This study investigates gas molecule adsorption on single-walled carbon nanotubes (SWNTs) and nanotube bundles using first principles methods. The equilibrium position, adsorption energy, charge transfer, and electronic band structures are analyzed for various gas molecules including NO₂, O₂, NH₃, N₂, CO₂, CH₄, H₂O, H₂, and Ar. The results show that most molecules adsorb weakly on SWNTs and can act as either charge donors or acceptors. Adsorption on nanotube bundles is stronger than on individual tubes. The electronic properties of SWNTs are sensitive to certain gases like NO₂ and O₂. Charge transfer and gas-induced charge fluctuations significantly affect the transport properties of SWNTs. Theoretical results are consistent with recent experiments. Gas molecules adsorb on SWNTs through physisorption, with weak interactions and minimal charge transfer. For NO₂ and O₂, charge transfer is more significant, leading to larger adsorption energies. The electronic band structures of SWNTs are not significantly changed by adsorption, but the degeneracies of energy bands are removed. NO₂ causes more pronounced band splitting than NH₃. The electronic density of states (DOS) of SWNTs adsorbed with NO₂ or NH₃ shows minimal changes, except for the slight modification due to band splitting. NO₂ adsorption significantly modifies the DOS shape, shifting the Fermi level into the valence band and making the SWNT p-type conductor. Adsorption of gas molecules on the surface or inside of nanotube bundles is stronger than on individual tubes. The interaction between nanotubes and gas molecules is weak, with minimal effect on the electronic structures of SWNTs. However, for NO₂ and O₂, the interaction is more pronounced, leading to significant changes in electronic and transport properties. The presence of NO₂ causes local charge fluctuations, which may affect the transport properties of metallic SWNTs. The study concludes that most gas molecules in air have weak effects on SWNTs, while O₂, as a charge acceptor, makes all nanotubes p-type conductors.This study investigates gas molecule adsorption on single-walled carbon nanotubes (SWNTs) and nanotube bundles using first principles methods. The equilibrium position, adsorption energy, charge transfer, and electronic band structures are analyzed for various gas molecules including NO₂, O₂, NH₃, N₂, CO₂, CH₄, H₂O, H₂, and Ar. The results show that most molecules adsorb weakly on SWNTs and can act as either charge donors or acceptors. Adsorption on nanotube bundles is stronger than on individual tubes. The electronic properties of SWNTs are sensitive to certain gases like NO₂ and O₂. Charge transfer and gas-induced charge fluctuations significantly affect the transport properties of SWNTs. Theoretical results are consistent with recent experiments. Gas molecules adsorb on SWNTs through physisorption, with weak interactions and minimal charge transfer. For NO₂ and O₂, charge transfer is more significant, leading to larger adsorption energies. The electronic band structures of SWNTs are not significantly changed by adsorption, but the degeneracies of energy bands are removed. NO₂ causes more pronounced band splitting than NH₃. The electronic density of states (DOS) of SWNTs adsorbed with NO₂ or NH₃ shows minimal changes, except for the slight modification due to band splitting. NO₂ adsorption significantly modifies the DOS shape, shifting the Fermi level into the valence band and making the SWNT p-type conductor. Adsorption of gas molecules on the surface or inside of nanotube bundles is stronger than on individual tubes. The interaction between nanotubes and gas molecules is weak, with minimal effect on the electronic structures of SWNTs. However, for NO₂ and O₂, the interaction is more pronounced, leading to significant changes in electronic and transport properties. The presence of NO₂ causes local charge fluctuations, which may affect the transport properties of metallic SWNTs. The study concludes that most gas molecules in air have weak effects on SWNTs, while O₂, as a charge acceptor, makes all nanotubes p-type conductors.