Air Quality at Heathrow Airport 2017 - Issue 1

Annual Report for 2017

Authors	Joanne Green
Compilation date	28 August 2018
Customer	Heathrow Airport Ltd
Approved by	Brian Stacey
Copyright	Ricardo Energy & Environment
EULA	http://ee.ricardo.com/cms/eula/

Contract reference

ED59405

Report reference

ED59405 V1

Executive summary

This report provides details of air quality monitoring conducted around Heathrow Airport during 2017. 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. Data from these four 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) and Oaks Road, on a residential location to the south west (505740 174500).

All sites monitored oxides of nitrogen (nitric oxide and nitrogen dioxide) and Particulate Matter (PM₁₀ and PM_2.5). PM₁₀ and PM_2.5 data for all sites in 2017 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. LHR2 registered 12 exceedances of this value during the year, and Harlington, Green Gates and Oaks Road registered no exceedances. All HAL sites met this objective for 2017.

The annual mean AQS objective for NO₂ is 40 μg m^-3. This was met at Harlington, Green Gates and Oaks Road. At LHR2, an annual mean of 48 μg m^-3 was registered for 2017. This value is slightly higher than those registered in 2016 and 2015 at 47 and 44 μg m^-3 respectively, showing a small increase in concentrations for this pollutant, however, similar increases were also observed throughout the south-east and London. The AQS objectives and EU limit values do not apply for this site, since LHR2 is located within the airport perimeter fence, where members of the public do not have access.

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. During 2017, only 3 to 7 exceedances to the limit value were registered at each site. 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 Harlington station met the AQS objective for ozone in 2016.

Average concentrations of NO, NO₂, PM₁₀, PM_2.5 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 PM_2.5 showed a much less pronounced seasonal pattern, which is quite common for particulates in urban areas. 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 direction data measured at the LHR2 location were used to investigate effects on pollutant concentrations and potential sources. 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 west. With regards to NO₂, there also appeared to be a contribution from the south south west at higher wind speeds, possibly indicating a major source further away, in this direction are; Terminal 5, the Central Terminal Area (CTA) and the M25. For both PM₁₀ and PM_2.5, high concentrations wre dominated by the regional episodes which brouhgt 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 2017. At all sites, particularly high concentrations of PM₁₀ were recorded in January (22nd to 24th), February (13th) and September (26th to 27th). Local emissions, combined with trans-boundary emissions from continental Europe, in conjunction with weather conditions are the origin of these high concentration episodes.

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 is less clear. The proportion of NOx measured as NO₂ has increased over the last decade, the decrease in 2016 has been reversed a small amount by a slight increase in 2017. The annual mean concentrations of PM₁₀ have remained similar to last year. 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 PM_2.5 which recorded lower annual averages than the comparison sites located in London. While the annual average of ozone (monitored at Harlington only) has returned to similar levels seen in 2012 the long term profile is still one of a slow increase in concentrations, likely due to its relationship with nitrogen oxides.

Although the airport is a material contributor to local air pollutant concentrations, there appears to be 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 the world’s busiest 2-runway international airport, handling approximately 78 million passengers in 2016 (Travel Stats 2018). The airport is situated approximately 12 miles to the west of 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 (NO_x), carbon monoxide (CO), oxides of sulphur (SO_x), 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 monitored at these sites:

• Oxides of nitrogen (nitric oxide (NO) and nitrogen dioxide (NO₂));
• Particulate matter (PM₁₀ and PM_2.5 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 2017, the 24th 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 2017.

In addition to this report, HAL has daily access to provisional data from its monitoring sites via their own Heathrow Airwatch website (Heathrow Airwatch 2018) and data from the UK’s national air quality monitoring network, through the Defra UK Air Information Resource (UK-AIR) (Defra UK-Air 2018).

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 (EC 2008/50 2008), and its update EU2015/1480 (EC 2015/1480 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.

1. 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.

2. 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. 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 PM_2.5 are used to describe particles with an effective size with a median aerodynamic diameter of 10 and 2.5 µm 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 2015 NAEI data, less than 0.1% of UK total PM₁₀ emissions are believed to originate from civil aircraft taking off and landing (Defra_3 2017).

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 that PM_2.5. 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 was carried out at four sites during 2017. 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) (Defra_2 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. 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

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 PM_2.5 - 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)(Defra_1 2009), full intercalibration audits of the HAL air quality monitoring sites take place at six-monthly intervals with service 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

Significant data gaps for periods > 24h for the stations are shown in Table 3.

