Pulsed-Cavity Ring down Spectroscopy(CRDS) is a promising technology capable of being self-calibrating besides being non-invasive and compact. This paper describes the basic principles of CRDS and summarizes the main application of it to gas detection at parts per billion level(ppb).In the present work, concentration of a gas is deduced basing on the Beer-Lamberts law and the time constant thus obtained for the three atmospheric gases NO 2, SO 2 and NO 3 are studied at 1 part per billion concentration. Variationsin time constant for the three gases with varying concentration of the Gas from 100 ppb to 3000 ppb are studied. It is observed that with increasing concentration the time constant difference increased showing a more observable value.