Photochemical smog is formed by the reaction among nitrogen oxides, hydrocarbons and ozone in sunlight, and contains:

Smog is a type of air pollution. The word was coined in the early 20th century by combining "smoke" and "fog". There are two main types of smog:

Classical smog (London smog or sulphurous smog): It occurs in cool, humid climates. It is a mixture of smoke, fog and sulphur dioxide, mainly from burning coal. It is chemically reducing in nature, so it is also called reducing smog. The worst example was the Great Smog of London in December 1952, which lasted five days and caused about 4,000 deaths immediately (later estimates are about 12,000). It led to the Clean Air Act of 1956 in Britain.

Photochemical smog (Los Angeles smog): It occurs in warm, dry and sunny climates with heavy motor traffic. It was first identified in Los Angeles in the 1940s. It forms when nitrogen oxides and hydrocarbons (volatile organic compounds), emitted mainly by vehicles, react in the presence of sunlight (hence "photochemical").

In photochemical smog, sunlight breaks down nitrogen dioxide (NO2) into nitric oxide (NO) and oxygen atoms. The oxygen atoms combine with oxygen molecules to form ozone (O3). Ozone, nitrogen oxides and hydrocarbons then react to form secondary pollutants such as peroxyacetyl nitrate (PAN), formaldehyde and acrolein. Because ozone and PAN are strong oxidising agents, photochemical smog is called oxidising smog. It appears as a brownish haze.

Effects of photochemical smog:

• Irritation of the eyes, nose and throat, coughing, headaches and breathing problems such as asthma.
• Damage to plants, rubber, plastics and paint; PAN is particularly harmful to plants.
• Reduced visibility.
• Ground-level (tropospheric) ozone is a harmful pollutant, unlike stratospheric ozone, which protects us from ultraviolet rays.

Delhi and other North Indian cities often suffer from severe smog in winter, caused by vehicle emissions, industries, construction dust and stubble burning, worsened by calm winds and low temperatures. Measures include the Graded Response Action Plan (GRAP) and the National Clean Air Programme (NCAP, 2019).

Why not the other options?

• ✗Sulphur dioxide and soot are the main components of classical (London) smog.
• ✗Carbon monoxide and water vapour alone do not make photochemical smog.
• ✗Chlorofluorocarbons deplete the ozone layer but are not components of smog.