Kiera Jefferson
Air pollution is a pressing issue that has claimed millions of lives, and its measurement is critical to understanding its impact and curbing its effects. The process of measuring air pollution involves collecting and assessing the levels of various pollutants, particularly gases and particulate matter. The concentration of pollutants in the air is typically expressed in parts per billion (ppb) or parts per million (ppm), as well as micrograms per cubic meter (μg/m³). These units help quantify the mass or volume of pollutants, including nitrogen dioxide, sulfur dioxide, and particulate matter like PM2.5 and PM10. To obtain these measurements, different devices and techniques are employed, ranging from simple diffusion tubes to advanced chemical and physical sensors, as well as satellite imaging. The data collected from these sources is used to generate air quality indexes, such as the Air Quality Index (AQI) used in the United States, which helps communicate the level of pollution and associated health risks to the public.
| Characteristics | Values |
|---|---|
| Units | Parts-per notation (ppb, ppm), micrograms per cubic meter (μg/m³) |
| Type of pollutants measured | Gaseous (nitrogen dioxide), sulfur dioxide, carbon monoxide, coarse/fine particulates (PM10, PM2.5) |
| Devices used | Active air quality monitors, static monitors, low-cost sensors, street-side monitoring stations, satellites, deposit gauges, Ringelmann charts, soot and dust collectors, absorbent test tubes, chemical and physical sensors |
| Index | Air Quality Index (AQI), a scale from 0 to 500, with 50 or below considered safe and above 100 deemed unhealthy |
| Databases | Air Quality System (AQS), World Air Quality Index project, UNEP and IQAir's real-time air pollution exposure calculator |
What You'll Learn
Air Quality Index (AQI)
The Air Quality Index (AQI) is a tool used to communicate about outdoor air quality and health. It is a scale from 0 to 500 that measures the amount of pollution in the air, with higher values indicating greater pollution and health risks. The AQI is divided into six colour-coded categories, each representing a different level of health concern. An AQI value of 50 or below is generally considered good air quality, while a value over 300 indicates hazardous air quality. These categories are assigned specific colours to help people quickly assess the air quality in their communities.
The AQI is established for five major air pollutants regulated by the Clean Air Act: ground-level ozone, airborne particles, sulfur dioxide, nitrogen oxides, and carbon monoxide. These pollutants are monitored by satellites orbiting the Earth, such as the GOES-R Series and JPSS satellites, which provide frequent measurements of particle pollution and aerosol concentrations.
The AQI is an essential tool for raising awareness about air quality and health risks. For example, during the COVID-19 lockdown in New York, there was a significant reduction in fine particle pollution (PM2.5), resulting in all hours falling within the "good" AQI category. This brief improvement highlighted the potential for more permanent reductions in pollution through the adoption of electric vehicles and cleaner energy sources.
Additionally, the AQI serves as a guide for protecting public health. When AQI values exceed 100, the air quality is considered unhealthy, initially for sensitive groups and then for everyone as values increase. To address this, Activity Guides are provided to help individuals take necessary precautions to safeguard their health when AQI levels indicate deteriorating air quality.
The AQI is a valuable tool for both informing the public about air quality and guiding actions to mitigate health risks associated with air pollution. By providing accessible information through colour-coding and health guides, individuals can make informed decisions to protect their well-being.
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Parts-per notation
The amount of pollutant in the air is usually expressed as a concentration, measured in either parts-per notation (ppb or ppm) or micrograms per cubic meter (μg/m³). Parts-per notation, also known as the volume mixing ratio, is a measure of the concentration of a pollutant in the air in terms of its mass or volume. It is commonly used for measurements of both gaseous pollutants, such as nitrogen dioxide, and coarse (PM10) and fine (PM2.5) particulates.
One of the most common methods for measuring air pollution using parts-per notation is through diffusion tubes. These are simple absorbent test tubes that are fastened to structures such as lamp posts to absorb specific pollutant gases. After a certain period, the tubes are sent to a laboratory for analysis.
