We have developed a low-cost, miniaturized laser heterodyne radiometer for highly sensitive measurements of carbon dioxide (CO 2) in the atmospheric column. In this passive design, sunlight that has undergone absorption by CO 2 in the atmosphere is collected and mixed with continuous wave laser light that is step-scanned across the absorption feature centered at 1,573.6 nm. The resulting radio frequency beat signal is collected as a function of laser wavelength, from which the total column mole fraction can be de-convolved. We are expanding this technique to include methane (CH 4) and carbon monoxide (CO), and with minor modifications, this technique can be expanded to include species such as water vapor (H 2 O) and nitrous oxide (N 2 O).

We present column CO2 measurements taken by the passive miniaturized laser heterodyne radiometer (Mini-LHR) at 1611.51 nm at the Mauna Loa Observatory in Hawaii. The Mini-LHR was operated autonomously, during the month of May 2013 at this site, working in tandem with an AERONET sun photometer that measures aerosol optical depth at 15-min intervals during daylight hours. Laser heterodyne radiometry has been used since the 1970s to measure atmospheric gases such as ozone, water vapor, methane, ammonia, chlorine monoxide, and nitrous oxide. This iteration of the technology utilizes distributed feedback lasers to produce a low-cost, small, portable sensor that has potential for global deployment. Applications of this instrument include supplementation of existing monitoring networks to provide denser global coverage, providing validation for larger satellite missions, and targeting regions of carbon flux uncertainty. Also presented here are preliminary retrieval analysis and the performance analysis that demonstrate that the Mini-LHR responds extremely well to changes in the atmospheric absorption.

Mounting concern regarding global warming and the increasing carbon dioxide (CO 2 ) concentration has stimulated interest in the feasibility of measuring CO 2 mixing ratios from space. Precise satellite observations with adequate spatial and temporal resolution would substantially increase our knowledge of the atmospheric CO 2 distribution and allow improved modeling of the CO 2 cycle. Current estimates indicate that a measurement precision of better than 1 part per million (1 ppm) will be needed in order to improve estimates of carbon uptake by land and ocean reservoirs. A 1-ppm CO 2 measurement corresponds to approximately 1 in 380 or 0.26% long-term measurement precision. This requirement imposes stringent long-term precision (stability) requirements on the instrument In this paper we discuss methods and techniques to achieve the 1-ppm precision for a space-borne lidar.

Presented here is a sensitivity analysis for the miniaturized laser heterodyne radiometer. This passive, ground-based instrument measures carbon dioxide (CO2) in the atmospheric column and has been under development at NASA/GSFC since 2009. The goal of this development is to produce a low-cost, easily-deployable instrument that can extend current ground measurement networks in order to (1) validate column satellite observations, (2) provide coverage in regions of limited satellite observations, (3) target regions of interest such as thawing permafrost, and (4) support the continuity of a long-term climate record. In this paper an uncertainty analysis of the instrument performance is presented and compared with results from three sets of field measurements. The signal-to-noise ratio (SNR) and corresponding maximum uncertainty for a single scan are calculated to be 329.4 ± 1.3 by deploying error propagation through the equation governing the SNR. Reported is an absorbance noise of 0.0024 for six averaged scans of field data, for an instrument precision of 0.14 ppmv for CO2.

We report on progress of our C02 laser sounder laboratory breadboard system the goal of which is to measure the integrated column abundance of C02 to better than 1 ppm from low Earth orbit globally, measuring at all latitudes and seasons through day and night. The challenge for an orbiting C02 instrument is to achieve high precision not high sensitivity. We have made simple yet significant improvements to our active, optical-sensing laser-sounder instrument and real-time data processing that now enables absolute absorption measurements to better than ± 0.050O for over 10 hours before re-calibration (equivalent to a lppm precision from orbit). Data from an eight day, 0.8 Km open path comparison test with a LICOR shows excellent agreement.12

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A passive Fabry-Perot based instrument is described for detecting column C02 through absorption measurements at 1 . 5 8~. In this design, solar flux reaches the instrument platform and is directed through two channels. In the first channel, transmittance fi5nges from a Fabry-Perot interferometer are aligned with C02 absorption lines so that absorption due to C02 is primarily detected. The second channel encompasses the same frequency region as the first, but is comparatively more sensitive to changes in the solar flux than absorption due to C02. The ratio of these channels is sensitive to changes in the total C02 column, but not to changes in solar flux. This inexpensive instrument will offer high precision measurements (error 4%) in a compact package. Design of this instrument and preliminary ground-based measurements of column C02 are presented here as well as strategies for deployment on aircraft and satellite platforms. 2

We report progress in assessing the feasibility of a new satellite-based laser-sounding instrument to measure CO2 concentrations in the lower troposphere from space.