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Tool 12 - Measuring City Air Quality
Objectives for Monitoring
Measurement of ambient concentrations of pollution is an integral component of air quality management. Air quality monitoring can provide information and help working groups to clarify a number of air quality issues, such as:
- To ascertain likely consequences, chiefly health, of ambient exposure
- To identify the contribution of specific sources/ activity sectors
- To measure the spatial distribution of pollutants
- To determine trends in concentrations of various pollutants
- To ascertain compliance with air quality standards
- To provide public information
Below you will find the generally accepted methodology and technology if sampling is to occur.
Essential Components in Establishing an Air Quality Monitoring Programme
1. Selection of pollutants
2. Averaging times
3. Mobile and stationary sources
4. Monitoring techniques
5. Network design and citing of stations
6. Data collection and storage
7. Quality assurance
8. Financial and manpower requirements
9. Meteorological data needs
Air Pollutants
Techniques for Monitoring
There are four instrumental methods for measuring ambient air concentration:
Each of these methods has its advantages and disadvantages. A monitoring technique may be assessed as most appropriate depending on the objective for which measurements are to be taken, as well as the resources available. At present, no methodology is appropriate for all circumstances and all requirements. State-of-the-art continuous analyzers and remote sensors are able to provide considerable amounts of raw data that can be used to produce excellent decision-relevant information. However, these instruments are expensive to purchase, operate and maintain and require considerable technical support. The problem is further exacerbated by the fact that skills required to make maximum use of continuous data outputs are not always available, particularly in developing countries. Consequently, the use of automated air quality measurement networks is either not appropriate or cost-effective for most developing countries.
Less sophisticated active and passive sampling techniques are not able to produce hourly continuous data, but are very reliable. They also require considerably low level of technical support than automated samplers. They are considerably cheaper to purchase, operate and maintain. Passive and active sampling methodologies are able to provide data to meet most monitoring objectives and are therefore more appropriate for most countries. As long as the monitoring methodology is able to meet the objectives of the AQM network, the technique adopted is not critical. The selection of equipment should therefore be based upon the availability of resources (both human and financial) to purchase and operate instruments most effectively.
Table 12.1 Instrumental Air Quality Monitoring Techniques
| Method |
Advantages | Disadvantages | Capital cost per sampler |
|
Collects an integrated sample of pollutant through diffusion of the pollutant along a path of air within a tube with one closed end where the gas is trapped on an absorbent substance which is subsequently analyzed. |
Very low cost and simple Useful for screening and baseline studies |
Unproven for some pollutants Integrated sample Laboratory analysis required |
US$ 2 - 4 |
|
Collects an integrated sample of pollutant by pumping air through the sampler and trapping the pollutant in a physical or chemical collecting medium that is subsequently analyzed to determine quantity present. |
Low cost Easy and reliable to operate Historical data set in some cities |
Integrated samples Labor intensive Laboratory analysis required |
US$ 2,000- 4,000 |
|
Uses a physical or chemical property of the pollutant to measure the concentration in continuously collected samples. Calibrations are performed using a standard of known concentration for comparison |
Proven high performance Continuous on-line measurement Low direct costs |
Complex and expensive High skills required to maintain and operate High recurrent costs. |
US$ 10,000 - 20,000 |
|
Determines the average concentration of the pollutant over a fixed path spectroscopically. |
Provides path or range resolved data Useful for near sources and for vertical measurements Multi-component measurements |
Difficult to support operate, calibrate and validate Not always comparable with fixed point sampling analyzers |
greater than US$ 200,000 |
Source: UNEP/WHO, 1994a
Table 12.2 Monitoring Methods for Various Air Pollutants
| Pollutant | Monitoring Method(s) | Brief Description of Method |
| Particulates | Hi-Volume Sampler | Used for measuring Total Suspended Particulates (TSP). Drawing air through a filter collects the particulate matter. The concentration is then determined from the mass collected. It is a laboratory based, highly sophisticated method and with a sampling period of 24 hours. It has a detection limit of about 5µg/m3. Sampling for PM10 and PM2.5 is more complicated and expensive. |
| Smoke Shade Reflections Method | Drawing air through a filter collects particulate matter. Concentration is determined from light reflectance of the darkened filter. It is a laboratory-based method with sampling period of 24 hours. The detection limit is 5µg/m3 | |
