Intensity Modulated Continuous Wave Laser Absorption Spectroscopy for Addressing Currently Unmet Carbon Monitoring Needs

Exelis has been developing a technique for monitoring CO₂ and CH₄ using high reliability telecom components and an Intensity Modulated Continuous Wave (IM-CW) implementation over the past decade. This measurement approach has been deployed in an airborne demonstration unit for evaluation by NASA Langley Research Center, and has demonstrated the ability to measure CO₂ concentrations to 0.67 ppm and standard deviations of <1.7 ppm. This initial development has recently been the subject of new development and activities related to applications for this technology in areas of interest with unmet measurement needs for CO₂ and CH₄, such as volcanos, carbon sequestration sites, and urban areas. We will present a detailed description of the IM-CW approach, along with a review of recent instrument development activities for a ground based carbon sequestration monitoring system and the evaluation of that system at the Zero Emissions Research and Technology site in Bozeman, Montana. We will also discuss the extension of the concept to a geostationary orbit-to-ground measurement, and the adaption of the current work, focused on CO₂, to CH₄. The laser-based technique, developed by Exelis, uses an intensity modulated continuous wave (IM-CW) approach, which allows multiple narrow-linewidth, fixed-wavelength, laser beams to be transmitted and received simultaneously. The approach uses a digital lock-in, or matched filter, method to provide a high precision measurement even in the presence of a large background signal. This method also allows for simultaneous transmission and reception of multiple narrow-linewidth lasers to probe specific atmospheric gas absorption features. The approach has been validated by NASA Langley Research Center for high accuracy and high precision measurements of CO₂ from an airborne platform, as compared to a high accuracy onboard in situ instrument, traceable to World Meteorological Organization (WMO) standards, since 2004. The carbon sequestration application instrument, Greenhouse gas Laser Imaging Tomography Experiment (GreenLITE), uses two laser-based transceivers and a series of retro-reflectors to measure the differential transmission over a number of overlapping chords, forming a plane above the surface. A cloud-based software system, developed by our teammates at AER, utilizes the measured differential transmissions of the overlapping chords, along with auxiliary data (T, P, RH, wind speed and direction) to retrieve the average dry air mixing ratio along each chord and to combine the multiple chords to derive and estimate of the 2D spatial distribution of the gas being monitored, and displays the retrieved data via a real-time web based service. GreenLITE is being developed under a cooperative agreement between Exelis and the National Energy and Technology Laboratory (NETL) under the Department of Energy (DOE), contract # DE-FE0012574. Atmospheric and Environmental Research, Inc. is a major partner in this development A geostationary orbit-to-ground measurement concept has also been developed, which is called the Laser Absorption Transmitter and Receiver-Network (LAnTeRN). This concept utilizes the increased energy that can be captured in a one way bistatic measurement approach, relative to the typical backscattered radiation used for both passive and active measurements from aircraft and space. The increased signal to noise and the continued measurement along the same path enables long term averaging and temporal resolution not available from other current or planned measurement approaches. The fixed path also enables detailed validation of the measurement using other ground and airborne methods (e.g. NASA's TCCON, NOAA's Aircore, etc.) with the potential to determine bias to the levels required for separating anthropogenic and biogenic sources and sinks, that is very difficult to achieve with current airborne and LEO based space measurements.