Instruments for measurements of air quality may vary in complexity and price from the simplest passive sampler to the most advanced and most often expensive automatic remote sampling system based upon light absorption spectroscopy of various kinds. Air monitoring methodologies can be divided into four main generic types, covering a wide range of costs and performance levels: continuous analyzers, active manual samplers, passive samplers and remote sensing devices. Each of these methodology types have advantages and disadvantages. Each type can be particularly useful in achieving certain monitoring objectives. Therefore it is important to consider each type of monitoring method in optimizing a monitoring network design.

The table below lists four typical types of instruments, their abilities and prices:

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Relatively simple equipment is usually adequate to determine background levels (for some pollutants), to check Air Quality Guideline values or to observe trends. Also for undertaking simple screening studies, passive samplers may be adequate. However, for complete determination of regional air pollution distributions, relative source impacts, hot spot identification and operation of warning systems more complex and advanced monitoring systems are needed.

When data are needed for model verification and performance, expensive monitoring systems are usually required.

Passive Samplers

Simple passive samplers have been developed for surveillance of time integrated gas concentrations. These types of samplers are usually inexpensive in use, simple to handle and have an adequate overall precision and accuracy dependent upon the air pollution concentration level in question. This method has been used in industrial areas, in urban areas and for studies of indoor/outdoor exposures for variety of pollutant like as ammonia (NH3), benzene-toluene-xylenes (BTX), sulfur dioxide (SO2), nitrogen oxides (NOx), ozone (O3), hydrogen fluoride (HF), hydrogen chloride (HCl), aldehydes, and volatile organic compounds (VOC).

Passive samplers include items such as diffusion tubes and badges. They tend to be simple and low cost, and can be deployed in large numbers with no reliance on access to electrical connections. This type of sampler is useful for screening studies, for mapping, and for baseline studies. While the samplers are often used for monitoring O3, NOx and SO2, the technology is unproven for some pollutants. Passive samplers are labor-intensive for their deployment and analysis. Passive samplers generally provide only monthly or weekly averages.

Passive samplers are an excellent tool for saturation sampling. This involves the collection of many samples in a small, well-defined area over a short duration, to provide an in-depth characterization. Saturation sampling is typically conducted to gather data necessary to properly site long-term monitoring devices. The passive sampler incorporates an adsorbing surface, pre-treated depending upon the target gas. The adsorbing surface is placed within a cylindrical enclosure that has a diffusive surface, allowing the target gas to reach the adsorbing surface. The sample is exposed for between 1 and 7 days or more depending upon the target gas and the expected ambient concentration. The sample is then extracted and analyzed using a variety of standard laboratory methods.

Active samplers

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Active samplers draw ambient air through a collecting medium for some specified time, typically 24 hours, with the volume of air being metered. The collecting medium is subsequently analyzed and the concentration of pollutant in the sampled air is determined. Active sampling methods are usually low cost and easy to operate. The active sampling methodologies offer reliable performance, with an extensive historical database because most of these methods have been in operation for many years. Active sampling methods require labor-intensive sample collection and analysis, and require laboratory analysis after the ambient air sample is collected.

Manual sampling is event-specific, that is, the sampler usually operates over a fixed period of time accumulating and integrating sample. Integrating measurement methods, although fundamentally limited in their time resolution, are useful for the assessment of long-term exposure, as well as being invaluable for a variety of area-screening, mapping and network design functions. Manual sampling is still widely used world-wide because manual methods offer a wider variety of pollutant monitoring and can be relatively straightforward.

In the past, active sampling of gaseous pollutants was typically carried out. This can be done by using wet absorption techniques, where sample air is introduced into a liquid reagent through impingers. The pollutant is absorbed in the reagent and the reagent is then analyzed using various methods (usually some sort of chromatography) to determine the concentration of pollutant in the batch sample. Another method of batch sampling is where sample air is drawn through a porous bed of solid adsorbent over a period of time. The pollutant is then extracted from the adsorbent and analyzed. Collection efficiencies from this type of sampling apparatus can often exceed 90%. Sampling for most ambient air toxics involves the collection of grab samples and subsequent analysis using gas chromatography–mass spectrometry (GC-MS) or high performance liquid chromatography. The grab sample is usually collected on an absorbent material or in a specially treated, chemically inert cylinder or bag. The sampling method is dependent upon the target compound, analysis method and sampling environment. Detailed information on the standard sampling methods used in the U.S. Environmental Protection Agency (U.S. EPA) air toxics sampling program is available online.[1]

Canister sampling

Canister sampling can be used for volatile hydrocarbons up to C9. Air samples are collected in stainless steel canisters by the aid of a pump or just by opening the valve of an evacuated canister. The canisters are sent to the laboratory for analysis and then cleaned by evacuating it.

