Photochemical Smog

Smog is nothing but a combination of smoke and fog. Smog occurs due to the factors like geography, sunlight, industrial activity, vehicular emissions, calmness of winds, burning of coal etc. Smog usually occurs at a place which is far away from the actual source of pollution.

Photochemical smog is the result of the action of ultraviolet radiation from sun on atmosphere polluted with pollutants such as nitrogen oxides and hydrocarbons. Photochemical smog can take place only in the presence of sunlight.

Why northern India is more vulnerable to photochemical smog than southern India?

Photochemical smog is a widespread problem in industrial cities all over the world. In India, northern India, especially Delhi suffers from photochemical fog especially at the time of stubble burning in neighbouring states like Punjab and Haryana.

However, in spite of similar urbanization, photochemical smog is witnessed in north Indian city like Delhi rather than southern Indian cities like Chennai and Mumbai. This is due to the following factors:

Stubble burning

According to IIT Kanpur, stubble burning is the third highest contributor to air pollution during winters, after construction dust and vehicular fumes.

Stubble is an 8-10 inches of straw that stands behind in the field after the harvest of crops like paddy and wheat. Farmers in states like Haryana, Uttar Pradesh and Punjab have the practice of burning the stubble to prepare the agricultural fields for the next sowing season.

Wind Circulation

Atmosphere above Delhi is the colliding place for two types of winds during winter- One which carries pollutants from stubble burning in neighbouring states and the other which carries moisture from Uttar Pradesh. This results in near-still wind conditions near the ground level that traps the pollutants,  resulting in smog.

Polluting Industries

Delhi has a large number of polluting industries which are primarily coal fired in its vicinity.

Geography and Climate

Delhi is a land locked continental city. So it cannot experience land and sea breeze which take away the pollutants away from the southern coastal cities. In addition, the duration of monsoon winds is short in Delhi compared to the southern Indian cities. Also, Delhi faces cold climate in winter which causes temperature inversion and results in pollutants to get trapped for longer duration.

Impacts of Smog

Human health

Photochemical smog results in reduced visibility, irritation of the eyes, and respiratory diseases in humans. The smog affects brain, eyes, nervous system and lungs. The smog contains toxic pollutants like PM2.5, PM10, carbon monoxide (CO), methane (CH4), carbon dioxide (CO2) and nitrogen oxide (NOx).

The following are the ill effects caused due to the inhalation of these pollutants in humans.

The PM 2.5 and PM10 results in eye irritation, asthma, bronchitis, heart and lung damage.

  • Carbon Dioxide (CO2): rapid breathing and rapid heart rate.
  • Methane (CH4): suffocation, fatigue and headaches.
  • Carbon Monoxide (CO): confusion, headache, and flu-like effects.
  • Nitrous Oxide (N2O): brain damage.
  • Nitrogen Oxide (NOx): airway infection, chest pain etc.
  • Sulphur Dioxide (SO2): eye irritation, chest tightness and asthma.

The pollutants present in the smog can be harmful to plants and animals as well. Photochemical smog can decimate sensitive crops, trees and other vegetation. Ground level ozone can hamper growth and productivity of trees.


It has been found that some plants like potato and tomato are very sensitive to ozone formed as a result of photochemical smog. Thus, the growth of these crops gets hampered due to the photochemical smog.

Why farmers are burning stubble?

For farmers, burning stubble is the easy and cheap way of disposing the stubble and prepare the field for next sowing season. For them, manual harvesting is not possible as the labour is expensive and not easily available.

Preventive Measures

The Environmental Pollution (Prevention and Control) Authority (EPCA) has suggested that the state governments should take incentive measures such as offering incentives to purchase no-till machines to farmers.

The local government agencies like patvaris and village sarpanchs should be roped in to stop the practice of stubble burning in farms.

Instead of burning, the farmers should be encouraged to sell their stubble to paper and cardboard factories. Similarly, government should make efforts to collect stubble from the farmers and use them for generating power.

Suggested Questions for GS Mains

What factors lead to formation of Photochemical Smog? Why northern India is more vulnerable to this problem than southern India? While discussing its impacts on agriculture, human health and overall ecosystem, suggest preventive measures against its formation.

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