NOx is shorthand for the nitrogen oxides, nitric oxide (NO) and nitrogen dioxide (NO2), both of which are produced during the combustion of hydrocarbons.
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Note that nitrous oxide (N2O) is not considered a NOx compound, while NOy compounds refer to those generated from the further oxidation of NOx, nitric (HNO3) or nitrous (HONO) acid, for example. NOx compounds are harmful to both human health and the environment, and thus monitoring NOx levels in the atmosphere, both around urban and rural environments, is vitally important to public health and environmental surveillance.
How Is NOx Produced?
At elevated temperatures, the endothermic reaction between nitrogen and oxygen becomes thermodynamically feasible, and thus NOx species are produced as a by-product of hydrocarbon fuel combustion in air. Within the troposphere and stratosphere, conversion between NO and NO2 species is rapid with the involvement of incident light and ozone, specifically with light acting as the photocatalyst in the reduction of NO2 to NO, and ozone oxidizing NO to NO2. The correlation between NOx and ozone concentration in the atmosphere is known as the Leighton relationship and determines the atmospheric concentration of each in association with the intensity of sunlight received.
Various volatile organic compounds present in the atmosphere can react with NOx in the presence of sunlight to produce diverse secondary pollutants, many of which are harmful to human health. For example, peroxyacetyl nitrate is produced from peroxyacetic acid radicals in the presence of NO2, and is considered an irritant to the eyes and lungs. Peroxyacetyl nitrate is also a major component of photochemical smog, dense fog/smoke that lingers in the atmosphere and infamously plagued large cities such as London, mainly in the 19th century.
NOx species also contribute to the generation of acid rain by multiple routes, one of which involves the photocatalyzed generation of a nitrate radical (NO3-) from reaction with ozone, followed by reaction with NO2 in dark conditions (at night) to form dinitrogen pentoxide (N2O5). Dinitrogen pentoxide dissolves rapidly in liquid phase water in the atmosphere to dissociate into two nitric acid (NO3) molecules. The now acidic rain is harmful to plants and aquatic life and can result in soil acidification over the long term.
While the majority of NOx produced each year is thought to be anthropogenic in origin, large quantities are also produced by natural processes since NOx species constitute a vital element of the nitrogen cycle. Ammonia and nitrate in soil can be converted to NOx by microorganisms and released into the atmosphere, particularly when nitrogen fertilizer is added to the agricultural industry and excess nitrogen species are present.
Another natural process producing NOx is lightning. Lightning strikes can generate sufficient local temperatures to catalyze the formation of NOx from atmospheric nitrogen and oxygen; a single lightning strike is thought to consume around 7 kg of atmospheric nitrogen in the reaction. Much of the NOx produced by lightning strikes linger in the upper atmosphere, while that originating from fuel combustion or biogenic sources remains closer to the ground, where it can more adversely affect human health.
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What Is the Influence of NOx on Human Health and the Environment?
NOx and associated chemical species have been implicated in several health conditions; NOx, ozone, volatile NOx compounds, and nitric acid vapor, for example, each implicated in triggering and exacerbating asthma symptoms.
Nitryl radicals produced from NOx may also be involved in causing DNA mutations, as radical species are known to potentially cause damage to the DNA molecule, resulting in cancer.
Importantly, NOx and ozone are greenhouse gases and contribute to the warming of the globe. NO and NO2 do not absorb light strongly in the atmosphere and thus are not considered direct greenhouse gasses like carbon dioxide (CO2), nitrous oxide (N2O), or ozone (O3).
Instead, due to involvement in generating other greenhouse gasses, mainly ozone, NOx is considered an indirect greenhouse gas. Conversely, NOx can also engage in chemical interactions within the atmosphere that lead to global cooling, chiefly via the generation of hydroxyl radicals in the atmosphere.
Hydroxyl radicals break down methane, a direct greenhouse gas and ozone reducer, into water and other harmless products. Additionally, hydroxyl radicals are involved in forming liquid aerosol droplets from sulfur dioxide, which is thought to provide a cooling effect by refracting light from the earth. However, huge additional quantities of NOx in the atmosphere from anthropogenic sources promote the dysregulation of ordinary atmospheric composition.
How Can NOx Levels Be Monitored?
NOx gasses may be mildly detectable by the human sense of smell, even at low concentrations, but intense or chronic exposure can be fatal. In environments where NOx gasses are expected at high concentrations, workers utilize detectors and breathing apparatus. However, such highly specialized equipment is expensive and generally unavailable to the average consumer, who may be interested in local NOx levels around the home or when walking the roadside, for example.
A small nanowire array sensor capable of sensitive NOx detection into the ppb range has recently been developed by the Australian Research Council Center of Excellence for Transformative Meta-Optical Systems. The device is only 0.2 mm wide and so can easily be fitted onto an existing silica chip or into other small wearable electronics, providing an alert to dangerous NOx levels.
The device utilizes a p-n junction between two types of semiconductor materials: doped zinc, which forms the base of the structure and represents the p-type semiconductor, and doped silicon, which forms the n-type semiconducting nanowires.
A non-doped inert section separates the zinc base and silicon wires, which forms the basis of how NOx is detected. p-n junctions are used in many electronic devices, such as diodes, where electrons belonging to the n-doped material move over to the p-doped material to fill electron holes, allowing a controllable flow of current through the diode.
In this case, electrons move through the inert section of the nanowire to fill holes with a specific current that can be measured unless affected by outside factors. NOx is a strong oxidizer capable of removing electrons from the inert section of nanowires, and thus in the presence of NOx fewer electrons reach the p-section, generating a lower current. Detectors for NOx and other toxic chemicals may become standard wearable technology in the future, integrated into devices such as cell phones or clothing to constantly monitor for spikes in gas concentration.
References and Further Reading
ARC Centre of Excellence for Transformative Meta-Optical Systems (2023). https://phys.org/news/2023-01-nanowire-sensors-internet.html
Lasek, J. A. & Lajnert, R. (2022). On the Issues of NOx as Greenhouse Gases: An Ongoing Discussion… Applied Science, 12(20). https://www.mdpi.com/2076-3417/12/20/10429
Lammel, G. & Grabl, H. (1995). Greenhouse effect of NOX. Envrion Sci Pollut Res Int., 2(1), pp. 40-45. https://pubmed.ncbi.nlm.nih.gov/24234471/
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