Air Quality Monitoring: Measuring Pollutants Accurately

Air pollution is a pressing issue that threatens human health and contributes to climate change, biodiversity loss, and pollution. According to the World Health Organization, 99% of the global population breathes unclean air, leading to approximately 7 million premature deaths annually. To address this crisis, governments and organizations are employing various methods to measure air pollution and improve air quality. This includes the use of smart technology, low-cost sensors, satellite imaging, and ground-based instruments to monitor and regulate pollutants such as particulate matter, ozone, carbon monoxide, and nitrogen dioxide. By understanding the causes and levels of air pollution, targeted actions can be taken to protect human health and the environment.

Air Quality Index

The Air Quality Index (AQI) is a tool used by the EPA to communicate about outdoor air quality and health. The AQI includes six color-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the more serious the health concern.

For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 indicates hazardous air quality. When the AQI value is 100 or below, the air quality is generally considered satisfactory. However, as the AQI value rises above 100, the air quality becomes unhealthy first for certain sensitive groups of people and then for everyone as the value continues to climb.

The AQI is calculated using data from air quality monitors, which are outfitted with sensors designed to detect specific pollutants. Some of these sensors use lasers to scan particulate matter density in a cubic meter of air, while others rely on satellite imaging to measure the energy reflected or emitted by the Earth. This data is then used by governments and countries to identify air pollution hotspots and take targeted action to protect and improve human and environmental well-being.

Low-cost air quality monitors have emerged as a viable public alternative in areas lacking government-operated stations or in remote regions, as they are easier to deploy and have significantly lower operational costs.

Active and passive measurement

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

Passive sampling is ideal for long-term environmental assessment as it relies on the slow diffusion of pollutants and does not provide real-time data. It is also influenced by environmental factors like temperature, wind speed and humidity, which can lead to potential inaccuracies.

Active measurement devices are automated or semi-automated and tend to be more complex and expensive. They use fans, pumps or sorbent tubes to suck in and collect air samples, which are then analysed in a laboratory or automatically on-site. Active sensors use either physical or chemical methods. Physical methods measure an air sample without changing it, for example, by seeing how much of a certain wavelength of light it absorbs. Active samplers can also detect lower pollutant concentrations, making them suitable for environments with low pollution levels.

Active and passive sampling both play crucial roles in environmental monitoring. Each method is suited to different applications and has unique advantages. For example, active sampling is ideal for rapid assessments during emergencies like chemical spills, while passive sampling is more suitable for long-term average pollutant concentration measurements.

Satellite monitoring

One of the key advantages of satellite monitoring is its ability to provide a broad spatial coverage. Satellites orbit the Earth, allowing them to collect data over vast areas, including remote regions that may be challenging to access by ground-based monitoring stations. This capability enables the identification of air pollution hotspots and the analysis of pollution transport over long distances.

Satellite imaging plays a vital role in satellite monitoring. Satellites can measure the energy reflected or emitted by the Earth's surface, including the atmosphere. By analysing this data, researchers can estimate the concentration and distribution of pollutants such as particulate matter (PM), ozone, nitrogen dioxide, and sulfur dioxide. This information is crucial for understanding the extent and impact of air pollution on both a local and global scale.

In addition to providing valuable data on air quality, satellite monitoring also assists in tracking the sources and movement of air pollutants. By continuously observing the Earth's atmosphere, satellites can detect emissions from various sources, such as industrial facilities, power plants, vehicles, and natural events like wildfires or volcanic eruptions. This real-time data aids in identifying pollution sources, monitoring their impact, and informing effective strategies for pollution control and mitigation.

The integration of satellite monitoring with other technologies further enhances our ability to measure air pollutants. For example, the collaboration between NASA's Jet Propulsion Laboratory (JPL) and local governments in California demonstrates the effective utilisation of satellite data. JPL has employed cutting-edge technologies, such as the Airborne Visible/Infrared Imaging Spectrometer-3 (AVIRIS-3), to measure the distribution of ash and particulate matter in the atmosphere following wildfires. This data guides recovery efforts and supports the development of innovative technologies to address environmental challenges.

Real-time measurements

Air Quality Monitoring Stations:

Many countries have established air quality monitoring stations that continuously measure pollution levels. These stations are equipped with advanced analysers that can detect various pollutants, including particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. The data collected from these stations is used to generate real-time air quality indexes, such as the Air Quality Index (AQI) in the United States.

Satellite Imaging:

Satellite technology plays a crucial role in real-time air pollution measurement. Satellites can measure the energy reflected or emitted by the Earth's surface, providing valuable data on atmospheric conditions and pollution levels. This method is particularly useful for monitoring large areas and remote regions that may not have ground-based monitoring stations.

Laser Particle Sensors:

Laser technology is employed to measure particulate matter density in the air. Devices like the GAIA air quality monitor use laser sensors to detect harmful particles like PM2.5 and PM10 in real-time. These sensors are relatively easy to set up and can provide instantaneous data on air pollution levels.

Handheld and Wearable Sensors:

The development of portable, handheld, and even wearable air quality sensors has revolutionized real-time air pollution measurements. These devices, such as the Air Quality Egg and PurpleAir, are often internet-connected and provide moderately accurate data that can be analysed by smartphone apps. They empower individuals and communities to monitor their local air quality and better understand their exposure to pollutants.

Urban Monitoring Systems:

Metropolitan areas have implemented comprehensive monitoring systems, such as the London Air Quality Network and the Automatic Urban and Rural Network (AURN) in the United Kingdom. These systems utilise a network of static street-side monitoring stations that constantly sample various pollutants in urban air. The data collected helps local authorities make informed decisions to improve air quality in cities.

Health impact studies

One key aspect of health impact studies is the measurement of air pollution levels and the identification of specific pollutants. This involves the use of various devices and techniques, ranging from passive to active measurement methods. Passive devices, such as diffusion tubes, are relatively simple and inexpensive. They collect air samples, which are then analysed in laboratories to determine the presence and concentration of specific pollutants. On the other hand, active measurement devices are often more complex and sophisticated, utilising fans to collect and analyse air samples in real time or store them for later analysis. These devices include static monitors installed in public places, such as railroad stations, providing immediate feedback on local air quality.

The choice between passive and active measurement methods depends on various factors, including cost, ease of deployment, and the specific pollutants being targeted. For example, low-cost air quality monitors have been suggested for use in developing nations or regions with sparse monitoring infrastructure, such as Africa, Central Asia, and Latin America. This approach can improve data reliability and help address the disproportionate impact of air pollution on these regions' populations.

Furthermore, health impact studies have revealed the importance of considering indoor microenvironments and time-activity patterns in assessing exposure to air pollutants. Studies have shown that people spend a significant amount of time indoors, and indoor air quality can significantly impact overall exposure to pollutants. This has led to a more comprehensive and coordinated approach to exposure assessment, taking into account factors such as commuting behaviour, time spent in critical microenvironments, and the presence of emission sources.

Frequently asked questions