Another method for measuring air pollution using parts-per notation is through active measurement devices. These are automated or semi-automated and tend to be more complex and sophisticated than passive devices. They provide almost real-time pollution measurements, which are used to generate air quality indexes.
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Micrograms per cubic meter
Air pollution is usually measured in either parts-per notation (parts per billion or parts per million) or micrograms per cubic meter (μg/m³). The former is also known as the volume mixing ratio. These units express the concentration of air pollution in terms of the mass or volume of the pollutant.
In the United States, the EPA maintains a repository of air quality data through the Air Quality System (AQI). The EPA also established an AQI for five major air pollutants regulated by the Clean Air Act. Each of these pollutants has a national air quality standard set by the EPA to protect public health. The AQI is divided into six colour-coded categories, each corresponding to a range of index values. An AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.
Air pollution measurements are now largely automated and carried out using many different devices and techniques. These range from simple absorbent test tubes known as diffusion tubes to highly sophisticated chemical and physical sensors that give almost real-time pollution measurements. Satellites orbiting above Earth can also monitor what's in our air.
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Particle number
The concentration of particles in the air is measured in terms of the mass or volume of the pollutant. This is a common method for measuring gaseous pollutants, such as nitrogen dioxide, and coarse (PM10) and fine (PM2.5) particulates. PM10 refers to particles larger than 10 micrometers in diameter, which are not regulated by the EPA. PM2.5 particles, on the other hand, are fine inhalable particles with diameters of 2.5 micrometers or less. These particles are so small that they can be inhaled and cause respiratory and cardiovascular issues.
The Air Quality Index (AQI) is a system used to communicate air quality to the public. It combines the concentrations of common air pollutants, including fine and coarse particulates, to produce a single number on a colour-coded scale. An AQI value of 100 corresponds to the national ambient air quality standard for the protection of public health. Values above 100 indicate unhealthy air quality, with higher values posing risks to everyone.
The World Health Organization (WHO) has published air quality guidelines, recognising that there is no safe level of particulate matter for human health. National and regional agencies, such as the USEPA and the EU, set targets and regulate PM pollution in their respective regions.
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Satellite imaging
In 2001, a 'World Atlas' of artificial light at night was published, providing the first global measurements of the amount of human-caused light. This was created using images taken on cloud-free nights without moonlight, and by calibrating the data with a model of how light is transmitted through the Earth's atmosphere.
Satellites can also measure the concentration of particles (aerosols) in the atmosphere by observing how much light reaches the surface of the Earth and how much is reflected off the aerosols. This measurement is called aerosol optical depth or aerosol optical thickness. This can be compared to ground measurements to determine if the pollution is on the ground or kilometers above it.
The Tropospheric Emissions Monitoring of Pollution (TEMPO) instrument is a recent development in satellite imaging for air pollution monitoring. It will provide hourly, neighbourhood-scale measurements of pollutants, helping scientists to trace them to sources such as smokestacks and exhaust pipes.
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Frequently asked questions
The Air Quality Index is a scale that runs from 0 to 500, with each category corresponding to a different level of health concern and represented by a different colour. An AQI of 50 or below is considered safe, while readings above 100 are deemed unhealthy.
Air pollution is measured using air quality monitors that are outfitted with sensors designed to detect specific pollutants. Some use lasers to scan particulate matter density in a cubic metre of air, while others rely on satellite imaging to measure energy reflected or emitted by the Earth.
The earliest devices used to measure pollution include rain gauges, Ringelmann charts for measuring smoke, and simple soot and dust collectors known as deposit gauges. Modern devices include the GAIA air quality monitor, which uses laser particle sensors to measure PM2.5 and PM10 particle pollution in real-time. Satellites such as GOES-R and JPSS are also used to provide particle pollution measurements.
The amount of pollutant present in the air is usually expressed as a concentration, measured in either parts-per notation (e.g. parts per billion or parts per million) or micrograms per cubic meter (μg/m³).
PM2.5 refers to a microscopic particle that is 2.5 microns in width and almost 30 times smaller than the diameter of a human hair. It is often responsible for causing air pollution-related health issues, and measuring its concentration is key to formulating policies to control its presence in the air.