Overall data capture statistics along with summary statistics for the four monitoring sites are provided in Tables 4 to 8 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) (Defra_2 2016). This is particularly important at Harlington, as data from this site feeds into the Automatic Urban and Rural Network (AURN), the UK’s main network used for compliance reporting against the Ambient Air Quality Directives. In 2017, data capture for all pollutants at all sites was above the 90% data capture requirement.

NO₂

Figure 7: NO₂ exceedance.

PM_2.5

PM₁₀

Figure 8: PM₁₀ exceedance.

O₃

Figure 9: O₃ exceedance.

Black Carbon

4.2 Time series plot

Below are time series plots of daily concentrations of pollutants at the four sites except for NO₂ which is presented as hourly concentrations. There is one tab per pollutant with all data from the four 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 peaks in PM₁₀ and PM_2.5 in January and February. These are discussed further below. The elevated peaks of NO₂ and BC appear, to some extent, during the same periods mentioned for PM, and also during winter months following a typical seasonal pattern. Ozone shows typical seasonal patterns with the highest concentrations recorded during the summer months.

NO₂ hourly

PM_2.5

PM₁₀

O₃

Black Carbon

4.3 Comparison with air quality objectives

The Details of UK air quality standards and objectives specified by Defra are provided in Appendix 1.

During 2017 there were exceedances of the NO₂ annual and 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 Harlington, Green Gates and Oaks Road, but not at the LHR2 site, which recorded an annual mean concentration of 48 μg m^-3. Although this value exceeds the AQS objective for NO₂, LHR2 is located within the airport perimeter where members of the public do not have access and so falls into the category “other” as defined by the Defra Technical Guidance on air quality monitoring LAQM.TG (16) (Defra_2 2016). These sites are defined as: “Any special source-orientated or location category covering monitoring undertaken in relation to specific emission sources such as power stations, car-parks, airports or tunnels” and are located where the public are not exposed.

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 12 hourly mean NO₂ measurements exceeding 200 μg m^-3 at Heathrow LHR2, which was within the allowed limit of the objective. The exceedances occured on 5th and 22nd January; 27th/28th March; 19th April; 15th, 24th, and 26th May; and 18th July.

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 were some exceedances of the 50 μg m^-3 24h mean value registered at all sites. At LHR2, Harlington, Green Gates and Oaks Road, seven, three, three and four exceedances respectively were recorded. The maximum value of exceedances of each site varies between 77 to 91 μg m^-3. The annual mean AQS objective for PM₁₀ is 40 µg m-3. All sites measured average annual values ranging between 13 and 15 μg m^-3, this objective was therefore met.

While no AQS objective exists for PM_2.5, 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 8 and 9 μg m^-3. This is less than half 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 over 44 days during 2017. Of these, 42 occurred between 14th June and 6th July while the remaining two of these measurements occurred on 28th August. These exceedances were common at monitoring sites acros 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 at 33 μg m^-3 and 17 μg m^-3 for LHR2 and Oaks Road respectively. This values are similar to the ones obtained in the previous year for the same sites (25 and 16 μg m^-3). 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.

4.4 Time variation plot

Figure 10 to Figure 14 below show the variation of monthly, weekly, daily and hourly pollutant concentrations during 2017 at each of the four sites.

Seasonal variation

Seasonal variations seem to follow similar trends for NOx, PM and BC (when measured) at all sites during 2017, as can be observed in the ‘month’ plots of the figures below. Elevated concentration peaks were registered for all these pollutants at all sites in January. This was driven by two episodes which are discussed in more detail below. As in previous years, PM₁₀ and PM_2.5 concentrations showed much less seasonal variation than oxides of nitrogen with highest concentations in January but December concentrations being amongst the lowest. NO and NO₂ concentrations at all the sites generally follow a typical seasonal variation for urban areas with the highest concentrations occurring during the winter months. This pattern was also observed in previous years and is typical of urban monitoring sites. The 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.

O₃ concentrations measured at Harlington continue to follow a typical seasonal variation for this pollutant, with higher concentrations being measured in April, May, and June. 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 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. Similar peaks as the ones registered for PM can be seen in May and October for this BC, and are comparable to regional episodes recorded at other UK stations.

