Air Pollution Measurement: Tools And Techniques

Air pollution is a pressing issue that affects the health and well-being of people worldwide. To address this crisis, it is crucial to measure air pollution levels and understand their sources to formulate effective policies. The measurement of air pollution involves collecting and analysing data on various pollutants, such as particulate matter (PM2.5), ground-level ozone, nitrogen dioxide, and sulfur dioxide, among others. This data is gathered through a range of methods, including passive and active measurement devices, with the former including deposit gauges and diffusion tubes, and the latter employing physical or chemical methods to measure air samples. Modern technology has introduced automated sensors and monitors that provide real-time pollution measurements, contributing to the development of air quality indexes. These sensors can be stationary or portable, with some even being wearable, providing individuals and communities with the ability to monitor their local air quality. Governments and organizations, such as the UNEP and EPA, utilize this data to identify pollution hotspots and develop strategies to mitigate the impact of air pollution on human health and the environment.

Passive measurement devices, e.g. diffusion tubes and deposit gauges

Passive measurement devices are relatively simple and low-cost tools used to measure air pollution. They work by passively collecting samples of ambient air, which are then analysed in a laboratory. Diffusion tubes and deposit gauges are two examples of passive measurement devices.

Diffusion tubes are simple, inexpensive devices that are commonly used to measure air pollution. They are similar in appearance to laboratory test tubes and are often fastened to objects such as lamp posts to absorb specific pollutant gases. After a certain period of time, the tubes are sent to a laboratory for analysis. Diffusion tubes are particularly useful for measuring high concentrations of gaseous pollutants in the workplace over a few hours. They can also be used to measure lower concentrations of pollutants if the exposure time is extended. This makes them suitable for indoor and outdoor air quality studies where the goal is to identify locations or conditions with unusually high or low pollution levels.

Deposit gauges, also known as soot and dust collectors, are one of the oldest forms of pollution measurement. They are large, funnel-like instruments used to capture and measure atmospheric particulates, especially soot, that are carried in air pollution and deposited back to the ground. The design includes a large circular funnel made of a material that resists corrosion from acid rain, such as anodized aluminium, mounted on a simple stand, which drains into a collection bottle below. The bottle is typically removed after a week to a month for analysis of its contents, including water, insoluble matter, and soluble matter. Deposit gauges played a crucial role in the systematic monitoring of air quality in Britain from 1914 onwards, contributing to the evaluation of pollution control efforts.

While deposit gauges have been superseded by more advanced instruments and methods, they remain in use today. Their introduction marked a significant advancement in the study and comparison of pollution levels across different times and locations. The standardised design of deposit gauges allows for the systematic study and comparison of pollution data collected from various locations.

Active measurement devices, e.g. automated sensors

Active measurement devices are automated or semi-automated and tend to be more complex and sophisticated than passive devices. They use fans to draw in air, filter it, and either analyse it immediately or store it for later analysis. They are often more sensitive and reliable than passive devices.

Active sensors use either physical or chemical methods to analyse air samples. 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 in some way, through a chemical reaction, and then 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 satellites. Small, inexpensive portable (and sometimes wearable) internet-connected air pollution sensors, such as the Air Quality Egg and PurpleAir, constantly sample particulates and gases and produce moderately accurate, almost real-time measurements that can be analysed by smartphone apps. These sensors can be used for both indoor and outdoor environments and the majority focus on measuring five common forms of air pollution: ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Some measure less common pollutants such as radon gas and formaldehyde.

The data from these sensors can be used in a crowdsourced way, either alone or with other pollution data, to build up maps of pollution over wide areas. Governments can use this data to identify air pollution hotspots and take targeted action to protect human and environmental well-being.

Physical vs. chemical methods

Air pollution measurement is the process of collecting and measuring the components of air pollution, notably gases and particulates. The earliest devices used to measure pollution include rain gauges (for studying acid rain), Ringelmann charts for measuring smoke, and simple soot and dust collectors known as deposit gauges.

