The most abundant noble gas in air is Argon.

Air composition
Air is a mixture of various gases, including nitrogen, oxygen, carbon dioxide, and trace amounts of other gases. The noble gases are a group of elements that are chemically inert and have low reactivity. They are called noble gases because they were once believed to be rare and noble, due to their lack of reactivity. There are six noble gases in total: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).

Noble gases in air
While noble gases are generally present in the atmosphere in trace amounts, some are more abundant than others. In the case of air, which is primarily composed of nitrogen and oxygen, the most abundant noble gas is argon (Ar). Argon makes up approximately 0.934% of the Earth's atmosphere by volume.

Reason for abundance
There are a few reasons why argon is the most abundant noble gas in air:

1. Abundance in the Earth's crust: Argon is a product of the radioactive decay of potassium-40, which is a common element found in the Earth's crust. As a result, there is a continuous source of argon being released into the atmosphere from the Earth's surface.

2. Density: Argon is denser than the other noble gases, which means it is less likely to escape into space. This allows it to accumulate in the Earth's atmosphere over time.

3. Lack of reactivity: Like other noble gases, argon is chemically inert and does not readily react with other elements or compounds. This stability allows it to persist in the atmosphere without being consumed or transformed into other compounds.

4. Production in industrial processes: Argon is also produced as a byproduct of various industrial processes, such as the production of oxygen and nitrogen. These processes contribute to the overall abundance of argon in the atmosphere.

Conclusion
In conclusion, the most abundant noble gas in air is argon. Its abundance can be attributed to its continuous release from the Earth's crust, its density, its lack of reactivity, and its production in industrial processes.