| Beta-Gauge Method | Particulate is collected on high frequency glass fiber filter tape, through which sample air is drawn. The filter tape runs through a radioactive source and a detection device. This analysis is semi-continuous. The monitoring is cyclic and permits a time resolution of hours or even minutes. The detection limit is 5µg/m3 | |
| NO2 | Chemilumin-escence Method | The analyzer measures the NO concentration of air. This is determined by measuring the light emitted when NO is reacted with the O3 that is generated within the instrument. In order to determine the NO2 concentration in the air, the NO2 is first thermally decomposed to NO. The concentration of NO is then derived by comparing the reading for NO from the primary sample with the reading for air containing NO and decomposed NO2. It is a continuous automatic type of analysis with high level of sophistication. Detection limit is about 1µg/m3 |
| Christie Arsenite Method | In this method, NO2 is collected by bubbling air through a sodium hydroxide-sodium arsenite solution to form a stable solution of sodium nitrite. The nitrite ion produced during sampling is reacted with color forming reagents. The concentration of NO2 is then determined calorimetrically. This analysis is laboratory based with a sampling period of 24 hours and of low sophistication. The detection limit is about 2µg/m3. It is low cost. | |
| Diffusion tube Method | This is a laboratory-based method with low sophistication in which NO2 is diffused along a plastic tube, then absorbed and converted to nitrite by triethanolamine. The NO2 concentration is then determined colorimetrically after the nitrite has reacted with a color-forming reagent. The sampling usually takes place over a seven to fourteen day period, from which daily averages can be calculated. It has a detection limit of about 2 µg/m3 and is a low cost method. | |
| Differential Optical Absorption Spectroscopy Method(DOAS) | DOAS technique makes use of the fact that gases (e.g. NO2) absorb light at precise wavelengths that are unique to themselves. In operation, a beam of light known as spectrum is transmitted through the atmosphere from a source to a receptor. The concentration of a number of gases can be determined by analysis of the spectrum of light incident at the receptor. This involves comparison of the intensity of light at that wavelength absorbed uniquely by a specific gas (e.g. NO2) with the intensity of light absorbed at a wavelength that is not absorbed by any gas. This analysis is semi-continuous automatic and is highly sophisticated. Data is integrated in five minutes and its detection limit is 1µg/m3 | |
| SO2 | Hydrogen Peroxide Acidimetric and Calorimetric Methods | These are laboratory-based methods with low sophistication. In the acidimetric method, SO2 is absorbed in an oxidizing solution (e.g. dilute hydrogen peroxide, H2O2) which converts it to H2SO4. The original concentration of SO2 is then inferred by determining the concentration of the free acid (H+ ion) by filtration or electrically, by conductivity of pH measurements. Alternatively, in the colorimetric method, the sulfate concentration can be determined by reaction with a color-forming reagent, followed by spectrophotometric measurement of the color or by ion chromatography. The methods have a 24-hour sampling period that allows calculation of averaging time of one day or greater and both have a detection limit of 5µg/m3. |
| Calorimetric PararosanilineMethod | In this method, air is bubbled through a solution of dipotassium tetrachloromercurate. SO2 is continuously absorbed to form the non-volatile dichlorosulphitromercurate ion, which then reacts with formaldehyde and bleached pararosaniline to form red purple pararosaniline methyl sulphonic acid. The concentration of SO2 is determined from the color intensity of the dye, which is measured at a wavelength of 560nm. This method is laboratory based with sampling period as little as 30 minutes although 24 hours is commonly used. Detection limit is 5µg/m3. | |
| Gas Phase Fluore-scence Method | Pulsed ultra-violet light with a wavelength of 214nm is used to irradiate the sample gas (air) flowing continuously through an optical cell. SO2 molecules absorb the radiation and remit part of the energy at a different wavelength. This ? florescence is then detected by a photomultiplier. SO2 has a florescence band centered near 340nm. The strength of the electrical signal generated in the photomultiplier indicates the concentration of the SO2 in the air sample. The detection limit is 1µg/m3 | |
| Flame Photometric Method | The SO2 content in the air can be determined by burning air samples in hydrogen rich flame. Light emissions are measured with a photomultiplier in a photometric detector. The concentration of SO2 can be derived from the intensity of light of the wavelength (394nm) characteristic of sulfur. This method is highly sophisticated and has a detection limit of 1µg/m3 | |
| O3 | Ultra-violet Photometry Method | This technique is based on the fact that O3 absorbs ultra-violet radiation at 253.7nm. Sample air is drawn through an absorption cell across the path of beam of ultra-violet light. The concentration of O3 is determined from the degree of absorption at 253.7nm. This instrument typically gives a reading every 30 seconds and has a detection limit of 6µg/m3 |
