Differential absorption LIDAR test bed facility for the detection and quantification of greenhouse gases
We are developing a method for accurately quantifying greenhouse gas emissions from natural and anthropogenic sources and sinks to meet the requirements for local, national, and international mitigation efforts. It will be based on a technology known as differential absorption LIDAR (Light Detection And Ranging), which can provide species specific concentrations of greenhouse gases with range resolution. When these measurements are coupled to precise wind velocity measurements, the flux of the greenhouse gases, the important quantity for regulators, can be determined. An indoor testing facility is also being developed to rigorously test hardware components (optical receiver, laser control, etc.) and software (analysis algorithms, data throughput).
The basic processes involved in elastic backscatter LIDAR are as follows. A laser source emits a pulse of light (typically a few nanoseconds), and as the pulse propagates, the photons interact with particles in the atmosphere. Some of these interactions, such as Mie and Rayleigh scattering, result in backscattered photons. The photons are collected by the detector and recorded as function of time. This time-of-flight data has a direct correspondence with the range (distance) at which the scattering event occurred.
Differential absorption LIDAR (DIAL) is based on the same principal, but operates at two wavelengths, one on resonance and one off resonance of the molecular absorption of the greenhouse gas of interest. Because the on resonance wavelength is more strongly absorbed by the greenhouse gas, the difference between both signals is proportional to its number density. Thus, this technology could provide regulators with quantity of greenhouse gases being released at a particular location and pinpoint their sources.
The cartoon above illustrates the DIAL system measuring two plumes of CO2 emissions separated by 500 meters. The panel on the right is a simulation of the raw signal return, the log of the range corrected signal, and the final DIAL result which clearly shows the concentration of CO2 in each plume of gas.
The DIAL system will span this spectral region shown above where several important greenhouse gases absorb. The absorption spectra of greenhouse gases and water were modeled using HITRAN 2008 parameters and the atmospheric abundance of each.
The DIAL system will be located in a room that connects to a 100 meter long tunnel facility. A 30 meter long section of the tunnel will have precise gas handling and flow control, and is being developed in collaboration with the Process Measurements Division. This section will be equipped with a variety of sensors (terahertz gas sensor, FTIR, NDIR, PITOT tubes, etc.) that will serve to validate the operation of the DIAL system and the corresponding analysis algorithms. Collaborators in the Statistical Engineering Division are developing new analysis methods with a well characterized uncertainty.
Schematic of DIAL test facility.
The DIAL system is being designed to span the near infrared spectral region where the greenhouse gases, carbon dioxide, methane, and nitrous oxide, absorb. The tunable near IR light will be generated using a specially designed optical parametric oscillator (OPO) and pumped with a 100 Hz Nd:YAG laser. The OPO is based on the Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) design which has demonstrated very high conversion efficiency, high per pulse laser energy, and excellent beam quality. The proposed DIAL system will implement heterodyne detection and takes advantage of acoustic-optical-modulator (AOM) and electro-optic-modulator (EOM) technology to recover multiple points across the absorption lineshape for each backscatter return signal using a single seed laser and a high speed detector (400 MHz InGaAs APD). The layout of the optical system is shown below.