Poor air quality, both indoors and outdoors, is one of the top environmental risks to public health in the UK. Numerous studies show a direct relationship between air pollution and cardiovascular disease, respiratory disease, and cancers.

On a national and international scale, NPL’s Air Quality and Aerosol Metrology Group ensures robust measurement science plays a crucial role in air quality monitoring, thereby helping protect the environment and public health.

Current monitoring techniques

Adherence to set limits for concentrations of toxic gases and particulates in our air is mapped using monitoring networks across the UK, particularly in urban areas. This is how we know, for example, that congested urban roads, like Marylebone Road in London, often exceed health exposure limits.

In order to capture a picture of our exposure, these networks use a variety of monitoring equipment, ranging from diffusion tubes – which are low cost, accessible and easy to deploy over wide geographical areas, and give a time integrated measurement – to top of the range, high accuracy, time resolved reference instruments employing spectroscopic techniques. This combination of technologies is important, enabling us to achieve a regular view of changing pollution levels across an area and direct the most accurate monitoring to those locations most in need of it.

NPL also works with manufacturers, government, academia and the public to improve their understanding of low-cost air quality sensors. Government-run monitoring stations provide valuable high-quality data on the air we breathe on busy streets, but it is impracticable to have them on every street corner. Some people are keen to have the ability to track their exposure to air pollution wherever they go, with portable air quality sensors.

Manufacturers have started to produce low-cost air quality sensors, which are increasingly heralded as a way for governments to add to their existing network of data points and assess what difference their policies are having on the environment. However, portable sensor measurement techniques are not yet fully mature, and different sensors can give differing readings in the same location, even if they are manufactured by the same provider. Furthermore, the quality and reliability of measurements from such units have been seen to deteriorate over time.

NPL has extensive knowledge of sensors and data quality and advises government, academia, industry and the public about how to best use the data produced by air quality sensors. NPL scientists also validate the performance of low-cost sensors and has developed extensive facilities to test a wide range of such devices.

One example is the work we do to test lower-cost ammonia sensors. Ammonia is an important pollutant that can adversely affect not only plant ecosystems, but also human health, since it can react with other chemicals to produce particulates (PM2.5). Measures to cut ammonia emissions, mainly from intensive agriculture, are believed to be an important component in reducing human exposure to this particulate material. NPL has tested ammonia sensors from different manufacturers in exposure chamber facilities. The results provided manufacturers with the tools to improve the traceability and accuracy of their ambient ammonia measurements and provided end users and regulators with information about which sensors work best in which conditions.

“the Royal College of Physicians has found that inhaling particulates annually causes around 29,000 deaths in the UK”

The Royal College of Physicians has found that inhaling particulates annually causes “around 29,000 deaths in the UK, which may rise to around 40,000 deaths when also considering nitrogen dioxide exposure.” These associated health effects have led to members of the public taking action to reduce air pollution and buying low-cost sensors to monitor air quality and their exposure to it. By providing guidance on what data from these sensors can tell us, and how that information can be used, we can ensure the public can make more informed choices about their exposure.

However, the regulations guiding how air quality is monitored are not comprehensive. Existing networks give a good view of which pollutants are present, but they don’t necessarily allow us to identify their sources or map small changes in concentrations. NPL is working on a number of projects that promise to make the way we monitor air quality smarter and contribute to better regulations in the future, allowing us to better tackle the issue.

“more accurate measurements of air pollutants such as oxides of nitrogen must be performed to fulfil the requirements of the EC Directive on Ambient Air Quality and Cleaner Air for Europe”

Identifying pollution hotspots

NPL is engaged in air quality measurements via the Breathe London project, which uses the world’s most advanced network of air quality monitors to better understand any Londoner’s exposure to air pollution around the city. The data collected through this project is helping to build up a detailed picture of London’s air quality and identify areas with appreciable toxins in the air.

Air pollution monitoring sensors

More accurate measurements of air pollutants such as oxides of nitrogen must be performed to fulfil the requirements of the EC Directive on Ambient Air Quality and Cleaner Air for Europe (Directive 2008/50/EC).

