Kiera Jefferson
Atmospheric pollutants are measured using a variety of methods and devices, ranging from simple passive devices to advanced active sensors. Air quality is typically assessed using the Air Quality Index (AQI), which provides a quantitative measure of the level of pollution in the air. This index is calculated by combining and comparing the concentrations of common air pollutants, such as ozone, carbon monoxide, nitrogen oxides, and particulate matter. Modern air pollution measurement techniques include the use of automated sensors, satellite imaging, and ground-based monitoring stations, which provide real-time data on pollutant levels. These measurements are crucial for understanding the impact of air pollution on human health and the environment, as well as for developing policies and regulations to mitigate its effects.
| Characteristics | Values |
|---|---|
| Measurement methods | Passively, actively, or visually |
| Passive devices | Diffusion tubes, deposit gauges |
| Active devices | Automated or semi-automated sensors |
| Visual measurement | Ringelmann charts |
| Modern measurement | Automated, using devices like diffusion tubes, chemical and physical sensors |
| Pollutants measured | Ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, PM2.5, PM10, nitrogen dioxide, and particulates |
| Global monitoring | Satellites, ground-level monitoring |
| Air Quality Index (AQI) | Scale of 0-500, with colour-coded categories |
| AQI categories | Green, Yellow, Orange, Red, Purple, Maroon |
| AQI values | 50 or below is safe, above 100 is unhealthy |
| Global data sources | World Bank's Little Green Data Book, Global Burden of Disease study, World Health Organization |
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What You'll Learn
Air Quality Index (AQI)
Air quality is measured using the Air Quality Index (AQI). The AQI works similarly to a thermometer, ranging from 0 to 500, indicating the changes in the amount of pollution in the air. It is based on the measurement of particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO) emissions. The AQI has six categories, each with a specific colour code and health advisory. Code Green and Yellow indicate safe air quality, while Code Orange is unhealthy for sensitive groups. Code Red and Purple indicate unhealthy air for all, and Code Maroon signals emergency conditions.
AQI values are computed by combining the concentrations of common air pollutants, including ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, and fine and coarse particulates. This results in a single number presented on an easy-to-understand scale. The AQI can be influenced by various factors, such as rush-hour traffic, upwind forest fires, or a lack of dilution of air pollutants.
Air quality measurement methods have evolved over time, ranging from early devices like rain gauges and Ringelmann charts to modern automated sensors. Passive measurement devices, such as diffusion tubes, absorb specific pollutant gases and are analysed in laboratories. In contrast, active measurement devices, like fans, collect and analyse air samples using physical or chemical methods. Satellites and ground-based instruments also play a crucial role in monitoring air pollution, providing real-time data and global perspectives.
The AQI is utilised by various countries, with each nation's definition reflecting the discourse surrounding national air quality standards. For example, Canada uses the Air Quality Health Index (AQHI), which provides a numerical value from 1 to 10+ to indicate the level of health risk associated with local air quality. Similarly, Hong Kong adopted the AQHI in 2013, considering four air pollutants: ozone, nitrogen dioxide, sulfur dioxide, and particulate matter (PM10 and PM2.5). Australia takes a consistent approach with its air quality indexes, using a simple linear scale where 100 represents the maximum concentration standard for each pollutant.
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Remote sensing
Passive solar sensors, for example, can measure the energy reflected from the Earth and display it as a digital image or photograph. However, they are limited to capturing data only during daylight hours due to their energy requirements. Remote sensing instruments can also be used to detect trace gases, aerosols, and greenhouse gases. For instance, NASA/JPL's California Laboratory for Atmospheric Remote Sensing (CLARS) uses remote sensing instruments to measure trace gases, aerosols, and greenhouse gases in the LA basin.
Satellite remote sensing is a useful method for mapping atmospheric concentrations of aerosols and pollutants. It is a lower-cost and less time-consuming alternative to traditional methods. Satellite data can be used to indicate criteria air pollutants (e.g., PM2.5 and NO2) and greenhouse gases (e.g., CH4 and CO2). Remote sensing has been used to estimate ambient PM2.5 concentrations using NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) data. AOD is a measure of light extinction (scattering + absorption) by atmospheric aerosols and can predict ambient PM2.5 concentrations.
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Primary and secondary pollutants
Air pollutants can be categorized into two main groups: primary pollutants and secondary pollutants.
Primary Pollutants
Primary pollutants are emitted directly into the atmosphere from a specific source. These pollutants are released in their harmful form and do not undergo any chemical changes before becoming pollutants. Sources of primary pollutants can be natural, such as volcanic eruptions or fires, or anthropogenic, such as carbon monoxide from vehicles and the burning of fossil fuels. Some common examples of primary pollutants include:
- Particulate matter: Tiny solid or liquid particles suspended in the air.
- Carbon monoxide: A colorless, odorless, and toxic gas produced by incomplete combustion.
- Nitrogen oxides (NOx): A group of highly reactive gases formed during the combustion process.
- Volatile organic compounds (VOCs): Organic compounds that can evaporate at room temperature.
- Sulfur dioxide: A colorless gas produced by the combustion of sulfur-containing fuels. Natural sources such as volcanoes also contribute significantly to sulfur dioxide emissions.
