Current Research in Lidar Technology Used for the Remote Sensing of Atmospheric Aerosols

Abstract

Lidars are active optical remote sensing instruments with unique capabilities for atmospheric sounding. A manifold of atmospheric variables can be profiled using different types of lidar: concentration of species, wind speed, temperature, etc. Among them, measurement of the properties of aerosol particles, whose influence in many atmospheric processes is important but is still poorly stated, stands as one of the main fields of application of current lidar systems. This paper presents a review on fundamentals, technology, methodologies and state-of-the art of the lidar systems used to obtain aerosol information. Retrieval of structural (aerosol layers profiling), optical (backscatter and extinction coefficients) and microphysical (size, shape and type) properties requires however different levels of instrumental complexity; this general outlook is structured following a classification that attends these criteria. Thus, elastic systems (detection only of emitted frequencies), Raman systems (detection also of Raman frequency-shifted spectral lines), high spectral resolution lidars, systems with depolarization measurement capabilities and multi-wavelength instruments are described, and the fundamentals in which the retrieval of aerosol parameters is based is in each case detailed.

Keywords: aerosol; lidar; review; technology.

Figure 1

Basic schematics of a lidar system: A laser transmitter emits light pulses to the atmosphere; an optical assembly, usually a telescope, collects part of the scattered radiation, which, after being filtered, is brought onto a photo-detector; the detected signal is then amplified, digitized and processed to retrieve atmospheric parameters.

Figure 2

Illustration of the principle of the Raman technique to measure the aerosol extinction. The range-corrected Raman signal for a purely molecular atmosphere is represented by the blue curve. The red curve represents the range-corrected Raman signal if there were two aerosol layers (between 500 m and 1000 m, and between 1500 m and 2000 m) with $5\times {10}^{-4}$ m⁻¹ aerosol extinction coefficient (green curve).

Figure 3

Example of retrievals of aerosol optical properties: (a) backscatter coefficient from KF (with a constant lidar ratio of 50 sr) and Raman algorithm; (b) extinction coefficient from Raman algorithm; and (c) lidar ratio from Raman algorithm. The data are from a Saharan dust intrusion detected in Barcelona, Spain, on 28 May 2016, with the multi-wavelength lidar system from the Universitat Politècnica de Catalunya (UPC).

Figure 4

Qualitative backscatter spectrum around the central backscatter frequency of an atmosphere containing aerosol. The frequencies are referred to the one emitted by the laser.

Figure 5

Principle of the HSRL using an etalon to separate the molecular backscatter from the aerosol one (adapted from [62]).

Figure 6

Color ratio vs. lidar ratio vs. depolarization ratio for different aerosol and cloud types [72] with data from [73].

Figure 7

Detection of Saharan dust with depolarization channel with the UPC multi-wavelength lidar during the 26 May 2016 intrusion: (a) Range-corrected total power as a function of time and height above ground level; (b) Volume depolarization ratio as a function of time and height.

Figure 8

Depolarization ratios measured with the UPC multi-wavelength lidar during the 26 May 2016 Saharan dust intrusion event: (a) quicklook of the volume depolarization ratio between 14:24 and 15:23 UT with a time resolution of 1 min.; (b) average vertical profile of the volume (green) and particle (red) depolarization ratios with their associated error bars (black).