Jeremy Dobler

Atmospheric CO 2 column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar

The 2007 National Research Council (NRC) Decadal Survey on Earth Science and Applications from Space recommended Active Sensing of CO(2) Emissions over Nights, Days, and Seasons (ASCENDS) as a midterm, Tier II, NASA space mission. ITT Exelis, formerly ITT Corp., and NASA Langley Research Center have been working together since 2004 to develop and demonstrate a prototype laser absorption spectrometer for making high-precision, column CO(2) mixing ratio measurements needed for the ASCENDS mission. This instrument, called the multifunctional fiber laser lidar (MFLL), operates in an intensity-modulated, continuous wave mode in the 1.57 μm CO(2) absorption band. Flight experiments have been conducted with the MFLL on a Lear-25, UC-12, and DC-8 aircraft over a variety of different surfaces and under a wide range of atmospheric conditions. Very high-precision CO(2) column measurements resulting from high signal-to-noise ratio (>1300) column optical depth (OD) measurements for a 10 s (~1 km) averaging interval have been achieved. In situ measurements of atmospheric CO(2) profiles were used to derive the expected CO(2) column values, and when compared to the MFLL measurements over desert and vegetated surfaces, the MFLL measurements were found to agree with the in situ-derived CO(2) columns to within an average of 0.17% or ~0.65 ppmv with a standard deviation of 0.44% or ~1.7 ppmv. Initial results demonstrating ranging capability using a swept modulation technique are also presented.

Atmospheric CO(2) column measurements in cloudy conditions using intensity-modulated continuous-wave lidar at 1.57 micron.

This study evaluates the capability of atmospheric CO2 column measurements under cloudy conditions using an airborne intensity-modulated continuous-wave integrated-path-differential-absorption lidar operating in the 1.57-μm CO2 absorption band. The atmospheric CO2 column amounts from the aircraft to the tops of optically thick cumulus clouds and to the surface in the presence of optically thin clouds are retrieved from lidar data obtained during the summer 2011 and spring 2013 flight campaigns, respectively. For the case of intervening thin cirrus clouds with an average cloud optical depth of about 0.16 over an arid/semi-arid area, the CO2 column measurements from 12.2 km altitude were found to be consistent with the cloud free conditions with a lower precision due to the additional optical attenuation of the thin clouds. The clear sky precision for this flight campaign case was about 0.72% for a 0.1-s integration, which was close to previously reported flight campaign results. For a vegetated area and lidar path lengths of 8 to 12 km, the precision of the measured differential absorption optical depths to the surface was 1.3 - 2.2% for 0.1-s integration. The precision of the CO2 column measurements to thick clouds with reflectance about 1/10 of that of the surface was about a factor of 2 to 3 lower than that to the surface owing to weaker lidar returns from clouds and a smaller CO2 differential absorption optical depth compared to that for the entire column.

A New Laser Based Approach for Measuring Atmospheric Greenhouse Gases

In 2012, we developed a proof-of-concept system for a new open-path laser absorption spectrometer concept for measuring atmospheric CO2. The measurement approach utilizes high-reliability all-fiber-based, continuous-wave laser technology, along with a unique all-digital lock-in amplifier method that, together, enables simultaneous transmission and reception of multiple fixed wavelengths of light. This new technique, which utilizes very little transmitted energy relative to conventional lidar systems, provides high signal-to-noise (SNR) measurements, even in the presence of a large background signal. This proof-of-concept system, tested in both a laboratory environment and a limited number of field experiments over path lengths of 680 m and 1,600 m, demonstrated SNR values >1,000 for received signals of ~18 picoWatts averaged over 60 s. A SNR of 1,000 is equivalent to a measurement precision of ±0.001 or ~0.4 ppmv. The measurement method is expected to provide new capability for automated monitoring of greenhouse gas at fixed sites, such as carbon sequestration facilities, volcanoes, the short- and long-term assessment of urban plumes, and other similar applications. In addition, this concept enables active measurements of column amounts from a geosynchronous orbit for a network of ground-based receivers/stations that would complement other current and planned space-based measurement capabilities.

Applications of fiber lasers for remote sensing of atmospheric greenhouse gases

In 2004 ITT Exelis developed the Multifunctional Fiber Laser Lidar (MFLL) for measuring atmospheric CO2. This lidar relies on high efficiency telecom laser components and Erbium Doped Fiber Amplifiers (EDFA’s) to implement a unique Continuous Wave (CW) Intensity Modulated (IM) differential absorption lidar measurement. This same approach has also been used to measure atmospheric O2 by replacing the EDFA’s with fiber Raman amplifier technology. The use of all fiber coupled components results in a highly reliable, flexible and robust instrument. The general architecture of the MFLL, its implementation for greenhouse gas measurements, and as a pseudorandom noise encoded altimeter system is reviewed. Results from a 2011 flight campaign on the NASA DC-8 aircraft which included CO2, O2, and PN altimetry using a single receiver for all three measurements are also discussed. In addition, an introduction to a novel variation of this approach that will enable greenhouse gas monitoring from a geostationary orbit is given. This paper provides a general overview of a set of applications for fiber lasers in the area of active remote sensing that have been developed by Exelis over the past several years.

