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Analytical methods, techniques and instruments
The government's Air Quality Strategy for England, Scotland,
Wales and Northern Ireland 2007 sets national air quality
standards to protect human health, and is the main policy
instrument for improving air quality. Policies set out in the strategy
support the achievement of national air quality objectives and
EU air quality limit values.
The Air Quality Strategy identifies air quality standards for nine
pollutants and a timescale for achieving these standards.
Objectives for seven of the pollutants have been set in regulations for
the purposes of the Local Air Quality Management Regime. Local
authorities are required to work toward achieving these objectives. The
government runs the national monitoring network for air quality and
publishes information on national and local air quality.
Local authorities must regularly review and assess air quality in their
area. If air quality objectives are being breached or are at risk, they
must declare an air quality management area and draw up an action
plan to work towards achieving the objectives.
Local authorities may also apply for powers to undertake roadside
emissions testing of vehicles. They are responsible for the regulation of
emissions from around 500 installations under the Pollution Prevention
and Control Regime, 17,000 small industrial processes under the Local
Authority Pollution Prevention and Control (LAPPC) regime and many
other sources under the Clean Air Act.
Air quality standards
Objectives based on the above national standards have been set for
certain pollutants. Local authorities must assess whether the air quality
standards will be met in their area by the specified target date. If the
objectives are not met, local authorities must establish Air Quality
Management Areas (AQMAs) and ensure that the standards will be
met. The decision as to where exactly the air quality objective applies
in any given case rests with the local authority.
Around 180 AQMAs have been designated so far by local authorities.
The majority of these are in respect of the objectives for nitrogen
dioxide and PM10 and are mainly as a result of emissions from road
transport. Fewer than ten AQMAs have been designated as a result of
emissions of sulphur dioxide from industrial sources regulated by local
authorities, the Environment Agency or the Scottish Environment
Protection Agency.
Sampling principles
The fundamental principle behind any sampling activity is that a small
amount of collected material should be representative of all the
material being monitored. The number and location of samples that
are needed to make up a representative sample depends on how
homogeneous the material is. If it is very homogeneous, only a few
samples may be required. If the material is heterogeneous, many more
samples will be required.
This fundamental principle applies as much to sampling stack gases
as it does to any other type of sampling. Although the gas in a stack
might be thought of as being more uniform than, for example, a
stockpile of coal, gases in stacks can become non-homogeneous.
This may be due to differences in chemical composition, or differences
in temperature and velocity, which may lead to stratification and
swirling. Where the gas is also carrying particulates along the duct,
there is likely to be even less homogeneity. Here, special measures
must be taken to ensure samples are representative.
For gases carrying particulates, the sampling approach has to address
two effects.
Firstly, inertial effects introduced by gravity and the duct geometry lead
to the particles being unevenly distributed in the duct. Samples must
be obtained from multiple sample points across the sampling plane to give an overall average of the
particulate emission. Rules have been developed specifying where
these sampling points should be located and they are provided in the
Environment Agency TGN M1. In the case of a cross-duct CEM monitoring particulates, the average particulate concentration is
obtained as an integrated measurement across the duct.
Secondly, for extractive methods, the sample must be collected
isokinetically (at the same speed as the flow in the duct).
Where the measurement is of concentrations of gaseous species
alone, a sampling location where the gases are well mixed should be
chosen. If gases are well mixed, it is possible to demonstrate that
sampling can be carried out from a single sampling point in the
sampling plane. However, if the mass emission rate is to be calculated,
the gas volumetric flowrate will need to be measured; this will require
velocity measurements to be made at several points across the
sampling plane. Some pollutants, for example metals and dioxins, are
present in both particulate and vapour phases. Other pollutants such
as hydrogen chloride may be present in an aerosol phase and vapour
phase. Aerosols are normally treated as particulates. In all such cases,
isokinetic multi-point sampling is required.
Choice of sampling method, technique
and instruments
There is a wide choice of monitoring approaches, analytical
techniques, published methods and equipment that can be used to
carry out stack emissions measurements. It is important that each of
these is chosen to be suitable for the application in question.
The sampling approach, technique, method and equipment that are
chosen can have different affects on the requirements for access,
facilities and services. Though the precise requirements can vary, the
following will always be required:
- A safe means of access to the sampling position
- A means of entry for sampling equipment into the stack
- Adequate space for the equipment and personnel
- Provision of essential services, such as electricity
The different approaches to monitoring
stack emissions
Stack emissions monitoring can be classified into two types:
Periodic measurements - a measurement campaign is carried out at
periodic intervals, for example, once every three months. The sample
is usually - but not always - withdrawn from the stack (extractive
sampling). An instrumental or automated technique may be used,
where the sampling and analysis of the substance is fed to an on-line
analyser. Alternatively, a technique may be used where a sample is
extracted on site and analysed later in a laboratory. Samples may be
obtained over several hours, or may be so-called 'spot' or 'grab'
samples collected over a period of seconds to several minutes.
