Nitrogen Dioxide NO₂

Nitrogen dioxide (NO₂) is one of six criteria air pollutants identified by the United States Environmental Protection Agency (US EPA) and is monitored extensively around the world by regulatory agencies.

NO₂ can be emitted directly from industrial process and combustion. NO₂ can also be formed as a secondary pollutant by reaction of nitric oxide (NO) and ozone (O₃). To learn more about NO_x, NO and NO₂, read our blog on A Beginner's Guide to the Nitrogen Oxide Family as Air Pollutants.

The nitrogen dioxide module used in Aeroqual’s range of fixed monitors uses a cutting-edge automatic baseline correction (ABC) design to measure nitrogen dioxide (NO₂) in ambient air. It combines an advanced electrochemical NO₂ sensor with an integrated O₃ filter and a sample/zero cycling operation to deliver near reference performance in ambient NO₂ monitoring.

The sensor used in the NO₂ module is an electrochemical gas sensor. The sensor generates an electrical current in nA which is proportional to the concentration of gas that comes in to contact with the sensor.

The module uses a patented design that creates two different flow paths, one of which passes through a proprietary scrubber to remove NO₂ and produce a baseline measurement. The module has a controlled flow rate across the sensor. 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 NO₂ concentration using a proprietary algorithm approximately once per minute.

The NO₂ module performance specifications are given in the table below.

An Aeroqual AQM 65 containing an NO₂ module was co-located in April 2022 at an air quality regulatory station in Riverside, Southern California and its data compared with the reference data downloaded from the US EPA AirNow website. The NO₂ module produced an R² of 0.96 and a mean absolute error (MAE) of 1.3 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 100 ppb, was calculated as 5.8%. Note that an expanded relative uncertainty of less than 15% is required for an NO₂ monitor to be considered regulatory grade (Directive 2008/50/EC). The NO₂ module meets this requirement.

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

Just like a regulatory nitrogen dioxide analyzer based on chemiluminescence, this sensor-based system can be fully field calibrated using standard calibration equipment and reference gases. This ensures the module calibration is fully traceable to NIST primary standards.

The NO₂ 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 NO₂ 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.

Aeroqual also offers 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 NO₂ module is specific towards NO₂ but other gases in the environment may also cause a response from the NO₂ sensor. These do not indicate a fault. These effects are part of the module behavior and should be considered when examining data.

The NO₂ 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 expected life of 2 years in climates with moderate humidity. In climates with very high humidity the expected life may be reduced.

The Aeroqual NO₂ sensor head uses a gas sensitive electrochemical (GSE) sensor, with an inbuilt ozone scrubber. GSE NO₂ sensors are often susceptible to ozone cross-interferences. By using a NO₂ sensor with a selective ozone filter, we can get a direct measurement of NO₂ without interference from ozone. 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 NO₂ concentration.

The NO₂ sensor head specifications are given in the table below.

Although the GSE sensor in the nitrogen dioxide sensor head is designed to measure NO₂, it does show cross-interferences to other gases. The table below shows the response in ppm of the NO₂ sensor head to different gases.

An Aeroqual S500 with a nitrogen dioxide sensor head was co-located for 4 days with an Ecotech EC9841 NOx analyzer to demonstrate performance outside the Aeroqual office in Auckland, New Zealand. The time series and scatter plots are shown below. The sensor head gave an R² value of 0.8, showing good correlation with the reference instrument.