Ozone O₃

Ozone (O₃) is one of six criteria air pollutants identified by the US EPA and is monitored extensively around the world by regulatory agencies.

Ozone is formed naturally in the stratosphere by the reaction of oxygen (O₂) and UV light. O₃ in the stratosphere absorbs harmful deep UV light and, protects the biosphere on the ground.

O₃ can also be formed at ground level by a complex reaction between UV light, NO_x, and volatile organic compounds (VOCs), which come from anthropogenic sources such as vehicles and industrial emissions. O₃ at ground level is harmful to vegetation and humans which is why it is monitored.

The sensor used in the ozone module is a Gas Sensitive Semiconductor (GSS) sensor manufactured by Aeroqual. The sensing material is a n-type semiconductor whose resistance increases when exposed to oxidizing gases such as O₃. Selectivity towards O₃ is achieved by careful control of the sensor material and operating temperature.

The module uses a patented design that creates two different flow states across the sensor: a zero-flow state and a measurement state. The sensor resistance is measured during both the zero and measurement states, and these values are used to calculate the O₃ concentration using a proprietary algorithm approximately once per minute. The flow rate is determined by a flow control orifice on the exhaust side of the sensor and measured at the inlet to the sensor module (expected to be approximately 130 mL min^-1).

An Aeroqual AQM 65 containing an O₃ module was co-located in April 2022 at an air quality regulatory station in Riverside, Southern California and its data compared with reference data downloaded from the US EPA AirNow website. The O₃ module produced an R² of 0.99 and a mean absolute error (MAE) of 2.1 ppb. The low MAE value is indicative that the module has low zero and span drift.

The total measurement uncertainty, based on the EU Guide to Demonstration of Equivalence, using an hourly limit value of 75 ppb, was calculated as 5.8%. Note that an expanded relative uncertainty of less than 15% is required for an O₃ monitor to be considered regulatory grade. The O₃ module meets this requirement.

Directive 2008/50/EC downloaded 10/02/2023 here.

Just like a regulatory ozone analyzer using UV absorption, the ozone module can be field calibrated using standard calibration equipment. This ensures the module calibration is fully traceable to NIST primary standards.

O₃ is not available in a gas cylinder due to its reactivity. The O₃ module span calibration is achieved using a reference grade O₃ source such as the 306 O₃ calibrator from 2B Technologies. The module is zero calibrated using a zero air source such as the AirCal 1000 or AirCal 8000. The zero calibration step of the O₃ module calibration takes approximately 15 minutes and the span calibration takes between 15 and 30 minutes.

Calibration frequency will be dependent on the user’s data quality objectives and Quality Assurance Project Plan (QAPP), but based on the low rate of field drift the recommended calibration frequency is 1-3 months.

Aeroqual can also offer its patented remote calibration procedure MOMA for this module which avoids the need for a site visit while also maintaining traceability to nearby regulatory air monitoring stations. See your Aeroqual representative for more information on this software tool.

The Aeroqual ozone module is very specific towards O₃, but other gases in the environment may also cause a response from the O₃ sensor. These do not indicate a fault. These effects are part of the module behavior and should be considered when examining data.

The O₃ module response to other ambient air pollutants is documented in the table below.

The ozone module contains a solenoid, which can fail occasionally. The lifetime of the solenoid is typically 2 to 3 years.

The GSS sensor does not contain materials which are consumed during its lifetime and O₃ modules have been observed to provide high quality readings in AQM 65s running continuously for up to 4 years in moderately polluted sites with regular instrument servicing. At sites with high pollutant levels and low service levels the lifetime can be reduced to as little as 1 year.

On average the lifetime of the O₃ module is between 2 and 3 years.

Aeroqual offers two different sensor types for ozone measurement in our sensor head range:

The GSS sensor is manufactured by Aeroqual. The sensing material is an n-type semiconductor whose resistance increases when the sensing layer is exposed to an oxidizing gas such as ozone. Selectivity towards ozone is achieved through careful control of the sensor material and operating temperature. Like the ozone module, the GSS ozone sensor head goes through a zero cycle to improve accuracy and baseline stability. The sensor resistance is measured during the zero and measurement states, and these values are used to calculate the O₃ concentration using a proprietary algorithm approximately once per minute. The GSS sensor head gives good accuracy, sensitivity, and stability at low ozone concentrations.

The GSE sensor head incorporates an electrochemical gas sensor. This sensor generates an electrical current in nA, which is proportional to the concentration of gas that comes in to contact with the sensor. The sensor current is converted to a voltage and this voltage is used to calculate the ozone concentration. The GSE sensor head has a faster response time than the GSS sensor head. The GSE sensor head can also be used across a wider concentration range. The GSS ozone sensor is affected by volatile organic compounds (VOCs) while the GSE sensors are cross sensitive to NO₂ and Cl₂.

The GSE sensor head is therefore more suitable for indoor applications, while the GSS sensor head is more suitable for outdoor applications.

The table below highlights the differences between the two sensors:

The table below contains the GSS sensor head specifications. These are suitable for outdoor applications and can be used on our Ranger and S Series products.

The table below contains the GSE sensor head specifications. These are suitable for indoor applications and can be used on our Ranger, S Series and Series 900 products.

The ozone OZU GSS sensor head was evaluated by the South Coast Air Quality Monitoring District (SC AQMD) as part of their AQ-SPEC program. Three S500 bases with OZU sensor heads were deployed at the SC AQMD monitoring site in Rubidoux, Southern California. The R² value was greater than 0.85 for all three units. You can view the full results here.