
Figure 22: PM2.5 Polar Plot for Heathrow sites
Figure 24: Average PM2.5 trends by month by site
PM10
PM10 and PM2.5:Figure 22 and Figure 23 show that the sites have very similar, almost identical, plots with high concentrations occuring when the wind is from an easterly direction. This is in contrast to the NO and NO2 plots and is mostly due to the impact of transbounday pollution events.
To investigate further Figure 24 and Figure 25 present the variation in concentrations as hourly averages plotted by month. This shows that the highest concentrations were measured in April and November coinciding with the regional episodes as discussed in Section 4.6. The polar plots are therefore picking up these regional eposides hence the plots are all very similar in nature.
Figure 23: PM10 Polar Plot for Heathrow sites
Figure 25: Average PM10 trends by month by site
BC
BC: The plots for black carbon show that both sites have registered the highest BC concentrations when wind speed was low, which suggests that the major sources of BC are local, likely local fuel combustion from residential,and local traffic sources. The yellow signature from the west to south quadrant at the LHR2 site may be contribution from the biomass boiler at Terminal 2.
Figure 26: BC Polar Plot for Heathrow sites
O3
Ozone: The pattern for ozone is similar to previous years. Lower ozone levels occur at low wind speeds, which shows that ozone was being scavenged by local emissions, most likely the local traffic sources. High levels of NO caused by the combustion of fossil fuels tend to react rapidly with O3 to produce NO2 (destruction of ozone by titration with NO). O3 levels tend to be higher at high wind speeds, where the effect of local NO emissions is not so well pronounced. The highest ozone concentrations seem to come from the East and west, for wind speeds above 5 ms-1.
Figure 27: O3 Polar Plot for Heathrow sites
4.8 Periods of elevated pollutant concentration
This section reviews the most significant periods of high air pollution concentrations for the whole year. It is important to stress that, despite there being some periods when pollutant concentrations exceeded the applicable air quality objectives, these were attributable to specific external sources. Analysis of episodes are sourced from the London Air Quality Network (Environmental Research Group, 2019) and DAQI maps from UK Air (“UK-AIR” 2019). The Air Quality Index presented at the Department of Environment, Food & Rural Affairs (Defra) UK-AIR website. calculates air quality index bands that go from 1 (Low) to 10 (Very High). Several elevated pollution episodes were recorded at Heathrow during 2020.
There was widespread moderate and high particulate pollution recorded across England and Wales between Wednesday 8th and Sunday the 12th of April in 2020. This episode is also evident as the highest daily mean particulate concentration recorded at Heathrow during 2020. The high PM was primeraly caused by long-range transport of pollution from continental Europe.
Figure 28: DAQI for 9th April 2020
Between Saturday 8th and Thursday 13th August the southern half of the UK saw a prolonged heat wave with temperatures over 30 degrees for several days. This led to wide spread moderate and high pollution levels of Ozone, culminating in the higest Ozone measurements of 2020 at the the London Harlington site. At the same time a combination of imported and local emmission saw little dispersion caused widespread moderate pollution levels.
Figure 29: DAQI for 12th August 2020
Dispite no official Guy Fawkes displays taking place due to Covid-19 restrictions moderate/ high/ very high pollution levels were seen betwen Thursday 5th and Sunday 8th November. This was caused by calm and foggy conditions reducing pollutant dispersion from bonfires and fireworks.
Figure 30: DAQI for 6th November 2020
4.9 Long term changes in pollutant concentrations
LHR2 has been in operation for 28 years (following installation in 1993). Three of the other four sites have all been in operation since 2003 or earlier. There is now a considerable amount of data which can be used to assess how pollutant concentrations have changed over this period. Annual mean concentrations of NOx, NO, NO2, PM10, PM2.5, and O3 are illustrated In Figures 31 to 36 below. BC measurements only started in 2014. The amount of data is still considered not to be enough for this type of analyses, and therefore the BC time series for black carbon annual mean is not presented on this report. Annual means are only shown for years in which data capture was at least 75%. Also shown is the mean result from an average of up to seven urban non-roadside monitoring sites in London. These are: London Bexley, London Bloomsbury, London Eltham, London North Kensington, London Teddington, London Haringey and London Westminster.
