Sulfur Dioxide SO₂

Sulfur dioxide (SO₂) is one of six criteria air pollutants identified by the US EPA and is monitored extensively around the world by regulatory agencies.

SO₂ is emitted directly from industrial processes and is also directly emitted by vehicles, especially if fuel quality is poor.

The SO₂ module uses a patented automatic baseline correction (ABC) design to measure SO₂ in ambient air. It combines an advanced electrochemical SO₂ sensor and a sample/zero cycling operation to deliver near reference performance in ambient SO₂ monitoring.

The sensor used in the SO₂ module is an electrochemical gas sensor. The sensor generates an electrical current in nA which is proportional to the concentration of gas which comes in to contact with the sensor.

The module design creates two different flow paths, one of which passes through a proprietary scrubber to remove SO₂ to produce a baseline measurement. The module has a controlled flow rate. 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 60 mL min^-1).

The sensor current is measured during both the baseline and sample measurement states and these values are used to calculate the SO₂ concentration using a proprietary algorithm approximately once per minute.

The SO₂ module performance specifications are given in the table below.

Just like a regulatory sulfur dioxide analyzer using UV fluorescence, the SO₂ module can be field calibrated using standard calibration equipment and reference gases. This ensures the module calibration is fully traceable to NIST primary standards.

The SO₂ module factory calibration is achieved using a zero-air source, certified calibration gas standards and a certified gas dilution calibrator. The Aeroqual AirCal 1000 or 8000 systems can be used to calibrate the SO₂ module in the field.

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.

To demonstrate performance, a six-point gas test, at known SO₂ concentrations, was carried out using the equipment described above. The module shows good linearly (R² = 0.99) over the 0 – 1000 ppb range studied.

The Aeroqual sulfur dioxide module is specific towards SO₂ but other gases in the environment may also cause a response from the SO₂ sensor. These do not indicate a fault. These effects are part of the module behavior and should be considered when examining data.

The SO₂ module response to other ambient air pollutants is documented in the table below.

The non-catalytic chemical scrubber has an expected life of 2 to 3 years. The internal solenoid has an expected life of 2 to 3 years. The GSE sensor has an average expected life of 1 to 2 years in climates with moderate humidity and particle loading.

The Aeroqual sulfur dioxide sensor heads use a gas sensitive electrochemical (GSE) sensor. The electrochemical gas sensor in the sensor head generates an electrical current in nA, which is proportional to the concentration of gas that comes in contact with the sensor. The sensor current is converted into SO₂ concentration.

The table below contains the sulfur dioxide sensor head specifications. These can be used on our Ranger, S Series and Series 900 products. The 0-10 ppm sensor is designed for outdoor environmental and indoor air quality monitoring. The 0-100 ppm sensor is designed for industrial health and safety monitoring.

Although the GSE sensor in the SO₂ sensor head is designed to measure SO₂, it does show cross-interferences to other gases. The table below shows the response in ppm of the SO₂ sensor head to different gases.