Secondary Pollutants
Secondary pollutants are formed in the atmosphere when primary pollutants react with each other or with other substances. They are harder to control because they have different ways of synthesizing, and their formation is not yet fully understood. Some examples of secondary pollutants include:
- Tropospheric ozone: One of the most well-known secondary pollutants, formed by the chemical reaction of sunlight, nitrogen oxides, and volatile organic compounds.
- Ground-level ozone: A harmful pollutant that can cause long-term respiratory issues.
- Acid rain: Formed when sulfur dioxide and nitrogen oxides combine with water vapor in the atmosphere.
- Peroxyacetyl nitrate (PAN): A component of photochemical smog that can cause eye irritation and respiratory issues.
Measurement of Primary and Secondary Pollutants
Particulate matter (PM) is the most commonly monitored air pollutant, with over 30,000 global measurement points reported by air quality databases. However, it is important to measure a range of pollutants to truly understand the dynamics of air pollution in a given area. By pairing measurements of various pollutants with our understanding of the chemical reactions that drive air pollution, we can develop effective strategies to improve air quality and minimize exposure.
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Passive and active measurement
Atmospheric pollutants are broadly measured in two ways: passively or actively. Passive devices are relatively simple and inexpensive. They collect ambient air samples, which are then analysed in a laboratory. Diffusion tubes, for instance, are fastened to lamp posts to absorb specific pollutant gases. After a set period, they are sent to a laboratory for analysis. Deposit gauges, large funnels that collect soot and other particulates, are another example of passive devices.
Active measurement devices, on the other hand, are automated or semi-automated and tend to be more complex and sophisticated. They use fans to suck in air, filter it, and either analyse it immediately or store it for later analysis. Active sensors use either physical or chemical methods. Physical methods measure an air sample without changing it, for example by observing how much of a certain wavelength of light it absorbs. Chemical methods, on the other hand, involve changing the sample through a chemical reaction and then measuring it. Most automated air-quality sensors are examples of active measurement. These sensors range from small handheld devices to large-scale static monitoring stations in urban areas. Remote monitoring devices used on aeroplanes and satellites also fall under this category.
Air quality is often measured using the Air Quality Index (AQI), which works like a thermometer that runs from 0 to 500. It tracks changes in the amount of pollution in the air. The AQI has six categories, each with a specific colour code, to communicate the level of health concern. For instance, Code Green and Yellow indicate that the air is generally safe, while Code Red and Purple indicate that the air is unhealthy for everyone.
Satellite-based measurements of atmospheric pollutants are important for monitoring air quality and climate on a large scale. They enable scientists to compute the concentration of atmospheric trace gases. However, there are limitations to satellite-based measurements, and they should be used in conjunction with ground-based measurements for calibration and evaluation.
To measure the chemical composition of atmospheric pollutants, various techniques are used, including mass spectrometry, gas chromatography, and column chromatography. Gravimetric analysis is also employed to measure particulate matter concentrations by monitoring the change in weight resulting from the accumulation of matter on a substrate.
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Biomonitoring
Bioindicators provide a qualitative assessment of the health of an ecosystem by measuring biotic responses to environmental stress. Lichens, for instance, indicate poor air quality, while the presence of certain plants can indicate the presence of specific pollutants. Lichens are particularly useful bioindicators because they have no roots or cuticle, and so acquire all their nutrients from direct exposure to the atmosphere. This means that they accumulate contaminants from the air, and their high surface-area-to-volume ratio further encourages the interception of these contaminants.
Biomonitors, on the other hand, provide a quantitative assessment of the impact of pollutants on an organism or ecosystem. They can be used to compare with instrument measurements, providing a 'snapshot' of an ecosystem's health. For example, the reduction in lichen chlorophyll content or diversity indicates the presence and severity of air pollution.
Plants can be used as both bioindicators and biomonitors. Different plants exhibit different symptoms when exposed to various air pollutants, and these symptoms can be used to judge the type of pollutant, its concentration, and the duration of exposure. For example, alfalfa and sesame plants show visible symptoms when exposed to SO2 concentrations of 3.4 mg/m3 for one hour, while spinach, cucumber, and oats show symptoms when exposed to concentrations of 0.14 ~ 1.4 mg/m3 for 8 hours.
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Frequently asked questions
The AQI is a scale that runs from 0 to 500, with each value indicating a different level of air pollution and health concern. An AQI value of 50 or below is generally considered safe, while values above 100 are deemed unhealthy.
Air pollution is broadly measured in two ways: passively and actively. Passive devices are simple and low-cost, collecting air samples that are then analysed in a laboratory. Active devices are automated or semi-automated, using physical or chemical methods to analyse air samples.
Passive devices include diffusion tubes and deposit gauges. Active devices include various sensors, such as those that use lasers to scan particulate matter or satellite imaging to measure energy reflected by the Earth.
Common air pollutants measured include ozone, carbon monoxide, nitrogen oxides, sulfur dioxide, and particulate matter (PM2.5 and PM10). These pollutants are tied to human and environmental health impacts, with PM2.5 posing the greatest health threat.
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