InfraredLIDARApplications in Atmospheric Monitoring

Abstract

The acronym LIDAR stands for LIght Detection And Ranging, an optical analog of RADAR (RAdio Detection And Ranging). The conventional version of LIDAR requires a laser transmitter to launch short pulses of coherent light, which are scattered from atmospheric targets of interest back to an optical receiver, with a time delay that is determined by the range of the target. Optical phenomena in the Earth's atmosphere (e.g. Rayleigh scattering, Raman scattering, Mie scattering, refraction, and resonant absorption) contribute to the amplitude of optical signals returning to the receiver and their characteristic wavelength dependence allows us to measure the concentration and velocity distributions of different atmospheric molecules and aerosol particles. LIDAR backscattering in the infrared (IR) region, on which this article concentrates, is well suited to detecting aerosols (as in clouds or industrial particulate emissions). IR DIAL (DIfferential Absorption Lidar), a variant in which two or more wavelengths are used simultaneously to separate resonant molecular signals from background, enables many molecular species to be monitored by means of their IR absorption spectra. Closely related approaches comprise long‐path IR laser absorption and IPDA (Integrated Path Differential Absorption), with retroreflection from a topographic target (e.g. a strategically located mirror or a hard target, such as the ground in air‐borne applications); these approaches sacrifice optical range information but gain sensitivity because they integrate over all molecules in the optical column between the transmitter/receiver and the reflector or hard target. All of these techniques are vitally dependent on pulsed IR laser technology.

Preliminary range-resolved detection of stable water isotopologues by differential absorption lidar using a 2 µm parametric source

“…Differential absorption lidar (DIAL) is a remote gas sensing technique based on the use of tunable narrow-linewidth sources emitting radiation in and out of tune with the absorption line of the species of interest. To perform sensitive, range-resolved gas measurements, these sources need to deliver high peak power in the nanosecond regime [1]. A potential application for such a device is the measurement of stable water isotopologues in the lower atmosphere, which would lead to improved understanding of processes governing the global hydrological cycle, with the prospects of improving the accuracy in the predictions made by atmospheric general circulation models [2].…”

Demonstration of range-resolved detection of stable water isotopologues by differential absorption lidar

“…The provision of such data with sufficient vertical resolution in the lower troposphere (0-3 km) has the potential to significantly increase the accuracy in the predictions made by atmospheric general circulation models [1]. For this purpose, differential absorption lidar (DIAL) is a promising technique which has been successfully demonstrated for water vapor and other greenhouse gases [2]. In order to quantify the relative abundance in HD 16 O (D), the method requires collocated measurements of both the main isotopologue H2 16 O and HD 16 O with an equivalent relative precision, which has not been demonstrated so far despite recent progress on multiple wavelengths DIALs [3], and the proven capability of some approaches to probe isotopic water vapor lines [4].…”