Furthering air quality modelling and research
Air pollution reduces life expectancy and increases infant mortality. That much is well documented in Europe and internationally.
The challenge is to understand better the characteristics and impact of polluting gases and particles, to measure and monitor them, and to introduce remedial measures that protect human health and the environment. UK legislation and EU directives have created new responsibilities for industry, which increases the importance of accurate measurement in order to test compliance. Many businesses also need support as they attempt to reduce pollution without damaging their economic viability.
A lot of research is focused on the specific properties of airborne particles, which are the most dangerous component of the air breathed in Western Europe and contribute annually to large numbers of premature deaths.
These particles are also known as particulate matter (PM), a term used to describe airborne solid particles and droplets. They can be manufactured or natural, ranging from vehicle emissions to sea spray. And they vary in size, composition and origin.
The chemical composition of PM may include a range of sulphates and nitrates, ammonium salts, sodium chloride, elemental and organic carbon, a wide range of heavy metals, other minerals and biomaterials
Paul Quincey, an air quality scientist at the UK’s National Physical Laboratory, says that despite much effort it has not yet been possible to identify which chemical constituents of PM are responsible for the adverse effects on health. “No-one knows what it is about particles in the air that causes health problems,” he says. “This is possibly the biggest unanswered question in relation to air quality.”
Significant risk to health
A recent World Health Organisation (WHO) review confirmed that exposure to particles and ozone posed a significant risk to human health at concentration levels common in Europe. And a recent study for the European Union attributed 32,000 premature deaths in the UK to airborne particles annually.
Concerns about these health effects have led to the implementation of regulations – at international, national, and local level – to reduce emissions of harmful air pollutants and their precursors.
European regulation starts to bite
EU policy on air quality aims to control emissions from vehicles and aircraft, improve fuel quality and promote environmental protection in the transport and energy sectors. Pollution is a key issue in the European Community’s Sixth Environment Action Programme (EAP) – “Environment 2010: Our future, Our choice” – which aims to achieve levels of air quality that protect human health as well as the environment.
Air quality scientists at the UK’s National Physical Laboratory (NPL) are at the forefront of developments in new monitoring technologies. They are also helping industry to comply with legislation such as the Large Combustion Plant Directive, which has growing requirements for continuous emissions monitoring.
Companies which don’t meet the requirements will ultimately have to pay fines and may have to shut down their production processes. In the long-term, however, the legislation may decrease company costs through self-monitoring and streamline processes for emissions monitoring.
“The EU directives are starting to bite,” says NPL’s Rod Robinson. “Air quality monitoring is becoming more and more important and companies are facing obligations to install new self-monitoring equipment.”
Networking is good for you
Networks of air quality measurement sites in the UK provide public health information, ensure compliance with UK and EU regulation, and enable further air quality modelling and research.
NPL runs several of these networks. It manages the UK Particle Counting Network of eight monitoring stations for the Department of the Environment, Food and Rural Affairs with a powerful consortium including Birmingham University and King’s College London. The network consists of a series of monitoring sites at locations across the UK, with instruments that measure different particle sizes and chemical species.
The consortium is improving the quality and volume of data available to government as well as health and climate change researchers. It will help to inform policy around air pollution mitigation and increase scientific understanding of the characteristics of particulate matter in the UK.
“We run the network like a public service with a lot of information available to academics, including data capture rates and uncertainties,” says Dr Melanie Williams, environmental group leader in NPL’s Quality of Life Division. “Good policy making requires good data and that requires accurate measurement with verifiable results – that is what we deliver to Defra.”
NPL has also operated the UK Heavy Metal Monitoring Network since September 2004. It has 17 sites at rural, urban and industrial locations throughout the UK, monitoring metals including lead, cadmium, arsenic, nickel and mercury. The network is part of the UK’s response to the EU 1st Daughter Directive on lead and the forthcoming 4th Daughter Directive on arsenic, nickel, cadmium and mercury.
In addition to running networks, NPL provides traceability and data quality assurance to other monitoring groups working with Defra and local authorities. It also audits the performance of instruments used for air quality measurement, helping to ensure that data from the networks is accurate and verifiable.
Local authorities have the task of improving local air quality in highpollution areas and are undertaking remedial action in Air Quality Management Areas (AQMAs). NPL advises councils on how accurate measurement can help to assess improvements.
Some of the characteristics of airborne particles that may affect health are:
- Composition – the quantity of certain toxic components such as specific metals or organic compounds
- Oxidising capacity – a broad measure of the reactivity of material which enters the human body
- Size – smaller particles reach further into the body than larger particles and tend to have larger effects in proportion to their mass (see ‘Size Matters’ box)
- Number concentration – health effects may be caused simply by the number of reaction sites triggered in the body
- Source – particulates may be more toxic from some sources than Others
None of these properties are currently regulated by the EU’s air quality directives or UK legislation.
For historical reasons, the parameter that appears in the legislation is the total mass of particles below a certain diameter. PM10 refers to the total mass of particles smaller than 10 micrometres within a certain volume of air, and PM2.5 refers to particles smaller than 2.5 micrometres.
The choice of total mass as the key quantity has several disadvantages that have only recently become apparent:
- When particles are collected onto a filter for weighing, their mass will vary with moisture content. Also, some of the collected material is semi-volatile, arriving as solid or liquid but then evaporating rapidly. Other material can react with gases and increase in mass. The collected mass is therefore ambiguous and difficult to define
- The mass of the filter itself will also vary with moisture content. This variation, and the relatively small mass of the collected particles, means that measuring the mass of particles by subtracting a presampled filter mass from a sampled filter mass is a significant measurement challenge
- The emphasis on mass gives the dominant role to the larger rather than smaller particles, as a nanometre-scale particle will have negligible mass compared to a micrometer-scale one. This runs contrary to the current thinking that it is the smaller particles that are more important
It’s in the source
Some sources of particulates, according to studies, generate a more toxic and harmful mix. This needs to be properly understood by those developing control measures.
