Network of Raman and dual-polarization LiDARs for the monitoring and characterization of atmospheric aerosols

Atmospheric aerosols play a key role on climate balance (direct, semi-direct and indirect effects), on human health (increase of breathing problems and lung cancer for pollution aerosols) and human activities (damage to aircraft engines by volcanic ashes, reduction of visibility by dust or pollution aerosols). In order to monitor and characterize this threat it is necessary to localize, characterize and trying to quantify the presence of aerosols in the atmosphere from the lowest layers (~100 m) up to the tropopause (18 km). Active remote sensors by LiDARs are the only one capable of providing range-resolved measurements and thus determine the height of aerosol layers. Furthermore, nitrogen Raman LiDARs offer the opportunity to detect aerosol particle and retrieve independently particle extinction and backscatter coefficients contrary to Rayleigh-Mie LiDARs that require an assumption on aerosol type. The use of a cross-polarized channel gives additional information on aerosol depolarization ratio and enables a finer classification of aerosol type. In this paper how it is possible to detect atmospheric layers, to retrieve their optical properties and to classify these layers with the new Raman and dual-polarization LiDAR R-Man510.

The R-MAN510 is the new Raman lidar super ceilometer developed by LEOSPHERE Company. This instrument combines the benefits of both ceilometer (low cost and high repetition rate) and lidar systems (low overlap and high measurement range). This automatic and stand-alone ultraviolet lidar (at 355 nm) has been especially designed for network applications with a low-maintenance diode-pumped laser. It is composed of three detection channels for the elastic co- and cross-polarizations and for the nitrogen Raman backscattered signal at 387 nm. Measurements are typically performed with a vertical resolution at 15 m and a temporal resolution between 30 seconds (for elastic channel) and 10 minutes (for Raman channel). Elastic channels are used for the detection of atmospheric structures (Convective Boundary Layer, Residual layer, Stable Boundary Layer, aerosol/cloud layers) using a 2D gradient algorithm. Then optical properties are retrieved using cross-polarization (for depolarization ratio) and Raman channel (for extinction-to-backscatter ratio so called lidar ratio). The final step of the algorithm is the classification of cloud (low, middle or high water/ice clouds) and aerosol (maritime, continental pollution, dust or volcanic ashes) structures using both depolarization and lidar ratio information. The combining of extinction coefficients profiles from Raman channel with ancillary measurements (size distribution, complex refractive index and density) gives access to mass concentration profiles with an overall uncertainty of ~30% for pollution aerosols and ~70% for volcanic aerosols.

A network of Raman and dual-polarization LiDARs, giving access to the localization, the characterization and identification of aerosol plumes in the atmosphere, would be a powerful tool or decision makers for pollution monitoring or in case of volcanic eruption.