Environmental Chemistry

Oxides of Sulphur and Nitrogen

Do you know Delhi is one of the most polluted cities in the world? The presence of toxic gasses and various air pollutants usually cross the permissible limit. Central Pollution Control Board suggests that the air quality of Delhi is in extremely poor condition and it is breaching all the permissible air quality standards. Oxides of sulphur and nitrogen, oxides of carbon, hydrocarbons, hydrogen sulphide, ozone and other oxidants gaseous along with particulate pollutants like fumes, dust, smoke, etc. are the air pollutants that are responsible for severe air pollution in Delhi as well as many other states in India.

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Tropospheric / Air Pollution

Oxides of Sulphur and Nitrogen

Example of Smog in Delhi During Winter Season (Source: Delhidailynews)

The cause of tropospheric pollution is the presence of gaseous and solid air pollutants in the air in an excess quantity than desirable. It is primarily of two types

  • Gaseous Air Pollutants– This category of air pollutants includes substances such as oxides of sulphur and nitrogen, hydrogen sulphide, hydrocarbons, ozone, oxides of carbon, and other oxidants.
  • Particulate Pollutants– It includes pollutants such as smoke, dust, mist, smog, fumes, etc.
Oxides of Sulphur and Nitrogen

Example of Air Pollution (Source: Wikipedia)

Oxides of Sulphur and Nitrogen

We know elemental nitrogen is one of the major components of earth’s atmosphere. Thus, it will participate in many natural processes. It is responsible for the formation of nitrogen dioxide entering the atmosphere. The classic example of the formation of nitrogen oxides is by the chemical reaction of atmospheric nitrogen and oxygen during the thunderstorm.

The chemical reaction occurs in the presence of lightning to form the nitric oxide which further undergoes reaction with oxygen to form nitrogen dioxide. Oxides of sulphur and nitrogen are responsible for many natural phenomena.

Similarly, oxides of sulphur such as sulphur dioxide enter the atmosphere by natural processes such as volcanic eruptions. Moreover, oxidation of hydrogen and sulphide during organic matter decomposition process in the absence of air leads to the formation of sulphur dioxide.

Since many decades the natural processes based on oxides of sulphur and nitrogen have been taking place and there was a global balance of these gases in the earth’s atmosphere. However, lately many anthropogenic or human activities led to the increase in the amounts of the compounds more than the desired quantity.

Thus, it is causing an imbalance in the earth’s atmosphere and exerting detrimental effects. Thus the excess quantity of the gases makes it air pollutants. We will study the negative impact of oxides of sulphur and nitrogen when present in excess quantity than the permissible limit.

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Oxides of Sulphur

The general term to refer to all oxides of sulphur is SOx. The major oxides of sulphur are sulphur dioxide and sulphur trioxide. Sulphur dioxide (SO2) gas has a characteristic pungent, irritating taste and odour but it is a colourless gas. Moreover, it is water soluble and forms sulphurous acid (weak acid).

Sulphur dioxide can slowly react with the oxygen present in the air to form sulphur trioxide (SO3). However, it is not a stable compound so it tends to react with water to form sulphuric acid. The oxidation process of SO2 is slow without the presence of a catalyst. However, particulate matter present in the polluted air act as the catalyst and speeds up the sulphur dioxide reaction to form sulphur trioxide.

2SO2 (g) +O2 (g) → 2SO3 (g)

Moreover, gaseous pollutants such as hydrogen peroxide and ozone can also promote the reaction.

SO2 (g) +O3 (g) → SO3 (g) + O2 (g)

SO2 (g) + H2O2 (l) → H2SO4 (aq)

Source of Oxides of Sulphur

The burning of sulphur-containing fossil fuel leads to the formation of oxides of sulphur. Although sulphur dioxide is the most common gas among oxides of sulphur it is highly poisonous to living organisms including plants and animals.

Effects of Sulphur Oxides

Human Beings

Reports suggest even low concentrations of oxides of sulphur especially sulphur dioxide can cause respiratory problems such as bronchitis, asthma, and emphysema in human beings. It can also cause eye irritation, redness, and tears.

Breathing of this air pollutant can cause irritation to nose and throat which can further give rise to constant coughing, wheezing, breathing difficulties and tightness in the chest. SO2 can cause bronchitis and lung cancer on exposure to higher levels.

Plants and Vegetation

Oxides of sulphur interfere in the chlorophyll synthesis process and damage plant cells. Moreover, interruption of chlorophyll synthesis process will slow the photosynthesis process of the plants. Higher concentration of sulphur dioxide stiffens flower buds. Thus, the flower buds will eventually fall off.

It causes excessive water loss in plants. This air pollutant decreases the quality of plant and affects plant yield. It reduces crop yields. Oxide of sulphur is more harmful if it combines with other pollutants such as a combination of oxides of sulphur and nitrogen, the combination with fluorides, etc.


Sulphur dioxide causes severe erosion to stones and statues, especially if the materials contain carbonate in them. It can also cause pitting, scarring, and discolouration of the materials. SO2 promotes corrosion reaction in different metals and metal surfaces in the presence of moisture. It causes discolouration of paper, paint, fabric, leather, etc.

Acid Rain

Sulphur dioxide and sulphur trioxide present in the atmosphere combines with water droplets during the time of rain. As a result of which the rain falls in the form of sulphurous acid (H2SO3) or sulphuric acid (H2SO4). This pH of this rain is much lower than the normal pH of rainwater. This type of rain is called acid rain. The acid rain damages soil surface, vegetation, land, etc. It causes leaching of soil and plant canopy and loss of nutrients. Acid rain disturbs the aquatic life.


Two primary component of Industrial smog or London smog is sulphur dioxide and particulate pollutants such as soot, dust, smoke, etc from burning of coal. The combustion of sulphur-containing materials such as coal or fossil fuels releases sulphur dioxide. This combines with water droplets and forms highly toxic particles. However, nowadays due to the adoption of advanced measures by industry and government, it is now a thing of past.

Download Oxides of Sulphur and Nitrogen Cheat Sheet PDF


Oxides of Nitrogen

Nitrogen in combination with oxygen can form a series of oxides such as nitric oxide (NO), nitrous oxide (N2O), dinitrogen tetroxide (N2O4), dinitrogen trioxide (N2O3), and dinitrogen pentoxide(N2O5). However, the oxides responsible for environmental pollution are NO, NO2 and N2O.

Collectively the oxides of nitrogen are known as NOx. The oxides of nitrogen can behave as the primary pollutant by producing toxic reaction themselves or they can act as secondary pollutants and combine with other pollutants to form acid rain, photochemical smog, aerosols.


Oxides of nitrogen can form from natural as well as human activities or anthropogenic sources. Nitrogen-containing compounds burn in industries and automobiles to form nitrogen oxides. This is also an example of anthropogenic sources.

The main components of air are Dioxygen and dinitrogen. They will do react together at a normal temperature. However, during the time of lightning the nitrogen and oxygen combine to form oxides of nitrogen at high altitudes. When NO2 oxidizes and form nitrate (NO3) ion that enters into the soil and acts as a fertilizer.

Dinitrogen and dioxygen combine during the burning of fossil fuel at high temperature, released from the automobile engines, to form significant amounts of nitrogen dioxide and nitric oxide.

N2 (g) + O2 (g) → 2NO(g) at 1483 K

The nitric oxide will rapidly react with oxygen to form NO2 (nitrogen dioxide).

2NO (g) + O2 (g) → 2NO2 (g)

The rate of nitrogen dioxide production will increase if the reaction occurs between stratospheric ozone and nitrogen oxide.

NO (g) + O3 (g) → NO2 (g) + O2 (g)

The irritant red haze usually seen in the traffic and congested regions is due to the presence of oxides of nitrogen.

Effects of Oxides of Nitrogen

Human Health

It is a harmful lung irritant and can cause respiratory diseases in children. The chemical can increase the chances of lung infections. It is also a powerful eye irritant. The symptoms of oxides of sulphur and nitrogen are quite similar. This can also cause coughing, wheezing, bronchitis but few additional symptoms are colds and flu.

Elderly people and children are more prone to having asthma on exposure for a long duration. Oxides of nitrogen can increase the chances of acute respiratory illness such as acute bronchitis in children. It can also increase airway resistance in adults. The gaseous pollutant is toxic to any living tissue.


The gaseous pollutant can damage vegetation. It can cause damage to plant leaves and in turn, affect photosynthesis rate. NO2 has the potential to suppress the growth of plant even at a very low concentration of 0.3ppm. It can make plants, vegetation, and trees prone to diseases. Higher concentration of oxides of nitrogen can decrease the production of chlorophyll and can cause chlorosis.

Acid Rain

Nitrogen dioxide can combine with water droplets/water vapour in the atmosphere and fall down in the form of acid rain. Acid rain is very harmful to materials, vegetation, soil surface, etc.


It is harmful and can cause discolouration and damage to the materials such as fibres, textile materials, metals, etc.

Photochemical Smog

Oxides of nitrogen in the presence of sunlight and hydrocarbons can form photochemical smog. This type of smog is very toxic and contain a bunch of different chemical compounds such as ozone, organic compounds (peroxo compounds, aldehydes, acetyl nitrate, ketones), and aerosols.

Solved Examples for You

Question: Select the gaseous pollutant that causes acid rain

  1. Ozone and dust
  2. SO and CO
  3. SO and NO
  4. CO and NO

Solution: Option C.

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