Robert T. Menzies’s research while affiliated with California Institute of Technology and other places

This Special Issue of Remote Sensing continues a long line of related research papers covering the use of optical and laser remote sensing for quantitative measurement and imaging of chemical species and physical parameters of the atmosphere [...]

Improved remote sensing observations of atmospheric carbon dioxide (CO2) are critically needed to quantify, monitor, and understand the Earth’s carbon cycle and its evolution in a changing climate. The processes governing ocean and terrestrial carbon uptake remain poorly understood, especially in dynamic regions with large carbon stocks and strong vulnerability to climate change, for example, the tropical land biosphere, the northern hemisphere high latitudes, and the Southern Ocean. Because the passive spectrometers used by GOSAT (Greenhouse gases Observing SATellite) and OCO-2 (Orbiting Carbon Observatory-2) require sunlit and cloud-free conditions, current observations over these regions remain infrequent and are subject to biases. These shortcomings limit our ability to understand and predict the processes controlling the carbon cycle on regional to global scales. In contrast, active CO2 remote-sensing techniques allow accurate measurements to be taken day and night, over ocean and land surfaces, in the presence of thin or scattered clouds, and at all times of year. Because of these benefits, the National Research Council recommended the National Aeronautics and Space Administration (NASA) Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission in the 2007 report Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. The ability of ASCENDS to collect low-bias observations in these key regions is expected to address important gaps in our knowledge of the contemporary carbon cycle. The ASCENDS ad hoc Science Definition Team (SDT), comprised of carbon cycle modeling and active remote sensing instrument teams throughout the United States (US), worked to develop the mission’s requirements and advance its readiness from 2008 through 2018. Numerous scientific investigations were carried out to identify the benefit and feasibility of active CO2 remote sensing measurements for improving our understanding of CO2 sources and sinks. This report summarizes their findings and recommendations based on mission modeling studies, analysis of ancillary meteorological data products, development and demonstration of candidate technologies, and design studies of the ASCENDS mission concept. The ASCENDS modeling studies have demonstrated that: 1. ASCENDS will resolve statistically significant differences in total column CO2 concentrations, resulting from foreseeable changes in surface flux over the entire globe. These flux changes could include identifying CO2 emissions from permafrost thaw at high latitudes, shifting patterns in regional fossil fuel emissions, the evolving nature of the Southern Ocean carbon flux, and/or changes to tropical and mid-latitude terrestrial sinks. 2. ASCENDS will substantially advance our understanding of the carbon cycle through improved flux estimates with reduced uncertainty at global to regional scales. Reduced flux uncertainties at regional scales are necessary for improved understanding of the processes controlling long-term carbon sinks. 3. ASCENDS measurements also have the potential to reduce biases due primarily to lower susceptibility to errors from atmospheric scattering and those due to changes in illumination geometry. This can contribute significantly towards improving constraints on surface fluxes beyond passive sensors such as GOSAT and OCO-2. During the past decade, NASA has invested in the development of several different Integrated Path Differential Absorption (IPDA) lidar approaches and associated technologies that are candidates for ASCENDS. The IPDA approach measures the range to the scattering surface and the column abundance of CO2 with increased sensitivity across the mid and lower troposphere. Several aircraft field campaigns using space simulator instruments demonstrated that: 1. Accurate CO2 column mixing ratios can be retrieved from airborne lidar data. 2. Evaluation against in situ aircraft observations show that CO2 column absorption measurements can be made with high precision and low bias over a wide range of surface types and between scattered clouds. 3. High-quality observations can be made to cloud tops and through thin clouds and aerosol layers. In addition, evaluation of the magnitude of errors in current meteorological reanalyses helped to clarify the need for ancillary atmospheric measurements and to define the error budget for ASCENDS measurements. Statistical analysis of meteorological products from three different atmospheric modeling centers shows that uncertainty in current surface pressure estimates from models is typically less than 0.1% except in high latitudes regions. These findings were used to eliminate the need for a coincident oxygen measurement to meet the desired CO2 mixing ratio accuracy for ASCENDS. These studies and field activities have greatly improved our understanding of the space-based capabilities required for ASCENDS, and represent significant progress toward meeting the demands of an active remote-sensing mission. Integrating results from the measurement campaigns and modeling studies, the ASCENDS ad hoc SDT developed a set of measurement requirements as well as a study of the ASCENDS mission that demonstrates the mission is feasible. The results of this study show that multiple commercially-available spacecraft buses should be able to accommodate an ASCENDS instrument. In addition, the Falcon 9 or other smaller launch vehicles can accommodate an ASCENDS observatory. An active CO2 mission would provide a unique complement to the expanding international constellation of passive CO2 missions, helping to fill critical gaps in coverage and to reduce biases in key areas through cross calibrations. This report also outlines areas where further research is needed. These include but are not limited to: 1. Modeling studies that examine the role of ASCENDS in a future international constellation of planned passive and active space sensors as well as studies to evaluate the impact of different orbit choices and vertical information on flux inference. 2. Aircraft campaigns targeting observations over high latitudes and the tropics, including those to coincide with CO2 observations from passive satellite instruments on orbit. 3. Improve techniques for the retrieval of vertical profile information. 4. Developing candidate mission approaches to have a lower cost to NASA in order to meet limitations of the Explorer class of missions recommended by the 2017 Decadal Survey. Potential approaches include pursuit of international partnerships or deployment to the International Space Station (ISS). It is hoped that the findings and activities of the ASCENDS ad hoc SDT will continue to advance mission readiness in coordination with the carbon cycle science community.

