Retrievals of Backscatter Coefficient and Mass Concentration of Particles with Coherent Doppler Lidars

Many industrial activities produce significant quantities of fugitive particles like dust, soot or any other particles, with potentially adverse effects on the local and neighbouring environments. Such industrial sites are more and more controlled in order to ensure that maximum levels of pollution are not overtaken. For both regulatory bodies and industries, it is becoming crucial to understand the spatial dispersion of particles with wind conditions and operations of the plants. Industrial sites are nowadays mostly equipped with in-situ sensors that provide accurate measurements of pollution with time. However, the number of locations monitored by such sensor remains too limited to have a 2D view of the dispersion over an area of interest.

If Coherent Doppler Lidars (CDL) have been developed initially to measure accurately wind speeds, they are more and more used for measuring aerosols and clouds. Indeed, the backscatter signal of CDLs can be also used to retrieve atmospheric optical properties like backscatter coefficient. The interests are to measure simultaneously winds and aerosols with one single sensor. In addition, CDLs can be equipped with scanning capabilities allowing the 2D monitoring and mapping of wind and particles.

Different techniques have been developed in the last ten years to retrieve backscatter coefficients and even mass concentrations of particles from the backscatter signal of CDLs. As for direct detection Lidars, the telescope function needs to be calibrated in order to provide a corrected backscatter signal. The effect of focalization to maximize the measurement range of CDLs will be taken into account by fitting a Lorentzian function on the profiles of backscatter signal. For CDLs with all fibered emission / reception chain, the telescope function is expected to be constant and thus such a calibration should be performed when the focus or any other internal parameter of the CDL is changed.

The current study will propose a methodology to calibrate the telescope function and present an analysis of its stability and its parameters with time. The attenuated backscatter coefficient obtained will be then compared to backscatter coefficients provided by direct detection Lidars. The systematic and random errors will be then analyzed based on two campaigns performed in West Europe and in the East coast of USA. An analysis of both wind and backscatter measurements at a coastal site will be presented showing how industrial chimney plumes and atmospheric turbulence affect the dispersion of aerosols. Then, backscatter signals of CDL are compared to mass concentration of particles. The correlation analysis doesn’t reveal a linear function. The explanations for these discrepancies will be given. Finally, the operational applications of such multi-function remote sensors for air quality purposes will be discussed through one practical example.