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CMES article September 2018
In New South Wales alone, there are up to 85 ambient air quality monitoring stations active at any one time. Most of them are able to detect particulates in the air and approximately half are also installed with gas analysers to measure pollutant gases such as NOx, SO2 and CO.
According to The National Environment Protection (Ambient Air Quality) Measure legislation the measurement of the pollutant gases must be carried out according to various national standards. For example, for the measurement of CO the AS/NZS 3580.7.1-2011/Amdt 1-2012 applies and this calls for the use of a direct read instrument such as an NDIR analyser. On the other hand, the measurement of NOx is prescribed by the AS/NZS 3580.5.1-2011 and use of chemiluminescence instrumentation is necessary.
Most of the instrumentation used for outdoor air quality monitoring is direct read instrumentation using detection methods such as NDIR or chemiluminescence. These analysers do not require the continuous use instrumentation gases such as carrier gases or detector gases for their operation. However, they do need a zero gas such as Nitrogen 5.0 grade or Zero Air and calibration gas mixtures which will often be prepared at 90% of the full scale deflection of the instrument. This single point high level calibration is generally adequate since these measurement techniques are quite linear.
Occasionally for R&D type activities, where a more broad spectrum of pollutants may be monitored, it would be appropriate to use an FTIR to investigate inorganic species or a gas chromatograph for VOC pollutants. Both setups will require a range of calibration gas mixtures. Additionally, the FTIR will require Nitrogen 5.0 grade as a purge gas and the gas chromatograph will require a carrier gas such as Helium 5.0 and perhaps detector gases such as Hydrogen 5.0 and Instrument grade Air if a flame ionisation detector is fitted, which would be typical when doing work with VOC pollutants.
A stainless steel regulator must be used for all calibration gases that contain ppm levels of corrosive components such as NO or SO2. When using calibration gas mixtures a single stage regulator will be suitable. The additional long term flow stability of a two stage regulator is not necessary this calibration event where the user is available to adjust the regulator pressure, if required. For further advice on cylinder pressure regulators, please email our specialty gases technical experts.
The most common gas that is monitored in indoor air quality monitoring is carbon dioxide, CO2. Sensors for CO2 are built into air conditioning units to turn them on when they detect the increased levels of CO2, which is assumed to be from the presence of humans who might appreciate a cool and comfortable room temperature. This is an energy saving application. Increasingly, formaldehyde is also monitored inside buildings on a frequent basis. A broad spectrum of VOCs are also monitored in occasional studies and their presence has been linked to something known as 'sick building syndrome'.
During the manufacture of the various indoor air quality monitoring gas sensors calibration gas mixtures are required to test and validate the sensor operation. For a comprehensive indoor air quality (IAQ) study it will be common to collect air samples in gas chambers and bring these to an environmental contract laboratory for analysis. The gas chromatographs used in this laboratory will be calibrated using sophisticated multi component VOC gas mixtures.
In a range of industrial settings it is common to measure both the factory emissions at the source (most likely a smoke stack) and also at the perimeter of the site. Air at the site perimeter should generally be similar to fresh natural air and free of toxic pollutants and nuisance odours. To determine that this is indeed the case perimeter air quality monitoring will often be undertaken. It is common to either use gas sensors installed on the perimeter fence, as might be the case for common BTEX pollutants in petrochemical processing and storage applications, or to gather air samples in gas-tight containers and transport these to an environmental contract laboratory for analysis.
The phrase BTEX is used to describe a collection of four volatile organic compounds: benzene, toluene, ethylbenzene and xylene. These are often representative of a wider VOC emissions profile are are among the most carcinogenic of the VOC compounds and are therefore commonly tested for.
In addition to petrochemical works and toxic BTEX emissions there are more benign reasons for conducting outdoor industrial air quality monitoring. For example almost every water treatment facility will have some level of hydrogen sulphide (H2S) emissions. The gas is produced by the bacteria that live in the dirty water and these bacteria are often engaged in the desirable breakdown of water pollutants. High levels of H2S are toxic to humans, but these levels are rarely encountered in the outdoor environment around a water treatment works. However, H2S is detectable by humans at extremely low levels and it simply smells bad with a characteristic stink of rotten eggs. To better understand and control the nuisance odours associated with these water processing facilities it is common to conduct perimeter monitoring of H2S using gas detectors, especially if they are close to urban neighbourhoods.
When BTEX sensors are used for perimeter air quality monitoring, or for higher concentration gas leak detection within the facility, they will be tested and calibrated using specialty gases calibration gas mixtures containing low levels of the BTEX components.
Sulphur dioxide perimeter monitoring can be conducted using gas detector sensors installed on the perimeter fence. These gas sensors require calibration prior to use and will generally be tested for functionality periodically during use. The gas detector sensors will be serviced at perhaps annual intervals at which point a highly accurate traceable calibration gas mixture will be used.
The gas chromatographs used in the laboratory, where ambient air samples are analysed, will be calibrated using sophisticated multi component VOC gas mixtures. They also require high purity instrumentation gases such as helium and nitrogen to function.