Air Pollution Measurement: Common Monitoring Methods

Air pollution is a pressing issue that poses a significant threat to human health and the environment. With the deteriorating air quality across the globe, it is crucial to measure air pollution to address this crisis effectively. A common way to measure air pollution is through the use of air quality sensors and monitors, which can be portable, wearable, or stationary. These devices constantly sample the air for particulates and gases, providing real-time measurements of air quality. The data collected by these sensors can be analyzed through smartphone apps or aggregated to create air quality maps. Additionally, the concentration of pollutants in the air is typically expressed in parts-per notation (ppb or ppm) or micrograms per cubic meter (μg/m³). This numerical data is then used to calculate air quality indices, such as the Air Quality Index (AQI) in the United States, which categorizes air quality into different levels of health concern. These measurements help governments and organizations identify areas of concern and develop strategies to improve air quality and protect public health.

Portable sensors

Air pollution is broadly measured in two ways: passively or actively. Deposit gauges, for example, are one of the oldest forms of passive pollution measurement. They are large funnels that collect soot or other particulates, which are then analysed in a laboratory. Active measurement devices, on the other hand, 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 immediately or store it for later analysis.

The data from portable sensors can be analysed by smartphone apps and used in a crowdsourced way to build maps of pollution over wide areas. Their low cost allows for the deployment of multiple sensors to cover a small area, capturing the spatial variability of pollution in ways that a single regulatory monitor cannot. For example, industries use small portable sensors to warn of hazardous work conditions or determine personal exposure to a particular pollutant.

While portable sensors are less accurate than Federal Reference Method (FRM) or Federal Equivalent Method (FEM) monitors, they can still be useful in providing a qualitative understanding of relative pollutant concentrations at a given location. They can also be used to supplement satellite-driven technologies, which provide large-scale exposure to air pollutants but have limitations due to factors such as seasonal and spatial variability, diurnal and nocturnal pollution levels, and cloud cover.

Examples of portable sensors include the Air Quality Egg, PurpleAir, and Atmotube PRO.

Deposit gauges

The introduction of deposit gauges was a turning point in environmental history, as they helped raise awareness about the impact of air pollution. The Great Smog of London in 1952, which led to thousands of deaths, brought about a radical shift in pollution control and the implementation of the Clean Air Act in the UK. Deposit gauges were instrumental in monitoring the effectiveness of these efforts, with significant reductions in deposited matter observed over time.

Active sensors

Physical methods measure an air sample without changing it. For example, a physical method might measure how much of a certain wavelength of light is absorbed by the air sample. Chemical methods, on the other hand, change the sample through a chemical reaction and then measure the result. Most automated air-quality sensors are active sensors.

Air quality sensors range in size and function. Some are small, portable, and wearable, such as the Air Quality Egg and PurpleAir. These sensors constantly sample particulates and gases and produce moderately accurate, almost real-time measurements. Data from these sensors can be analysed by smartphone apps and used to build maps of pollution over wide areas. Other sensors are large, expensive, and static, constantly sampling the air for local authorities. These sensors make up metropolitan monitoring systems such as the London Air Quality Network and the Automatic Urban and Rural Network (AURN) in the UK. In the US, the EPA maintains data from over 10,000 monitors in its Air Quality System (AQS).

Low-cost air pollution sensors are also available. These sensors can be used to detect, monitor, and report on specific air pollutants such as particulate matter (PM) or carbon dioxide, as well as environmental factors like temperature and humidity. These sensors may have a visual display that uses colours or graphics to indicate the concentration of pollutants. However, it is important to note that there are currently no widely accepted air concentration limits for most indoor pollutants, so the levels that trigger an alert are determined by the manufacturer.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a system for measuring and reporting on the health effects of the five most common air pollutants. It is designed to help people understand the impact of air quality on their health and protect themselves from the negative effects of air pollution. The AQI is computed by combining or comparing the concentrations of a "basket" of common air pollutants (typically ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, and both fine and coarse particulates) to produce a single number on an easy-to-understand (and often colour-coded) scale.

The AQI is reported by various organisations and agencies around the world, including the Environmental Protection Department in Hong Kong, which uses a scale of 1 to 10+ to indicate the level of health risk associated with local air quality. On this scale, the higher the number, the greater the health risk. The AQI can also be used to provide advice on how to improve air quality, such as proposing behavioural changes to reduce environmental footprint.

In Canada, the Air Quality Health Index (AQHI) is a similar scale-based system that is designed to help Canadians understand the impact of air pollution on their health. Each province in Canada has its own AQHI, which is reported by the relevant provincial government agency. The AQHI in Canada also provides health advice for each of the six AQI bands, with recommendations for people who are particularly sensitive to air pollution.

The AQI is usually calculated using data from air pollutant monitors or models, which measure the concentration of pollutants in the air over a specified period. This data, combined with the time of exposure, represents the dose of the air pollutant. The health effects corresponding to a given dose are then established through epidemiological research. It is important to note that air pollutants vary in potency, and the function used to convert from air pollutant concentration to AQI also varies depending on the pollutant.

The AQI can be influenced by various factors, including an increase in air emissions, such as during rush-hour traffic or when there is an upwind forest fire. Additionally, the lack of dilution of air pollutants can also contribute to a higher AQI. While the AQI is a useful tool for understanding and managing air quality, it is important to recognise that air pollution measurement is a complex field that requires ground-level monitoring, sophisticated equipment, sustained funding, technical expertise, and regulatory support.

Satellite data

Satellite remote sensing provides data on air quality, particularly in areas where there are no ground monitors. Satellites can provide information on criteria air pollutants such as PM2.5 and NO2, and greenhouse gases such as CH4 and CO2. For example, NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) data has been used to estimate ambient PM2.5 concentrations in California from 2006 to 2012. PM2.5 is a microscopic particle that is often responsible for causing air pollution-related health issues, and measuring its concentration is crucial for formulating policies to control its presence in the air.

However, there are limitations to satellite data. For instance, satellite observations are limited to cloud-free days, and in winter, when pollution is typically higher, there are fewer cloud-free days. Additionally, mineral dust can lead to anomalies in the data, as seen in the high levels of PM2.5 in desert countries. Furthermore, accessing, processing, and interpreting observational data requires a degree of technical skill, which can be a barrier for organizations with limited resources.

Despite these limitations, satellite-driven technologies are a powerful supplementary tool for measuring large-scale exposure to air pollutants. For example, the California Air Resources Board (CARB) has funded research projects using remote sensing techniques, and CARB staff have participated in collaborative research to estimate PM2.5 species concentrations using satellite data in Southern California from 2001 to 2015. This research has enhanced our understanding of PM2.5 spatial distribution and its relative contribution to total PM2.5, strengthening our knowledge of its sources.

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