The air that we breathe needs to be regularly monitored and assessed to ensure ongoing quality of life. There are proven links between pollution and adverse effects on our health as well as on environmental sustainability. Therefore, we need to ensure that the amounts of pollutants in the air do not go above levels where they become dangerous to our health or to the environment.

This is an issue that concerns the government, business and the public, and with just cause. A study by the European Commission estimated that air pollution was responsible for 310,000 premature deaths in Europe each year, while the World Health Organization estimates that out door air pollution causes 1:3 million premature deaths per year worldwide.

Having a better knowledge of how many toxins there are in our airspace helps to underpin environmental and health research, and support the modelling of any future impact. This provides an infrastructure which can quickly respond to changes, such as the specification of new pollutants, or events which lead to sudden increases in harmful emissions.

Highly accurate measurement science is instrumental to this whole process. It is critical to understanding the levels of man made and naturally occurring pollutants, and essential to developing strategies to mitigate short and long term impacts. It provides the basis for setting the limits and the ability to enforce them.

In the UK, the government has established several Air Quality Networks, through the Department for Environment, Food and Rural Affairs (Defra) that are organised into networks that target a particular kind of pollutant, using a particular measurement method. This information informs government policy, validates emission reductions, and provides warnings if pollutants reach dangerous levels.

The science behind air quality

Airborne particles are complicated and difficult to measure. They have the most serious effect on human health of all air pollutants. They also have a significant, but poorly quantified effect on climate change. Both of these factors are highly relevant to the current interest in the manufacture of nanoparticles for industrial purposes.

Airborne particles in outdoor air are a varied and changing mixture of natural particles (such as dust and sea salt), man made particles (such as carbon rich particles from engine exhaust) and particles formed within the atmosphere by reactions between gases (such as ammonia and nitrogen dioxide, which form ammonium nitrate). They vary in size from a few nanometres to tens of micrometres. Many different techniques are used to measure them. Standard methods for measuring airborne particles are generally classed in two groups: PM10 (particles of 10 micrometers in diameter or less) and PM2.5 (particles less than 2.5 micrometres in diameter).

A range of techniques is available for measuring airborne particles. The reference method involves weighing filters before and after they are exposed to the air being sampled. Some common automatic methods of measurement include:

• Tapered Element Oscillating Microbalances (TEOM), which measure the increasing mass of a filter by its effect on the resonant frequency of a vibrating support

• Beta attenuation monitors, which measure the increasing amount of material on a filter by its absorption of electrons emitted by a weak beta source

• Optical monitors, which can gauge the size of individual particles from signals scattered from a light beam and integrate this into a total volume of particles

NPL helped the UK’s Facility for Airborne Atmospheric Measurements (FAAM) to monitor the progress of the volcanic ash cloud from Iceland in 2010. The TEOM was incorporated to conduct real time measurements of particle mass concentration. The TEOM is a well established instrument that directly monitors airborne particle mass concentration, which could form the basis for determining whether levels, now used by the Civil Aviation Authority as safety limits, 0.2, 2 and 4 mg particles per cubic metre, are being exceeded.

Many of these instruments give different results from the reference method to varying degrees in differing circumstances, and have many variations. New technologies which give ever more accurate results are being investigated. A variation of the TEOM called the Filter Dynamic Measurement System (FDMS) is currently receiving a great deal of attention in the UK.

DIAL

A remote sensing system called a Differential Absorption Lidar facility (DIAL) provides rapid, accurate measurements of airborne atmospheric pollutants. The system is a self contained mobile laboratory with additional measurement equipment to monitor meteorological parameters and ambient gas concentrations.

The DIAL system can monitor atmospheric pollutants remotely at ranges of up to 3 km. The measurements produced are traceable to primary standards of gas concentration and free from interference and contamination. DIAL is particularly useful for measurements of emissions from tanks, flares and diffuse sources e.g. landfill sites.

DIAL uses a high energy laser source of tuneable wavelength that is transmitted over the measurement region. A small fraction of this light is scattered back by the aerosols and particulates that are present in the atmosphere. This is collected with a telescope and a very sensitive detector. The extent of the absorption is known from accurate laboratory data and this enables the concentration, and spatial distribution of the atmospheric pollutants to be determined directly.

The Differential Absorption Lidar provides rapid, accurate measurements of airborne emissions from industrial plant.