Covid-19 pandemic restrictions saw very tight restrictions on international and local travel, with multiple lockdowns imposed and stay at home orders placed. These actions saw NO2 pollution levels across the country drop sharply at the end of March. Overall primary pollutants saw a big decrease in their annual means and this is seen can be seen in the below graphs.
NO2
NO2: Note that between 2015 and 2019 Harlington and Green Gates have the same concentrations and the trace for London Harlington is behind the one for Green Gates. The general average trend of concentrations at all sites over the last 10 years has seen a reduction of pollution level with individual sites showing yearly variability. All sites showed a significant drop in 2020, Green Gates dropped 37%, LHR2 41%, Oaks Road 36% and London Harlington 34% when compared to 2019.
Figure 31: Time series for annual mean NO2
NO
NO: Annual mean concentrations of total NO have generally decreased at LHR2 since it came into operation. There was a clear decrease throughout the 1990s at the LHR2 site although since the turn of the millennium the decreasing trend is less obvious with the annual mean fluctuating between approximately 33 μg m-3 and 50 μg m-3. At the other three sites, an overall decrease in annual mean NO has occurred during the period 2007-2015, although considerable variations have occurred from one year to the next. As reported during 2016 all the Heathrow sites and the London sites recorded the highest annual mean of this decade. This trend has reversed in 2017 and again in 2018 with concentrations reducing on those recorded during 2016. As with NO2 significant drops have been seen at all sites in 2020 due to the Covid-19 restrictions.
Figure 32: Time series for annual mean NO
NO2 as a % of NOx
NO2 as a percentage of total NOx: From the early 1990s to about 2006 NO2 accounted for an increasing percentage of total NOx at LHR2. Since then, it has fluctuated between 40% and 50%. The proportion of NOx measured as NO2 at the other three sites has been consistently higher, but has followed broadly similar yearly variations to those seen at LHR2. This stable percentage saw a sharp decrease in 2016 but with subsequent increases in the following years with the exception of 2019. This is also seen in the average of the other London sites.
Figure 33: Time series for annual mean NO2 as a percentage of total NOx
PM2.5
PM2.5: Please note thatin several instances the trend one for one site cant been seen because it is behind that of another. All sites show a general decline in levels since they commensed opperating, though there have been periods of stability and occasional upwards spikes. As has been seen with NO2 a significant reduction of PM2.5 was seen in 2020 and again can largely be put down to Covid-19 restrictions.
Figure 34: Time series for annual mean PM2.5
PM10
PM10: PM10 data was measured with a TEOM up until 2013 (at Harlington there was a TEOM FDMS from 2009 to 2013). From then up until 2014 the data was VCM corrected. From 2014 onwards all data is from FIDAS instruments and therefore requires no correction factor. The annual means of PM10 recorded in 2016 are similar to those recorded in 2015. However, a step change in the trend can be seen at all sites, which appears to coincide with the installation of the new Fidas analysers. Further to this the annual means of the four sites now all sit well below the other averaged London sites. A study of PM concentrations using FDMS and Fidas analysers was undertaken at Harlington, where over 30 months of co-located data is available for review. These studies concluded that annual mean Fidas and FDMS PM concentrations agree to within 1 μg m-3 of each other.
Figure 35: Time series for annual mean PM10
O3
O3: Ozone was only measured at Harlington. A slight upward trend can be detected since measurements began. Annual means of NO and NO2 have been slightly decreasing since 2013, and significantly in 2020, which can probably indicate that ozone increase is caused by the reduction of concentration of combustion sources in the area, mainly NO - responsible for the fast consumption of O3 to form NO2. The balance of production and loss reactions combined with atmospheric air motion determines the global distribution of ozone on timescales of days to many months. A further possibility for the gradual increasing trend is a change in formation rate constants due to climate changes influence on factors such as temperature. The same trends can be seen at other London sites. The increase seen in 2020 will be strongly influenced by both the Covid-19 restrictions and the long period of high ozone concentrations during August as discussed in Section 4.6.
