Sensitive and accurate dual wavelength UV-VIS polarization detector for optical remote sensing of tropospheric aerosols

TL;DR

This paper presents a highly sensitive UV-VIS polarization lidar system capable of accurately measuring particle depolarization ratios in the troposphere, including urban pollution, with minimal systematic bias.

Contribution

The study introduces a novel dual wavelength UV-VIS polarization detector with a synthetic transfer matrix, enabling robust calibration and measurements over a wide depolarization range in real atmospheric conditions.

Findings

Measured backscattering coefficients as low as 5×10⁻⁸ m⁻¹.sr⁻¹

Depolarization ratio detection limit of 0.6%

Effective polarization calibration under real atmospheric conditions

Abstract

An UV-VIS polarization Lidar has been designed and specified for aerosols monitoring in the troposphere, showing the ability to precisely address low particle depolarization ratios, in the range of a few percents. Non-spherical particle backscattering coefficients as low as 5 {\times} 10-8 m-1.sr-1 have been measured and the particle depolarization ratio detection limit is 0.6 %. This achievement is based on a well-designed detector with laser-specified optical components (polarizers, dichroic beamsplitters) summarized in a synthetic detector transfer matrix. Hence, systematic biases are drastically minimized. The detector matrix being diagonal, robust polarization calibration has been achieved under real atmospheric conditions. This UV-VIS polarization detector measures particle depolarization ratios over two orders of magnitude, from 0.6 up to 40 %, which is new, especially in the UV where molecular scattering is strong. Hence, a calibrated UV polarization-resolved time-altitude map is proposed for urban and free tropospheric aerosols up to 4 kilometres altitude, which is also new. These sensitive and accurate UV-VIS polarization-resolved measurements enhance the spatial and time evolution of non-spherical tropospheric particles, even in urban polluted areas. This study shows the capability of polarization-resolved laser UV-VIS spectroscopy to specifically address the light backscattering by spherical and non-spherical tropospheric aerosols.