Ozone is a molecule that contains three oxygen atoms that are constantly destroyed and formed in the stratosphere. The ozone layer is of great significance as it is responsible for absorbing the harmful radiation from the sun, preventing it from reaching the Earth's surface.
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Several layers compose the Earth's atmosphere. The troposphere is the lowest layer that extends from the Earth's surface up to about 10 kilometers in altitude, comprising most activities.
The stratosphere is the next layer, which lies between 10 to 50 kilometers above the Earth's surface, and is the region where most commercial airplanes fly. The stratosphere is also the layer that contains most atmospheric ozone.
The ozone layer absorbs a portion of radiation known as Ultraviolet B (UVB); UVB has shorter wavelengths and is linked to effects like cataracts, skin cancers, damage to some crops, and a danger to aquatic life.
Why Monitor the Ozone?
The ozone plays an important role in protecting life on the Earth's surface by shielding the sun's harmful rays from reaching the surface. Because of the catastrophic exposure ground-level ozone has on life on Earth, various protection agencies must accurately measure ozone gas.
A critical component of air quality management involves ozone monitoring in the atmosphere and ambient air. Different electrochemical and gas sensor technologies have been developed to monitor ozone concentrations. Ultraviolet absorption has been the basis of conventional monitoring devices in the past.
Ozone sensors can be expensive, energy inefficient, and usually at fixed locations limiting the portability of such sensors
How Do You Detect Ozone?
Because of the importance of knowing ozone concentration in air quality and the stratosphere, several types of investment are placed on improving ozone sensors.
In a recent study, an electrochemical sensor (OXB421, Alphasense) was used in a miniaturized ozone sensor that was combined with a lab jack. This ozone sensor was compared to conventional systems used to measure or monitor ozone concentrations.
Electrochemical sensors were able to produce a voltage signal proportional to the ozone concentrations between the ranges of 5ppb-10ppm. The influence of gas sample flow and the relative humidity (RH) were also put into consideration and investigated separately.
A rapid change in the RH of approximately 20%/min generated significant and instant changes in the signals of the ozone sensors. In contrast, slow changes In the relative humidity of 0.1%/min had a little or negligible effect on the response from the ozone sensor. The miniaturized ozone sensor was tested for real-world applications.
In another instance, air quality was monitored over 18 days. It was discovered that sensor measurements were close to those recorded from previous studies using conventional sensors.
For the 18 days of air quality monitoring, the data collected by the sensor was in agreement with data collected by the reference UV ozone analyzer.
This study aimed to investigate miniaturized electrochemical sensors to measure ozone in ambient air quality monitoring and laboratory conditions to have this device be used as an alternative to conventional devices. This miniaturized electrochemical ozone sensor is cheaper, consumes less power, and is portable, making it more convenient to use.
Limitations of Ozone Sensors
Today, commercially available ozone sensors are based on either semiconductor ozone sensors or electrochemical principles. These sensors, in principle, have sufficient sensitivity to sense ozone in parts per billion (ppb) and are suitable for outdoor air pollution and quality monitoring.
These ozone sensors are low-cost, allowing them to be deployed in denser networks to measure air quality networks and give a more improved insight into human exposure levels. Ozone monitoring is vital to air quality management and experimental research regarding atmospheric chemistry, such as ozone interaction with chemicals in the atmosphere and the surface of the Earth.
Traditional photometric ozone monitoring instruments that have existed until now were based on UV absorption, requiring a high sampling gas flow of greater than 1L/min and consuming too much power.
These types of ozone sensors were often too expensive and were limited by environmental conditions, such as during smog chamber simulation experiments where the volumes or flow rate of the gases were not high enough to be measured by the gas sensors.
In such incidents, optical methods such as cavity ring-down spectroscopy (CRDS) are employed; however, these air quality monitors are costly and specialized equipment is not often found in monitoring facilities.
Another challenge faced is that such equipment or instruments are bound to their set locations, making them immobile and connected to the AC power supply in secure facilities.
Ground-level ozone is not directly emitted into the ambient air but is formed due to chemical reactions between volatile oxides (VOCs) and nitrogen oxides (NOXs).
These air pollution particles are emitted by cars, power plants, refineries, boilers, and chemical plants. Ground-level ozone increases on hot sunny days in urban environments and can also reach rural areas due to transportation by the wind.
Ozone present in the air we breathe can be harmful, causing respiratory and other diseases to humans. High exposures can affect ecosystems and vegetation, including parks, forests, and wilderness areas. Therefore, it is essential that protection agencies assess ozone levels regularly; this information can be accessed from daily air quality or weather monitoring.
Future of Ozone Sensors
More research has been undertaken to produce affordable, mobile, less cumbersome ozone sensors which do not require high flow rates to sense ozone concentrations.
Doing so will improve air quality monitoring, improve our understanding of atmospheric chemistry, and prevent damage to the ozone layer due to chemicals being released from human activities.
References and Further Reading
US EPA. (2022) Basic Ozone Layer Science | US EPA. [online] Available at: https://www.epa.gov/ozone-layer-protection/basic-ozone-layer-science
Mt.com. (2022) Dissolved Ozone Sensor | pureO3. [online] Available at: https://bit.ly/3JKD6Ho
Ripoll, A., Viana, M., Padrosa, M., Querol, X., Minutolo, A., Hou, K.M., Barcelo-Ordinas, J.M. and García-Vidal, J., (2019) Testing the performance of sensors for ozone pollution monitoring in a citizen science approach. Science of the total environment, 651, pp.1166-1179. https://www.sciencedirect.com/science/article/pii/S004896971833701X
Pang, X., Shaw, M.D., Lewis, A.C., Carpenter, L.J. and Batchellier, T., (2017) Electrochemical ozone sensors: A miniaturised alternative for ozone measurements in laboratory experiments and air-quality monitoring. Sensors and Actuators B: Chemical, 240, pp.829-837. https://www.sciencedirect.com/science/article/pii/S092540051631437X
US EPA. (2022) Ground-level Ozone Basics | US EPA. [online] Available at: https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics