Nitrogen oxides

Nitrogen oxides (NOx = NO + NO₂) are an important part of the atmospheric chemistry that occurs in the air we breathe. Human-generated sources (combustion) represent most global emissions, though emissions from soil, fires and lightening are non-negligible components to the global budget. Nitric oxide (NO) at typical ambient levels is considered harmless, though nitrogen dioxide (NO₂) is understood to cause respiratory irritation through oxidative stress of tissue and reduces lung function. The detrimental health effects associated with exposure to NO₂ are greatest for the most vulnerable groups of people – pregnant women, children and the elderly.

Concentration limits for the protection of human health often exist for NO in occupational health settings, overseen by national health and safety agencies (in the UK, this is the Health and Safety Executive). These limit values and averaging periods differ by country.

Concentration limits for NO₂ exist for occupational health and for ambient air, for the protection of human health. In recent years there has been increased focus on reducing NO₂ emissions by national governments, national and international organisations including the Committee on the Medical Effects of Air Pollutants, and the World Health Organisation. The ever-growing popularity of motor vehicles has stifled some of the emissions reductions from the automotive industry.

also known as nitrogen oxide, nitrogen monoxide

Nitric oxide (NO) is formed at high temperatures, where molecular nitrogen and oxygen may be broken apart in to their atomic constituents, and these atoms can then bond as NO.

NO is an unstable molecule (it has an unpaired electron), also referred to as a free radical, meaning it is highly reactive and therefore has a very short atmospheric lifetime (on the order of seconds). For this reason, it is anticipated that the distribution of NO at appreciable concentrations are confined close to emission sources. Nonetheless, emissions of NO have a substantial impact on global, regional and local atmospheric chemistry and air quality.

The Zephyr measures NO with an electrochemical sensor provided by AlphaSense. The Zephyr NO algorithm counteracts the effects of temperature and relative humidity with a mixed linear model. Each sensor is calibrated with a period of field collocation with independently calibrated reference instrumentation (Teledyne T200).

Nitric oxide is not to be confused with the greenhouse gas nitrous oxide (N₂O) which is emitted from agricultural soils from nitrogen-rich fertiliser use.

Nitrogen dioxide (NO₂) is the first oxidation product of NO. NO₂ can also revert back to NO by interaction with sunlight by photolysis (a chemical reaction which sees photons of light break apart a molecule). The atmospheric lifetime of NO₂ varies depending on solar intensity and overall levels of NOx and ozone (O₃).

The Zephyr measures NO₂ with an electrochemical sensor provided by AlphaSense. The Zephyr NO₂ algorithm counteracts the effects of temperature and relative humidity with a mixed linear model. Each sensor is calibrated with a period of field collocation with independently calibrated reference instrumentation (Teledyne T200).

During daytime conditions, there is rapid creation and destruction of NO₂ back to NO. For this reason, it is often convenient to sum the two gases together, which is called nitrogen oxides (NOx, pronounced en-oh-ex or knocks). NOx emitted from vehicle exhausts is mostly NO, then this rapidly dilutes and oxidises to NO₂.