Air Pollution Measurement Methods For Geographers

Air pollution is a pressing issue that poses a threat to the environment, biodiversity, and human health. With increasing emissions contributing to climate change, it is crucial to measure air pollution to identify its causes and implement effective solutions. The process of measuring air pollution involves collecting and analyzing the components of air pollution, such as gases and particulates. This can be done through passive or active devices, ranging from simple diffusion tubes to advanced chemical and physical sensors. These sensors provide near real-time data, generating Air Quality Indexes (AQIs) that help assess the health risks associated with air pollution. By understanding the levels and sources of air pollution, governments and organizations can take targeted action to protect human well-being and the environment.

Air quality indexes (AQIs)

Air pollution is caused by many factors, and notably includes gases and particulates. The measurement of air pollution is the first step in identifying its causes and then reducing or regulating them to keep the air quality within legal limits.

AQIs are generated from measurements made by sensors, which are becoming increasingly accurate and near real-time. Sensors range from small handheld devices to large-scale static monitoring stations in urban areas, and remote monitoring devices used on aeroplanes and satellites. The data from these sensors is used to generate maps of real-time air pollution for more than 80 countries. The GAIA air quality monitor, for example, uses laser particle sensors to measure PM2.5 and PM10 particle pollution, which is one of the most harmful air pollutants.

The AQI scale used for indexing real-time pollution is based on the latest US EPA standard, using the Instant Cast reporting formula. The AQI includes five major air pollutants regulated by the Clean Air Act: particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO) emissions.

Passive and active measurement devices

Air pollution is broadly measured in two ways: passively and actively. Passive devices are relatively simple, low-cost, and work by soaking up or otherwise passively collecting a sample of the ambient air, which is then analysed in a laboratory. Active devices, on the other hand, are automated or semi-automated and tend to be more complex and sophisticated than passive devices, though they are not always more sensitive or reliable.

Passive Measurement Devices

One of the most common forms of passive measurement is the diffusion tube, which looks similar to a laboratory test tube. It 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, one of the oldest forms of pollution measurement, are another type of passive device. They are large funnels that collect soot or other particulates and drain them into sampling bottles, which are then analysed in a laboratory.

Active Measurement Devices

Active measurement devices are automated or semi-automated and are generally more complex than passive devices. They use fans to suck in the air, filter it, and either analyse it automatically or collect and store it for later analysis in a laboratory. 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. 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 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. Active sensors can also be mounted on vehicles, such as taxis, buses, and trams, for drive-by air pollution sensing.

Satellite monitoring

Satellite remote sensing has several advantages over traditional ground-based monitoring. Firstly, it offers a much wider spatial coverage, allowing for the observation of air pollution in areas that may not have ground monitoring stations. This is especially useful for monitoring large geographical areas, such as continents or the entire planet, and can provide a more global perspective on air quality. Secondly, satellite data can be used to identify air pollution hotspots, track air pollution trends, and study the health effects of air pollution. By collecting data on criteria air pollutants, such as PM2.5 and NO2, and greenhouse gases like CH4 and CO2, satellites provide valuable information for researchers and policymakers.

One example of a satellite-based air pollution monitoring system is the Tropospheric Emissions Monitoring of Pollution (TEMPO) instrument. TEMPO is a NASA sensor mounted on a commercial communications satellite that orbits in sync with the Earth's rotation. It provides near-real-time monitoring of air pollution across most of North America, from Mexico City to northern Canada. TEMPO's sensors can detect tiny differences in the light reflected when sunlight strikes molecules in the atmosphere, allowing it to track pollutants such as ozone, nitrogen dioxide, sulfur dioxide, and aerosols.

Another example of satellite air pollution monitoring is South Korea's Geostationary Environmental Monitoring Spectrometer (GEMS). Launched in 2020, GEMS provides daily insights into nitrogen dioxide pollution across much of Asia. It has tracked air-fouling dust storms from northern China and monitored sulfur dioxide emissions from volcanic eruptions. GEMS has revolutionized air quality observations, offering a more comprehensive understanding of air pollution patterns and their impacts.

