Editorial Feature

An A to Z of Environmental Sensors

Environmental sensors are instruments used to identify and monitor the quality of the environment. Depending on the observables measured – such as temperature, humidity, the presence of contaminants – and the application, there are several types of environmental sensors available.

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Environmental monitoring is paramount since factors such as water and air quality, soil pollution, and chemical exposure are linked to various diseases and health problems. Pesticides, heavy metals and other contaminants are also cause of concern.

Other applications of environmental sensors include the monitoring of phenomena like weather and storms. Information can be accurately measured, and the data can be tested, recorded and stored.

The principles behind environmental sensors are often based on the detection of objects and changes in their environment following the interaction with radiation such as visible light, infrared, or laser. When choosing environmental sensors, sensitivity, accuracy and cost should be considered.

What Can Environmental Sensors Do?

Environmental sensors can measure contaminants that pose risks to human health and the environment. The detection of metals, volatile organic compounds (VOC), and biological contaminants is used to monitor water purity, air quality and soil composition. The data is used by governments, regulatory bodies and local communities to establish the status of an environment or to identify trends in environmental parameters.

Some environmental sensors are used to monitor weather and storms. They measure temperature, humidity, atmospheric pressure, and precipitation, to name a few. Air quality is monitored through the measurement of temperature, humidity, and emissions, and integrating a network of sensors and geographic information system (GIS) models provides information on pollution levels.

Currently, smart building monitoring makes use of environmental sensors to help with providing insights about living conditions as well as the safety of living and working environments.

Environmental Sensors for Weather Monitoring

Weather sensors are devices that measure data that can be used for extreme weather preparedness (in case of major weather events like hurricanes), or to ensure public and worker safety and minimize business disruption.

Common environmental sensors for sensors include temperature and humidity sensors, which are used to calculate the heat index. Another example is road weather sensors that measure surface conditions and temperatures to help identify dangerous road conditions (i.e., ice or flooding).

Sunlight and UV sensors aid in quantifying the amount of solar radiation reaching a specified environment and can be used to calculate the UV index.

Wind speed and direction sensors help to understand air movement, which is very useful in the maritime world for plotting optimal courses and preserving fuel, or for engineers and architects to inform the design and placement of buildings.

Other commonly used sensors are those that monitor atmospheric pressure and precipitations. Multiple sensors can be built into a weather station, measuring all the relevant parameters in a given application.

Pollution Monitoring – from Gas Sensors to Nanomaterials

A key application of environmental sensors is the determination of pollutants to inform and monitor the pollution exposure of individuals and communities. Particularly important are air pollution sensors, classified into two main categories: one to measure the concentration of gases and the other to measure the concentrations of particulate matter (PM).

Gas phase components such as carbon monoxide (CO), nitrogen dioxide (NO2) and volatile organic compounds (VOC) are determined by measuring how they interact with the sensing material (e.g., an electrochemical cell or a metal oxide semiconductor). Other gas sensors measure the absorption of light at visible or infrared wavelengths (e.g., for O3 and CO2).

PM concentrations have an impact on visibility and climate change. Continuous monitoring of PM – essential to reduce air pollution – is achieved with light scattering methods that use the relationship between scattered light and particle characteristics, especially size.

Recent technological developments in nanomaterials are leading to environmental sensors able to detect contaminants at the nanomolar to sub-picomolar level. This type of environmental sensor consists of a nanomaterial, a recognition element, and a signal transduction method.

The high surface area-to-volume ratios make nanomaterials highly sensitive, leading to extremely low detection limits. Moreover, the comparable sizes of the nanomaterial and the analyte of interest (metal ions, biomolecules, pathogens) contribute to increased sensitivity.

The main nanomaterials used for this new class of environmental sensors are quantum dots (QDs), metal nanoparticles, and carbonaceous nanomaterials, and they find applications in the detection of pesticides, pathogens, and particularly heavy metals such as mercury, lead, cadmium, and chromium.

Sensors for the Monitoring of Buildings

Environmental monitoring in buildings has become crucial as it provides relevant stakeholders with important information regarding the energy performance, safety and cost-effectiveness of buildings. The use of environmental sensors has therefore been increasingly popular, where the parameters generally measured are temperature, carbon emissions, and humidity.

Temperature sensors include thermocouples; resistance temperature detectors (RTDs); thermistors; Integrated Circuit (IC) sensors. Thermocouples are common thanks to their wide measurement range, fast response times, and low cost. However, the other types of temperature sensors might be more accurate or stable towards oxidation.

CO2 sensors provide useful information regarding the presence, location, and user activity within a building and help maintain indoor air quality. The most common of these types of environmental sensors are NDIR (non-dispersive infrared) devices, where light intensity is used to measure the concentration of CO2 in the atmosphere.

CO and VOC are pollutants in indoor environments that have gained significant attention. The two most common ways of detecting CO and VOC are via metal oxide semiconductor field-effect transistor (MOSFET)-based sensors and electrochemical-based sensors.

Humidity sensors are generally classified into capacitive and resistive, and measure the amount of water vapor in air. This is generally measured as relative humidity (as a function of the temperature), or absolute humidity (also known as PPM).

With significant advancements in environmental monitoring over the last decades, new exciting technologies are emerging. With the field in constant expansion, the ultimate goal is to develop environmental sensors that are small, low-cost, sensitive, and durable.

What Are The Causes of Water Pollution?

References and Further Reading

Ho, C. K., et al. (2005). Overview of Sensors and Needs for Environmental Monitoring. Sensors (Basel), 5, pp. 4-37. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3909362/

Snyder, E. G., et al. (2013). The changing paradigm of air pollution monitoring. Environmental Science & Technology, 47, pp. 11369-77.10.1021/es4022602. Available at: https://www.ncbi.nlm.nih.gov/pubmed/23980922

Willner, M. R. & Vikesland, P. J. (2018). Nanomaterial enabled sensors for environmental contaminants. Journal of  Nanobiotechnology, 16, p. 95.10.1186/s12951-018-0419-1. Available at: https://www.ncbi.nlm.nih.gov/pubmed/30466465

Hayat, H., et al. (2019). The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings. Sensors (Basel), 19.10.3390/s19173648. Available at: https://www.ncbi.nlm.nih.gov/pubmed/31443375

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