Calibration of backscattering coefficients with coherent heterodyne lidar utilizing molecular scattering.

Coherent heterodyne lidars are typically used for windspeed and attenuated backscattering measurements. The lack of molecular backscattering detection capability has limited the calibrated backscattering measurements until recent advances in coherent lidar technology. In this work, the simultaneous detection of aerosol and molecular backscattering is demonstrated with coherent heterodyne lidar, and the results are compared with a state-of-the-art Raman lidar PollyXT as a reference in a long-range for the first time. The molecular scattering is measured up to 3 km altitude and the extinction of laser power is calculated based on the attenuation. The essential corrections for the molecular scattering data are described and discussed. The backscattering coefficients are calculated with simple high spectral resolution lidar algorithms and the low altitude overlap is calibrated with on-site ceilometer data. A successful high spectral resolution measurement within coherent heterodyne lidar enables the self-calibration of the measured data and removes guessing of the backscatter to extinction ratio and initial values for the typical inversion algorithms. The self-calibration also enables basic aerosol classification based on the measured backscatter to extinction ratios.