The DIAL technology can be used for a number of atmospheric campaigns including: • Remote measurements into inaccessible, hazardous or elevated areas • Wide area surveys of ambient air quality • Measurement of total industrial site emissions • Boundary fence monitoring • Identification and quantification of leaks • Plume tracking and source identification from complex industrial plant • Validation of emission estimates or modelling techniques

DIAL can measure simultaneously in the infrared, visible and ultraviolet spectral regions. It can monitor real time data for any gaseous species with characteristic absorptions in UV-VIS-NIR spectral regions including: SO2, NO2, NO, Ozone, Benzene, Toluene, Xylene and higher aromatics, Alkanes, Alkenes, petroleum and diesel vapours, Hg, HCl, N2O, HF and H2S.

DIAL is particularly useful in measuring and assessing ‘fugitive’ emissions. Emissions from known sources such as plant chimneys can be measured reasonably well; however, there can be, and are, significant fugitive emissions from other parts of a plant. This includes leaks, storage tanks and waste treatment or emissions from area sources like landfill. Studies in the United States and Canada, and historical data from Sweden shows measured total site emissions from refineries can be up to ten times higher than calculated.

Benefits of optical remote sensing

Conventional ambient monitoring techniques measure gases at a single point, whereas optical remote sensing uses a beam of ultraviolet, visible or infrared radiation to measure gas concentration within a line-of-sight path.

The benefit of using an optical technique like DIAL is that it can measure total site emissions, and take account of any upwind sources. It can also monitor diffuse sources such as landfill or lagoons. The results generated from optical techniques are therefore better to use for measuring regulatory compliance for industrial sites, evaluating impact and for early warning of any issues.

DIAL in operation

DIAL has been used to carry out many measurements of industrial emissions, since the technology was developed in the late 1980s.

Recent measurements, in the last four years, have included large scale studies of emissions of VOCs and benzene from refineries and other petrochemical plant in Texas, Netherlands and Belgium, measurements of methane losses from many landfill sites in the UK, USA, France and Ireland, and studies of emissions from other industrial sources in USA, Canada and Ireland.

A wide breadth of industrial emissions can be sampled including landfill gas, using passivated containers or pumped sorbent sampler tubes in series, to trap light, heavy and substituted VOCs.

Using automated thermal desorption and GC Mass Spectrometry with a flame ionisation detector, a range of complex sulphur, oxygen, and chlorine substituted light to heavy VOCs can be both identified and quantified.

There are also specialised services available for sampling contaminated land using soil probes, designed to allow VOCs to diffuse into sorbent sampler tubes inserted into the ground. These soil probes have been validated at a number of trials at commercial filling stations and refineries. The VOCs are then desorbed from the sorbent and analysed by GC Mass Spectrometry.

IR-DIAL

An infrared (IR) differential absorption Lidar (DIAL) (also capable of ultra violet measurements) has been developed to make field measurements of the distribution of total site emissions (controlled and fugitive) from petrochemical, landfill and other industrial installations.

The IR-DIAL was validated through a series of controlled field experiments. This included comparison to GC analysis and tests against controlled methane releases from a test stack, all detailing agreements on the order of ±20%.

The results were published in June 2011 and showed that in VOC measurements at a UK petrochemical site the American Petroleum Institute’s methodology of the time for calculating the emitted flux underestimated by a factor of 2.4. Also, in a similar field trial it was found that scaling traditional point measurements at easily accessible flanges and valves to represent all flanges and valves on a site led to an underestimation by a factor of 6.

In addition to petrochemical examples, field measurements can also be taken from a landfill site to demonstrate the advantages of the DIAL technique for monitoring area emission sources. In a case study it was found that active (still being filled) cells resulted in significantly greater methane emission rates (30 kg h-1) than closed (≤ 10 kg h-1).

Environmental measurement in the real world

Air quality refers to the levels of pollutants in air that are relevant to human health or ecosystems. The most important pollutants have changed over recent decades. Historically, coal burning in towns led to high levels of sulphur dioxide (and particles – see below), but this pollutant is now only a localised problem.

The most important gaseous pollutants for human health effects are currently ozone and nitrogen dioxide. Various government initiatives exist to monitor and control levels of these pollutants, or their precursors. For example NPL manages four of Defra’s Air Quality Networks. These Networks are based on aspects of particle measurement. Of all the particulate measurement techniques, black smoke appears to be the one most consistently associated with health effects. The Black Smoke

Network provides a valuable historical dataset on the impacts of the Clean Air Acts. It involves the measurement of carbon containing particulate matter from the burning of solid fuel and vehicle emissions. Black smoke sampling used an eight port sampler to draw air at a constant flow rate through a paper filter.

Suspended particulate matter was collected on the filter forming a stain, the darkness of which is measured by a reflectometer. These measurements are now made using an aethalometer.

