Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx). Other nitrogen oxides include nitrous acid and nitric acid. NO2 is used as the indicator for the larger group of nitrogen oxides.
Nitrogen dioxide is a yellowish-brown gas with a characteristic pungent acrid odour. NO2 is soluble in water at which time it reacts to form nitric acid.
NO2 primarily gets in the air from the burning of fuel. NO2 forms from emissions from cars, trucks and buses, power plants, and other road equipment.
Effects of NO2:
Health effects
Breathing air with low concentrations of NO2 can cause irritation of the eyes and throat, headache, nausea, and a gradual loss of strength. Such exposures over short periods can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms (such as coughing, wheezing or difficulty breathing). Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. Extreme concentrations can cause pulmonary edema (water in the lungs) and potential death.
Exposure Limits:
Time Weighted Average (TWA): 3ppm*
Short Term Exposure Level (STEL): 5ppm
*Australia and other countries are looking to reduce the TWA to 0.2ppm https://www.cacgas.com.au/blog/permissible-limits-for-gases-in-the-workplace
Environmental effects
NO2 and other NOx interact with water, oxygen and other chemicals in the atmosphere to form acid rain. Acid rain harms sensitive ecosystems such as lakes and forests. The nitrate particles that result from NOx make the air hazy and difficult to see though. NOx in the atmosphere contributes to nutrient pollution in coastal waters.
Applications:
The major cause of NO2 in the air is a result of incomplete combustion of fossil fuels with the highest concentrations resulting from diesel vehicles. Coal mining, buses and other diesel vehicles create the greatest hazard for ambient air exposure in mines, tunnels and enclosed spaces such as repair garages.
NO2 along with nitric oxide, carbon dioxide and sulphur dioxide are all pollutants from power generation.
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Calibration Gas Solutions:
The main applications for calibration of NO2 instruments are for personal exposure gas monitoring, fixed area monitoring, emissions (EPA, CEMS) monitoring and exhaust gas measurement.
Personal Exposure Monitoring:
As the TWA and STEL values are so low, the requirement for nitrogen dioxide calibration gas for personal exposure gas monitoring has typically been either 5ppm or 10ppm. Nitrogen dioxide has been a challenging gas for many users of gas detection as the highly reactive gas has an extremely short shelf life in smaller non-refillable cylinders (7 months). This creates issues with having the gas readily available for immediate requirements.
Smaller cylinders of 34L, 60L or 112L have been the norm for this application primarily because of the short shelf life. In most cases customers ask for the smallest cylinder because of the shelf life.
Fortunately, there are two solutions available today which can assist in resolving this issue.
Electronic gas generators
Electronic gas generators can often be a good solution. They can provide gas concentrations of NO2 down to as low .05ppm to 5ppm. Gas generators use an electrochemical cell which is long lasting. The same base unit can be used with other gas cells (CL2, CL02, HCN, H2S), providing flexibility for measuring other gases. Although an initial capital cost is required, they are very cost effective over time and provide a long term solution. The electrochemical cell can be stored when not in use providing customers with NO2 gas when they need it, even for very infrequent requirements.
White Top Specialty Gas Cylinder
The 36-month shelf life “white top” specialty gas mixture cylinder is an excellent solution for those customers with multiple instruments. High pressure cylinders are available in larger capacities of 900 and 4000 litres of gas. These cylinders are specifically treated and conditioned to provide stability over this extended time period. Based upon using only two litres of gas per monthly calibration a customer would only need to have 10 instruments to consume 900 litres within the shelf life. That does not include bump testing/response testing which should be done daily.   Â
Fixed flow, on-demand flow or pressure regulators are available to connect these cylinders to your gas detection instruments or docking stations.
These cylinders are also available for our EPA and CEMS standards providing long term environmental monitoring solutions for all NO2 applications.
Material requirements for NO2:
Nitrogen dioxide is extremely reactive and requires stainless steel regulators and Teflon-lined tubing for proper calibration and response testing.
Never use standard “tygon” tubing for NO2 as the gas will adsorb onto the tubing and provide incorrect readings. The video below demonstrates this using chlorine rather than nitrogen dioxide, but a similar reaction would occur with NO2.
As has been demonstrated, there are multiple options for calibrating and bump testing NO2 instruments and sensors which can meet customers specific requirements. For further information on our calibration gas solutions for NO2 or any other application, contact us at cac@cacgas.com.au
