Greenhouse gases trap outgoing terrestrial (infrared) radiation, warming the lower atmosphere. Which of the following gases does not exhibit a significant greenhouse effect under Earth tropospheric conditions?

Introduction / Context:
Greenhouse gases (GHGs) absorb and re-emit infrared radiation, reducing the efficiency with which Earth loses heat to space. The major naturally occurring GHGs include water vapour, carbon dioxide, methane, nitrous oxide, and ozone; human activities have increased several of these concentrations.

Given Data / Assumptions:

• We evaluate gases for significant IR absorption features in the thermal infrared window.
• Background gases like N2 and O2 are largely IR-inactive (no strong dipole moment changes in fundamental vibrational modes).
• We seek the non-GHG among common atmospheric gases.

Concept / Approach:
Molecules absorb IR when their vibrations change the dipole moment. Linear homonuclear diatomics such as N2 and O2 lack a permanent dipole and have vibrational modes that are IR-inactive, making them negligible greenhouse contributors despite their abundance. Conversely, CO2 (asymmetric stretch/bend), CH4 (multiple vibrational modes), N2O, and O3 have strong IR absorption bands and contribute to the greenhouse effect.

Step-by-Step Solution:

Verification / Alternative check:
Atmospheric spectroscopy data and climate texts consistently classify N2 (and O2) as non-GHG for IR absorption; any greenhouse influence is indirect (e.g., pressure broadening), not primary absorption.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming abundance alone (N2) implies greenhouse effect; confusing UV absorption (O3) with lack of IR activity—ozone does both.

Final Answer:
Nitrogen (N2)