Continuous emissions monitoring systems (CEMs) - automated
measurements carried out continuously, with few if any gaps in the
data produced. Measurement may be carried out in situ in the stack
(for example, cross-duct monitoring), or extractive sampling may be
used with an instrument permanently located at or near the stack.
CEMs are also referred to as Automated Monitoring Systems (AMS),
particularly in mainland Europe.
The main characteristics of the two approaches are summarised in the
table below. One approach is not inherently superior to the other; both
have their own strengths and weaknesses depending upon the
application. In general, however, CEMs provide increased confidence
for both regulatory purposes and process control.
Periodic sampling of gases
For monitoring gases, the range of sampling equipment and apparatus
is very wide. However, they can be grouped conveniently into
automated techniques and manual techniques. For automated
techniques, the sampling and quantification stages are conventionally
considered to take place almost simultaneously, within an analyser.
With manual techniques, the sample is taken and then the
quantification and analysis takes place as a discrete, later stage.
The main steps in measuring gaseous pollutants are as follows:
- Stage 1: representative sample of source gas extracted through
a probe and filtered
- Stage 2: gases collected in an appropriate medium
- Stage 3: the sampled substance is analysed using an
appropriate technique
When gases are measured using automated/instrumental techniques,
such as gas analysers, Stage 2 is omitted, and the sample goes
directly to the analysis stage (Stage 3). In contrast, when gases are
measured using manual techniques, Stage 3 is usually carried out
away from the site at an analytical laboratory.
The location requirements for measuring gas concentrations are less
exacting than for particulates, as variations in velocity tends not to
affect the homogeneity of the gas concentration. This means that the
proximity to bends, branches, obstructions and fans is less important.
However, sampling after the ingress of dilution air must be avoided.
However, sometimes it is necessary to report mass emissions rates,
such as g s-1, to demonstrate environmental compliance, or for
pollution inventory reporting or emissions trading purposes.
Calculation of mass emissions rate requires the measurement of gas
volumetric flow rate through the duct. This requires velocity
measurements to be taken at different points across the sampling
plane. Measurements to determine stack gas velocity and volumetric
flow rate should be made in accordance with ISO 10780:1994 or BS
EN13284-1. A suitable sampling location should therefore conform to
the particulate monitoring flow stability requirements.
Isokinetic sampling
Due to the wide range of particle sizes normally present in process
emission streams, it is necessary to sample isokinetically to ensure
that a representative sample of the particulate emission is obtained.
Only very fine particles below 5 microns aerodynamic diameter behave
like a gas and do not normally require isokinetic sampling. Isokinetic
sampling is achieved when the gas enters the sampling nozzle at the
same velocity and direction as the gas travelling in the stack or duct.
If the sampling velocity is less than the isokinetic ratio (usually
expressed as a percentage), the actual volume sampled will be less
than it should be. At first sight, it would appear that the emission will
be underestimated. However, because the sampling rate is too low,
there is a divergence in flow around the sampling inlet.
Small particles are able to follow the flow and a percentage of them
will not be sampled. Larger particles, on the other hand, are not able to
follow the flow because of their greater inertia, and more of these
particles will enter the sampler. Thus a sub-isokinetic sampling ratio will
lead to a bias in the sampled particle size distribution towards the
larger particles. This could lead to an overestimate of the particulate
concentration depending on the original size distribution.
Sampling at a rate in excess of the isokinetic ratio will lead to a bias in
the sampled particle size distribution towards the smaller particles.
This could lead to an underestimate of the emission concentration
depending on the original size distribution.
In situ or extractive monitoring
As the term suggests, monitoring can be done in situ (e.g. within the
stack itself) or extractive (sample withdrawn from the stack). Both
periodic and continuous monitoring can be performed using in situ or
extractive techniques.
Remote sensing
Remote sensing methods allow measurements to be made directly in
the atmosphere without obtaining samples. The average concentration
of a specifically targeted pollutant is determined over an extended
measurement path, rather than at a localised point. Some methods
allow the concentration to be spatially resolved. Others give the
average concentration over the whole path length, which finds
application in assessing the transfer of pollutants across site
boundaries and along roads and runways, but the difficulty of
interpreting integrated-path data should be recognised.
Differential optical absorption spectroscopy (DOAS) instruments use a
double-ended system, which measures the average concentration
between the instrument and a reflector up to hundreds of metres away.
