A method is developed whereby number densities of several of the atmospheric gas constituents can be determined as a function of altitude. The method employs two laser signals of different wavelengths, one occurring in an absorption region of the gas in consideration and one occurring in a spectral region where no absorption takes place. Returned scattered signals are collected and used along with absorption line strength and line width to determine the number density. The method described affords an indirect way of determining number densities of a specific constituent of the atmosphere by electromagnetic probing.