Figure 36: Time series for annual mean Ozone at Harlington
4.10 Relationship with airport activity and impact of Covid-19
In this section, the potential for correlation between airport activity and pollutant concentrations is investigated by comparing pollutant concentrations with Aircraft Transport Movements (ATM) at Heathrow from the Heathrow website (Travel Stats, 2020).
Figure 37 shows monthly mean NOx concentrations at the four monitoring sites, together with monthly total ATMs. ATMs have risen steadily at Heathrow from 1995 to 2007, after which there was a decline until 2011. Since then, ATMs have remained steady at around 470,000. Local ambient concentrations in NOx have fluctuated over the same period, but there is no obvious relationship between NOx concentrations and airport activity. Seen in Figure 37 the number of ATM’s dropped significantly in April of 2020 and remained low for the rest of the year due to Covid-19 restrictions. At this time drops in NO2 are also seen before ATM’s and NO2 levels slowly increase again over the year as restrictions are eased before dropping again when a second national lockdown was announced. It must be noted however that these restrictions affected the public as well as the airport and so reductions are not just due to reduced airport activity.
Figure 38 shows the same comparison for PM10, there has been no clear relationship between annual mean PM10 and changes in air transport movements for many years, and this continues to be the case when looking in detail at the last year with no significant drop in readings when Covid-19 measures were introduced. This does not mean that the airport is not a major contributor to local ambient PM10, but suggests that variations in ambient PM10 concentrations are also dependent on other factors. This simple analysis of air traffic movements indicates that annual variation in pollutant concentrations (i.e. the periods of high and low concentration) around Heathrow are influenced to a greater extent by general meteorological factors than by air traffic movement.
Figure 37: Time series for monthly ATM and monthly mean NOx concentrations
Figure 38: Time series for monthly ATM and monthly mean PM10 concentrations
5 Conclusions
The following conclusions have been drawn from the results of air quality monitoring around Heathrow Airport during 2020.
Oxides of nitrogen and particulate matter (as PM10 and PM2.5) were monitored throughout 2020 at four sites around Heathrow Airport (LHR2, London Harlington, Green Gates and Oaks Road). Ozone was measured at Harlington. BC was measured at LHR2 and Oaks Road. In November 2019 a new site at Bath Road was installed however due to power issues, resulting from Covid-19 closing the fast food restraunt that supplies the power, the site was off for more than 75% of the year and so has not been analysed for this report. The conclusions of the 2020 monitoring programme are summarised below.
Data capture of at least 90% was achieved for all pollutants at all the monitoring sites.
Oxides of nitrogen were monitored at all four sites. No sites exceeded the AQS objective of 200 μg m-3 for hourly mean NO2 more than the 18 permitted times per year during 2020.
No site exceeded the annual mean AQS objective of 40 μg m-3 for NO2 in 2020.
All four sites met the AQS objective for 24-hour mean of 50 μg m-3 (not to be exceeded more than 35 times a year) and annual mean of 40 μg m-3 for PM10. The particulate matter was measured using a FIDAS instrument with no correction required for PM10 or PM2.5.
Ozone was measured at Harlington only, this site exceeded the AQS objective for ozone on 20 days in 2020 which was an exceedance of the AQS objective. Exceedances occured each month between April and September, during warm sunny days.
Seasonal variations in pollutant concentrations at all sites were similar to those observed in previous years and at other urban background sites. Both NO, NO2 and BC exhibited higher concentrations during the winter months. PM10 and PM2.5, which have both primary and secondary components, showed a much less pronounced seasonal pattern. Ozone levels were highest during the spring and summer, as is typical.