We describe the data processing and analysis algorithms used for high-precision retrievals of CO ₂ weighted-column mixing ratio and 2-μm surface reflectance from the Carbon Dioxide Laser Absorption Spectrometer (CO2LAS). The CO2LAS at the Jet Propulsion Laboratory, Pasadena, CA, USA, is one of the instruments designed to demonstrate capabilities needed for the NASA Active Sensing over Nights, Days, and Seasons mission concept. The integrated path differential absorption technique is used in CO ₂ retrieval. The along-track spatial resolution for the airborne measurements described here ranges from 10 m to 1 km. Our approach employs heterodyne detection of two laser signals reflected off earth's surface. Frequency domain processing enables return signal peak detection and high-fidelity power estimation. We compensate for range to ground variations using a digital elevation model and emphasize the importance of reflectance weighting in time averaging of the surface elevation. Quality control filters are applied, as well as a statistical methodology to filter laser speckle fluctuations. Reflectance is also retrieved on the scale of a few meters of ground track. Our data processing workflow and example retrievals are presented.

Modeling of a space-based high-energy 2-μm triple-pulse Integrated Path Differential Absorption (IPDA) lidar was conducted to demonstrate carbon dioxide (CO2) measurement capability and to evaluate random and systematic errors. A high pulse energy laser and an advanced MCT e-APD detector were incorporated in this model. Projected performance shows 0.5 ppm precision and 0.3 ppm bias in low-tropospheric column CO2 mixing ratio measurements from space for 10 second signal averaging over Railroad Valley (RRV) reference surface.

Sustained high-quality column carbon dioxide ( CO 2 ) atmospheric measurements from space are required to improve estimates of regional and continental-scale sources and sinks of CO 2 . Modeling of a space-based 2 μm, high pulse energy, triple-pulse, direct detection integrated path differential absorption (IPDA) lidar was conducted to demonstrate CO 2 measurement capability and to evaluate random and systematic errors. Parameters based on recent technology developments in the 2 μm laser and state-of-the-art HgCdTe (MCT) electron-initiated avalanche photodiode (e-APD) detection system were incorporated in this model. Strong absorption features of CO 2 in the 2 μm region, which allows optimum lower tropospheric and near surface measurements, were used to project simultaneous measurements using two independent altitude-dependent weighting functions with the triple-pulse IPDA. Analysis of measurements over a variety of atmospheric and aerosol models using a variety of Earth’s surface target and aerosol loading conditions were conducted. Water vapor ( H 2 O ) influences on CO 2 measurements were assessed, including molecular interference, dry-air estimate, and line broadening. Projected performance shows a < 0.35 ppm precision and a < 0.3 ppm bias in low-tropospheric weighted measurements related to column CO 2 optical depth for the space-based IPDA using 10 s signal averaging over the Railroad Valley (RRV) reference surface under clear and thin cloud conditions.