Diurnal variation

The diurnal variation analyses viewed in the ‘hour’ plots below show typical urban area daily patterns for NO₂ at all sites. 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 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 PM_2.5 are determined by two main factors. 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 PM_2.5 can be seen at all four sites, but these were less pronounced than those for oxides of nitrogen. PM₁₀ at Heathrow LHR2, and to a lesser extent at the other sites, shows an interesting pattern with peaks during the night time hours. This most likely indicates emissions from runway maintenance activities and the associated airside construction depot which was located near LHR2 which occured during 2017.

BC diurnal variation appears to follow the same trend pattern of NOx with two peaks measured at the same periods (07:00 AM and 20:00 PM) suggesting a strong primary componenet from vehicle exhaust.

Weekly variation

The analyses of the weekly variation for NO₂, PM_2.5, BC and O₃ show that the same type of diurnal patterns occur 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. PM₁₀ at Heathrow LHR2 shows that the night time peaks are during weekday nights with the highest concentrations on a Monday night, again most likely emissions from night time runway maintenance.

NO₂

Figure 10: Temporal variation NO₂.

PM_2.5

Figure 11: Temporal variation PM_2.5.

PM₁₀

Figure 12: Temporal variation PM₁₀.

O₃

Figure 13: Temporal variation O₃.

BC

Figure 14: Temporal variation BC.

4.5 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 15 to Figure 22 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 (as indicated by the scale).

These plots therefore show how pollutant concentration varies with wind direction and wind speed.

NO₂

NO₂:Figure 15 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. 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 (around 5-10 m s^-1) 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.

At LHR2 higher concentrations of NO₂ were associated with two sets of conditions, winds from the north east quadrant and from the south west quadrant. This can then be broken down into several sections. Calm conditions and light winds (<5 ms-1) from the east brought pollutants from the nearest roads (Bath Road and Northern Perimeter Road and associted junctions) and the built-up area of Harlington. As in previous years, other high NO₂ concentrations are associated with a wind direction of south south west for high wind speeds, (>10 ms-1), possibly indicating a major source further away. In this direction the airports departures and arrivals area along with the Central Terminal Area (CTA) can be found.

Figure 15: NO₂ Polar Plot for Heathrow sites

NO

NO:Figure 16 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 north east and south west indicting some emissions from the airport to the south east and both Bath Road and the Northern Perimter Road to the north east.

Figure 16: NO Polar Plot for Heathrow sites

PM_2.5

PM₁₀ and PM_2.5:Figure 17 and Figure 18 show that the sites have very similar, almost identical, plots with high concentrations occuring when the wind is from an easterly direction. To investigate further Figure 19 and Figure 20 present the variation in concentrations as hourly averages plotted by month. This shows that the highest concentrations were measured in January 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. Interestingly Figure 20 shows that there are additional peaks in PM₁₀ at LHR2 during nighttime from September through to December. These are most likely due to local runway maintenance activities but are not picked up by the polar plot, probably masked by the higher concentrations measured during the episode periods.

Figure 17: PM_2.5 Polar Plot for Heathrow sites

Figure 19: Average PM2.5 trends by month by site

PM₁₀

PM₁₀ and PM_2.5:Figure 17 and Figure 18 show that the sites have very similar, almost identical, plots with high concentrations occuring when the wind is from an easterly direction. To investigate further Figure 19 and Figure 20 present the variation in concentrations as hourly averages plotted by month. This shows that the highest concentrations were measured in January 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. Interestingly Figure 20 shows that there are additional peaks in PM₁₀ at LHR2 during nighttime from September through to December. These are most likely due to local runway maintenance activities but are not picked up by the polar plot, probably masked by the higher concentrations measured during the episode periods.

Figure 18: PM₁₀ Polar Plot for Heathrow sites

Figure 20: 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 sognature from the SSW at the LHR2 site may be contribution from the biomass boiler at Terminal 2.

Figure 21: 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 south, for wind speeds above 10 ms^-1.

Figure 22: O₃ Polar Plot for Heathrow sites

4.6 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.

The Air Quality Index presented at the Department of Environment, Food & Rural Affairs (Defra) UK-AIR website³. calculates air quality index bands that go from 4 (Moderate) to 10 (Very High). Several elevated pollution episodes were recorded at Heathrow during 2017. These corresponded to elevated concentrations for most of the UK regions around the 22^nd January, 13^th February and 27^th September. These pollution episodes are consistent with the period of elevated PM and NO_x concentrations measured at the monitoring stations at Heathrow, and explanations for these pollution events follow below. In addition PM₁₀ and PM_2.5 concentrations were elevated on 2^nd and 3^rd of November around the Guy Fawkes celebrations.

