A method for improving the accuracy of estimating concentration path length of a target molecule using a differential absorption LIDAR (DIAL) system. In particular, this method allows improved detection of plumes containing the target molecule against inhomogeneous background, such as uncovered ground or ground with various types of cover. In an embodiment of the present invention, spectral surface reflectivity variations are systematically corrected based on interpolation of surface reflectivity measurements of multiple offline beams of different wavelengths, which are relatively close to the online wavelength. In another embodiment, the signal to noise ratio of the received online pulse energy is improved by using multiple laser beams having the online wavelength and the signal to noise ratio of the received pulse energies at an offline wavelength is improved by using multiple laser beams having that offline wavelength.