UGC NET Questions (Paper – 1)

Air pollutants are often classified as primary when they are emitted directly, and as secondary when they form through chemical reactions in the atmosphere. Tropospheric ozone is not emitted directly in large quantities; instead, it is generated when nitrogen oxides and volatile organic compounds react in the presence of sunlight. This process is central to photochemical smog formation in cities. Thus, tropospheric ozone produced photochemically is a classic example of a secondary pollutant.

Option A:
Option A is incorrect because sulphur dioxide released from stacks is a primary pollutant; it is emitted in that form and may later transform into sulphates, which are secondary.

Option B:
Option B is incorrect as carbon monoxide is directly emitted from incomplete combustion in engines and is therefore a primary pollutant, not a secondary one.

Option C:
Option C is incorrect since lead particles are directly discharged from certain industrial processes or old fuels. They do not primarily form from atmospheric reactions and therefore are considered primary pollutants.

Option D:
Option D is correct as it acknowledges that ozone in the lower atmosphere results mainly from photochemical reactions. Its formation depends on sunlight and precursor gases, fitting the definition of a secondary pollutant.