Understanding how gas molecules such as water, oxygen, or carbon monoxide interact with nanoparticles NPs remains an important challenge in the studies of atmospheric chemistry and catalysis reactions. When gas molecules are directly adsorbed onto Au nanodisks (direct nanoplasmonic sensing DNPS) or when they interact with NPs supported on Au nanodisks (indirect nanoplasmonic sensing INPS), they modify the local dielectric properties of the surrounding media. Such a modification induces a wavelength shift of the localized surface plasmon resonance (LSPR) of Au nanodisks which can be easily detected by UV-Vis spectroscopy. We measured water adsorption isotherms by DNPS following the Au LSPR shift when water molecules adsorb/desorb on Au nanodisks [1]. With high sensitivity, we used INPS to follow also the interaction of water molecules on soot NPs that are a significant contributor to global warming in the atmosphere. Then, CO and oxygen adsorption as well as CO oxidation, on Pt NPs, have also been followed by INPS coupled with mass spectrometry [2]. We obtained a quantitative and very sensitive probe even for a very low Pt NPs density. For both DNPS and INPS, we could detect small molecule quantities such as a few hundredths of an adsorbed monolayer. Moreover, we can use nanoplasmonic sensing in a large range of pressure and temperature. FDTD calculations are performed to interpret LSPR spectra and to optimize the physical parameters of our plasmonic nanosensors.