Figure 21: DAQI for 22nd January 2017

• There was widespread moderate/high pollution recorded across Southern England and Wales between Sunday 22^nd January and Tuesday 24^th January 2017. This was due to a high pressure system resulting in calm, settled conditions bringing about high PM₁₀ and NO₂. Calm conditions would have led to poor dispersion of local traffic emissions. Furthermore, elevated PM concentrations occured due to high PM_2.5 levels which were seen at all the Heathrow sites and across the UK. This was the highest of any PM_2.5 episode since the DAQI was set up in 2012. All of the Heathrow sites saw PM₁₀ concentrations breach the high index and in addition Heathrow LHR2 breached the moderate index for NO₂.

Figure 22: DAQI for 13th February 2017

• Moderate PM₁₀ and PM_2.5 levels were recorded at all 4 Heathrow sites on Monday 13^th February 2017. This can be seen in the wider DAQI above with other areas of Eastern England showing an index of high. Air masses originated from Northern Europe including areas such as Poland and Germany, this demonstrates a contribution of PM from further afield.

Figure 23: DAQI for 26th September 2017

• Moderate pollution was recorded across parts of Southern England on 26^th September 2017. This was mainly attributed to polluted air masses adding to existing UK emissions. These air masses brought with them agricultural, industrial and traffic related emissions, causing elevated levels of PM as seen on this date and the day after at all Heathrow sites.

Some Ozone episodes were recorded over the summer period which coincided with O₃ concentrations at London Harlington reaching an index of high:

• Widespread moderate ozone was recorded on Saturday 17^th transformation of nitrogen oxides and other ozone precursors into ozone.
• Similarly, on the 20^th to 22^nd June more hot weather contributed to elevated ozone levels in much of England.

4.7 Long term changes in pollutant concentrations

LHR2 has been in operation for 25 years (following installation in 1993). The other three 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, PM_2.5, O₃ and black carbon are illustrated In Figures 26 to 31 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 six urban non-roadside monitoring sites in London. These are: London Bexley, London Bloomsbury, London Eltham, London North Kensington, London Teddington and London Westminster.

NO₂

NO₂: There was a clear decrease from 2003 to 2015 at the LHR2 site. However the last two years have seen increases in NO₂ concentration. This was not mirrored at the other London sites which on average saw a decrease year on year since 2011. The annual mean concentrations at Harlington, Green Gates and Oaks Road have followed a general downward trend since installation. 2016 saw a slight increase in annual means at all sites, however 2017 concentrations have reduced to concentrations measured in 2015 or lower.

Figure 24: 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, a 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 with concentrations reduced on those recorded during 2016.

Figure 25: 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 percentage seems to have stabilised in all sites since 2012 with a sharp decrease in 2016 but an increase the following year in 2017. This is also seen in the average of the other London sites.

Figure 26: Time series for annual mean NO₂ as a percentage of total NOx

PM_2.5

PM_2.5: For Green Gates and Oaks Road, where trends can be observed over several years, concentrations initially decreased and had remained stable since 2008/2009. 2017 saw a further reduction at both sites. Concentrations at Harlington have been more erratic but this year as last year concentrations remain similar to the other Heathrow sites. Please note that London Harlington and Green Gates had the same average concentrations the last three years and so the Harlington line is not seen.

Figure 27: 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 28: 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, 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.

Figure 29: Time series for annual mean Ozone at Harlington

4.8 Relationship with airport activity

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 2018).

Figure 30 shows annual mean NOx concentrations at the four monitoring sites, together with annual total ATMs. ATMs rose 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. Figure 31 shows the same comparison for PM₁₀, with no clear relationship being apparent between annual mean PM₁₀ and changes in air transport movements. 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 30: Time series for annual ATM and annual mean NOx concentrations

Figure 31: Time series for annual ATM and annual mean PM10 concentrations

5 Conclusions

The following conclusions have been drawn from the results of air quality monitoring around Heathrow Airport during 2017.

Oxides of nitrogen and particulate matter (as PM₁₀ and PM_2.5) were monitored throughout 2017 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. The conclusions of the 2017 monitoring programme are summarised below.