The World Health Organisation reported that 4.2 million deaths every year are a direct result of exposure to a

mbient air pollution such as NO2, SO2, NH3, CO2 and CO. One of the most dangerous pollutants, NO2 gas, is produced by burning fossil fuels e.g. in diesel engines. Therefore, accurate measurements are essential to understand population level exposure, improve air quality models and emission inventories, to discern long-term trends in concentrations and to enforce air quality and vehicle emission legislation.

“the unique electronic structure makes graphene a sensitive detector, but not a selective one”

This is essential for the timely evaluation of air pollution mitigation policies, and to improve our understanding of the influence of man-made emissions on the climate system.

NPL experts, working with partners from the Graphene Flagship, Chalmers University of Technology, the Advanced Institute of Technology, Royal Holloway University and Linköping University, have created a low-cost, low-energy consuming NO2 sensor that detects NO2 levels in real time.

The graphene based NO2 sensor detects pollutant levels based on changes in its electrical resistance. The high sensitivity of graphene to the local environment has been shown to be highly advantageous in sensing applications, where ultralow concentrations of absorbed molecules induce a significant response in the electronic properties of graphene. The unique electronic structure makes graphene a sensitive detector, but not a selective one. The challenge for future research in this area is to dramatically improve the selectivity to deliver practical alternatives to current devices. Significant progress has been achieved here using straightforward functionalisation techniques tailored to minimise cross interferences for ultimate application in environmental monitoring and air quality.

There is a well-demonstrated global need for high sensitivity, low-cost, low-energy consumption miniaturised NO2 gas sensors to be deployed in a dense monitoring network which can be used to pinpoint and avoid high pollution hot spots. Such sensors, operating in real-time, can help to visualise pollution in urban areas with unprecedently high local resolution.

“the future of air quality monitoring is smart. We are already seeing the introduction of smart buildings that monitor air quality and adapt ventilation in line with toxic gas and particulate concentrations”

With the data provided by a dense network of sensors, governing bodies could adopt smart and flexible restrictive measures in specific areas recognised as being highly polluted. For example, city councils might dynamically restrict and divert traffic away from schools or hospitals. Furthermore, the public could use an app to plan their commutes around NO2 hotspots.

The future of methane monitoring

The amount of methane in the atmosphere today is higher than at any point in at least 800,000 years. Concentrations have grown by 150% since the Industrial Revolution, with levels surging particularly fast in the past decade. This is a big problem for climate change, as methane is a much more powerful greenhouse gas than carbon dioxide, causing 28 times more warming compared to CO2 over a 100-year timeframe.

Whilst methane doesn’t directly impact upon human health, the global increase in methane emissions is responsible for half of the observed rise in tropospheric ozone levels. Scientists at the University of York reported that Ozone has been responsible for about one million premature respiratory deaths globally per year.

To reduce methane emissions, we first need to understand where the methane is coming from. Current technologies for monitoring methane emissions are largely aircraft based. NPL leads in the development of some of these technologies, such as the DIAL system: a transportable laboratory that can accurately map emissions sources from individual sites.

Alongside experts from GHGSat, NPL is working on a European Space Agency (ESA) funded project to develop a ground-based calibration service for methane satellites. This project is investigating the feasibility of using a UK landfill site as a calibrated source for methane measurements from satellites. The NPL ‘Differential Absorption Lidar’ (DIAL) facility and Fugitive Emission Measurement System (FEDS) would be deployed to the site to characterise and quantify the emissions, providing the reference values for the methane emissions.

Smarter air quality monitoring through standardisation

The future of air quality monitoring is smart. We are already seeing the introduction of smart buildings that monitor air quality and adapt ventilation in line with toxic gas and particulate concentrations. As previously mentioned, people are able to have a personal exposure record by using portable sensors to inform decisions about their own exposure, such as changing their route to work.

Standardisation is crucial to ensuring that the information the sensors and equipment give us is reliable and can be used to inform important decisions. NPL has developed standard test protocols and helped developers and users evaluate instrument performance and demonstrate conformance with standard protocols.

As a result of these projects and the many others that NPL is currently actively engaged in, we are helping to meet the UK Government’s climate targets and tackle the growing issue of poor air quality. This work is increasing our understanding of emissions and pollution, which will support the safeguarding of nature and the health of the public.