The diurnal patterns of concentrations of all pollutants were mostly typical of urban monitoring sites. Peak concentrations of NO, NO2 and BC coincided with the morning and evening rush hour periods, and to lesser extent, particulates. Levels of ozone peaked in the afternoons.
Several periods of elevated PM10 and PM2.5 concentrations (daily mean concentrations in the Defra “Moderate” band) occurred during April, August and November 2020. As in previous years, other urban background monitoring sites in London showed a similar pattern of elevated particulates concentrations during these periods. This indicates that the higher concentrations measured at Heathrow reflected regional variations in PM10 concentration, rather than any emission sources specific to the airport.
Meteorological measurements are made at LHR2, allowing the effect of wind direction and speed to be investigated. The polar plots plotting hourly mean pollutant concentrations against the corresponding wind speed and wind direction shows significant source contribution from local traffic and potentially residential sources for NO2, NO and BC. There is also contribution from the airport although this emissions source is less significant. PM concentrations are dominated by regional episodes as demonstrated by the by high concentrations from an easterly direction.
Mean annual concentrations of pollutants at the four Heathrow sites analysed were comparable with those measured at other suburban and urban background monitoring sites in London.
Long-term annual mean concentration data from this monitoring program show a gradual downward trajectory in levels of NO with some yearly variation. Covid-19 restrictions brought in in March 2020 heavily influenced NO2 causing a 34% to 42% reduction in levels compared to 2019. PM10 measurements at all sites decreased slightly during 2020. O3 concentrations measured at Harlington increased by over 20%.
Covid-19 restrictions saw big reductions in both ATM’s and public movements. This can be seen in the large drop in both seen in the NO2 monthly recordings, however on a monthly scale the impact on PM readings is not obvious.
Neither seasonal patterns, nor long-term trends, in pollutant concentration at the Heathrow sites showed any obvious relationship to annual aircraft transport movements. Although the airport is likely to be a contributor to local air pollution, ambient concentrations are also influenced by meteorological and other factors. Specifically aircraft emissions contribute little to either ambient NO2, PM10 or PM2.5, these are influenced to a greater extent by local vehicle emissions.
6 References
Appendix I - Air Quality objectives and Index bands
Appendix II - Monitoring apparatus and techniques
Monitoring Equipment
The following continuous monitoring methods were used at the Heathrow air quality monitoring stations:
- NO, NO2: chemiluminescence with ozone.
- PM10 and PM2.5: Fine Dust Analysis Systems (FIDAS).
- O3: UV absorption analyser, Harlington only.
- Black Carbon (BC): Aethalometer, LHR2 and Oaks Road only.
These methods were selected in order to provide real-time data. The chemiluminescence and the UV absorption analysers are the European reference method for ambient NO2 and O3 monitoring.
Each analyser provides a continuous output, proportional to the pollutant concentration. This output is recorded and stored every 10 seconds, and averaged to 15 minute average values by the on-site data logger. This logger is connected to a modem and interrogated twice daily, by telephone, to download the data to Ricardo Energy & Environment. The data are then converted to concentration units and averaged to hourly mean concentrations.
The analysers for NOx and O3 are equipped with an automatic calibration system, which is triggered daily under the control of the data logger. Fully certificated calibration gas cylinders are also used at each site for manual calibration.
Aethalometers quantify black carbon on filter samples based on the transmission of light through a sample. The sample is collected on a quartz tape, and the change in absorption coefficient of the sample is measured by a single pass transmission of light through the sample measured relative to a clean piece of filter. The aethalometers operate most commonly at two wavelengths, 880 nm and 370 nm. The 880 nm wavelength is used to measure the black carbon (BC) concentration of the aerosol, while the 370 nm wavelength gives a measure of the “UV component” of the aerosol.
The FIDAS unit employs a white light LED light scatter method that offers additional information on both particle size distribution from 0.18 to 30 microns (PM1, PM2.5, PM4, PM10 and Total Suspended Particles (TSP). This analyser has demonstrated equivalence to EN12341:2015, and is certified for use in UK monitoring networks under the MCERTS for UK PM certification scheme.