Advancements towards active remote sensing of CO2 from space using intensity-modulated, continuous-Wave (IM-CW) lidar

The Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) CarbonHawk Experiment Simulator (ACES) is a NASA Langley Research Center instrument funded by NASA’s Science Mission Directorate that seeks to advance technologies critical to measuring atmospheric column carbon dioxide (CO2) mixing ratios in support of the NASA ASCENDS mission. The ACES instrument, an Intensity-Modulated Continuous-Wave (IM-CW) lidar, was designed for high-altitude aircraft operations and can be directly applied to space instrumentation to meet the ASCENDS mission requirements. Airborne flight campaigns have been used to demonstrate ACES’ advanced technologies critical for a spaceborne instrument with lower platform consumption of size, mass, and power, and with improved performance. ACES recently flew on the NASA DC-8 aircraft during the 2017 NASA ASCENDS/Arctic-Boreal Vulnerability Experiment (ABoVE) airborne measurement campaign to test ASCENDS-related technologies in the challenging Arctic environment. Data were collected over a wide variety of surface reflectivities, terrain, and atmospheric conditions during the campaign’s eight research flights. ACES also flew during the 2017 and 2018 Atmospheric Carbon and Transport – America (ACT-America) Earth Venture Suborbital - 2 (EVS-2) campaigns along with the primary ACT-America CO2 lidar, Harris Corporation’s Multi-Frequency Fiber Laser Lidar (MFLL). Regional CO2 distributions of the lower atmosphere were observed from the C-130 aircraft during the ACT-America campaigns in support of ACT-America’s science objectives. The airborne lidars provide unique remote data that complement data from more traditional in situ sensors. This presentation shows the applications of CO2 lidars in meeting these science needs from airborne platforms and an eventual spacecraft.

GreenLITE: a new laser-based tool for near-real-time monitoring and mapping of CO2 and CH4 concentrations on scales from 0.04-25 km2

In 2013, Harris and Atmospheric and Environmental Research developed the greenhouse gas laser imaging tomography experiment (GreenLITE™) under a cooperative agreement with the National Energy Technology Laboratory of the Department of Energy. The system uses a pair of high-precision, intensity-modulated, continuous-wave (IMCW) transceivers and a series of retroreflectors to generate overlapping atmospheric density measurements from absorption of a particular greenhouse gas (e.g. CO2 or CH4), to provide an estimate of the two-dimensional spatial distribution of the gas within the area of interest. The system can take measurements over areas ranging from approximately 0.04 square kilometers (km2) to 25 km2 (~200 meters (m) × 200 m, up to ~5 km × 5 km). Multiple GreenLITE™ CO2 demonstrations have been carried out to date, including a full year, November 04, 2015 through November 14, 2016, deployment over a 25 km2 area of downtown Paris, France. In late 2016, the GreenLITE™ system was converted to provide similar measurements for CH4. Recent experiments showed that GreenLITE™ CH4 concentration readings correlated with an insitu instrument, calibrated with World Meteorological Organization traceable gas purchased from the NOAA Earth Systems Research Laboratory, to within approximately 0.5% of CH4 background or ~10-15 parts per billion. Several experiments are planned in 2017 to further evaluate the accuracy of the CH4 and CO2 retrieved concentration values compared to the calibrated in situ instrument and to demonstrate the feasibility of GreenLITE™ for environmental and safety monitoring of CO2 and CH4 in industrial applications.

Impact of ambient O2(a1Δg) on satellite‐based laser remote sensing of O2 columns using absorption lines in the 1.27 µm region

Determination of CO2 mixing ratio columns from space using Laser Absorption Spectroscopy (LAS) requires simultaneous measurements of CO2 number density columns and knowledge of the dry atmospheric surface pressure. One approach to determining the surface pressure is to make an LAS column measurement of O2 number density in the 7857.3–7921.7 cm−1 (1.27 µm) region of the O2(1Δ) state. A complicating factor in the LAS O2 measurement is the presence of a permanent but spatially variable natural source of airglow from the O2(1Δ) state. In addition, the laser radiation can induce stimulated emission from the ambient O2(1Δ) state and also cause stimulated absorption and emission from the ground state O2 molecules as the laser beam passes through the atmosphere. Finally, the upwelling surface-reflected solar radiation is an additional source of background radiation. The effects of these additional radiation sources on the LAS measurement of O2 are examined. The surface-reflected solar radiation produces the largest background at 3 orders of magnitude more intense than the laser backscatter signal, while the airglow is of the same order of magnitude as the laser backscatter. The stimulated emission from ambient O2(a1Δg) is found to be about the same order of magnitude as the laser radiation. These effects are evaluated under noon, twilight, and midnight conditions at midlatitudes, the equator, and the pole. The stimulated emission is in the same direction and in phase with the laser signal, its contamination of the LAS O2 measurement prevents a full sunlight determination of surface pressure.

Raman Amplification of a Narrow Linewidth Continuous Wave Signal for Spectroscopic Remote Sensing Applications Using Longitudinally Varying Core Fibers

We report on the development of a Raman amplifier using fibers with longitudinally varying cores. Approximately 3.0W of a 1.26?m narrow linewidth continuous wave signal has been demonstrated for remote sensing of atmospheric oxygen levels.