Air Pollution Measurement: Tools And Techniques

Air pollution is a critical global issue, with 99% of the world's population breathing unclean air, according to the World Health Organization. To address this crisis, air pollution levels are measured using various tools and techniques, such as air quality monitors, satellites, and sensors. These devices detect specific pollutants, including particulate matter, gases, and chemical compounds, to assess the cleanliness or pollution level of the air. The data collected is then used to generate air quality indexes, like the Air Quality Index (AQI), which help communicate the level of health concern to the public. These measurements are vital for governments and public health officials to develop strategies, regulations, and recommendations to mitigate the impact of air pollution and protect public health.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a scale that runs from 0 to 500, with 0 being the least polluted and 500 being the most polluted. The AQI is a way of showing changes in the amount of pollution in the air and is often used as a metric in legal air quality standards. An AQI value of 100 generally corresponds to the level of the short-term national ambient air quality standard for protection of public health. Values at or below 100 are generally thought of as satisfactory, while values above 100 indicate that the air quality is unhealthy for certain sensitive groups of people, such as children, senior citizens, and people with heart and lung diseases. As the AQI value increases further, the air quality becomes unhealthy for everyone.

The AQI is divided into six colour-coded categories, each representing a different level of health concern. These categories include:

• Code Green and Yellow: Air is generally safe for everyone.
• Code Orange: Unhealthy for sensitive groups, including children, senior citizens, and people with heart and lung diseases.
• Code Red and Purple: Unhealthy for everyone.
• Code Maroon: A health warning of emergency conditions.

The AQI is calculated using data from air quality monitors, which are outfitted with sensors designed to detect specific pollutants. Some common pollutants measured include PM2.5, PM10, ground-level ozone, nitrogen dioxide, and sulfur dioxide. These pollutants can come from various sources, including human-caused emissions, such as fossil fuel use, and natural sources, such as dust storms and wildfires.

The EPA, or the Environmental Protection Agency, established the AQI for five major air pollutants regulated by the Clean Air Act. The AQI is used to communicate about outdoor air quality and associated health effects, helping people understand the potential risks and take appropriate actions to protect their health. Websites like AirNow.gov provide air quality data and resources, including interactive maps, calculators, and activity guides, to help individuals stay informed about their local air quality and take necessary precautions.

Passive and active measurement devices

Air pollution is broadly measured in two ways: passively and actively. Passive devices are relatively simple, low-cost, and work by collecting samples of ambient air, which are then analysed in a laboratory. Diffusion tubes, fastened to lamp posts, are one of the most common forms of passive measurement. They absorb specific pollutant gases, which are then analysed in a laboratory. Deposit gauges, which are large funnels that collect soot or other particulates, are another type of passive device.

Active measurement devices are automated or semi-automated, and tend to be more complex and sophisticated than passive devices. They use fans to suck in the air, filter it, and either analyse it automatically 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 measuring how much of a certain wavelength of light it absorbs. Chemical methods change the sample through a chemical reaction and then measure that. Most automated air-quality sensors are examples of active measurement. These range from small handheld devices to large-scale static monitoring stations in urban areas.

There are also direct-reading air quality monitors, which provide information at the time of sampling, enabling rapid decision-making. These monitors are particularly useful for identifying point-source contamination or emissions, such as gas leaks. Velocity meters (hotwire anemometers) are handheld devices used to measure air speed (velocity). They can be used to monitor the effectiveness of ventilation systems. The Mercury Vapour Indicator is a popular handheld device that quickly and accurately measures mercury vapour.

Personal sensors can empower individuals and communities to better understand their exposure to air pollution. For example, a research group led by William Griswold at UCSD handed out portable air pollution sensors to 16 commuters and found "urban valleys" where buildings trapped pollution. The Kunak AIR Lite is another example of a new air pollution measuring device that provides real-time information. It is equipped with two cartridges with GasPlug technology that can be combined for the measurement of different gases.

Monitoring stations

Air Quality Monitoring Stations

Air quality monitoring stations are fixed or mobile installations equipped with sensors and instruments designed to measure and record air pollution levels. These stations are strategically placed in locations that represent a specific area's air quality, such as urban centres, industrial zones, or remote natural environments. The placement of these stations follows specific guidelines and standards to ensure accurate and representative measurements.