1. Data capture of at least 90% was achieved for all pollutants at all the monitoring sites.
2. 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 2017.
3. One site, LHR2, exceeded the annual mean AQS objective of 40 μg m^-3 for NO₂ in 2017, with an annual mean of 48 μg m^-3, although the EU limit values and AQS objectives do not apply at the LHR2 site, because it is within the airport boundary where there is no public exposure. The other three HAL sites did not exceed this objective.
4. 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. PM_2.5 data is divided by 1.06 to ensure gravimetric equivalence.
5. Ozone was measured at Harlington only, this site exceeded the AQS objective for ozone on 8 days during 2017, entering into the “Moderate” band for 7 times during that period and the “HIgh” band on 1 occasion. These results are less than the permitted maximum of 10 days per calendar year. The AQS objective was therefore met in 2017.
6. 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 PM_2.5, 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.
7. 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 levels of ozone peaked in the afternoons. The exception was for PM concentrations. Peak concentrations, particularly at Heathrow LHR2 were seen during the night time with the higest concentrations on Mondays between 10pm and 4am. This was likely construction work in the vicinity of the site.
8. Several periods of elevated PM₁₀ concentration (daily mean concentration in the Defra “High” band) occurred during 2017. As in previous years, other urban background monitoring sites in London and the south east of England showed a similar pattern of elevated PM₁₀ concentrations during the above 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.
9. 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 regional episodes as demonstrated by the by high concentrations from an easterly direction.
   
10. Mean concentrations of pollutants at the four Heathrow sites in 2017 were comparable with those measured at other suburban and urban background monitoring sites in London.
11. Long-term annual mean concentration data from this monitoring program show a gradual downward trajectory in levels of NO with some yearly variation. The rise seen in 2016 has not continued with all sites seeing a reduction in 2017 although in most cases concentrations are still above those in 2015. A small increase is observed in annual mean concentrations of NO₂ at LHR2 for 2017 continuing the upward trend seen in 2016. At all other sites NO₂ reduced compared to 2016 levels. PM₁₀ measurements at all sites except Harlington saw a reduction in concentration from 2016. O₃ concentrations continue to increase over time.
12. 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 significant contributor to local air pollution, ambient concentrations are also influenced by meteorological and other factors.

6 References

Air Quality Expert Group. 2004. “Nitrogen Dioxide in the United Kingdom.” http://uk-air.defra.gov.uk/library/aqeg/publications.

Defra UK-Air. 2018. “UK-AIR.” UK-AIR, Air Quality Information Resource. http://uk-air.defra.gov.uk/.

Defra_1. 2009. “QA/QC Procedures for the UK Automatic and Urban Rural Air Quality Monitoring Network (AURN).” Department for Environment, Food; Rural Affairs; the Devolved Administrations. http://uk-air.defra.gov.uk/reports/cat13/0910081142_AURN_QA_QC_Manual_Sep_09_FINAL.pdf.

Defra_2. 2016. “Local Air Quality Management - Technical Guidance LAQM.TG (16).” Department for Environment, Food; Rural Affairs in partnership with the Scottish Executive, Welsh Assembly Government; Department of the Environment Northern Ireland. https://laqm.defra.gov.uk/documents/LAQM-TG16-April-16-v1.pdf.

Defra_3. 2017. “UK Informative Inventory Report (1990 to 2015).” https://uk-air.defra.gov.uk/assets/documents/reports/cat07/1703161205_GB_IIR_2017_Final_v1.0.pdf.

EC 2008/50. 2008. “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.” http://data.europa.eu/eli/dir/2008/50/oj/eng.

EC 2015/1480. 2015. “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).” https://eur-lex.europa.eu/eli/dir/2015/1480/oj.

Heathrow Airwatch. 2018. Heathrow Airwatch - Air Quality Information in the Heathrow Area. http://www.heathrowairwatch.org.uk/.

Travel Stats. 2018. Investor Centre Traffic Statistics Heathrow. http://www.heathrow.com/company/investor-centre/results-and-performance/traffic-statistics.

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 PM_2.5: 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, PM_2.5, 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 (Defra_1 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 (EC 2008/50 2008).

For further information, please contact:

Name	Nick Rand
Address	Ricardo Energy & Environment, Gemini Building, Harwell, Didcot, OX11 0QR, United Kingdom
Telephone	01235 753484
Email	nick.rand@ricardo.com