Measuring And Tracking Pollutants: Methods And Technologies

The Air Quality Index (AQI) is a widely used system for measuring and tracking pollutants. The AQI works like a thermometer that runs from 0 to 500, indicating the level of air pollution in a given region. The higher the number, the worse the air quality. This system is important because it helps to identify areas with poor air quality, which can have significant impacts on public health and the environment. Additionally, air quality monitors are used, which are outfitted with sensors designed to detect specific pollutants. These sensors use lasers to scan particulate matter density or rely on satellite imaging to measure energy reflected or emitted by the Earth. Global satellite data is an advanced method for measuring PM2.5 levels, which are microscopic particles often responsible for air pollution-related health issues. However, on-the-ground monitoring of PM2.5 requires sophisticated equipment and sustained funding, which may not be available in all countries.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a tool used to communicate information about outdoor air quality and health. It is a numerical system that measures the level of air pollution in a given region. The higher the number, the worse the air quality. The AQI is calculated using data from air quality monitors, which are outfitted with sensors designed to detect specific pollutants. These sensors may use lasers to scan particulate matter density, or rely on satellite imaging to measure energy reflected or emitted by the Earth. This data is then processed by air quality databanks, which produce an aggregated AQI reading.

The AQI is divided into six categories, each corresponding to a different colour and level of health concern. The categories are as follows:

• Green (0-50) — Good: Air pollution is of little to no risk.
• Yellow (51-100) — Moderate: While air quality is acceptable, some may be at risk, particularly those who have heightened sensitivity to air pollution.
• Orange (101-150) — Unhealthy for Sensitive Groups: People with sensitivities may experience negative health effects, but the general public will less likely be affected.
• Red (151-200) — Unhealthy: The general public may experience health effects, and those with sensitivities may experience more serious effects.
• Purple (201-300) — Very Unhealthy: Everyone is at an increased risk of health effects, and a health alert is enacted.
• Maroon (301 and Higher) — Hazardous: A health warning of emergency conditions is enacted, meaning that everyone is more likely to be impacted.

The AQI is an important tool for public health officials, who can use it to make recommendations on specific behaviours, such as the level of outdoor activity or the length of time that sensitive people should spend outdoors. By understanding the AQI and taking appropriate actions, individuals and communities can help to improve air quality and protect their health.

Ground-level monitoring

Air Quality Index (AQI)

The Air Quality Index, or AQI, is a standard system for measuring and communicating air quality. It works similarly to a thermometer, with a scale from 0 to 500, indicating the level of pollution in the air. The AQI is divided into six colour-coded categories, ranging from Code Green (good) to Code Maroon (hazardous), to help the public understand the health risks associated with different levels of air pollution.

Air Quality Monitors

These are instruments equipped with sensors designed to detect specific pollutants. Some use lasers to scan particulate matter density, while others use satellite imaging to measure energy reflected or emitted by the Earth. This data is then processed to generate an AQI reading.

Ambient Air Quality Monitoring

This type of monitoring involves collecting and measuring samples of ambient air pollutants. It helps evaluate the current state of the atmosphere by comparing it to clean air standards and historical data. This information is crucial for determining whether a region meets the National Ambient Air Quality Standards (NAAQS) and for developing strategies to improve air quality.

Stationary Source Emissions Monitoring

This type of monitoring focuses on individual stationary sources of emissions, such as facilities, manufacturing plants, and industrial processes. It involves collecting and analysing data on emissions levels, control device performance, and work practices. This information is used to demonstrate compliance with regulatory requirements and to guide corrective actions if needed.

Continuous Monitoring Systems (CMS)

CMS are instruments that continuously measure actual emissions levels from stationary sources. There are three main types: Continuous Emission Monitoring Systems (CEMS), Continuous Opacity Monitoring Systems (COMS), and Continuous Parametric Monitoring Systems (CPMS). CEMS directly measure the pollutant of concern or a surrogate pollutant. COMS measures opacity, indicating the amount of light attenuated by particulate matter in emissions. CPMS measures key parameters that indicate system performance, such as temperature, pressure, or flow rate.

Biological Tools and Biosensors

Biological tools, such as biomarkers, can be used to monitor environmental pollution. Biomarkers are biological responses measured in organisms naturally exposed to a polluted site, serving as indicators of the presence and/or effects of pollutants. Biosensors, such as antibodies and endocrine-disrupting chemicals, are also used to detect specific pollutants and provide early warning signals.

Remote Sensing and Satellite Technology

Remote sensing and satellite missions equipped with high-performance synthetic aperture radars play a crucial role in monitoring pollution, especially at large spatial scales and over long periods. They provide complementary data to ground-based monitoring systems, offering timely and cross-boundary information that can be analysed using geographic information systems.

These ground-level monitoring methods provide valuable data for assessing and managing pollution risks, informing policy decisions, and ultimately protecting public health and the environment.

Satellite-driven technologies

For example, NASA's Terra and Aqua satellites, which orbit the Earth from pole to pole, capturing images of the entire Earth every day, are used to identify pollution caused by wildfires and dust storms. Similarly, the Moderate Resolution Imaging Spectroradiometer (MODIS), an instrument that flies on NASA's Terra and Aqua satellites, is used to access daily true-colour images of much of the world and identify pollution caused by factors such as smoke, dust, or haze.

