Cavity Ring Down Spectroscopy (CRDS) technique has a wide range of applications in atmospheric sciences as this single instrument is capable of measuring different gases with very high accuracy and sensitivity. Observations of CO2, CO and CH4 are being made at Nainital using CRDS. The diurnal pattern of these trace gases is mainly influenced by upslope and downslope mountain winds which is an important phenomena in high altitude sites. The boundary layer processes and role of OH chemistry is also playing an important role.