Air pollution measurement
Air pollution measurement is the process of collecting and measuring the components of
Importance of measurement
Air pollution is caused by many things. In urban environments, it can contain many components, notably solid and liquid particulates (such as
Types of measurement
Air pollution is (broadly) measured in two different ways, passively or actively.[5]
Passive measurement
Passive devices are relatively simple and low-cost.[6] They work by soaking up or otherwise passively collecting a sample of the ambient air, which then has to be analyzed 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, one of the oldest forms of pollution measurement, are another type of passive device.[7] They are large funnels that collect soot or other particulates and drain them into sampling bottles, which, again have to be analyzed in a laboratory.[7]
Active measurement
Active measurement devices 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.[6] They use fans to suck in the air, filter it, and either analyze it automatically there and then or collect and store it for later analysis in a laboratory. Active sensors use either physical or chemical methods.[8] 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.[5]
Air quality sensors
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.
Personal air quality sensors
At one end of the scale, there are small, inexpensive portable (and sometimes wearable),
Sensors like this were once expensive, but the 2010s saw a trend towards cheaper portable devices that can be worn by individuals to monitor their local air quality levels, which are now sometimes informally referred to as low-cost sensors (LCS).[9][15] A recent review by the European Commission's Joint Research Center identified 112 examples, made by 77 different manufacturers.[16]
Personal sensors can empower individuals and communities to better understand their exposure environments and risks from air pollution.
Small-scale static pollution monitoring
Unlike low-cost monitors, which are carried from place to place, static monitors continuously sample and measure the air quality in a particular, urban location. Public places such as busy railroad stations sometimes have active air quality monitors permanently fixed alongside platforms to measure levels of nitrogen dioxide and other pollutants.[19] Some static monitors are designed to give immediate feedback on local air quality. In Poland, EkoSłupek air monitors measure a range of pollutant gases and particulates and have small lamps on top that change colour from red to green to signal how healthy the air is nearby.[20]
Large-scale pollution monitoring
At the opposite end of the spectrum from low-cost sensors are the large, very expensive, static street-side monitoring stations that constantly sample the various different pollutants commonly found in urban air for local authorities and that make up metropolitan monitoring systems such as the London Air Quality Network[21] and a wider British network called the Automatic Urban and Rural Network (AURN).[22] In the United States, the EPA maintains a repository of air quality data through the Air Quality System (AQS), where it stores data from over 10,000 monitors.[23] The European Environment Agency collects its air quality data from 3,500 monitoring stations across the continent.[24]
The measurements made by sensors like these, which are much more accurate, are also near real-time and are used to generate air quality indexes (AQIs). Between the two extremes of large-scale static and small-scale wearable sensors are medium-sized, portable monitors (sometimes mounted in large wheelable cases) and even built into "smog-mobile" sampling trucks.[25]
Recently, drive-by air pollution sensing systems have emerged as a promising approach for air quality monitoring, utilizing sensors mounted on taxis, buses, trams, and other vehicles.[26] In particular, buses have garnered considerable attention as a mobile sensing platform due to their widespread availability and extensive geographical coverage.[27]
Remote monitoring
Air quality can also be measured remotely, from the air, by
Methods of measurement for different pollutants
Each different component of air pollution has to be measured by a different process, piece of equipment, or chemical reaction. Analytical chemistry techniques used for measuring pollution include gas chromatography; various forms of spectrometry, spectroscopy, and spectrophotometry; and flame photometry.
Particulates
Until the late 20th century, the amount of soot produced by something like a
In modern pollution monitoring stations, coarse (PM10) and fine (PM2.5) particulates are measured using a device called a
The atomic composition of particulate samples can be measured with techniques such as
Nitrogen dioxide
It can also be measured automatically much more quickly, by a chemiluminescence analyzer, which determines nitrogen oxide levels from the light they give off. In the UK, for example, there are over 200 sites where NO
2 is continuously monitored by chemiluminescence.[44]
Sulphur dioxide and hydrogen sulphide
Sulphur dioxide (SO2) is measured by fluorescence spectroscopy. This involves firing ultraviolet light at a sample of the air and measuring the fluorescence produced.[45] Absorption spectrophotometers are also used for measuring SO2. Flame photometric analyzers are used for measuring other sulphur compounds in the air.[46]
Carbon monoxide and carbon dioxide
Carbon monoxide (CO) and carbon dioxide (CO2) are measured by non-dispersive infrared (NDIR) light absorption based on the
Ozone
Ozone (O3) is measured by seeing how much light a sample of ambient air absorbs.[49] Higher concentrations of ozone absorb more light according to the Beer-Lambert law.
Volatile organic compounds (VOCs)
These are measured using gas chromatography and flame ionization (GC-FID).[50]
Hydrocarbons
4), NMHC (non-methane hydrocarbons), and THC (total hydrocarbon) emissions (where THC is the sum of CH
4 and NMHC emissions).[51]
Ammonia
Ammonia (NH
3) can be measured by various methods including chemiluminescence.[53]
Natural measurements
Air pollution can also be assessed more qualitatively by observing the effect of polluted air on growing plants such as lichens and mosses (an example of biomonitoring).[54][55][56] Some scientific projects have used specially grown plants such as strawberries.[57]
Measurement units
The amount of pollutant present in air is usually expressed as a concentration, measured in either parts-per notation (usually parts per billion, ppb, or parts per million, ppm, also known as the volume mixing ratio), or micrograms per cubic meter (μg/m³). It's relatively simple to convert one of these units into the other, taking account the different molecular weights of different gases and their temperatures and pressures.[58]
These units express the concentration of air pollution in terms of the mass or volume of the pollutant, and they are commonly used for measurements of both gaseous pollutants, such as nitrogen dioxide, and coarse (PM10) and fine (PM2.5) particulates. An alternative measurement for particulates,
Urban air quality index (AQI) values are computed by combining or comparing the concentrations of a "basket" of common air pollutants (typically ozone, carbon monoxide, sulphur dioxide, nitrogen oxides, and both fine and coarse particulates) to produce a single number on an easy-to-understand (and often colour-coded) scale.[61]
History
Air pollution was first systematically measured, in Britain, in the 19th century. In 1852, Scottish chemist Robert Angus Smith discovered (and named) acid rain after collecting rain samples that turned out to contain significant quantities of sulphur from coal burning. According to a chronology of air pollution by David Fowler and colleagues, Smith was "the first scientist to attempt multisite, multipollutant investigations of the chemical climatology of the polluted atmosphere".[62]
In the early 20th century, Irish physician and environmental engineer John Switzer Owens and the Committee for the Investigation of Atmospheric Pollution, of which he was secretary, greatly advanced the measurement and monitoring of air pollution using a network of deposit gauges. Owens also developed a number of new methods of measuring pollution.[63]
In December 1952, the Great Smog of London led to the deaths of 12,000 people.[64] This event, and similar ones such as the 1948 Donora smog tragedy in the United States,[65] became one of the great turning points in environmental history because they brought about a radical rethink in pollution control. In the UK, the Great Smog of London lead directly to the Clean Air Act, which may have had consequences even more far reaching than it originally intended.[66] Catastrophic events like this led to pollution being measured and controlled much more rigorously.[62]
See also
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