Clean air is vital to human health and well-being, and the adverse health effects of exposure to poor air quality are now well understood and have been widely reported in the mainstream press.
In the United Kingdom (UK), poor air quality is predominantly caused by motor vehicle emissions, with exposure particularly marked in densely populated urban areas with high volumes of traffic. Emissions from the near-continent, along with domestic activities such as industry, farming and heating and cooking within homes, also contribute to the United Kingdom’s emissions burden and the quality of the air we breathe.
Monitoring ambient air quality is therefore as important as ever for understanding the temporal profile and spatial extent of air pollution in our towns and cities and for implementing effective strategies to mitigate its effects.
In the UK, there is a long history of air quality monitoring. At a national level, the Automatic Urban and Rural Network (AURN)1, run by the Department for Environment, Food & Rural Affairs (Defra), has been in operation since the early 1970s and is now the UK’s largest automatic air quality monitoring network. At a local level, air quality monitoring to various degrees of coverage and sophistication is undertaken by large numbers of local authorities to support requirements under the Local Air Quality Management (LAQM) regime2.
At the individual project level, air quality monitoring is often commissioned by developers and planners to support baseline studies for Environmental Impact Assessments (EIA), to assist with the verification of air quality models or to discharge air quality-related planning conditions.
Current methods of measuring air quality
Continuous monitoring stations
Continuous (or automatic) Monitoring Stations (CMS) form the backbone of air quality measurement in the United Kingdom. These stations run near-continuously, commonly providing hourly or 15-minute average data and are usually strategically placed in roadside, background or industrial areas to monitor concentrations of key, regulated air pollutants at ‘breathing height’ (nominally 1.5 m to 2 m above ground level). Many CMS include sophisticated analytical ‘reference’ instruments that meet well-defined international standards for the quality of the measurement data they produce.
CMS are often operated by local authorities to measure air quality in designated Air Quality Management Areas (AQMAs)3 or to track long-term trends in air quality at background locations away from major sources of emissions. Many regulated industrial operators with significant sources of combustion or other process emissions also have CMS installed within, or on, facility boundaries, or adjacent to nearby sensitive uses (e.g. residential dwellings or protected ecological sites).
CMS are relatively expensive to purchase and operate, and require a secure weatherproof cabinet, continuous power supply and air conditioning units to keep the equipment cool in the warmer summer months. Pumps, filters and inlet heads also form part of the overall system. CMS can house an array of reference or non-reference instruments capable of measuring various pollutants, including:
- Particulate Matter (PM): PM2.5 and PM10 are particles suspended in the air, often originating from combustion processes and industrial emissions.
- Nitrogen Dioxide (NO2): A reddish-brown gas primarily emitted from vehicles and industrial processes.
- Sulphur Dioxide (SO2): A pungent gas released from burning fossil fuels containing sulphur.
- Carbon Monoxide (CO): A colourless, odourless gas produced by incomplete combustion of carbon-based fuels.
- Ozone (O3): A secondary pollutant formed by chemical reactions between nitrogen oxides (NOX) and volatile organic compounds (VOCs).
- Volatile Organic Compounds (VOCs): Organic chemicals that can vaporise into the air. They can originate from sources like industrial processes and vehicle emissions.
Diffusion tubes/passive samplers
Diffusion tubes, also known as passive samplers or passive monitors, are small, usually plastic air quality monitoring devices. The tubes operate on the principle of passive diffusion, which allows gases to move from an area of higher concentration (outside the tube) to an area of lower concentration (inside the tube), with the lower concentration inside the tube maintained through the retention of the target gas onto an absorbent matrix. Diffusion tubes facilitate the measurement of a variety of airborne gases, including NO2, SO2 and VOCs, but they cannot be used to measure particulate matter.
Being small and lightweight, diffusion tubes are significantly cheaper and easier to install than CMS, and are therefore frequently used by local authorities and consultants to measure pollutant concentrations (most notably NO2) adjacent to roads, construction sites and industrial facilities. As diffusion tubes are usually deployed for a period of 2-4 weeks before they are sent to a laboratory for analysis, only long term (monthly or annual mean) pollutant concentrations can be derived from these devices. Short-term peaks in pollution concentrations cannot be captured.
