A sensor to detect environmental ozone concentrations

A new method developed by Brazilian researchers is cheaper and easier to use than other instruments for measuring the quantity of the toxic compound in the air.

By Elton Alisson

Agência FAPESP – Researchers from the Chemistry Institute at Universidade Estadual Paulista’s Araraquara Campus (Unesp-Araraquara) in Brazil have developed a new method for measuring the ozone concentration in the environment. Ozone is toxic to humans, plants and animals, can damage materials such as rubber and colorants, and contributes to the greenhouse effect.

Portable, cheaper and easier to use than other equipment designed for this purpose, the new ozone-measuring device was initially developed during a FAPESP-funded research project. The patent for the device was also filed with the assistance of another FAPESP program, the Program to Support Intellectual Property (PAPI/Nuplitec).

“We conducted a study on analytical systems to determine the ozone levels in the environment, and we saw that there were few chemical methods available for this purpose,” explained Arnaldo Alves Cardoso, Chemistry Institute Professor and lead researcher on the project.

“Based on this realization, we sought to develop a new chemical method to determine the concentration of ozone in the environment. This method is recommended for rapid measurements in internal environments, for example, when it is not feasible to acquire the sophisticated and expensive electronic equipment that is available today,” said Cardoso.

The sensor the team developed is based on ozone’s reactivity to indigo. Indigo is used as a colorant in the textile industry to dye fabrics, such as denim, and in the foodstuffs industry to add aniline blue to candy, gum or powdered beverage mixes. In the presence of ozone, the indigo becomes oxidized and changes color.

A round indigo-soaked cellulose filter (2 cm in diameter) is inserted into a disposable plastic syringe containing ethylene glycol – a substance that increases the humidity of the filter and thus facilitates the reaction between ozone and the material surface.

Drawing air through the syringe causes any ozone in the air to contact and discolor the indigo in the cellulose filter. The higher the concentration of ozone in the air sample collected from a given environment, the lighter the color of the paper.

In early versions of the assay, the indigo remaining after the reaction was extracted, and its intensity (i.e., the amount of indigo remaining) was determined using a colorimeter. However, the method was recently adapted to permit reading directly from the filter through comparison to a scale of 30 shades of blue. This visual scale was developed by doctoral student Gabriel Garcia based on digitized images of the indigo filters and can be printed on any commercial printer.

After some hours of testing, the final color of the filter is compared to the scale. Each interval corresponds to a concentration of approximately 3 ppb (parts per billion) of ozone. If the difference between the initial and the final color of the cellulose filter corresponds to 20 shades on the scale, for example, then the ozone concentration in the air sample is approximately 60 ppb, which is considered high.

“Using the table of indigo shade variations that Gabriel created, it is possible to make visual comparisons to determine the ozone concentration in a given environment,” said Cardoso.

Comparisons of equipment

The researchers compared the performance of the new chemical method with that of equipment used by government bodies responsible for monitoring the air quality – such as São Paulo’s environmental sanitation technology company, Cetesb – to determine ozone concentrations in the atmosphere in open environments. These agencies use devices similar to the equipment of the Unesp-Araraquara Chemistry Institute, which was acquired with FAPESP funding.

The experiments conducted on the Unesp-Araraquara campus revealed that the measurements conducted with the team’s chemical sensor were very close to those obtained using the electronic equipment, which costs almost US $10,000.

One difference between the two devices is that while the electronic equipment performs the analysis and detects ozone concentration in real time, the chemical sensor, which costs R$ 400, provides the average concentration over a given day and requires several hours of testing.

“The sensor that we developed will not totally replace the electronic equipment used to gauge the ozone concentration in the atmosphere,” emphasized Cardoso.

“The electronic equipment indicates the ozone concentration in the environment minute by minute and allows the detection of rapid variations. Our method can be used to determine whether the ozone concentration is high or low in a certain location as well as to calculate the median value,” he said.

Because it is cheap and portable, according to the researchers, the sensor can be considered a viable alternative for monitoring sites that are far away from large urban centers – because ozone pollution is no longer a problem that is exclusive to metropolises – and indoor environments, which also generate the compound.

Ozone has anti-bacterial properties and can be used to eliminate microorganisms in the water and the air, and it also reacts with organic compounds that generate unpleasant odors, such as cigarette smoke. Because of these properties, ozone has been used in a broad array of appliances, including air purifiers, washing machines, ventilators and germicidal bulbs. Furthermore, it is often mixed in swimming pool and aquarium water and is utilized in room deodorizers and granular fungicides.

Because ozone is present in so many materials, indoor environments also contribute to increasing the concentration and exposure to the gas. In external environments, ozone is formed in the atmosphere by chemical reactions involving sunlight, nitrogen oxides and volatile organic compounds emitted in the evaporation of fuel, vehicle exhaust and even vegetation such as eucalyptus trees.

“The major problem with ozone generation in indoor environments is that it is added to the ozone produced in external environments and increases the concentration of the gas in the air. And this does not only apply to pollutants,” confirmed Cardoso.

Lack of legislation

Another problem highlighted by Cardoso is that while cities such as São Paulo have strict laws to limit the generation of ozone in external environments so as not to affect the air quality, there are still no statutes regulating the generation of the toxic gas in closed environments, despite the increasing domestic use of the compound.

Additionally, according to Cardoso, no one knows exactly what quantity of ozone appliances generate or the residual concentration of the compound in closed environments after these types of equipment have been used.

“The lack of technical specifications for these types of equipment and laws to control ozone generation in closed environments are very serious problems that need to be discussed,” he affirmed.

The researcher then suggested that the target indoor ozone concentrations should be the same as those established by the organizations responsible for monitoring the quality of air in closed environments.

For example, at the end of April, Cetesb released a state decree that reduced the maximum permitted concentration of ozone in the atmosphere of São Paulo State from 70 ppb to 50 ppb over an 8-year interval.

In the measurements conducted with chemical sensors in Araraquara, however, Unesp researchers detected ozone concentrations in the air of up to 85 ppb and found that the quantity of the gas in the region increases in the middle and at the end of summer, as well during sugarcane-burning season. “The burning of biomass increases the emission of compounds in the atmosphere that favor ozone formation,” explained Cardoso.

In the city of São Paulo, which has the country’s largest vehicle fleet, in 2012 alone the ozone concentration around Ibirapuera Park surpassed the 81 ppb mark 36-fold, according to Cetesb data.

“Although cars have catalyzers, more ozone-forming compounds will be emitted into the city’s atmosphere as years pass because the auto fleet grows daily and the average driving speed declines,” he noted.

The sensor developed by Unesp has already sparked interest among companies working on the issue of ozone generation, although the device is not yet being commercialized in part due to the lack of legislation establishing the ozone generation limits in internal environments.

To facilitate its use in closed environments, an alternative system has been developed in which the indigo-soaked filters are placed inside open containers for a few hours and then removed for color analysis and ozone level measurement. In this case, the use of equipment to draw in the air is not required because the air present in the environment with the indigo is sufficient to cause discoloration.

“The sensor for closed environments has not yet been made commercially available due to a lack of market demand. However, we are still seeking to develop chemical methods that will make determining ozone levels simpler, cheaper and more accessible,” said Cardoso.