Sources of nitrogen oxides

Nitrogen oxides, such as nitrogen(II) oxide (NO) and nitrogen(IV) oxide (NO₂), can be produced naturally and through human activities.

Natural sources:

• Lightning strikes - The high temperatures of lightning provide enough energy for nitrogen and oxygen to react.

Man-made sources:

• Formed in vehicle engines - Air enters the engine and the high temperature and pressure allows nitrogen and oxygen to react, producing nitrogen oxides that are emitted in the exhaust fumes.

Nitrogen oxides are pollutants, and can contribute to both acid rain and photochemical smog.

Roles of nitrogen oxides in acid rain

Acid rain is rainfall containing nitric acid (HNO₃) and sulfuric acid (H₂SO₄) that causes harm to plants, animals, infrastructure and bodies of water.

Nitrogen oxides have two roles in the formation of acid rain:

1. Direct role of NO In the atmosphere, NO is oxidised to NO₂:

• 2NO_(g) + O_2(g) ➔ 2NO_2(g) NO₂ dissolves in and reacts with water and oxygen in the atmosphere to produce nitric acid, a component of acid rain:

• 2NO_2(aq) + H₂O_(l) + 1⁄2O_2(g) ➔ 2HNO_3(aq)

1. Catalytic role of NO In the atmosphere, NO catalyses the oxidation of sulfur dioxide (SO₂) gas to sulfur trioxide (SO₃) gas:

• SO_2(g) + NO_2(g) ➔ SO_3(g) + NO_(g) The NO₂ that acted as the catalyst is regenerated from the NO product by reaction with oxygen:

• NO_(g) + 1⁄2O_2(g) ➔ NO_2(g) This NO₂ molecule can then catalyse the oxidation of another SO₂ molecule into SO₃.

Sulfur trioxide gas reacts with water to produce sulfuric acid, another component of acid rain:

• SO_3(g) + H₂O_(l) ➔ H₂SO_4(aq)

Role of nitrogen oxides in photochemical smog

Photochemical smog is a visible haze containing secondary pollutants such as peroxyacetyl nitrate (PAN) that causes harm to lungs, eyes and plants.

Secondary pollutants form from chemical reactions between primary pollutants (pollutants emitted directly from human activity) in the atmosphere.

Nitrogen oxides have a key role in the formation of photochemical smog:

1. Nitrogen oxides are primary pollutants directly released into the atmosphere from sources like vehicle exhausts and power plants.
2. In sunlight, nitrogen oxides can react with other primary pollutants like volatile organic compounds (VOCs) from unburnt fuel hydrocarbons.
3. This reaction produces secondary pollutants like PAN which make up photochemical smog.
4. Since sunlight provides the energy to drive the chemical reaction that produces PAN and other secondary pollutants, this type of smog is termed "photochemical" smog.

Reducing nitrogen oxides

The amount of nitrogen oxides pollution from vehicles is reduced by using catalytic converters containing platinum, palladium, and rhodium catalysts. These convert nitrogen oxides to harmless nitrogen gas before release:

2CO_(g) + 2NO_(g) ➔ 2CO_2(g) + N_2(g)