The JPL airborne Laser Absorption Spectrometer instrument has been flown several times in the 2007-2011 time frame for the purpose of measuring CO2 mixing ratios in the lower atmosphere. The four most recent flight campaigns were on the NASA DC-8 research aircraft, in support of the NASA ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) mission formulation studies. This instrument operates in the 2.05-μm spectral region. The Integrated Path Differential Absorption (IPDA) method is used to retrieve weighted CO2 column mixing ratios. We present key features of the CO2LAS signal processing, data analysis, and the calibration/validation methodology. Results from flights in various U.S. locations during the past three years include observed mid-day CO2 drawdown in the Midwest, also cases of point-source and regional plume detection that enable the calculation of emission rates.

We report airborne measurements of lidar directional reflectance (backscatter) from land surfaces at a wavelength in the 2.05 μm CO 2 absorption band, with emphasis on snow-covered surfaces in various natural environments. Lidar backscatter measurements using this instrument provide insight into the capabilities of lidar for both airborne and future global-scale CO 2 measurements from low Earth orbit pertinent to the NASA Active Sensing of CO 2 Emissions over Nights, Days, and Seasons mission. Lidar measurement capability is particularly useful when the use of solar scattering spectroscopy is not feasible for high-accuracy atmospheric CO 2 measurements. Consequently, performance in high-latitude and winter season environments is an emphasis. Snow-covered surfaces are known to be dark in the CO 2 band spectral regions. The quantitative backscatter data from these field measurements help to elucidate the range of backscatter values that can be expected in natural environments.

The objectives of NASA's ASCENDS mission are to improve the knowledge of global CO sources and sinks by precisely measuring the tropospheric column abundance of atmospheric CO and O. The mission will use a continuously operating nadir-pointed integrated path differential absorption (IPDA) lidar in a polar orbit. The lidar offers a number of important new capabilities and will measure atmospheric CO globally over a wide range of challenging conditions, including at night, at high latitudes, through hazy and thin cloud conditions, and to cloud tops. The laser source enables a measurement of range, so that the absorption path length to the scattering surface will be always accurately known. The lidar approach also measures consistently in a nadir-zenith path and the narrow laser linewidth allows weighting the measurement to the lower troposphere. Using these measurements with atmospheric and flux models will allow improved estimates of CO fluxes and hence better understanding of the processes that exchange CO between the surface and atmosphere. The ASCENDS formulation team has developed a preliminary set of requirements for the lidar measurements. These were developed based on experience gained from the numerous ASCENDS airborne campaigns that have used different candidate lidar measurement techniques. They also take into account the complexity of making precise measurement of atmospheric gas columns when viewing the Earth from space. Some of the complicating factors are the widely varying reflectance and topographic heights of the Earth's land and ocean surfaces, the variety of cloud types, and the degree of cloud and aerosol absorption and scattering in the atmosphere. The requirements address the precision and bias in the measured column mixing ratio, the dynamic range of the expected surface reflected signal, the along-track sampling resolution, measurements made through thin clouds, measurements to forested and slope surfaces, range precision, measurements to cloud tops, knowledge of the laser spot position, and off-nadir pointing. These requirements are independent of the measurement approach, and are consistent with the initial mission simulation studies performed by the formulation team. This presentation will summarize the requirements along with examples that have guided their selection.

A single transverse/longitudinal mode, compact Q-switched Ho:YLF laser has been designed and demonstrated for space-borne lidar applications. The pulse energy is between 34-40 mJ for 100-200 Hz operation. The corresponding peak power is >1 MW.

This paper provides atmospheric CO2 column abundance measurement results from a summer 2011 series of flights of a 2.05-mu m laser absorption spectrometer on the NASA DC-8 research aircraft. The integrated path differential absorption (IPDA) method is used for the CO2 column mole fraction retrievals. This instrument and the data analysis methodology developed to achieve retrievals over complex terrain and variable atmospheric conditions provide insight into the capabilities of the IPDA method for both airborne measurements and future global-scale CO2 measurements from low-Earth orbit pertinent to the proposed NASA Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission. Demonstrated in this paper is the capability to measure CO2 drawdown caused by crop activity during a midday flight over the U.S. upper Midwest area. In addition, an example is provided of high spatial resolution measurements of CO2 plumes from individual stack clusters of the Four Corners Power Plant in northwestern New Mexico. Complex terrain, the spectral properties of the aboveground scatterers, and potential cloud contamination are factors that complicate the column abundance retrieval. The impacts of these factors and various means of minimizing these influences in the retrievals are discussed.