Modern air pollution measurement is largely automated and carried out using various devices and techniques. These include simple absorbent test tubes (diffusion tubes) and highly sophisticated chemical and physical sensors that provide near-real-time pollution measurements, which are used to generate air quality indexes.

Physical methods measure an air sample without altering it. This can be done by observing how much of a certain wavelength of light it absorbs. Physical methods also include gravimetric analysis, which involves monitoring weight changes due to matter accumulation on a substrate, such as a filter.

Chemical methods, on the other hand, alter the sample through a chemical reaction and then measure the result. Analytical chemistry techniques used for measuring pollution include gas chromatography, various forms of spectrometry, spectroscopy, spectrophotometry, and flame photometry. Mass spectrometry, for example, can be combined with gas chromatography to identify thousands of organic species in the atmosphere.

Both physical and chemical methods are used in active sensors, which can be small handheld devices or large-scale static stations in urban areas. These sensors constantly sample particulates and gases, producing moderately accurate, near-real-time measurements that can be analysed by smartphone apps. They can also be used to create maps of pollution over wide areas, empowering individuals and communities to understand their exposure to air pollution.

Ground-level vs. satellite monitoring

Air pollution measurement is the process of collecting and measuring the components of air pollution, such as gases and particulates. The earliest devices used to measure pollution were simple, such as rain gauges, Ringelmann charts, and soot and dust collectors. Modern air pollution measurement is largely automated and carried out using a variety of devices and techniques, from simple diffusion tubes to advanced chemical and physical sensors.

Ground-level monitoring plays a crucial role in air pollution measurement. Ground-based sensors can be handheld, stationary, or wearable, providing real-time data on air quality. These sensors can be deployed in both indoor and outdoor environments, measuring pollutants such as ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The data collected by these sensors can be used to create maps of pollution levels over wide areas and identify air pollution hotspots. For example, the United Nations Environment Programme (UNEP) has supported the deployment of low-cost sensors in countries like Kenya, Costa Rica, Ethiopia, and Uganda to enhance ground-level monitoring capabilities.

Satellite monitoring is a more recent development in air pollution measurement. Satellites provide a global perspective on air quality and are particularly useful for filling in spatial gaps where ground monitors are sparse or non-existent. For instance, satellite remote sensing has been used to estimate ambient PM2.5 concentrations and identify air pollution hotspots. Satellites can also monitor the movement of pollution and track indicators of climate change, such as greenhouse gases. The Tropospheric Emissions: Monitoring of Pollution (TEMPO) is a satellite instrument developed by NASA and the US Environmental Protection Agency (EPA) that will revolutionize air quality forecasts by observing pollutant emissions and their interaction with weather patterns.

Both ground-level and satellite monitoring have their advantages and limitations. Ground-level monitoring provides detailed, localized data, but it may be limited in spatial coverage, especially in regions with sparse monitoring networks. Satellite monitoring offers a global perspective and consistent measurements across time and space, but it relies on ground-based data for calibration and validation. By combining these two approaches, scientists can improve the accuracy and comprehensiveness of air pollution measurements, leading to more effective air quality management and policies to protect public health and the environment.

Wearable sensors

There are various types of wearable sensors available for measuring air pollution. One of the most well-known sensors is the AirBeam, created by HabitatMap. This sensor measures local concentrations of air pollution particles (known as particulate matter), humidity, and temperature. It is a low-cost, palm-sized tool that is easy to use and can be clipped to clothing. The data collected by AirBeam sensors is visualised on the AirCasting platform, which maps out air pollution levels.

Another example of a wearable sensor is the Wynd Air Quality Tracker, a wearable device produced by a Silicon Valley startup. This sensor detects airborne particulate matter, including dust, allergens, and industrial pollution, in real time through a self-made light scattering sensor. It is connected to an iOS and Android application that allows users to interact with the device and access data.

The TZOA is another promising wearable commercial product for environmental monitoring. It measures PM10 and PM2.5 through a self-designed PM sensor that uses light scattering, atmospheric pressure, humidity, temperature, ultraviolet exposure, and ambient light to collect data. The TZOA is compact, lightweight, and can be clipped to clothing.

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