Particle sensors are small air monitors. They measure airborne particles smaller than 10 micrometres (PM10) or 2.5 micrometres (PM2.5). These sensors are also known as:
- optical particle counters
- optical sensors
- particulate matter sensors
- air quality sensors
- air pollution sensors.
Relatively cheap and easy to operate, particle sensors are a popular way to monitor air quality. They are simpler and less accurate than most other types of particle monitors. However, they can provide useful information about air quality, particularly as part of a network of sensors.
How sensors work
Particle sensors use a light-scattering technique to measure the concentration of particles in the air. A beam of light inside the sensor is passed through a sample of air, and particles in the sample scatter the beam of light. The scattered light is measured and used to calculate the concentration of particles in the air sample.
Although these sensors all use similar technology, they can calculate particle concentrations in different ways. They vary in size and can look very different, from a smartphone attachment to a much larger device with its own enclosure and solar panel.
What sensors are used for
Due to their simplicity and low cost, useful applications for particle sensors include:
- to supplement existing air monitoring networks by increasing their geographical coverage
- to help identify localised air pollution problems
- to increase people’s understanding of air quality and air pollution
- for personal exposure monitoring, especially for people who are more sensitive to air pollution.
Across the world, researchers, industry, government agencies and community groups have set up networks of particle sensors to monitor their local air quality.
We use particle sensors to measure smoke and dust. This supplements our main air monitoring network. We generally do not use information from these sensors for:
- regulation purposes
- compliance purposes
- providing health advice.
Interpreting information from particle sensors
Particle sensors are not as accurate as traditional or more sophisticated types of air monitors. Different types of sensors – and even sensors of the same type – can perform differently.
This means readings from these sensors should only be taken as a guide, rather than as a precise measurement of air pollution.
The information you get from these sensors provides a general indication of air quality. For example, a high reading could indicate the air is smoky. You could use this information to decide whether to go for a run or do another type of strenuous outdoor activity.
The low accuracy of particle sensors means you cannot compare information from particle sensors with air quality standards.
Air quality information from an individual particle sensor is more useful when it's part of a network of sensors. This provides a more complete picture of air quality across an area.
Issues with sensor accuracy
Due to their simplicity, particle sensors may have these issues:
Inconsistent measurement of particles from different sources
Particles scatter light beams differently depending on the type and mixture of particles. This means that a particle sensor may give a different measurement depending on what type of particle it's detecting. For example, a sensor detects particles from vehicle emissions differently to particles from smoke.
Increased likelihood of false high readings when relative humidity is high
Particle sensors are sensitive to high relative humidity. This can affect sensors whether you place them outdoors or indoors.
To overcome this issue, some particle sensors have relative humidity and temperature sensors. These apply a correction to the original measurement before the sensor displays its reading. However, there's generally no way of knowing the specific correction factors used in different types of sensors.
Inconsistent air flow
Ventilation and wind speed may affect the air flow inside the particle sensor. If you put your sensor in a box or near a wall, it may show a lower concentration of particles than if you put it in an open space.
This also means that when a sensor is moving – for example, if you take it on a bicycle – it responds differently to when it’s stationary.
Ageing of sensors
As a particle sensor ages, its fan gets dirty and slows down. Over time, this slows down air flow, which changes the response of the sensor and reduces the accuracy of its measurements.
Inconsistent calculations pre-programmed in sensors
The calculations pre-programmed into particle sensors can include some of the effects above or none. There's no way of knowing which issues, if any, apply to an individual sensor – even if you're comparing 2 sensors of the same type.
As such, each sensor typically needs to be calibrated or tested individually. This variability makes it difficult to judge the accuracy of air quality information from a sensor.
Changing technology
New types of particle sensors are becoming available, and the technology is evolving quickly. This makes it difficult to know about all the different models and their potential limitations. You might not know:
- whether a sensor reading has been corrected
- which calculations have been used.
This affects the level of confidence you can have in the quality of information you collect.
How to improve accuracy of sensors
Practical steps to help improve the accuracy and reliability of a particle sensor include:
- doing regular, ongoing calibration, ideally at the sensor location
- making sure the sensor has sufficient ventilation by placing it in an open space, away from walls and buildings
- selecting an averaging time period of at least one hour.
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