Adsorbent tubes

Adsorbent tubes can be used for sampling of a wide number of volatile organic compounds. The tubes can be filled with different kinds of adsorbents, depending of which components of interest. When used as a passive sampler, there is no need for any extra equipment. To decrease the minimum sampling period or to improve the detection limit, the tube can be connected to a pump. Adsorbent tubes are not suitable for some of the most volatile hydrocarbons.

Absorption bottles

The most commonly used active device for gaseous sampling has been the bubbler with an absorption solution, often together with a filtration system. A chemical solution is used to stabilize the pollutant for subsequent analysis with minimum interference by other pollutants. Samplers have also been used with impregnated filters based on the iodide absorption method. The flow is set with a restrictor and measured with a mass flow meter. In the sequential version of these samplers the desired start time can be set to start sampling at the same start time every day at 24 hour intervals.

Impregnated filter sampling

A relatively simple alternative to the use of solutions for absorption and chemical reaction is to use chemically impregnated filters. These filters are prepared by dipping filters into a solution of the selected chemical and drying them before sampling commences. This sampler consists of a glass bulb with an impregnated filter inside. The impregnated filter bulb is connected to a calibrated pump that draws a steady airflow through the filters. After exposure, the filter and the pollutant of interest react with the chemical on the filter. The filter is sent to the laboratory for analysis. The detection limit is better than for the other methods but the method is more labor intensive and depends of extra sampling equipment such as a high precision electric pump.

High and low volume sampler

For measurements of ambient suspended particles the most accurate way to determine aerosol mass concentration is to pass a known volume of air through a filter. Each filter has to be weighed unexposed, before being installed in the sampler. The weighing should be performed in weighing, the filter is placed in the plastic bag with zip tightening and marked with station identification and/or number.
Size selective samplers
A variety of sampling devices are available that segregate collected suspended particulate matter into discrete size ranges based on their aerodynamic diameters. These particle samplers may employ one or more fractionating stages. The physical principle by which particle segregation or fractionation takes place is inertial impaction. Therefore, most such devices are called impactors. Other impactors have been developed to fractionate suspended particles into two size fractions, i.e., coarse (from 2.5 to 10 μm) and fine (less than 2.5 μm). Although these virtual or dichotomous impactors operate like a typical inertial unit, large particles are impacted into a void rather than an impervious surface.

High volume PUF-sampler

The high volume PUF-sampler can be used for sampling of a wide range of organic pollutants like poly-aromatic hydrocarbons (PAH), dioxins, pesticides (like DDT), etc. The sampler consists of a glass cylinder and a filter holder. The glass cylinder holds two polyurethane foam (PUF) plugs for trapping the gas phase of the pollutants. The filter holder in front contains a glass fiber for collecting pollutants condensed on particles. The air is drawn through the sampler by a pump and 500 m3 of air would be a typical sample volume for a 24-hour sample.

Continuous monitors

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The analyzers are connected to a data acquisition system and an automatic gas calibration unit to provide regular quality control checks for the data. Continuous analyzers provide high resolution measurements (typically hourly averages or better) at a single point for most of the "Criteria Pollutants" (SO2, NO2, CO, O3 and PM) as well as for other important species such as VOC.

The sample is analyzed on-line and in real time, usually by electro-optic methods: UV or IR absorption, fluorescence, or chemiluminescence are common detection principles. To ensure that data from continuous emission monitoring systems are accurate and reliable, a high standard of maintenance, operational and quality assurance and quality control procedures is invariably required.

The advantages of continuous analyzer systems are that they offer a proven technology, high performance, hourly data, and/or on-line information. Disadvantages of continuous emission monitoring systems include the complexity and cost of the instrumentation, the requirement for a high level of skill in the operation of the instrumentation, and high recurrent costs.