In addition to TEMPO and GEMS, there are other satellite missions dedicated to air pollution monitoring. For instance, the European Space Agency is expected to launch Sentinel-4 in 2024, focusing on tracking air pollution over Europe. These satellite missions, along with advancements in technology, are ushering in a new era of highly spatially resolved and long-term data availability, enabling a better understanding of planetary-scale environmental changes.

Particulate matter (PM2.5 and PM10)

Particulate matter (PM) is a mixture of solid particles and liquid droplets found in the air. PM2.5 and PM10 refer to particulate matter with diameters less than or equal to 2.5 and 10 micrometres, respectively. These particles are so small that they can be inhaled, causing serious health issues, especially for vulnerable groups such as infants, children, and older adults with pre-existing heart or lung diseases.

PM2.5 particles are fine inhalable particles, generally with diameters of 2.5 micrometres or smaller. To put this into perspective, an average human hair is about 70 micrometres in diameter, making it 30 times larger than the largest fine particle. These particles are emitted directly from sources such as construction sites, unpaved roads, fields, smokestacks, or fires. They can also form in the atmosphere through complex chemical reactions involving pollutants like sulfur dioxide and nitrogen oxides.

PM2.5 is associated with a high proportion of adverse health effects related to air pollution. Long-term exposure has been linked to premature death, particularly in individuals with chronic heart or lung diseases. It also impacts lung growth in children, as evident from studies showing reduced lung function and smaller lung sizes in children from communities with high PM2.5 levels. Additionally, PM2.5 is the main cause of reduced visibility (haze) in certain regions.

PM10 particles, with diameters of 10 micrometres or less, are also inhalable and can induce adverse health effects. Short-term exposures to PM10 have been linked to the worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). While the long-term effects of PM10 exposure are less clear, several studies suggest a potential link to respiratory mortality.

PM10 is commonly found in air pollution and can be inhaled with every breath. It may also be deposited in the mouth, throat, or nose and subsequently ingested. Natural sources of PM10 include bushfires, dust storms, pollen, and sea spray. Additionally, vehicles contribute to PM10 levels through direct emissions from fuel combustion, tyre wear, and vehicle-generated air turbulence.

Measuring PM2.5 and PM10 concentrations is crucial for monitoring air quality and ensuring compliance with regulatory standards. This can be done using various methods, including fixed-site monitors, portable devices, and satellite technology. By understanding the levels of particulate matter in the air, authorities can implement measures to reduce emissions and improve air quality, ultimately mitigating the health risks associated with PM exposure.

Nitrogen dioxide (NO2) and sulfur dioxide (SO2)

To measure the concentration of these gases in the atmosphere, passive and active sampling methods can be employed. Passive sampling is a relatively simple and low-cost method. It involves using devices such as diffusion tubes or deposit gauges to collect pollutant gas samples over a period of time. These devices are then sent to a laboratory for analysis. Passive samplers have been shown to be precise, low-cost, and technically straightforward. They are also easy to deploy in different locations.

Active sampling, on the other hand, utilizes automated or semi-automated devices that are more complex and sophisticated. These devices use fans to collect and filter air samples, and they can either analyze the samples immediately or store them for later laboratory analysis. Active sensors can be physical or chemical. Physical methods measure the absorption of specific wavelengths of light by the gas sample, while chemical methods involve reacting the sample with another chemical to measure the resulting product.

In the context of measuring NO2 and SO2, studies have utilized passive sampling techniques coupled with ion chromatographic and potentiometric analyses. This involves deploying passive samplers in different locations for specific exposure times, typically 14 days. The extracted ions are then analyzed using ion chromatography or ion-selective electrodes, and the corresponding gas pollutant concentrations are calculated. This method has been applied in various cities to determine the presence of NO2, SO2, O3 (ozone), and NH3 (ammonia).

It is important to note that meteorological parameters such as humidity, wind speed, and temperature can influence the accuracy of gas concentration measurements. For example, temperature has been shown to have a significant effect on SO2 concentrations, while humidity and wind speed have insignificant effects on NO2 and SO2 concentrations at a 95% confidence level.

By employing these measurement techniques, we can better understand the levels of NO2 and SO2 pollution in different geographical areas and implement strategies to reduce their negative impacts on human health and the environment.

Frequently asked questions