Nitrogen Oxides NO_x

Nitrogen oxides (NO_x) are a group of gases (predominantly NO and NO₂) that contribute to air pollution processes such as smog formation and ground level ozone production, both of which have detrimental effects on air quality and human health.

NO and NO₂ are produced by combustion processes. The primary source of NO_x emissions is the burning of fossil fuels in vehicles, power plants and industrial operations.

The sensor used in the NO_x module is a Gas Sensitive Semiconductor (GSS) manufactured by Aeroqual. The sensing material is a n-type semiconductor whose resistance increases when exposed to oxidizing gases such as NO₂. A converter converts NO to NO₂ which allows NO and NO₂ to be measured and for NO_x to be reported. The sensor also responds to ozone, so the air stream is passed through an ozone scrubber before reaching the sensor.

The NO_x module uses Aeroqual’s patented automatic baseline correction (ABC) to measure NO_x. The module uses two different flow paths, one of which passes through a proprietary scrubber to remove NO₂ and produce a baseline measurement. 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 resistance is measured during both the zero and measurement states, and these values are used to calculate the NO_x concentration using a proprietary algorithm, once per minute.

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