| Cheminulumi-nescence Method | Sample air is mixed with ethylene (supplied from a cylinder) in flow cell at atmospheric pressure. The two gases react rapidly with accompanying emissions in the 350 to 600nm wavelength regions. These emissions are monitored with a sensitive photomultiplier and the concentration of O3 is inferred from the intensity of the signal. The detection limit is about 2µg/m3 | |
| Differential Optical Absorption Spectroscopy Method(DOAS) | The DOAS technique makes use of the fact that gases e.g. O3 absorb light at precise wavelengths which are unique to themselves. In operation, a beam of light with a known spectrum is transmitted through the atmosphere from a source to a receptor. The concentration of a number of gases can be determined by analysis of the spectrum of light incident at the receptor. This involves comparison of the intensity of light at that wavelength absorbed uniquely by a specific gas (e.g. O3) with the intensity of light at a wavelength that is not absorbed by any gas. The data is integrated over a period of 5 minutes. This method has a detection limit of 3µg/m3 | |
| CO | Infra-red Absorption Method | This technique is based on the fact that CO absorbs light at a characteristic wavelength (4.67nm) in the infrared spectrum. When the air sample is irradiated with such light, the CO concentration can be determined from the extent to which the sample absorbs the radiation. There are two main variants in analyzer design, referred to as the non-dispersive infrared and gas filter correlation techniques. The detection limit is less 0.5mg/m3 |
| Electro-chemical Cell Method | The concentration of CO is determined electronically during the oxidation of CO to CO2. It has detection limit of less that 0.5mg/m3 | |
| VOC | Total Hydrocarbon/ Non-methane Hydrocarbon Analyzer Method | Hydrogen is mixed with the sample air and combusted. An electric current produced from charged ions in the flame indicates the total hydrocarbon content of the air. The methane content can be determined by catalytically decomposing all other hydrocarbons before measurement. The non-methane hydrocarbon content can be inferred by subtracting the methane value from the total hydrocarbon value. The method has a detection limit of 5µg/m3 |
| Specific Hydrocarbons/ Halocarbons/ Oxygenates Method |
Specific compounds are separated by gas chromatography and detected individually by an appropriate method (i.e flame ionization, electron capture). The analyses are cyclic and generally have a period of about one-hour. The detection limit for an individual compound is less than 1µg/m3 | |
| Benzene | Gas Chromato-graphy Method | Benzene is collected from ambient air on a porous polymer adsorbent. It is then thermally desorbed into a gas chromatography, separated and measured using a flame or photo-ionization detector. The analysis is generally laboratory based and the collection by adsorption usually takes place over a period of about one-hour. It has a detection limit of less than 0.1µg/m3 |
| Differential Optical Absorption Spectroscopy Method | The DOAS technique makes use of the fact that gases e.g. benzene absorb light at precise wavelengths which are unique to themselves. In operation, a beam of light with a known spectrum is transmitted through the atmosphere from a source to a receptor. The concentration of a number of gases can be determined by analysis of the spectrum of light incident at the receptor. This involves comparison of the intensity of light at that wavelength absorbed uniquely by a specific gas (e.g. benzene) with the intensity of light at a wavelength that is not absorbed by any gas. The data is integrated over a period of 5 minutes. This method has a detection limit of 5µg/m3 | |
| PAH | High Performance Liquid Chromato-graphy (HPLC) Method | PAH are collected on filters (particulate PAH) followed by polymeric adsorption (gaseous PAH). Both are the extracted and partitioned chromatographically using high performance liquid chromatography. The individual PAHs are detected using florescence or ultra-violet absorption collection techniques. This method is laboratory based and the sampling period is about 24 hours. The detection limit depends upon the particular compound, but generally is less than 0.1µg/m3 |
| Lead & Cadmium | Atomic Absorption Spectroscopy Method | Lead and Cadmium particles are collected on filters, and then extracted into strong acid solution and analyzed by atomic absorption spectroscopy. (The metal ions in the solution can be atomized electrothermally or by chemical reduction in a flame, prior to spectroscopic analysis). This method is laboratory based and has a detection limit of about 0.01µg/m3 for lead and 0.001µg/m3 for cadmium. |
| X-Ray Fluorescence Method | Drawing air through filter paper collects samples. After preparation, the samples are placed in an X-ray Spectrometer. When irradiated with X-ray, metal pollutants absorb energy and then re-emit the energy at wavelengths characteristic of the individual metals. The metal concentrations are determined from the intensity of secondary X-rays emitted at these characteristic wavelengths, as observed with a spectrometer. The method is laboratory based with a sampling period of 24 hours. |
Source: World Bank-URBAIR, 1997