Another network is the Particle Counting and Speciation Network, which was established to research the nature of particulate matter in ambient air. Particulates are known to exacerbate symptoms of cardiovascular and respiratory diseases, and are also known to contribute to climate change. Particle number has also been associated with health effects.

The Network provides information on the chemical composition of particulate matter, thereby providing information on sources and allowing the relevant chemical processes to be modelled more reliably. It also generates reliable datasets of airborne particle number, concentration and size at selected sites. Long time series measurements of particle number are important for studies into the cause of disease.

The Heavy Metals Network measures ambient levels of arsenic, cadmium, chromium, copper, iron, manganese, nickel, lead, platinum, vanadium, zinc, and mercury, in particulate matter, at 24 sites around the UK. Accurately measured concentrations of the most toxic metals are requirements of the new EC Air

Quality Directive and the Fourth Air Quality Daughter Directives. Sites are predominantly located downwind of large metals’ processing facilities and in areas with a high population. The PAHs (Polycyclic aromatic hydrocarbons) Network (PAHs) is a group of persistent organic compounds, some of which are toxic and/or possible or proven human carcinogens; they are produced via incomplete combustion of carbon containing fuels, especially coal.

Validating the air monitoring technology of tomorrow

My colleague Nicholas Martin has written before (AWE International, June 2008) about how NPL is helping small and medium enterprises in the development of the next generation of novel continuous emissions monitors (CEMs). This is part of the work we undertake through our dedicated suite of test and calibration facilities. Our work helps to provide regulators and industry with the best basis with which to monitor releases from industrial processes and to meet the quality requirements of the Environment Agency’s Monitoring Certification Scheme – MCERTS.

NPL also works with third parties to validate the research and technology they produce aimed at providing better quality air monitoring.

NPL, through its various projects, is constantly striving to deliver benefit to the government and customers through detailed understanding of air quality. Air quality monitoring techniques and strategies lead to improved legislation, reduced environmental impact and better health. But the process is continuingly evolving. New technology and greater understanding of measurement methods will lead to ever more accurate and consistent air quality measurements across the UK and Europe, and will guide decisions from healthcare to climate change mitigation in future. NPL, along with its partners and other measurement institutes across the globe, is continually working to ensure better air quality through measurement.

About the National Physical Laboratory

The National Physical Laboratory (NPL) is the UK’s National Measurement Institute. NPL occupies a unique position, sitting at the intersection between scientific discovery and real world application. Its expertise and original research have underpinned quality of life, innovation and competitiveness for UK citizens and business for more than a century.

Through this role NPL has developed considerable expertise in the measurement of airborne particles which affect air quality.

NPL’s primary role is to provide highly accurate techniques to underpin requirements for measurements made elsewhere. It has the expertise and access to equipment to help businesses assess their emissions and underpin the air quality monitoring innovations of the future.

There are areas where NPL also has considerable experience of the practical aspects of the measurements. Air quality measurements are one such area. NPL’s expertise has been called upon to play leading roles in the World Meteorological Organisation (WMO), and the European Air Quality Reference Laboratories (AQUILA). We are entrusted by Defra, the Environment Agency and NERC to lead UK monitoring networks as well as the provision of traceable measurements for climate change models.

These measurement projects include detecting gases such as ozone, water vapour, airborne particles from 1 nm to 10 mm in size, and VOCs, or volatile organic compounds, which are chemical compounds with significant vapour pressures that can be dangerous to health and the environment, e.g. ‘smog’.

NPL offers a complete service, accredited by UKAS to ISO 17025, for the sampling and analysis of complex VOC mixtures. Air quality surveys are applicable to many industrial and air quality applications, including: groundwater and soil contamination assessment, tanker loading emissions monitoring, detection of leaks from storage facilities and vehicle emissions.

Accurate monitoring of air quality is also vitally important for regulatory purposes and industrial impact assessment. Ambient air quality surveys at NPL include characterising industrial emissions in accordance with UK and EC legislation, monitoring roadside and urban air quality, validating pollution abatement measures and supporting corporate environmental policies.

In addition to the various commercial services offered by NPL, we are closely involved in a number of UK and European projects to monitor air quality.

NPL is also playing a major role in the new EU co-funded project AirMonTech, which will make recommendations to the European Commission on how to revise legislation covering air quality in Europe, and on what research is needed to help with this decision. Author Dr Melanie Williams has been Group Leader for Environmental Measurements at NPL since 2004. She has worked in technical marketing and spent 13 years at BP developing new catalysts for petrochemical and refinery processes.

For more information visit www.npl.co.uk

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Published: 01st Sep 2011 in AWE International