Errors in the Lidar Measurement of Atmospheric Gases by Differential Absorption

Abstract

An analysis is presented of the lidar measurement of the distribution of atmospheric gases by the differential absorption of scattered energy. The error involved in the measurement of the spatial distribution of the gas density is investigated in terms of the uncertainties associated with the absorption coefficient, the power measurement, the atmospheric parameters, and the laser frequency. An application of the error analysis is made to the measurement of the density of water vapor by a ruby lidar tuned to the 694.38-nm line of water vapor. It is shown, using parameters of existing lidar systems, that the uncertainty in the measurement of water vapor is governed by instabilities in the laser frequency for observations below 2 km. At greater altitudes, the uncertainty in the measurement of the density arises from the variance associated with the power measurement. The fractional standard deviation of the water vapor measurement is less than 6% for altitudes below 3 km for the distribution of water vapor studied (February, Washington D. C.).