Satellite data can also be used to indicate criteria air pollutants (e.g. PM2.5 and NO2) and greenhouse gases (e.g. CH4 and CO2). For instance, the California Air Resources Board (CARB) staff estimated ambient PM2.5 concentrations using NASA's MODIS Aerosol Optical Depth (AOD) data and land use regression in California for 2006-2012. This research enhanced the understanding of PM2.5 spatial distribution, which might not be shown only with ground PM2.5 monitors.

In addition to NASA, other organisations such as GHGSat and Carbon Mapper are also using satellite technology to monitor methane and carbon dioxide emissions. GHGSat, for instance, has identified over 50 'super-emitters' in central Asia, the Middle East and the southwestern US, which tend to be in the fossil fuel, waste or agricultural sectors. Carbon Mapper, on the other hand, has helped detect natural gas pipeline leaks and inform strategies on landfill gas management.

Despite the benefits of satellite-driven technologies, they also have certain limitations. Satellite-based platforms often come with the cost of lower spatial and temporal resolution and larger measurement uncertainties (i.e. interference from other compounds in the atmosphere). Therefore, satellite-based remote sensing systems should be used in conjunction with surface measurements for calibration and evaluation.

Air quality forecasting via interactive maps

One example of this is the Air District in the Bay Area, which provides interactive maps showing air quality forecasts, monitoring data, facilities, and overburdened communities. The Air Quality Forecast Map is the default view, using the U.S. EPA's Air Quality Index scale to predict air pollution levels for the next five days. The Air Quality Monitoring Map displays data from the Air District Air Monitoring Network sites, including measurements of ozone, oxides of nitrogen, carbon monoxide, sulfur dioxide, and particulate matter. Ground-level monitors are placed near large industrial facilities, such as oil refineries, to ensure accurate data collection.

Google Maps also provides air quality information by combining data from various sources, including governmental reference monitoring stations, commercial sensor networks, and global and regional dispersion models. Google's model calculates pollutant concentrations on a 500m x 500m grid, providing a comprehensive view of air quality.

IQAir is another organization that offers real-time air quality data based on the guidelines set by the World Health Organization (WHO). This data assists in predicting trends and developing strategies to improve air quality.

These interactive maps and forecasting tools are essential for public health officials and communities to understand current air quality, identify areas of concern, and make informed decisions to protect the health of residents and the environment.

Air emissions monitoring

Ambient Air Quality Monitoring:

This type of monitoring involves collecting and measuring samples of ambient air pollutants to evaluate the current state of the atmosphere in relation to clean air standards and historical data. It provides valuable information to regulators, scientists, industries, and the public about the levels of air pollutants in a given region. The data collected helps identify areas that are not meeting National Ambient Air Quality Standards (NAAQS) and are considered non-attainment areas. Additionally, it helps assess trends in air quality and the impact of pollution from various activities. The pollutants monitored include criteria pollutants and toxic air pollutants, such as Carbon Monoxide (CO), Particulate Matter (PM), Sulfur Dioxide (SO2), Nitrogen Dioxide (NO2), Ozone (O3), and Lead (Pb).

Stationary Source Emissions Monitoring:

This type of monitoring focuses on individual stationary sources of emissions, such as facilities, manufacturing plants, and processes. It collects and uses measurement data to assess the level of air pollutant emissions from a specific source, evaluate the performance of emissions control devices, and verify work practices. The data obtained helps demonstrate compliance with regulatory requirements and provides valuable information to facility operators for necessary corrective actions. Stationary source emissions monitoring is typically required by Federal or State rules, such as the Clean Air Act (CAA), and is an essential component of State Implementation Plans (SIP) to ensure that regions meet the NAAQS.

Continuous Monitoring Systems:

Continuous monitoring systems (CMS) are employed in both ambient air quality monitoring and stationary source emissions monitoring. These include Continuous Emission Monitoring Systems (CEMS), which directly measure actual emissions levels from stationary sources; Continuous Opacity Monitoring Systems (COMS), which measure opacity as an indicator of particulate matter emissions; and Continuous Parametric Monitoring Systems (CPMS), which measure key parameters that influence emissions levels and control device efficiency.

Air Quality Index (AQI):

The Air Quality Index, or AQI, is a numerical system that measures air pollution levels and communicates them to the public. It ranges from 0 to 500, with higher values indicating worse air quality. The AQI is categorized into six levels, each associated with a different colour and level of health concern:

• Good (0-50) - Air pollution poses little to no risk.
• Moderate (51-100) - Air quality is acceptable, but sensitive individuals may be at risk.
• Unhealthy for Sensitive Groups (101-150) - People with sensitivities may experience negative health effects.
• Unhealthy (151-200) - The general public may experience health effects, and sensitive groups may be more severely affected.
• Very Unhealthy (201-300) - Everyone is at an increased risk of health issues, and a health alert is activated.
• Hazardous (301 and above) - A health warning of emergency conditions is issued, indicating a high likelihood of health impacts for everyone.

Air Pollution Calculators:

These tools allow officials to focus on specific pollutants, inputting the pollutant and its AQI level to obtain the concentration level, AQI category, and relevant health impact statements.

Air Quality Forecasting via Interactive Maps:

Real-time information on air quality standards, provided by organizations like IQAir, helps predict trends and develop proactive strategies to improve air quality.

Global Efforts:

The United Nations Environment Programme (UNEP) has developed the first real-time air pollution exposure calculator, combining data from 6,475 locations in 117 countries to provide hourly population exposure estimates. UNEP also supports the deployment of low-cost sensors and provides technical assistance to countries to enhance their air quality monitoring capabilities.

Frequently asked questions