A method and apparatus for measuring target gas concentrations in an atmosphere. The method and apparatus emit in the atmosphere a laser beam tuned to a molecular absorption line of a target gas, receive a reflected signal affected by gas absorption of the target gas in the atmosphere, divide and direct the received signal into a first optical path and a second optical path including in one of the paths a correlation gas cell filled with a predetermined concentration of the target gas, detect transmitted signals through the first optical path and the second optical path, and calculate a target gas concentration by comparing a first signal transmitted through the first optical path to a second signal transmitted through the second optical path. The apparatus includes a laser source tunable to a specific molecular absorption line of a target gas and configured to emit in the atmosphere a laser beam having a spectral bandwidth greater than a full width of the molecular absorption line of the target gas, a receiver configured to receive a reflected signal affected by gas absorption of the target gas in the atmosphere, and at least one detector configured to detect transmitted signals through the first optical path and the second optical path.