Air pollutants can be measured directly when they are emitted, for example, by placing instruments on factory smokestacks – or as concentrations in the ambient outdoor air.
To track ambient concentrations, researchers create networks of air-monitoring stations, which can be ground-based or mounted on vehicles, balloons, airplanes, or satellites.
In the laboratory, scientists use tools including laser spectrometers and electron microscopes to identify specific pollutants. They measure chemical reaction rates in clear plastic bags (“smog chambers”) that replicate the smog environment under controlled conditions, and observe emission of pollutants from combustion and other sources.
Measurement of Air Quality: Measurements of Air Pollution
There are many ways to measure air pollution, with both simple chemical and physical methods and with more sophisticated electronic techniques. There are four main methods of measuring air pollution.
1. Passive sampling methods
This method provides reliable, cost-effective air quality analysis, which gives a good indication of average pollution concentrations over aperiod of weeks or months. Passive samplers are so-called because the device does not involve any pumping. Instead the flow of air is controlled by a physical process, such as diffusion.
Diffusion tubes are simple passive samplers, which provide very useful information regarding ambient air quality. They are available for a number of pollutants, but are most commonly and reliably used for nitrogen dioxide and benzene.
The tubes, which are 71 mm long with an internal diameter of 11 mm, contain two stainless steel gauzes placed at one end of a short cylinder. The steel gauzes contain a coating of triethanolamine, which converts the nitrogen dioxide to nitrite.
The accumulating nitrates are trapped within the steel gauze, ready for laboratory analysis. The tube is open to the atmosphere at the other end, which is exposed downwards to prevent rain or dust from entering the tube.
To ensure that all the nitrogen dioxide originates from the testsite, the tubes are sealed before and after exposure. The tubes are manually distributed and collected, and are analyzed in a laboratory.
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2. Active sampling methods
This method use physical or chemical methods to collect polluted air, and analysis is carried out later in the laboratory. Typically, a known volume of air is pumped through a collector (such as a filter, or a chemical solution) for a known period of time.
The collector is later removed for analysis. Samples can be collected daily, providing measurements for short time periods, but at a lower cost than automatic monitoring methods.
3. Automatic methods
The advantage of this method is that it produces high-resolution measurements of hourly pollutant concentrations or better, at a single point. Pollutants analyzed include ozone, nitrogen oxides, Sulphur dioxide, carbon monoxide and particulates.
The samples are analyzed using a variety of methods including spectroscopy and gas. The sample, once analyzed is downloaded in real-time, providing very accurate information.
4. Remote optical/long path-analyzers
This method use spectroscopic techniques. Real-time measurements of the concentrations of a range of pollutants including nitrogen dioxide and Sulphur dioxide can be done using these analyzers.
The amount of pollution in the air, however sampled, is usually measured by its concentration in air. The concentration of a pollutant in air may be defined in terms of the proportion of the total volume that it accounts for.
Concentrations of pollutant gases in the atmosphere are usually measured in parts per million by volume (ppmv), parts per billion by volume (ppbv) or parts per trillion (million million) by volume (pptv).
Pollutant concentrations are also measured by the weight of pollutant within a standard volume of air, for example microgrammes per cubic metre (µgm-3) or milligrammes per cubic metre (mgm-3).
5. Air Quality Index (AQI)
The AQI is a nationally uniform color-coded index for reporting and forecasting daily air quality.
It is used to report on the most common ambient air pollutants that are regulated under the Clean Air Act: ground-level ozone, particle pollution (PM10 and PM2.5), carbon monoxide (CO), nitrogen dioxide (NO2), and sulfur dioxide (SO2).
The AQI tells the public how clean or polluted the air is and how to avoid health effects associated with poor air quality.
The AQI focuses on health effects that may be experienced within a few hours or days after breathing polluted air and uses a normalized scale from 0 to 500; the higher the AQI value, the greater the level of pollution and the greater the health concern.
An AQI value of 100 generally corresponds to the level of the short-term National Ambient Air Quality Standard for the pollutant. AQI values at and below 100 are generally considered to be satisfactory.
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When AQI values are above 100, air quality is considered to be unhealthy, at first for members of populations at greatest risk of a health effect, then for the entire population as AQI values get higher (greater than 150).
The AQI is divided into six categories that correspond to different levels of health concern. The breakpoints between these categories are selected based on a review of the health effects evidence. Some individuals are much more sensitive to air pollution than others.
Checking the AQI each day will help these people notice at what levels they begin to experience effects. The levels of health concern listed below are general guidelines used as a reference so that people can figure out their own sensitivity to air pollution.
Good:Air quality is good and poses little or no risk.
Moderate:Air quality is acceptable; however, there may be some health concern for a small number of unusually sensitive people. While EPA cannot identify these people, studies indicate that there are people who experience health effects when air quality is in the moderate range.
Unhealthy for Sensitive Groups: When air quality is in this range, people who are in sensitive groups, whether the increased risk is due to medical conditions, exposure conditions, or innate susceptibility, may experience health effects when engaged in outdoor activities.
However, exposures to ambient concentrations in this range are not likely to result in effects in the general population. For particle pollution, the sensitive groups include people with heart and lung disease, older adults, children, people with diabetes and people of lower SES.
Unhealthy: When air quality is in this range, everyone who in active outdoors may experience effects. Members of sensitive groups are likely to experience more serious effects.
Very Unhealthy: When air quality is in this range, it is expected that there will be widespread effects among the general population and more serious effects in members of sensitive groups.
Hazardous: Air quality in this range triggers health warnings of emergency conditions by media outlets. The entire population is more likely to be affected by serious health effects.
Air quality indices have two main purposes:
To relay necessary air quality information to the public so that people can modify their behaviour and stay healthy.
By monitoring pollution levels, countries can assess the effectiveness of their policies and adjust as needed to achieve better air quality in the future.
When air quality indices record high levels, government agencies deal with such instances through any of the following ways:
Sensitive groups, including children, the elderly and people with respiratory and cardiovascular problems are advised not to take part in any outdoor activities
Factories that emit large amounts of pollutants are ordered to shut down or cut production. People are also encouraged to use public transport to reduce vehicle emissions.
This has happened severally in China where the authorities have increased their efforts to bring China pollution to manageable levels.
People are asked to wear protective masks to avoid breathing in pollutants, particularly PM2.5 and ground ozone.
In summary, air quality measurement enables effective monitoring of air pollution, this helps to advise the public on the possible effects of air pollution on public health and measures to be taken.
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