Oxides of Nitrogen

Oxides of Nitrogen: Structure

Nitrogen dioxide and nitrogen monoxide are the most toxicologically significant gases of the group, so we're going to focus on their structure.

Nitrogen Dioxide (NO₂)

NO₂ is a covalent compound. It's composed of a central nitrogen atom and two oxygen atoms.

One oxygen atom is single bonded to the nitrogen; the other oxygen atom is double bonded.

O=N−O

Nitrogen Monoxide (NO)

Nitrogen monoxide is composed of one nitrogen atom and two oxygen atoms. The molecule is a free radical – the atoms share a double bond, but the nitrogen has an unpaired electron (indicated by a dot).

O=N•

Due to their unpaired electron, free radicals are highly unstable and reactive. Free radicals affect human health – they break down cells over time. As the body ages, it loses its ability to fight free radicals, resulting in oxidative stress.

Oxidative stress leads to cell damage, degenerative processes, and ageing.

Examples of Oxides of Nitrogen

Earlier, we heard about four different gases that are classed as oxides of nitrogen. Let's go into more detail about each one.

Causes of Oxides of Nitrogen

Where do oxides of nitrogen come from?

Natural Sources

The majority of NO_x are produced naturally. Sources include:

• Volcanic eruptions

• Biological decay

• Microbial activity in the ocean

• Lightning strikes

Combustion Processes

Hydrocarbon fuels are burnt in hot engines. In these hot conditions, typically between 75 and 105ºC, nitrogen and oxygen molecules in the air combine to form nitrogen monoxide.

N₂ + O₂ → 2NO

When nitrogen monoxide is released from vehicle exhausts or flue chimneys, it reacts with oxygen in the air to form nitrogen dioxide.

2NO + O₂ → 2NO₂

Figure 2: Fossil fuel power plants burn hydrocarbons, leading to NOx pollution. unsplash

Fertiliser Use

Farmers add chemical fertilisers to their crops. Fertilisers contain important plant nutrients such as nitrogen, phosphorus, and potassium. Plants need these nutrients to grow and function effectively.

Fertilisers contain nitrogen because it is used by plants to produce chlorophyll.

Without nitrogen and chlorophyll, plants cannot grow.

Using too much fertiliser results in an excess of nitrogen-rich organic matter in the soil. Microbes feeding on excess nitrate matter produce large amounts of NO_x, polluting the air.

Fertiliser use doesn't only lead to NO_x pollution. Excess fertiliser runs into waterways, causing eutrophication.

Eutrophication leads to algal overgrowth. Too much algae limits photosynthesis elsewhere in the water, leaving anaerobic 'dead zones' below.

The Effects of Oxides of Nitrogen

Oxides of nitrogen are pollutants, affecting the environment and human health.

Global Climate Change

Nitrogen dioxide is the third most potent greenhouse gas, trapping heat in the atmosphere and contributing to global climate change.

Furthermore, NO_x destroys stratospheric ozone. The ozone layer plays an essential role in protecting our planet from harmful UV radiation. Without it, UV radiation would kill plants and damage the DNA of living organisms.

Acid Rain

Oxides of nitrogen contribute to acid rain. When they react with water and air, they form nitric acid (HNO₃). Nitric acid mixes with water and falls to the ground as acid rain. Consequences include damage to forests and erosion of physical structures.

Photochemical Smogs

NO_x pollution contributes to the formation of photochemical smogs.

Photochemical smogs are visible as a brown haze. They're most prominent in warm, densely populated areas.

Smogs worsen respiratory illnesses and affect visibility.

Figure 3: A photochemical smog in Hong Kong. unsplash

Health Problems

Short-term exposure to oxides of nitrogen results in symptoms such as headaches, irritation of the eyes and noise, difficulty breathing, and abdominal pain. Long-term exposure can cause asthma and other respiratory conditions.

Exposure to very high concentrations of NO_x can impact fertility, harm developing fetuses, and even cause death.

Controlling Oxides of Nitrogen

How can we prevent NO_x from impacting the environment and living organisms? There are three primary methods: catalytic converters, urea sprays, and managing fertiliser use.

Catalytic Converters

Catalytic converters are installed in car engines to reduce harmful emissions such as NOx, carbon monoxide, and particulate matter.

The converters use redox reactions to reduce harmful emissions.

Catalytic converters are made of rare metals platinum and rhodium. They are positioned in a honeycomb shape to create a large surface area for the oxidation and reduction of harmful emissions.

The converters reduce nitrogen atoms from NO_x molecules.

Free, highly reactive oxygen atoms collide and form O₂ molecules.

Nitrogen atoms attached to the catalyst react with each other, forming N₂ molecules.

2NO → N₂ + O₂

2NO₂ → N₂ + 2O₂

Oxidation reactions use the newly formed O₂ molecules to convert carbon monoxide and methane into less harmful products.

CO + O₂ → CO₂

CH₄ + 2O₂ → CO₂ + 2H_2O

Urea Sprays

Urea is commonly used in controlling NO_x emissions from burning fossil fuels.

Flue chimneys containing oxides of nitrogen are sprayed with an aqueous urea solution.

Any oxides of nitrogen dissolve in the solution, forming nitrous acid (HNO₂).

Then, the nitrous acid reacts with the urea to form nitrogen, carbon dioxide, and water.

2HNO₂ + NH₂CONH₂ → 2N₂ + CO₂ + 3H₂O

Managing Fertiliser Use

To use fertiliser as effectively as possible, and limit NO_x emissions, farmers follow the “four R's”:

• Right application rate

• Right formulation (fertiliser type)

• Right timing of application

• Right placement of application

Using urea-based fertilisers as an alternative to ammonia-based fertilisers may reduce emissions of NO_x.

I hope that this article has explained oxides of nitrogens to you. Remember that they're a group of polluting gases, made of oxygen and nitrogen atoms. They cause global climate change, photochemical smogs, and health problems.

^{1. Adam Voiland, NASA Researchers Explore Lightning's NOx-ious Impact on Pollution, Climate, NASA, 2009}

^{2. Australian Government, Oxides of Nitrogen, Department of Climate Change, Energy, the Environment and Water, 2022}

^{3. Autodoc, Causes of Overheating, 2022}

^{4. H. J. Emeléus, Advances in Inorganic Chemistry and Radiochemistry, 1964}

^{5. PubChem, Nitrous Oxide, National Library of Medicine, 2022}