Construction dust monitoring
Constructing works can have a significant, temporary impact on local air quality. The most common impacts are increased particulate matter (PM) concentrations and dust soiling4. On large construction projects, or where sensitive uses such as residential properties, protected ecological sites or even some commercial operations that require clean air for a particular product or process are in close proximity to a site, monitoring may need to be carried out to ensure that any mitigation measures employed are effective in controlling dust emissions.
Particulate matter and dust monitoring for construction works tends to take place on the site boundary, and usually at a minimum of two locations: upwind and downwind of the site. In some instances, monitoring may be required at multiple points on the boundary, or at a specific location, such as an adjacent school or other sensitive use.
Particulate matter concentrations can be measured using several different instruments of varying cost and complexity. In the UK, the most common approach is to use what are termed ‘indicative’, usually light-scatter/optical devices, which have been deemed, under the Environment Agency MCERTS scheme5, to be equivalent to a reference, gravimetric/filter-based method. These indicative instruments are much cheaper to purchase than reference monitoring solutions and usually include remote communication capabilities and access to a data management/viewing platform that allows near real-time access to the measurement data for contractors working on site, consultants working on their behalf, or local regulators. Having access to near real-time data is key for construction monitoring as it allows for reactive mitigation to be deployed on site if agreed particulate matter ‘trigger’ or ‘action’ thresholds are breached.
Dust soiling is normally measured using a relatively low-cost dust deposition gauge, which may include a directional ‘sticky pad’ to help determine the location of any source(s) of dust emissions. Dust deposition gages are usually deployed for a period of a couple of weeks to a month before samples are sent back to a laboratory for analysis to determine the dust flux (mass of dust per unit time per area) or dust soiling (effective/actual area coverage, or EAC/AAC). Samples obtained from dust deposition monitoring can also be subject to further analysis to determine the chemistry and/or morphology of the sampled particles.
Emerging methods of air quality measurement
The emergence of ‘low-cost’ air quality sensor technologies over the last five or so years has enabled cheaper air quality monitoring devices to come to market, offering a similar range of target air pollutants to ‘traditional’ CMS but often at improved temporal resolution.
Air quality monitors fitted with low-cost sensors are generally much smaller than CMS, enabling mobile, hyper-local, or widely distributed monitoring networks, such as the Breathe London6 project, to be established, as well as providing monitoring solutions that are more affordable to ‘citizen scientists’ concerned about ambient air quality in their neighbourhood or, indeed, within their own homes.
Low-cost sensing solutions can range from simple, single pollutant sensors in units that are sold for a few tens of pounds to relatively sophisticated multi-pollutant devices that include communications and meteorological capabilities and may cost several thousand pounds, but which differ from reference methods/CMS because of their compactness, mobility and lower power consumption7.
Remote sensing employs satellites and other aerial platforms to monitor air quality on a regional or global scale. Instruments aboard these platforms measure atmospheric composition, allowing for the estimation of pollutant concentrations at ground/earth surface level. Key regulated pollutants, such as NO2 and PM2.5, and also greenhouse gases, such as methane (CH4) and carbon dioxide (CO2), can be estimated using remote sensing techniques8. Remote sensing is particularly useful for tracking large-scale trends, assessing pollution in remote or inaccessible areas, or for filling spatial gaps in direct, ground-based monitoring.
Mobile monitoring involves equipping vehicles with air quality sensors to collect data whilst driving. This approach provides dynamic, and near real-time information about air quality, which is especially useful for identifying pollution hotspots and tracking pollution sources in urban environments.
This technique was trialled in 2017 by the City of London Corporation using a ‘smog mobile’ – a Nissan e-NV-200 electric van (zero tailpipe emissions) operated by Enviro Technology Services Ltd9. The vehicle was equipped with a variety of instruments to measure gas and particulate matter pollution as the vehicle travelled around the City of London. Sample air was fed into the analysers via a manifold and dynamic valve that allowed switching between outside (ambient) and inside (inside the cab of the vehicle) sampling modes.
A subsequent analysis of the ‘smog mobile’ data by the City of London Corporation and Connected Places Catapult in 20216 found that the mobile monitoring technique produced a similar city-wide average NO2 concentration to that derived from City of London Corporation diffusion tube measurement data. Overall, the study demonstrated significant potential and value in using mobile air quality measurements to support assessment of air quality over large areas by local authorities.
Challenges in air quality measurement
Despite advancements in air pollution monitoring technology, particularly the emergence of low-cost sensing solutions, challenges persist in accurately assessing air quality in terms of temporo-spatial variability and measurement accuracy.
