Primary pollutants are directly emitted into the atmosphere from sources such as industries, vehicles, and natural processes. They can have harmful effects on human health and the environment. In contrast, secondary pollutants are formed in the atmosphere through chemical reactions involving primary pollutants and other compounds.

The substance that is not a primary pollutant among the options given is sulphuric acid (option D). Let's understand why sulphuric acid is considered a secondary pollutant.

Primary pollutants:
a) Particulate matter: Particulate matter (PM) refers to a mixture of solid and liquid particles suspended in the air. These particles can be emitted directly from sources such as smokestacks, construction sites, and road dust. PM includes both fine particles (PM2.5) and coarse particles (PM10). They can have adverse effects on respiratory health and visibility.

b) Sulphur dioxide: Sulphur dioxide (SO2) is a gas produced primarily from burning fossil fuels, particularly coal and oil. It is released into the atmosphere through industrial processes such as power generation, manufacturing, and transportation. SO2 can contribute to respiratory problems, acid rain formation, and the formation of secondary pollutants like sulphuric acid.

c) Nitrogen oxide: Nitrogen oxide (NOx) refers to a group of gases, including nitrogen dioxide (NO2) and nitric oxide (NO). They are produced during high-temperature combustion processes, such as in vehicle engines and power plants. NOx emissions contribute to the formation of smog, acid rain, and the production of secondary pollutants like ozone.

Secondary pollutants:
d) Sulphuric acid: Sulphuric acid (H2SO4) is formed in the atmosphere through the oxidation of SO2 and the subsequent reaction with other compounds. It is a strong acid and a major component of acid rain. Sulphuric acid can have detrimental effects on ecosystems, soil fertility, and infrastructure.

In conclusion, sulphuric acid is considered a secondary pollutant as it is formed in the atmosphere through chemical reactions involving primary pollutants like sulphur dioxide.

Primary pollutants are pollutants which are directly injected into the atmosphere. Sulphuric acid is not a primary pollutant. It is a secondary pollutant.