Types of Sensors and Instruments

Real-Time Data and Mapping

Many monitoring stations provide real-time data, allowing for immediate assessment of air quality. This data is often integrated into interactive maps, such as the World Air Quality Index (WAQI) map, which displays air pollution levels for over 10,000 stations worldwide. Real-time data enables governments, researchers, and the public to make informed decisions and take appropriate actions to mitigate the health impacts of air pollution.

Air Quality Index (AQI)

The data collected by monitoring stations is often used to calculate the Air Quality Index (AQI). The AQI is a standardised scale that translates complex air quality data into a simple and understandable format. It typically ranges from 0 to 500, with higher values indicating increased pollution levels and associated health concerns. The AQI is colour-coded, making it easy for people to interpret the air quality in their area and take necessary precautions.

Addressing Monitoring Gaps

Despite the importance of air quality monitoring, there are regions, particularly in Africa, Central Asia, and Latin America, that lack sufficient monitoring infrastructure. This gap in monitoring can lead to a lack of awareness and inadequate policy responses to air pollution issues in these regions. To address this, organisations like the United Nations Environment Programme (UNEP) have developed low-cost air quality monitors and advocated for legislation that makes air quality monitoring a legal requirement worldwide.

Calculators and databases

Air Quality Index (AQI) Calculators

The Air Quality Index (AQI) is a widely adopted metric that quantifies air pollution levels. It is designed to provide a comprehensive and easily interpretable indication of air quality. The AQI typically ranges from 0 to 500, with higher values indicating increased pollution and associated health concerns. These values are categorised into colour-coded levels, such as green for safe air and red or maroon for hazardous air quality. AQI calculators use data from ground instruments and satellite technology to determine these values.

Global Databases

Various organisations maintain databases that compile and present air quality data from diverse sources. For example, the World Health Organization (WHO) has developed the Global Database of Household Air Pollution Measurements, which aggregates data from 196 studies across 43 countries. This database offers insights into both household and outdoor air pollution levels worldwide. Users can directly query this database using Microsoft Excel or export the data for further analysis in statistical programmes. Additionally, the WHO has also established the WHO Ambient Air Quality Database, which specifically focuses on ground measurements of annual mean concentrations of nitrogen dioxide (NO2) and particulate matter (PM10 and PM2.5). This database aims to represent air quality in cities and towns rather than individual stations, providing a comprehensive view of human exposure to air pollution. The database currently covers over 600 human settlements in more than 100 countries and is regularly updated.

Real-Time Calculators

Real-time calculators play a crucial role in providing up-to-date information on air pollution exposure. In 2021, UNEP, in collaboration with IQAir, developed the first real-time air pollution exposure calculator. This calculator utilises global readings from validated air quality monitors in 6,475 locations across 117 countries, territories, and regions. By prioritising PM2.5 readings and employing artificial intelligence, the calculator estimates the population's exposure to air pollution on an hourly basis.

These calculators and databases are invaluable tools for governments, researchers, and policymakers in their efforts to monitor, understand, and address air pollution. They provide the quantitative foundation necessary for making informed decisions and implementing effective measures to improve air quality and safeguard public health.

Satellite imaging

One of the key advantages of satellite imaging is its ability to identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, industrial sources, and the long-range transport of human-made pollution. For example, satellite images captured by NASA in 2007 showed the transport of smoke from wildfires in the western United States across the country and over the Atlantic Ocean. Similarly, satellite data can be used to identify pollution caused by agricultural fires, which, despite being small, can significantly impact regional air quality.

Satellite remote sensing provides valuable data on criteria air pollutants, such as PM2.5 and NO2, as well as greenhouse gases like CH4 and CO2. This information is particularly useful for air quality studies, including the identification of air pollution hotspots, health effect studies, and tracking air pollution trends. For instance, satellite data has been used to estimate ambient PM2.5 concentrations, a critical pollutant that poses significant health risks when inhaled.

To measure air pollution, satellites observe how much light reaches the Earth's surface and how much is reflected off aerosols or particles in the atmosphere. This measurement, known as aerosol optical depth or thickness, is the same as that taken from the ground using a sun photometer. By comparing satellite and ground measurements, scientists can attribute pollution to specific sources identified in satellite images.

Additionally, satellite data can be fused with ground measurements and artificial intelligence to create multi-modal AI models, such as AQNet, which can predict air quality and output an "air-quality index" for comparing air quality across different regions. This integration of satellite imaging with other technologies offers a promising approach to addressing the challenges of detecting and monitoring air pollution.

Frequently asked questions