Which of the following gases is NOT considered one of the primary greenhouse gases in the Earth's atmosphere?

Introduction / Context:
This environmental chemistry question concerns greenhouse gases, which trap outgoing infrared radiation and contribute to the greenhouse effect and climate change. The question asks you to identify which listed gas is not one of the primary greenhouse gases in the Earth atmosphere.

Given Data / Assumptions:

• We are considering the major greenhouse gases in the troposphere and lower atmosphere.
• Options include methane, ozone, nitrous oxide, hydrogen and carbon dioxide.
• We assume standard climate science definitions of greenhouse gases.
• Water vapour is another major greenhouse gas but is not listed as an option here.

Concept / Approach:
Primary greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and water vapour, along with man made gases such as CFCs. These gases absorb infrared radiation and warm the atmosphere. Hydrogen gas (H2), on the other hand, does not significantly absorb infrared radiation in the way greenhouse gases do and is not classified as a primary greenhouse gas. Therefore, among the options, hydrogen is the one that does not belong to the list of major greenhouse gases.

Step-by-Step Solution:
1) Identify known greenhouse gases: carbon dioxide, methane, nitrous oxide, ozone and water vapour are all well established greenhouse gases.2) Methane (CH4) is a potent greenhouse gas with a strong warming effect per molecule compared with CO2.3) Ozone (O3) in the lower atmosphere acts as a greenhouse gas by absorbing infrared radiation, even though stratospheric ozone plays a different protective role.4) Nitrous oxide (N2O) is a long lived greenhouse gas emitted by agricultural activities and combustion.5) Hydrogen (H2) is a light gas that does not have significant infrared absorbing bands in the atmospheric window and is not counted among primary greenhouse gases.6) Carbon dioxide (CO2), although not listed as one of the first three options, is also an important greenhouse gas and is definitely not the correct answer here.7) Therefore, the gas that is not a primary greenhouse gas among the options is hydrogen.

Verification / Alternative check:
Climate science sources routinely list the main greenhouse gases as water vapour, carbon dioxide, methane, nitrous oxide, ozone and some halogenated gases like CFCs. Hydrogen is not on this list. Hydrogen does have some indirect climate effects when it leaks into the atmosphere and affects chemical pathways, but it is not itself a primary greenhouse gas. In contrast, methane, ozone and nitrous oxide are explicitly cited as greenhouse gases, confirming that hydrogen stands out as the non greenhouse gas in the options given.

Why Other Options Are Wrong:
Methane (CH4): A strong greenhouse gas produced from natural sources and human activities such as livestock and fossil fuel extraction.
Ozone (O3): In the troposphere, ozone contributes to greenhouse warming and air pollution, even though in the stratosphere it shields us from UV radiation.
Nitrous oxide (N2O): A potent greenhouse gas with long atmospheric lifetime, often associated with agriculture and combustion processes.
Carbon dioxide (CO2): The most discussed greenhouse gas because of its large anthropogenic emissions from burning fossil fuels and deforestation.

Common Pitfalls:
Some students may confuse importance with abundance and think that only very abundant gases like nitrogen and oxygen matter for climate. However, these major constituents do not absorb infrared radiation in the way greenhouse gases do. Another mistake is to think that any gas present in the atmosphere must be a greenhouse gas, which is not true. The key is whether the gas has vibrational modes that interact with infrared radiation. Remember the core group: CO2, H2O, CH4, N2O and O3, and you will quickly recognise that hydrogen is not among them.

Final Answer:
The gas that is NOT a primary greenhouse gas in the Earth atmosphere is hydrogen (H2).