Air quality can vary substantially within short distances due to localised sources of pollution and rapidly changing meteorological conditions (dispersion profiles). This makes it important to have a spatially dense network of monitoring stations to capture these variations. Low-cost sensors and diffusion tubes provide the option of good spatial coverage without excessive cost, although the later can only provide long-term, indicative measurement data.
Pollution levels can change throughout the day and across seasons, influenced by factors like traffic patterns, industrial activity, meteorological conditions and regional emissions. Continuous monitoring is therefore necessary to capture these temporal fluctuations. CMS and low-cost sensor devices can offer temporal resolution down to minutes, or in the case of low-cost sensors, seconds. However, as noted above, CMS are relatively expensive in terms of both capital and operational costs.
The accuracy of monitoring instruments is essential for capturing reliable data on which technical or policy interventions on local air quality/pollution can be made. Regular calibration and maintenance, particularly of CMS, is necessary to ensure precise measurements. For diffusion tubes, co-location studies (with CMS) can assist with validating diffusion tube measurements as can the use of supporting tools, such as Defra’s national diffusion tube bias adjustment factors spreadsheet10.
For low-cost air quality sensors a number of limitations have been reported, including:
- Accuracy and precision (low-cost sensors tend to be less accurate than reference instruments and at low concentrations may struggle measuring specific pollutants).
- Sensor drift (measurements becoming less accurate over time due to environmental conditions, exposure to pollutants and sensor ageing).
- Cross sensitivity (non-target species/pollutants are sometimes measured in error).
- Temperature and humidity (many low-cost sensors are sensitive to changes in temperature and humidity. Extreme environmental conditions can lead to erroneous readings).
The road to cleaner air
Clean air is vital to human health and well-being, and monitoring air quality is critical to understanding air pollution and improving local air quality through evidence-based technical and policy interventions. Various ‘traditional’ air quality monitoring techniques, including continuous monitoring stations (CMS), diffusion tubes and construction dust deposition and particulate concentration monitors, have been in existence for quite some time and are widely used by national and local authorities, industrial operators, developers, consultants and site contractors to quantify air pollution, assess risks and mitigate impacts where necessary.
Emerging air quality monitoring solutions, including low-cost sensors, remote sensing and mobile monitoring techniques, all have a role to play in measuring and assessing air quality now, and in the future. However, some of the limitations in air quality monitoring around spatial and temporal variability and measurement accuracy still pose challenges to overcome.
1 Automatic Urban and Rural Network (AURN) - Defra, UK
2 Local Air Quality Management (LAQM) Support Website | DEFRA
3 Air Quality Management Areas (AQMAs) - Defra, UK
4 guidance_monitoring_dust_2018.pdf (iaqm.co.uk)
5 Monitoring emissions to air, land and water (MCERTS) - GOV.UK (www.gov.uk)
6 Breathe London
7 AQEG advice on the use of 'low-cost' pollution sensors - Defra, UK
8 Air Quality Research Using Satellite Remote Sensing | California Air Resources Board
9 Atmosphere | Free Full-Text | Mobile Monitoring for the Spatial and Temporal Assessment of Local Air Quality (NO2) in the City of London (mdpi.com)
10 National Bias Adjustment Factors | LAQM (defra.gov.uk)
James Ferguson-Moore is a Principal Environmental Consultant in the Phlorum Air Quality Team.
James has worked in the field of Environmental Sciences for more than fifteen years and is a Chartered Scientist and a member of the Institute of Air Quality Management and Institution of Environmental Sciences.
The Phlorum Air Quality Team provides expert technical air quality consultancy services to a wide variety of clients predominantly in the land/property development sector. Services include impact assessments for planning, detailed dispersion modelling of road and industrial emissions, construction dust management plans and monitoring, odour assessments, including odour surveys and modelling, and ambient and indoor air quality monitoring.
Phlorum is an award-winning, Brighton-based, multi-disciplinary environmental consultancy that prides itself on providing transparent and honest professional services. Phlorum is UKAS certified by LRQA for ISO45001 (Safety), ISO14001 (Environment) and ISO9001 (Quality). Phlorum is also CHAS accredited, members of Constructionline, and are SMAS Worksafe accredited, leaving you safe in the knowledge that your requirements will be dealt with in a professional manner from start to finish.
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