Appendix III - Quality assurance and Quality control
Ricardo Energy & Environment operates air quality monitoring stations within a tightly controlled and documented quality assurance and quality control (QA/QC) system. These procedures are documented in the AURN QA/QC manual (“QA/QC Procedures for the UK Automatic and Urban Rural Air Quality Monitoring Network (AURN)” 2009).
Elements covered within this system include: definition of monitoring objectives, equipment selection, and site selection, protocols for instrument operation calibration, service and maintenance, integrity of calibration gas standards, data review, scrutiny and validation.
All gas calibration standards used for routine analyser calibration are certified against traceable primary gas calibration standards at the Gas Standards Calibration Laboratory at Ricardo Energy & Environment. The calibration laboratory operates within a specific and documented quality system and has UKAS accreditation for calibration of the gas standards used in this survey.
An important aspect of QA/QC procedures is the regular six-monthly inter calibration and audit check undertaken at every monitoring site. This audit has two principal functions: firstly to check the instruments and the site infrastructure, and secondly to recalibrate the transfer gas standards routinely used on-site, using standards recently checked in the calibration laboratory. Ricardo Energy & Environment’s audit calibration procedures are UKAS accredited to ISO 17025.
In line with current operational procedures within the Defra AURN, full inter calibration audits take place at the end of winter and summer. At these visits, the essential functional parameters of the monitors such as noise, linearity and, for the NOx monitor, the efficiency of the NO2 to NO converter are fully tested. In addition, the on-site transfer calibration standards are checked and re-calibrated if necessary, the air intake sampling system is cleaned and checked and all other aspects of site infrastructure are checked.
All air pollution measurements are reviewed daily by experienced staff at Ricardo Energy & Environment. Data are compared with corresponding results from AURN monitoring stations and with expected air pollutant concentrations under the prevailing meteorological conditions. This review process rapidly highlights any unusual or unexpected measurements, which may require further investigation. When such data are identified, attempts are made to reconcile the data against known or possible local air pollution sources or local meteorology, and to confirm the correct operation of all monitors. In addition, the results of the daily automatic instrument calibrations (see Appendix 2) are examined to identify any possible instrument faults. Should any faults be identified or suspected, arrangements are made for Ricardo Energy & Environment personnel or equipment service contractors to visit the site as soon as possible.
At the end of every quarter, the data for that period are reviewed to check for any spurious values and to apply the best daily zero and sensitivity factors, and to account for information which only became available after the initial daily processing. At this time, any data gaps are filled with data from the data logger back-up memory to produce as complete a data record as possible.
Finally, the data are re-examined on an annual basis, when information from the six-monthly inter calibration audits can be incorporated. After completion of this process, the data are fully validated and finalised, for compilation in the annual report.
Following these three-stage data checking and review procedures allows the overall accuracy and precision of the data to be calculated. The accuracy and precision figures for the pollutants monitored at Heathrow are summarised in Table 2.
All of the air quality monitoring equipment at both sites is housed in purpose-built enclosures. The native units of the analysers are volumetric (e.g. ppb). Conversion factors from volumetric to mass concentration measurement for gaseous pollutants are provided below:
- NO 1 ppb = 1.25 μg m-3
- NO2 1 ppb = 1.91 μg m-3
In this report, the mass concentration of NOx has been calculated as follows: NOx μg m-3 = (NO ppb + NO2 ppb) x 1.91.
This complies with the requirements of the Air Quality Directive and is also the convention generally adopted in air quality modelling (“Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on Ambient Air Quality and Cleaner Air for Europe” 2008).
| Name | Nick Rand |
| Address | Ricardo Energy & Environment, Gemini Building, Harwell, Didcot, OX11 0QR, United Kingdom |
| Telephone | 01235 753484 |
| nick.rand@ricardo.com |