Some monitoring networks incorporate mobile monitoring stations to improve spatial scales. It is possible continuous analyzers are fitted into a special enclosure on the back of a truck or on a trailer. Mobile stations can be good for special studies including complaint investigation. However, there can be durability and stability issues with the instruments, particularly when driven over rough roads. It can also take time to ensure the instruments are stable and therefore, it may not be possible to move the station on a daily basis. A good source of power supply is necessary and it may be difficult to ensure stable and continuous power at all locations.

Methods and instruments for measuring continuous air pollutants must be carefully selected,
evaluated and standardized. Several factors must be considered:
  • Specificity: respond to the pollutant of interest in the presence of other substances
  • Sensitivity: range from the lowest to the highest concentration expected
  • Stability: remain unaltered during the sampling interval between sampling and analysis
  • Precision: accurate and representative for the true pollutant concentration in the atmosphere where the sample is obtained
  • Response time: short enough to record accurately rapid changes in pollution concentration
  • Ambient temperature and humidity: no influence on the concentration measurements

Remote sensors

Remote sensors have recently been developed. They use long-path spectroscopic techniques to make real-time concentration measurements of a range of pollutants. The data are obtained by integrating along a path between a light source and a detector. Long-path monitoring systems can have an important role in a number of monitoring situations, particularly in proximity to sources. Remote sensing systems provide path or range-resolved data with multi-parameter measurements and are useful near emission sources.

However, the remote sensing systems are very complex, expensive and difficult to support, operate, calibrate, and validate. Data from remote sensing systems are not readily comparable with point data, and the operation of remote sensing systems is susceptible to problems due to atmospheric visibility and other interferences.

Typical air pollutant concentrations and methods of measurement

The table below lists the typical air pollutant concentration of interest involved in monitoring air quality:

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The most commonly used methods for automatically monitoring air pollutants such as those above are:
  • Sulfur dioxide (SO2): Measured by the fluorescent signal generated by exciting SO2 with UV light.
  • Nitrogen oxides (NOx): Measured by the chemilumiscent reactions between NOx and O3.
  • Ozone (O3): Measured by an ultraviolet absorption analyzer which determines the ozone concentration by the attenuation of 254 nm UV light along a single fixed path cell.
  • Particulate matter (PM-10, PM-2.5 and TSP): Measured by gravimetric methods including true micro weighing technology.For automatic monitoring an instrument named "Tapered Element Oscillating Microbalance (TEOM)" has been most frequently used. Measurement on filter tape using the principles of beta attenuation for estimating 30 minute or one hour average concentrations of PM-10 or PM-2.5 has also been used.
  • Carbon monoxide (CO): In urban air pollution studies, a non-dispersive infrared photometer utilizing gas filter correlation technology and state-of-the-art optical and electronic technology is used to measure low concentrations of CO accurately and reliably.
  • Hydrocarbons (Methane and NMHC): Measured using a flame ionization detector (FID). However, problems in power supplies may interrupt these continuous measurements.
  • Volatile Organic compounds (VOC): Measured by gas chromatography and photo-ionization detector (PID).

Meteorological data

Meteorological data are important input data to a system used for information, forecasting and planning purposes. Meteorological data are needed from the surface, normally collected from that are 10 m high, as well as up to the top of the atmospheric boundary layer.

Meteorological surface data such as winds, temperatures, stability, radiation, turbulence and precipitation are automatically measured and transferred to a central computer via radio communication, telephone or satellite. Measuring such data requires sensors for at least the most important parameters such as:
  • Wind speeds
  • Wind directions
  • Relative humidity
  • Temperatures or vertical temperature gradients
  • Net radiation
  • Wind fluctuations or turbulence
  • Atmospheric pressure
  • Precipitation

References

  1. Air Toxics - Monitoring Methods, U.S. EPA, Ambient Monitoring Technology Information Center (AMTIC).

Further reading

Publishing Note:

This article was written by Massoud Estiri, a member of this wiki, and uploaded by him in a pdf format. It was then transformed into this wiki's format using the Wikitext markup language and the Wikitext Editor by Milton Beychok, the organizer of this wiki.