Combustion sources are particularly important for health because toxicological studies have shown that primary combustion-derived particles have a higher toxic potential.
These particles are often rich in transition metals and organic compounds, and have a relatively high surface area. By contrast, several other single components of the particulate mixture (e.g. ammonium salts, chlorides, sulfates, nitrates and wind-blown dust such as silicate clays) have been shown to have a lower toxicity in laboratory studies.
Particles entering human lungs may be quite different to those entering the air. This is because emitted primary particles can change by reactions in the air and by agglomerating together. There can also be a substantial contribution from secondary particles, those that are not emitted but which form within the atmosphere from gaseous precursors such as ammonium sulphate.
Road transport is a primary source of particulate pollution, though power stations and industrial plants are also contributors to particulate matter, sulphur dioxide and nitrogen oxides.
Industrial emissions pose a tricky problem for particulate monitoring. What is measured in the stack bears little relationship to the resultant ambient levels of particulate due to secondary particulate formation in the atmosphere. Heavy particulates may fall out of the plume relatively quickly and smaller particulate may form subsequent to emission. This would not be observed in the stack.
NPL monitors emissions from industrial and other sites. Accurate evaluation of stack emissions is important for regulatory and process control requirements, so it provides a comprehensive stack monitoring and compliance testing service.
The service is delivered by a sophisticated self-contained mobile laboratory with a range of on-site measurement capabilities and the ability to provide direct measurement and characterisation of a wide range of gaseous and particulate emissions.
NPL also provides advice on emission abatement, process optimisation and state-of-the-art process monitoring techniques, with all measurements traceable to NPL’s primary gas calibration standards.
The particulate problem
Particles also play an important role in climate change. Their direct impact is through absorption and reflection of solar radiation. But they also have an indirect impact through their effect on the formation of water droplets, which has a far more local and immediate impact on climate than greenhouse gases such as carbon dioxide.
The importance of atmospheric chemistry to the concentration of a pollutant, rather than simply the quantity emitted by its sources, is not unique. Ground level ozone, the next most serious pollutant in terms of health effects, is almost entirely formed within the atmosphere by photochemical processes.
Nitrogen dioxide, another major pollutant, is primarily formed within the atmosphere by the oxidation (by ozone) of nitrogen monoxide, which is emitted during combustion processes in larger quantities. However, the situation with airborne particles is distinguished by being considerably more complicated.
The nature of particles entering the air is extremely diverse. Their size varies from a few nanometres for carbon particles from diesel engines, to tens of micrometres for windblown dust, which is typically rich in silicon.
NPL has a rich legacy in environmental monitoring and is today developing new measurement techniques to support future industrial needs.
One of the challenges comes from particulates in the nano to micro range, which are becoming more common as nanotechnology finds its way into more technologies and industrial processes.
Nano-particulates are subject to increasing scrutiny as a potentially significant contributor to health problems. A landmark 2004 report by the Royal Society and Royal Academy of Engineering (Nanoscience and nanotechnologies: opportunities and uncertainties) recommended that ‘until more is known about the environmental impact of nanoparticles…. the release of manufactured nanoparticles into the environment be avoided as far as possible’.
As part of its response to the Royal Society recommendations, DTI is funding research into the standardisation of measurements of ambient nanoparticles. This is being carried out by the Valid Analytical Measurement (VAM) programme at NPL, which focuses on particulate measurement and provides traceability for measurements in the air quality networks.
New measurement techniques
The VAM research involves development of new calibration methods for two particulate measurement techniques: condensation-particle counters (CPC) and the size-mobility particle sizers (SMPS). NPL collaborates in this research with Birmingham University and research institutes in Germany and Switzerland.
NPL has also anticipated future measurement needs by developing a laser based instrument, DIAL (Differential Absorption Lidar). DIAL provides remote rapid and accurate measurements of airborne emissions up to 3km from an industrial plant.
The DIAL technology is particularly useful for remote measurements into inaccessible areas, measurement of total industrial site emissions, quantification of leaks and plume tracking from complex industrial plants. As attention shifts to assessing the overall impact of industrial sites, this technique is available to make the necessary measurements. NPL is also developing an airborne DIAL system which will improve the UK’s atmospheric research capabilities. There are several technologies used to automatically monitor airborne particles. In the UK, the most common is the Tapered Element Oscillating Microbalance (TEOM). Techniques based on betaattenuation and optical scattering are also used. Perhaps unsurprisingly, given the nature of the pollutant being measured, these techniques produce results that can differ greatly from each other.
Remote measurement of emissions from refineries using DIAL Calibration and comparison is made difficult by the lack of a realistic “standard particle source”. Wind tunnels can be used, but do not reproduce the complex and variable mixture of particles found in ambient conditions, notably the secondary and semi-volatile particles. Most evaluation is therefore carried out by parallel field trials with different instruments sampling the same outdoor air.
We know a lot about air pollution and how to measure it, yet significant challenges remain. The characteristics of particulates need regulation, and further study is required into the effects of nano-particles on human health. Businesses need help complying with new legislation and implementing self-monitoring.
In all of this, pollution monitoring networks have a vital and growing role. NPL will continue to provide data quality assurance, manage the networks and contribute its expertise to a pollution measurement and control regime that can protect human health and the environment.
Published: 10th Sep 2005 in AWE International