Previous Year Question

Oxygen (O₂) constitutes approximately 20.95% of the Earth's atmosphere, but it is not a Greenhouse Gas. Greenhouse Gases contribute to atmospheric warming by absorbing and re-emitting Infrared Radiation emitted from the Earth's surface. Oxygen is a Diatomic Molecule composed of two identical atoms. Its normal molecular vibrations do not produce a change in the Electric Dipole Moment, so it does not effectively absorb thermal infrared radiation emitted by the Earth.

Carbon Dioxide (CO₂) and Methane (CH₄) are major Greenhouse Gases. CO₂ emissions increase primarily through fossil fuel combustion, cement production, and deforestation. Methane is emitted from livestock farming, rice cultivation, wetlands, waste decomposition, and natural gas production. These gases enhance the Greenhouse Effect by absorbing Infrared Radiation. Over a 100-year period, the Global Warming Potential (GWP) of Methane is approximately 27–30 times greater than that of Carbon Dioxide.

Nitrogen Dioxide (NO₂) is an air pollutant emitted primarily from vehicles, Thermal Power Plants, and industrial fuel combustion. It belongs to the Nitrogen Oxides (NOₓ) group, which also includes Nitric Oxide (NO). In the presence of sunlight, reactions involving NOₓ and Volatile Organic Compounds (VOCs) produce Tropospheric Ozone (O₃), which is itself a Greenhouse Gas. Therefore, NO₂ has an indirect influence on climate, although it is not classified among the major direct greenhouse gases. NO₂ and Nitrous Oxide (N₂O) are different chemical compounds; N₂O is a powerful direct Greenhouse Gas.

The status of NO₂ also became a subject of dispute in the 2016 UPPSC examination question. Candidates argued that both NO₂ and O₂ could be considered possible answers, whereas the Uttar Pradesh Public Service Commission had identified O₂ as the answer. On 9 December 2016, the Allahabad High Court, in Sunil Kumar Singh and Others v. State of U.P., considered expert evidence explaining that NO₂ contributes to the formation of Tropospheric Ozone and is therefore associated with an indirect greenhouse effect. The Court accepted the Commission's decision to identify O₂ as the answer. The distinction between direct Greenhouse Gases and pollutants with indirect climatic effects is important in scientific classification.