By Isabelle Robinson, M.Sc.Sep 16 2018Recently, many industries have been investing in low-cost air pollution sensors. As the name suggests, these sensors are cheaper than conventional methods of air pollution monitoring, making them more attractive to companies, while also making it possible to monitor in a variety of locations.
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New Standard to Evaluate Sensors
However, a study by the European Commission found that the data provided by these low-cost sensors may be of a lower standard or even false. This was found by comparing the data from the low-cost sensors with the data collected from official monitoring stations by EU Member States; in accordance with European legislation and international standards methods.
This has sparked some development in the field: if the quality of the low-cost sensors could be improved, then there could be a significant increase in the number of devices used around the world. This could be especially useful in developing countries. Monitoring for air pollution, traffic management, citizen science and health could all be enhanced.
Currently, a standard to evaluate sensors based on a uniform criterion is under development by the European Standardization Organization.
Air Quality Sensors
There are a number of air quality sensors available on the market at the moment. These include;
- Electrochemical sensors - these are sensors that use chemical reactions between gases in the air and the electrode situated in a liquid inside a sensor, to detect air quality.
- Metal oxide sensors - these work by monitoring gases in the air as they react on the sensor surface. This allows it to monitor the resistance.
- Photoionization detectors - used to detect when volatile organic compounds (VOCs) ionize in the sensor. The detector measures the resulting electrical current from the reaction.
- Optical particle counters - use light to detect pollution. The sensor measures the scattering of light by the pollutant particles. Some specialized optical sensors are used to detect lethal gasses like carbon monoxide, or pollutants like carbon dioxide, by measuring the absorption of infrared light rather than the scattering of visible light.
Low-Cost Sensors Provide Low-Quality Data
There are numerous reasons why low-cost sensors provide lower quality data. The machines are highly sensitive, and so the signals can be intercepted by variety of environmental factors; such as other compounds, temperature, humidity, pressure and signal drift. This means that weaker signals from the air pollutant can be lost. It is known that when there are higher concentrations of the pollutant, the data is much more accurate.
These sensors can also be sensitive to weather conditions. High wind-speeds, temperatures and humidity can all have a negative impact on the sensor, making it difficult to detect air pollutants.
Therefore, to get accurate results from an air quality sensor, the application, location, site conditions and technology must all be reviewed upon set-up to ensure the sensor is appropriate for use. This means that portable sensors are usually less accurate as the location and conditions vary. This is made more apparent by the fact that simple corrections or recalibrations are not always possible, or may have to be done too frequently. This is because of the influence of meteorological parameters on a sensor signal.
On the other hand, low-cost sensors can provide more accurate data if chosen specifically for the needs of the site and can be useful for many industries. To ensure that each air pollutant sensor is working as expected, there are a number of influences that should be considered.
Currently, official EU monitoring stations use air pollution sensors which have been rigorously tested and are chosen based upon a standardized selection process, to determine which sensor is the most appropriate given the location and environmental factors. It should be noted that these sensors are regularly checked, results validated and calibrated if needed.
Conclusion
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It is also important to understand that sensors may produce accurate data on the pollutant concentrations, but this is only representative of the area the sensor is inhabiting. The means that the data may actually only represent a limited section, because the data generated depends upon the pollutant, source and surroundings. Therefore, if organisations are investing in an air pollutant sensor, care and understanding is needed to interpret the results accurately.
References
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