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Water Monitoring Assocation is a division of the STA

  What Do We Mean

What Do We Mean?

Explanation of common terms used in emissions monitoring

1. The difference between measurement and monitoring

These terms are often used interchangeably, but they have specific and different meanings:

·         Measurement of an environmental parameter involves assigning a numerical quantity its magnitude.

·         Monitoring involves interpretation of a number of measurements carried out over a period of time.  It usually involves an element of comparison, either with itself or with some external benchmark, e.g. an environmental quality standard or a guideline value. 

 Some other terms and concepts requiring clarification include:

·         Monitoring technique – the scientific principle behind the sampling and analysis.

·         Monitoring method – the documented procedure describing in detail how the monitoring technique should be used for a particular application.  Official standard methods are published by the European Committee for Standardisation (CEN), the International Standards Organisation (ISO), the British Standards (BS) Institute, and others.

·         Monitoring equipment – some standard monitoring methods specify in detail the exact instruments and apparatus that must be used, but many do not. 

·         Quantitative Monitoring – measures how much of it is present.

·         Semi-quantitative Monitoring – whether it is present in trace, minor or major amounts.

·         Qualitative Monitoring – measures what it is composed of.

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 2. Sampling and analysis

An environmental measurement includes both a sampling stage and an analysis stage (though it can sometimes be difficult to say where one ends and the other begins).  Sampling must be representative both spatially and temporally.  Sampling is itself a very specialised subject.

The fundamental principle of sampling is that a small amount of collected material should be a reasonable estimate of the overall character of the material.  For this to apply, the sample must be a representative sample.  The number and locations of samples needed depends on how homogeneous the bulk material is, in both spatial and temporal terms.  For a very homogeneous material, only a few samples may be required; if it is non-homogeneous, many more samples will be required.

 Some other terms and concepts requiring clarification include:

·         Analyte – the chemical species being investigated (qualitatively or quantitatively).

·         Measurand – often, the analyte is not measured directly, but by measuring another parameter that is proportional to it.  In such cases the term measurand (or sometimes detrerminand) is used.  For example, in quantifying the temperature (the measurand) of gases emitted by a chimney stack using a thermocouple, the actual quantity measured is millivolts generated at the thermocouple junction.

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3.  Different types of emissions monitoring

There are many ways of characterising different types of monitoring.  A convenient classification is: 

·         Periodic monitoring - measurements carried out at periodic intervals, e.g. once every three months throughout the year.

·         Continuous monitoring - measurements carried out continuously, with few if any gaps in the data produced.  Equipment used for this purpose are referred to as continuous emissions monitors (CEMs) or automatic monitoring systems (AMS). 

 Periodic monitoring can be carried out automatically or manually.  Automatic monitoring uses an instrument that (usually) gives a direct readout of pollution levels in real time.  Alternatively, a manual technique may be used, where the pollutant is sampled on site but is usually analysed later in the laboratory.  Samples may be obtained over fairly lengthy periods of several hours, or may be so-called spot samples or grab samples collected over a period of seconds to a few minutes.

CEMs, on the other hand, nearly always use automatic techniques.  Some techniques measure the pollutant concentration in situ. Examples are opacity meters where the reduction in intensity of an optical beam is measured after it has travelled across the duct.  Other systems are extractive, where a sample of the exhaust gas is drawn continuously from the stack to a nearby instrument. An example is a chemiluminescence analyser for measuring NOx.  

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4.  Different categories of CEMs

CEMs - especially those for monitoring particulate emissions - fall into three categories.  It is important to distinguish between the types:

i)                    Quantitative CEMs - fully quantitative instruments where a valid relationship exists between the measured parameter (e.g. opacity) and the particulate concentration, a calibration can be made, and the concentration is given in units of mg/m3.

ii)                   Indicative CEMs – these are semi-quantitative.  The readout/ output of the instrument increases as the particulate concentration increases, but the relationship will not necessarily be linear and the readout will not necessarily be in mg/m3 (it could be, for example, percentage obscuration). These CEMs are indicative of the arrestment plant performance and can show trends in levels of particulate emissions, etc.

iii)                 Arrestment failure CEMs – these are simple go/no-go indicators, used to show arrestment plant failure, e.g. bag-filter bursting.  Such a significant increase in particulate levels produce a jump in the output of the instrument which then triggers an alarm.  Although this type of CEMs is itself sometimes referred to as an “alarm”, it should be pointed out that audible and visual alarms can be linked to all three types of CEM.

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5.  Different types of emissions to air

There are two main categories of industrial emissions to atmosphere:

§         Controlled releases – the emissions are managed in some way, either as part of a process or as part of a control/ abatement mechanism and the emissions are therefore quantifiable.

§         Fugitive releases – these are, literally, releases that cannot be captured.  They are uncontrolled and often dependent on external conditions (e.g. wind) which makes them difficult to quantify with any reasonable degree of certainty.  Another definition of a fugitive emission is a release that is unintentional.  An oil refinery may have a quarter of a million pumps, valves and flanges that can potentially leak, making it impractical to measure the emissions from every source.

Emissions can also be described in terms of their spatial characteristics, usually as a point source, line source or area source.  It is important to recognise that that each of these can be either controlled releases or fugitive releases, as shown in the table below.  It is controlled point sources (e.g. chimney stacks and vents) that are most commonly monitored. 

Table 1.  Categories of emissions to atmosphere


Point source

Line source

Area source

Controlled release

Emissions from fixed-location plant and often (but not always) released to atmosphere via a vent, duct or chimney stack

Tail-pipe emissions from vehicles driving along a road

Open process tanks, storage tanks with floating roofs

Fugitive release

Intermittently leaking valve

Dust re-suspended in a vehicle’s wake; wind whipping of dusty material on an open conveyor belt.

Wind-whipping of a stockpile of dusty material

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Last modified: December 01, 2008

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