The means to measure area-averaged flux in the atmospheric surface layer is provided since large aperture scintillometer (LAS) was applied. The measurement technique of water vapor area-averaged flux is an important problem which is not solved so far. The behavior of low frequency range in the scintillation spectrum is probably related to fluctuations of water vapor. The spectra measured by LAS are different from the measurements of Nieveen. Another corner frequency was measured which is related to outer scale of absorb medium, so it is more reasonable. The structure parameter of refractive index imaginary part for the different latent heat flux was calculated by means of simulation theory. Analysis indicates that the low frequency range in the scintillation spectrum is also probably related to absorption and scattering fluctuations of aerosol.