The system is able to measure many common pollutants including
SO2, NO, NO2, H2S, O3, benzene, toluene, xylenes and formaldehyde.
Laser interferometry detection and ranging (LIDAR) can measure
aerosol particles.
Measurement by remote sensing techniques tends to be expensive
because of the complexity and sophistication of the equipment and
data handling facilities.
Remote methods lend themselves to mobile sampling: this may be
vehicle-mounted instruments for carrying out measurements at a large
number of locations, or for measuring the pollution concentration
profile along a given route.
Airborne systems using in situ continuous analysers have been used
for some specialist applications, such as tracking power station
plumes across the North Sea. Such systems have the advantage of
greater freedom of movement, three dimensional capability and higher
speed of traverse, but are of course so expensive as to be only
justified for specialist investigations.
Microscopic analysis
Samples of particulates obtained from smoke, dust, grit or fumes can
be analysed microscopically to aid identification and characterisation,
which in turn may help in identifying the source, if unknown.
Optical Light Microscopy (OLM) uses the visible, or near visible, portion
of the electromagnetic spectrum, whereas Scanning Electron Microscopy
(SEM) analyses the surface of solid objects, producing images of higher
resolution than optical microscopy. SEM produces representations of
three dimensional samples from a diverse range of materials.
Gas liquid chromatography
Gas liquid chromatography involves a sample being injected onto the
head of the chromatographic column. The sample is transported
through the column by the flow of inert, carrier gas. The column itself
contains a liquid stationary phase, which is adsorbed onto the surface
of an inert solid. The principle is that the gaseous components of the
air sample injected take different lengths of time to pass through the
column depending upon their chemical structure.
By using known gases as standards, it is possible to identify the
components within the unknown sample based upon the time it takes
them to pass through the column. Different columns are used to
separate different components.
Mass spectrometry
The modern gas chromatograph is a fairly complex instrument,
mostly computer controlled. The samples are mechanically injected,
the analytical results are automatically calculated and the results
Gas Chromatography printed out, together with the pertinent operating conditions in a
standard format.
In air pollution analysis, gas chromatography can be used for odour
analysis, along with mass spectroscopy.
Mass spectrometry is an analytical tool used for measuring the
molecular mass of a sample. For large samples such as biomolecules,
molecular masses can be measured to within an accuracy of 0.01% of
the total molecular mass of the sample.
For small organic molecules, the molecular mass can be measured to
within an accuracy of 5 ppm or less, which is often sufficient to confirm
the molecular formula of a compound. In environmental work, mass
spectrometry is often used to identify organic pollutants such as Poly
Aromatic Hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs).
Mass spectrometers can be divided into three fundamental parts,
namely the ionisation source, the analyser and the detector.
The sample has to be introduced into the ionisation source of the
instrument. Once inside, the sample molecules are ionised and the
ions extracted into the analyser region of the mass spectrometer,
where they are separated according to their mass-to-charge ratios.
The separated ions are detected and this signal sent to a data system
where the mass-to-charge ratios are stored together with their relative
abundance for presentation in the format of a spectrum.
The analyser and detector of the mass spectrometer - and often the
ionisation source too - are maintained under high vacuum to give the
ions a reasonable chance of travelling from one end of the instrument
to the other without any hindrance from air molecules. The entire
operation of the mass spectrometer - and often the sample
introduction process also - is under complete data system control on
modern mass spectrometers.
Atomic absorption spectrophotometry
Atomic absorption spectrophotometry is an analytical technique used
to measure a wide range of elements in materials such as metals,
pottery and glass.
The sample is accurately weighed and then dissolved, often using
strong acids. The resulting solution is sprayed into the flame of the
instrument and atomised. Light of a suitable
wavelength for a particular element is shone through the flame, and
some of this light is absorbed by the atoms of the sample.
The amount of light absorbed is proportional to the concentration of
the element in the solution and hence in the original object.
Measurements are made separately for each element of interest in turn
to achieve a complete analysis of an object and thus the technique is
relatively slow to use. However, it is very sensitive and it can measure
trace elements down to the part-per-million level, as well as being able
to measure elements present in minor and major amounts.
Author
Andrew Taylor is currently a Chartered Safety Practitioner working with SHEilds Ltd
as a tutor on the NEBOSH Diploma in Environmental Management. He has
extensive experience in Health, Safety and Environmental Management, most
recently in consultancy and construction.
The NEBOSH Diploma in Environmental Management is delivered via
e-learning and is accessible worldwide. This benefits students who have work
and family commitments as a cost effective way in which to develop in an
environmental career.
For more information contact us at SHEilds Ltd by calling +44 (0)1482 806805,
where you can speak to course advisors directly.
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