Air Pollution Measurement: Instruments And Their Uses

Air pollution is a pressing issue that affects the health of people and the planet. To address this, various instruments and methods are used to measure air quality and pollution levels. These instruments range from simple, low-cost sensors to large-scale static monitoring stations, with the shared goal of providing accurate and timely data on air pollution. The selection of the right instrument depends on factors such as durability, maintenance requirements, and ease of transportation, sensitivity, and the range of particle sizes it can measure. This includes the use of satellites in orbit, mobile sensing technology, and advanced sensors that provide real-time pollution measurements. Governments worldwide have also introduced Air Quality Indexes (AQIs) to monitor and report on ambient air quality. These AQIs help in understanding the impact of pollution on health and the environment, enabling collaborative efforts to improve air quality for future generations.

Active and passive measurement devices

Air pollution is broadly measured in two ways: passively and actively. Passive measurement devices are relatively simple, low-cost, and collect ambient air samples which are then analysed in a laboratory. One of the most common forms of passive measurement is the diffusion tube, which looks similar to a laboratory test tube and is fastened to something like a lamp post to absorb one or more specific pollutant gases of interest. After a period of time, the tube is taken down and sent to a laboratory for analysis. 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.

Active measurement devices, on the other hand, are automated or semi-automated and tend to be more complex and expensive. They use fans, pumps, or sorbent tubes to collect air samples, which are then analysed in a laboratory or automatically on-site using 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. Chemical methods change the sample in some way, through a chemical reaction, and measure that. Most automated air-quality sensors are examples of active measurement. Air quality sensors range from small handheld devices to large-scale static monitoring stations in urban areas, and remote monitoring devices used on aeroplanes and space satellites.

There are also portable, wearable, and Internet-connected air pollution sensors, such as the Air Quality Egg and PurpleAir. These constantly sample particulates and gases and produce moderately accurate, almost real-time measurements that can be analysed by smartphone apps. Their data can be used in a crowdsourced way, either alone or with other pollution data, to build maps of pollution over wide areas. They can be used for both indoor and outdoor environments and can measure common forms of air pollution such as ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide, as well as less common pollutants like radon gas and formaldehyde.

In addition to the above, there are some specialised devices used to measure air pollution. For example, velocity meters (hotwire anemometers) are handheld devices used to measure air speed (velocity) and monitor the effectiveness of ventilation systems. The Mercury Vapour Indicator is another handheld device that quickly and accurately measures mercury vapour contamination. Direct-reading air quality monitors provide information at the time of sampling, enabling rapid decision-making. They are particularly useful for identifying point source contamination, such as gas leaks, and for performing screening surveys to determine areas where additional evaluation is needed.

Mobile and stationary monitoring

Mobile monitoring is a flexible approach to measuring air pollution that can cover large spatial areas and focus on specific communities of interest. It is particularly useful for capturing "spatial variability" (differences in pollution levels across various locations) and "transient emissions" (short-term spikes in pollutants). The mobility of this approach reduces wind dependency, which is a factor in stationary monitoring.

Vehicles equipped with specialised air quality devices and sensors can measure air pollution along public roads, especially in disadvantaged communities facing a disproportionate burden of environmental exposure. These sensors can measure air pollution block-by-block and identify sources of pollution. Mobile monitoring can also be conducted by volunteers carrying handheld instruments.

However, mobile monitoring may not capture the temporal variability (changes in pollution levels over time in a single location). It is also resource-intensive, requiring complex vehicles and highly trained staff to manage the data.

Stationary monitoring, on the other hand, is essential for long-term measurement of air pollution in a specific location. It involves setting up permanent or semi-permanent sites with various instruments and methods to collect data. For example, a regulatory air monitoring station collecting data over many years is a permanent site, while a portable trailer stationed in a community for several months is a semi-permanent site.

Stationary monitoring data provides insights into air quality trends and the effectiveness of pollution control measures. It can also inform actions to reduce emissions and improve production practices. Fenceline monitoring, a type of stationary monitoring, enables sources of air pollution to take proactive measures and potentially reduce the amount of pollution and product loss.

Both mobile and stationary monitoring approaches play a crucial role in addressing air pollution challenges and protecting public health and the environment.

Air quality indexes

Air pollution is caused by many factors, and in urban environments, it may include solid and liquid particulates (such as soot from engines) and numerous different gases (most commonly sulfur dioxide, nitrogen oxides, and carbon monoxide). The quality of the air is extremely important as it directly affects our health in both the short and long term and can also impact the survival of the flora and fauna on the planet.

The AQI is a tool that measures how much pollution is in the air and helps us understand how it affects people's health and the environment. The AQI helps everyone understand how polluted the air around them is and how poor air quality affects people's health and the environment. It also helps lawmakers, scientists, and people work together to make the air cleaner and healthier for everyone, thus helping to make the world more sustainable for future generations.

There are many tools to measure particulate matter, and the right method and tool depend on different factors. For example, we need to consider if the tool can measure particulate matter in real-time, if it needs gas dilution, and how sensitive the tool is and what range of particle sizes it can measure. Most automated air-quality sensors are examples of active measurement, and they range from small handheld devices to large-scale static monitoring stations in urban areas. At one end of the scale, there are small, inexpensive portable (and sometimes wearable) Internet-connected air pollution sensors, such as the Air Quality Egg and PurpleAir. These constantly sample particulates and gases and produce moderately accurate, almost real-time measurements that can be analyzed by smartphone apps.

The U.S. Environmental Protection Agency (EPA) maintains a repository of air quality data through the Air Quality System (AQS), where it stores data from over 10,000 monitors. The European Environment Agency collects its air quality data from 3,500 monitoring stations across the continent.

Remote sensing

Satellite remote sensing can provide detailed information on the contaminants present in our atmosphere, including particulate pollution, ground-level ozone, carbon monoxide, and other major contaminants. Examples of satellite monitoring systems include the Geostationary Operational Environmental Satellites-R (GOES-R) and the Joint Polar Satellite System (JPSS), operated by the National Oceanic and Atmospheric Administration (NOAA) in the USA.

Overall, remote sensing provides a valuable tool for addressing global air pollution challenges and filling in spatial gaps in ground monitoring resources.

Low-cost sensors

The data from these sensors can be used to build maps of pollution over wide areas, both indoors and outdoors, and they are especially useful in low- and middle-income countries where traditional, more expensive monitors are lacking. They can also supplement existing reference-grade monitors in high-income countries by providing more localized, near real-time air quality information.

The accuracy of low-cost sensors has been evaluated by organisations such as the EPA, but generally based on outdoor conditions which may not reflect indoor use. There are currently no widely accepted indoor performance criteria or air concentration limits for most pollutants indoors, so the levels that trigger an alert are determined by the manufacturer. If you are concerned about the health impacts of the levels of pollutants measured by a low-cost sensor, it is recommended to discuss this with a healthcare provider.

Overall, low-cost sensors represent a key tool for improving air quality monitoring and filling gaps in existing global and local monitoring networks, contributing to policy-relevant air quality strategies.

Frequently asked questions